| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
What's this? |
Reproductive Biology |
Department of Plant Biology, 2502 Plant Sciences Building, University of Georgia, Athens, Georgia 30602 USA
Received for publication August 19, 2005. Accepted for publication November 18, 2005.
ABSTRACT
One of the major evolutionary trends in flowering plants is the evolution of unisexual flowers (male or female) from perfect flowers. This transition has occurred repeatedly in many taxa and has generated a wonderful array of variation in sexual expression among species. Theoretical studies have proposed a number of mechanisms to explain how this level of variation could be maintained in natural systems. One possible mechanism is the female compensation hypothesis, which predicts that female mutants require an increase in their seed fitness in order to invade a hermaphroditic system. Using Geranium maculatum, I tested this hypothesis and showed that female mothers produced more and larger seeds than hermaphroditic mothers even though they were indistinguishable in their vegetative traits and the flower production. Seeds from females were also more likely to germinate and produced seedlings with larger above- and belowground biomass. These seedlings were more likely to flower than those from hermaphrodites in at least one of the two populations studied. Combined, these results indicated that females in G. maculatum did compensate for their loss of male function by producing more and better seeds than hermaphrodites. This provides a mechanism for the maintenance of female plants in this species.
Key Words: female compensation • fitness • Georgia • Geraniaceae • gynodioecy • male sterility • progeny quality • spring ephemeral
One of the major evolutionary transitions observed in flowering plants is the evolution of unisexual flowers (male or female) from perfect flowers. This transition has occurred more than 100 times in many different taxa (Charlesworth, 2002
) and has generated an incredible amount of variation in both the sexual expression and morphology of flowers (Richards, 1997
). The variation observed in sexual systems of flowering plants has fascinated biologists since Darwin (1877)
in part because of the influence these systems have on the mating patterns of a species. In addition, by affecting mating patterns, the sexual systems of a species can also influence the likelihood of local adaptation, the prevalence of genetic drift and, ultimately, the propensity of a population to evolve. Today, understanding how variation has been generated and maintained in sexual systems remains a central question in plant evolutionary biology (Barrett, 2002
).
One sexual system that has proven to be an ideal subject for examining the impact of sexual system on a population is gynodioecy. In gynodioecious systems, a population may contain both hermaphroditic individuals that produce perfect flowers as well as females that produce flowers lacking pollen. Females in gynodioecious species usually arise as the result of a mutation, present either in the nuclear or cytoplasmic genomes, that cause male sterility (Hanson, 1991
; Saumitou-Laprade et al., 1994
; Schnable and Wise, 1998
; Webb, 1999
; Budar et al., 2003
).
Gynodioecy has received a great deal of attention from evolutionary biologists in part because of the striking difference in pollen production seen in hermaphrodites (pollen-producing) and females (pollen-lacking) within a population. Significant variation in traits that have a strong effect on fitness, such as pollen production, is unlikely to be maintained in a population for long periods because alleles that convey lower fitness would be quickly eliminated by natural selection (Futuyma, 1998
). In the case of gynodioecy, females can only reproduce through ovules, and their average potential for sexual reproduction is half that of their hermaphroditic counterparts, which reproduce through both ovules and pollen. Females, therefore, would not be expected to persist in a population containing hermaphrodites, and gynodioecious populations should occur infrequently. However, far from being rare, gynodioecy is seen in almost 7% of all angiosperms (Richards, 1997
). This observation suggests that females in gynodioecious populations possess reproductive advantages that enable them to persist longer than would be expected.
Theoretical studies suggest that one critical requirement for the establishment of gynodioecy is that females need to somehow compensate for their loss of pollen production (Lewis, 1941
; Lloyd, 1975
, 1976
; Charlesworth and Charlesworth, 1978
, 1979
; Charlesworth, 1980
, 1981
, 1992
; Schultz, 1994
; de Hann et al., 1997
). One possible mechanism for such compensation would be through increasing seed fitness, often referred to as female compensation (Darwin, 1877
). The amount of female compensation can determine whether female plants can successfully invade a hermaphroditic population (Lewis, 1941
; Lloyd, 1975
; Charlesworth and Charlesworth, 1978
; Charlesworth, 1981
). In addition, the magnitude of female compensation could also influence the frequency that females could reach in a population (Lloyd, 1974
; Jacobs and Wade, 2003
). Therefore, characterizing whether and how female compensation occurs is important in understanding the maintenance of females in a gynodioecious system.
Previous studies have largely attempted to estimate female compensation by examining the number of seeds produced by females and hermaphrodites grown under natural conditions. Results from these studies suggest that females generally produce more seeds than hermaphrodites (e.g., Gouyon and Couvet, 1987
; Kohn, 1989
; Olson, 2001
; Ramsey and Vaughton, 2002
; Schultz, 2003
, but see exceptions in del Castillo, 1993
; Dinnetz and Jerling, 1997
; Medrano et al., 2005
), but the magnitude of this difference depends on many biotic and abiotic factors (e.g., Wolfe and Shmida, 1997
; McCauley and Brock, 1998
; Delph and Carroll, 2001
; Marshall and Ganders, 2001
; Ramula and Mutikainen, 2003
; Ashman et al., 2004
; Asikainen and Mutikainen, 2005a
, b; Rosas et al., 2005
). The number of seeds produced by a plant, however, is only a partial measurement of fitness. Other factors, such as the quality of the seeds produced and their ability to germinate and produce healthy seedlings, can also have a significant impact on an individual's reproductive success.
A few studies have examined differences in progeny quality between females and hermaphrodites in gynodioecious species (e.g., Eckhart, 1992
; del Castillo, 1993
; Jordano, 1993
; Puterbaugh et al., 1997
; Sakai et al., 1997
; Delph et al., 1999
; Alonso and Herrera, 2001
; Weller and Sakai, 2005
). These studies have largely used the mass of the seeds produced as a surrogate measurement to estimate progeny quality, and the results are in conflict. Puterbaugh et al. (1997)
found a significant difference in the mass of seeds produced by females and hermaphrodites. However, no differences were found in other studies (e.g., Kohn, 1988
; Eckhart, 1992
; Sakai et al., 1997
; Delph et al., 1999
; Alonso and Herrera, 2001
; Ramula and Mutikainen, 2003
; Weller and Sakai, 2005
). Interpretation of results from such studies is further complicated by the fact that seed mass does not always correlate with early seedling performance in gynodioecious plants. For example, hermaphrodites and females produce seeds of similar mass in Sidalcea oregana (Ashman, 1992
), Silene acaulis (Delph et al., 1999
; Delph and Carroll, 2001
), and Schiedea salicaria (Weller and Sakai, 2005
), but germination was higher for seeds produced by females. These findings suggest that seed mass may not accurately reflect progeny quality. A small number of studies had examined progeny quality directly, but measurements were limited to very early life stages such as germination and early plant size (Eckhart, 1992
; Puterbaugh et al., 1997
; Wolfe and Shmida, 1997
; Alonso and Herrera, 2001
; Delph and Mutikainen, 2003
; Weller and Sakai, 2005
). Thus far, there are only a handful of studies that examined progeny fitness beyond an early stage of seedling growth (Ashman, 1992
; Avila-Sakar and Dominguez, 2000
; Ramsey and Vaughton, 2002
; Delph and Mutikainen, 2003
). Therefore, the extent to which overall progeny performance may contribute to female compensation remains an area that requires more empirical data.
The goal of the present study was to obtain a more thorough estimate of female compensation over multiple life history stages in the wild geranium, Geranium maculatum. This project builds on a previous study by Ågren and Willson (1991)
who investigated differences between females and hermaphrodites in G. maculatum using a natural population in Illinois, USA. Ågren and Willson found that females produced more seeds than hermaphrodites, but they did not detect any difference in the size of seeds produced nor did they examine subsequent seedling performance. In this study, I measured the number and size of seeds produced as well as progeny performance in two populations of G. maculatum over two seasons. A second objective of the current study was to examine the potential impact of population sex ratios on female compensation by comparing results from this study to Ågren and Willson's study (1991). The frequency of females in a population can potentially influence seed production in females (pollen availability) and therefore overall female fitness (McCauley and Brock, 1998
; Graff, 1999
; Ashman and Diefenderfer, 2001
; Ramula and Mutikainen, 2003
; Rosas et al., 2005
). The population studied by Ågren and Willson had a low frequency of females (9.4%), while I examined female compensation in populations with much higher female frequencies (35% and 50%). If pollen availability is one of the factors contributing to the reproductive success in female plants, the female compensation observed in this study should be lower than that was observed by Ågren and Willson (1991)
.
The specific questions addressed in this study are (1) Do females in mixed populations in Geranium maculatum compensate for their loss of male function by producing more and/or heavier seeds than hermaphrodites? (2) Do progeny from female plants perform better in seedling and adult (reproductive) stages than progeny from hermaphrodites? (3) Are the levels of female compensation lower in populations with higher frequencies of female plants? The first question was addressed using seeds collected from two field populations over 2 years, the second by growing the field-collected seeds in a greenhouse for two seasons, and the third by comparing results from this study to the earlier estimate by Ågren and Willson (1991)
.
MATERIALS AND METHODS
Study organism
Geranium maculatum is a common spring ephemeral species that is often found in alluvial woods and coves. It is widely distributed in eastern North America; north to Newfoundland and Manitoba in Canada, south to Georgia, and west to Kansas and the Dakotas in the U.S.A. (Radford et al., 1968
). Geranium maculatum is a perennial species with a thin rhizome ca. 1 cm in cross-section diameter. In my northern Georgia (GA) study sites, ramets emerge in late February and flowers appear within the following month. Individual plants may flower for 2–3 weeks, but the flowering season of the population usually lasts until early June with the peak in mid April. Each ramet is an annual structure that dies back in August and individuals overwinter as dormant rhizomes.
Flowers of G. maculatum are pink to lavender with diameters ranging from 1.5 to 3.0 cm. The most commonly observed insect visitors in GA include bumblebees, honeybees, halictid bees, and butterflies. Flowers are protandrous, where anthers dehisce soon after the petals open, but the five stigmatic lobes remain tightly pressed together and are nonreceptive until 1–2 days later. In natural populations, the nearly non-overlapping sexual phases within a flower make autogamous selfing a rare event (S.-M. Chang, unpublished data). However, plants often have more than one open flower at a given time, allowing for the possibility of selfing between flowers of the same plant (geitonogamy).
Among natural populations surveyed in northern GA, half (7/16) of them contained some female plants. The sex ratios for mixed populations ranged from 0.1 to 50% females (this study and Ågren and Willson, 1991
). In this study, I focused on two populations, MP and OT, in Clarke County, GA, that have about 1800 and 1200 reproductive plants and about 35 and 50% of female plants, respectively.
Adult morphological traits
To determine if adult female and hermaphroditic plants in natural populations differed from each other in their morphological traits, plants were measured in their natural environment. Two 1-m wide transects through the main portion of the two study populations were examined in April 2003, and the following traits of all reproductive plants within the transects were measured: (1) three vegetative traits (number of rosette leaves, length of the largest rosette leaf, and length of the largest cauline leaf) and (2) two reproductive traits (height of the inflorescence and size of one petal randomly selected from one open flower).
Gender-specific success in seed production
Progeny quantity
To compare reproductive success through seed production between females and hermaphrodites, the number and the size of seeds they produced in the two study populations were measured for 2 years. In 2003, 50 (25 female [F] + 25 hermaphrodite [H]) plants were originally sampled in both populations, but the sample size was reduced to 28 (10F + 18H) and 31 (15F + 16H) for MP and OT, respectively, due to deer browsing. In 2004, a deer fence was assembled to prevent deer damage and a total of 289 (112F + 177H) and 117 (59F + 58H) plants were sampled from MP and OT. In both years, I counted the total number of flower buds on the determinant inflorescence for each plant sampled. In late May after all target plants had finished flowering and fruits had started to develop, entire inflorescences of the sampled plants were bagged using fine-mesh bridal veil. This procedure was necessary to collect mature seeds because fruits of G. maculatum have an automatic seed dispersal mechanism that forcefully throws seeds to as far as 3 m from the maternal plants (Stamp and Lucas, 1983
). Plants were examined every week in May and June to record the number of fruits produced and to collect the mature seeds. Fruits were collected and stored in paper envelopes until they were processed later in the laboratory. The total number of seeds produced by each plant was counted to compare the quantity of seeds produced by females and hermaphrodites.
Progeny quality
Seed mass—To make up for seeds lost to deer browsing, additional seeds from nontarget plants were collected in 2003. Because not all seeds were collected from these plants, they were used to study the quality but not the quantity of the progeny. In total, 792 seeds from 117 (42F + 75H) and 86 (35F + 51H) maternal plants from MP and OT were collected and weighed in 2003. In 2004, the sample size was increased to 2909 seeds from 184 (97F + 87H) and 134 (71F + 63H) individuals from MP and OT, respectively. Seeds collected in both years were weighed individually using a digital balance with the precision of 0.01 mg.
Seedling performance—To evaluate the quality of progeny beyond seed mass, I planted most of the seeds collected in 2003 and followed them throughout their first two growing seasons. A total of 436 seeds from MP (41F + 75H maternal plants) and 305 seeds from OT (35F + 51H maternal plants) were planted. Prior to planting, seeds were first scarified by nicking the seed coat with a sharp razorblade and then soaking them in water for 14 days in a cold room (5°C). Geranium maculatum seeds are notoriously difficult to germinate, and an earlier study by Martin (1965)
showed that germination occurred slowly and over a long period of time when planted without scarification. To facilitate the uniformity of germination, I removed the seed coat carefully without damaging the cotyledons and embryos inside after the seed coat was softened by soaking in water for 3 days. This method has been shown in a pilot study to be effective in breaking the physical dormancy of the seeds.
Two weeks later, seeds were removed from the cold room and planted individually into 2-inch pots containing Faford 3B mix (Conrad Faford, Awawam, Massachusetts, USA). For the following 7 days, these pots were kept on a greenhouse bench with misting spray every 10 min and indirect sunlight before being moved into a regular greenhouse bench. Limited by the space available and the low number of seeds in many families, seeds from the two populations were planted in separate temporal blocks to maximize the power of detecting sex differences within a population. This design, unfortunately, sacrificed the ability to detect any difference between populations.
Seedling size was measured as the total leaf area produced at 1 mo (early size) and 4 mo (late size) after they were moved into the greenhouse. I used leaf width as a simple and nondestructive way to measure leaf area. A very strong relationship was found between leaf areas and leaf widths (area = 1.03[width] + 0.32[width]2; R2 = 0.99) using 100 leaves from nonexperimental plants at the same developmental stages. This equation was used to convert the measurements of leaf width into leaf areas for all of the study plants.
After 4 mo of growth, a length of time similar to the growing period in the field for the first growing season, ramets had distinct signs of senescence: the leaves turned brown or red. The amount of water given to the plants was then gradually reduced over the course of 2 wk, and all the wilted plant material above ground was removed at the end of the drying period.
To obtain the belowground biomass, soil was carefully washed away from the roots and the rhizomes. All experimental plants were processed on the same day, and were air dried on a lab bench for 24 h before being weighed. Because these plants would be studied through the next growing season, the rhizomes' wet mass was measured, instead of killing plants to measure dry mass. After weighing, the rhizomes were wrapped in moist sphagnum mosses and kept in unsealed plastic bags for 1 mo in a 5°C cold room before they were planted back into soil at room temperature (approximately 25°C). This method has been previously tested and shown to be effective to stimulate G. maculatum rhizomes to continue regular growth in the greenhouse.
Most plants resumed growth within 1 wk after the rhizomes were brought back to the ambient temperature in the greenhouse. Two months after new ramets emerged, I again measured the total leaf area using the method described. Some inflorescences also started to grow within 2 wk after emergence, but the first flower did not appear until 25 d after the emergence. Plants were examined everyday, and the flowers on each plant were counted until all flowering had ceased.
Data analyses
Adult morphological traits
Analyses of variance (ANOVA) were carried out (PROC GLM in SAS [SAS, 1999
]) to analyze the maternal sex and population differences in the morphological and reproductive traits measured in the field. Gender, population, and their interactions were all treated as fixed variables in the analyses.
Progeny quantity
An ANOVA was carried out (PROC GLM in SAS) to evaluate the effects of gender, population, year, and their interactions on the total seed number produced. To further determine at what stage the difference in seed number might have arisen, individual ANOVA was carried out for each of the three components of total seed production: flower number, fruit set (number of fruits/number of flowers) and seed set (total number of seeds/number of fruits), using the same set of predicting variables as in the ANOVA described previously. Least square means were plotted for each of the four traits (total seed production and the three components) to illustrate the differences between the main effects.
Progeny quality
Seed mass—An ANOVA (PROC MIXED in SAS) was carried out to evaluate the effects of gender, population, year, and their interactions on individual seed mass. Because multiple seeds were collected from many maternal plants, maternal identity (ID) was also included as a random variable in this analysis to remove maternal effects unrelated to their gender. Maternal ID was nested under the interaction of year, population, and maternal gender. In addition to seed mass, progeny quality was also evaluated by their performance in the life history stages at or after seed germination. Logistic regressions (PROC CATMOD in SAS) were used to analyze the effect of gender on the probability of germination among seeds planted. Germination was treated as a categorical variable (0 for an ungerminated and 1 for a germinated seed). Because of the planting design (described earlier), populations were analyzed separately and, therefore, the predicting variables for each analysis only included the maternal gender and the initial seed mass. Maternal plant was not included in the analysis because of the small number of progeny in some maternal families.
Seedling performance—For the progeny traits measured in the first growing season, a preliminary MANOVA was first carried out to analyze the overall effect of maternal gender and maternal individual on the early seedling performance traits, including early and late seedling sizes in season one and the end-of-first-season belowground biomass. Maternal identity (ID) was included as a random variable. To further interpret any effects detected in the MANOVA, individual characters were subsequently analyzed using univariate ANOVAs (PROC MIXED in SAS) to identify the specific trait(s) that could account for the differences between the two maternal genders. Residuals for these traits were examined, and the variables were transformed when necessary to satisfy the assumption of normality. Significance tests were done using transformed data, but the means were reported using the original data.
The progeny quality in the second season was evaluated by four performance traits: reemergence, aboveground plant size, probability to flower, and flower production. The two continuous traits, plant size and the flower production, were analyzed using two separate ANOVAs. All the plants that reemerged after stratification were included in the plant size analysis, but only the ones that flowered were included in the flower production analysis. Reemergence and flowering probabilities were analyzed similar to germination described because of their categorical nature. PROC CATMOD in SAS was used for these logistic regression analyses with maternal gender as the predicting variable.
Female compensation
Female compensation (FC) was defined as the increase in female fitness when compared to hermaphrodites. With this definition, FC was calculated using the following equation: FCi = (WFi/WHi) – 1, where WFi and WHi were the average fitness measurements of female and hermaphroditic plants focusing on a particular fitness component i, such as seed number or survival probability. Because female compensation was most often reported for seed number, one FC was calculated for seed number in each of the two populations studied. To summarize the fitness difference over all the life stages examined in this study, an accumulative female compensation was calculated by multiplying the relative fitness (WFi/WHi) of four fitness components—seed number, germination probability, survival probability from first to second season, and reproductive probability in the second season-together and then subtract one from this value (i.e., accumulative FC = 
(WFi / WHi) – 1).
RESULTS
Adult morphological traits
Overall, within each population, female and hermaphroditic plants did not differ significantly in any of the vegetative traits measured (Table 1). However, populations differed significantly for all of these traits. Specifically, plants in MP tended to have more but smaller leaves, consistent with a typical response to higher light intensity experienced in MP than in OT. In contrast to the vegetative traits, petal lengths differed significantly between the two sexes. Petals of hermaphrodites were on average 37.5 and 37.24% longer (Table 1) than petals of females in MP and OT, respectively. Plants in MP tended to have larger flowers than OT, but the difference was not significant. The pattern of significant gender effect on reproductive but not vegetative traits was consistent with the study by Ågren and Wilson (1991).
|
|
Progeny quality
Seed mass
Among the seeds collected, there was a significant gender effect (F1,524 = 17.18, P < 0.001) on seed mass although the magnitude of the difference was rather small. More specifically, females produced seeds that were on average 6.0 and 10% heavier than seeds produced by hermaphrodites in MP (contrast for the difference between means of the two genders: t = 1.24, P = 0.21) and OT (contrast t = 2.01, P = 0.045) in 2003, respectively (Fig. 2). In 2004, the difference increased to 17.6% in MP (contrast t = 4.37, P < 0.001) but dropped to –0.5% in OT (contrast t = 0.26, P = 0.54) (Fig. 2). With regard to population difference, MP plants produced seeds that weighed slightly less than those of the OT plants (F1,524 = 10.27, P < 0.005) (Fig. 2) probably due to the significantly more seeds they produced than the OT plants. The effect of year was not significant (F1,524 = 0.09, P = 0.77).
|
2 = 6.04, P < 0.01) and seeds from female mothers germinated better than seeds from hermaphroditic mothers (2 = 3.85, P < 0.05) (Table 2). Because both seed mass and gender were included simultaneously in the model, the significant gender effect suggested that the higher germination in female families was not only due to seed provisioning mediated through seed mass but also could be due to other factors.
|
The only second-season traits that differed between maternal genders were plant size and flowering probability, but only in OT. This was most likely due to the more significant gender difference in the plant size at the end of the first season (Table 2) in OT than in MP, including both the above- and belowground portions of the plants. The probabilities for re-emergence in the second season did not differ between the maternal genders in either MP or OT. Among the plants that produced flowers in the second growing season, the number of flowers they produced also did not differ between the maternal gender groups.
Female compensation
The female compensation for seed number was calculated to be 0.41 for the OT population and 0.50 for the MP population in 2003 and 0.25 and 0.14 in 2004, respectively (Fig. 1a). The FC for the seed mass was 0.06 and 0.1 for MP and OT in 2003, and 0.18 and –0.005 (not different from 0) in 2004, respectively (Fig. 2). The FC for the seedling performance from germination to reproduction was 0 for MP and 0.5 for OT (Table 2). The accumulated FC was 0.50 for MP and 0.88 for OT.
DISCUSSION
I show here the evidence to support the female compensation hypothesis in G. maculatum. Female compensation was observed in both seed number and progeny quality. However, the magnitude of FC varied between populations and years. A moderate level of accumulative FC was found in both study populations. I discuss each of these points in greater details.
Female compensation in seed number
Theoretical studies have shown that female compensation is a necessary condition for male sterile mutants to invade and persist in an originally hermaphroditic population (Lewis, 1941
; Lloyd, 1974
, 1975
; Charlesworth and Charlesworth, 1978
; Charlesworth, 1981
). Results from this study showed that female plants of G. maculatum achieved such compensation by producing more and larger seeds and better-performing seedlings than coexisting hermaphroditic plants. On average, females produced from 13 to 50% more seeds than hermaphrodites in the same population. This higher seed production in females was not due to differences in the original reproductive potential (i.e., the flower number; see Fig. 2 and Ågren and Willson, 1991
) or differences in the fruit number but, rather, female plants appeared to be able to convert more of the ovules into seeds within a fruit (Fig. 2).
There are at least three possible factors that might be responsible for the higher seeds per fruit in females than in hermaphrodites in the same population. First, higher seed production could be due to resource reallocation from floral organs to seed production. As shown in this study, the main morphological traits that distinguish females from hermaphrodites in G. maculatum are the flowers that they produce. Hermaphroditic flowers are 1.45 times longer in petal, 3 times longer in anthers, and twice as long in the filaments as female flowers. These measurements were similar to an earlier report by Ågren and Willson (1991)
on two Illinois populations (H : F petal lengths of 19.2 : 13.5 mm; H/F = 1.42). The reduced size of floral organs in females should allow these plants to conserve carbon, nitrogen, phosphorus, and other nutrients that would otherwise have been needed for production of petals (Schemske, 1978
), pollen (Rabie et al., 1983
; Ashman, 1994
; Bacha et al., 1997
), and nectar (Southwick, 1984
; Ashman and Stanton, 1991
; Pyke, 1991
; Harder and Barrett, 1992
; Ashman and Schoen, 1997
). Furthermore, several species that maintain smaller petals require less water (Nobel, 1977
; Galen et al., 1999
). Combined, these saved resources could be reallocated to either make more seeds or better-provisioned seeds by the female plants.
The second possibility is that inbreeding depression reduces the number of mature seeds produced by hermaphrodites. Geranium maculatum is self-compatible and Ågren and Willson (1991)
found evidence for a high selfing rate (>60%) and a high level of inbreeding depression (ranging between 0.5 and 0.75). Though the high selfing rates were estimated using an IL population, a similar pattern is suspected in the GA populations because of low pollinator activity (e.g., as low as 0.05 visits/flower in 5 h in OT). If a proportion of hermaphrodite seeds were the product of selfing but all female seeds were from outcrossing, inbreeding depression could lead to the reduced seed number and seed mass observed in this study. The gender difference might be smaller if females tended to receive pollen from genetically related individuals. This is a phenomenon likely to occur in species with limited seed dispersal such as G. maculatum. Further studies that examine selfing, biparental inbreeding rates, and their impacts on inbreeding depression would allow a better evaluation on the contribution of inbreeding depression to the maintenance of females in G. maculatum.
Finally, females might have received more pollen grains per flower than the hermaphrodites, resulting in higher seed production for the former. This possibility seems unlikely considering the smaller petal size and the lack of pollen source nearby (within the same flower). Indeed, Williams et al. (2000)
found that female plants in G. richardsonii generally received lower pollinator visitation and fewer pollen grains than hermaphrodites. Other studies had shown that such a deficit in pollen receipt could intensify when the female frequency increased in the population (Aizen, 2001
; Ashman and Diefenderfer, 2001
).
Progeny quality
Progeny quality was examined in two ways, the more commonly used measure of seed mass and a more direct measurement of seedling performance at multiple life stages. Seed mass was 6 and 10% higher in females than in hermaphrodites of MP and OT populations. Though progenies from female mothers performed similarly to hermaphrodites in MP, they outperformed progenies from hermaphrodites by 50% in OT in terms of the cumulative probability to flowering in the second season (49 vs. 32%). It is difficult to reconcile the small difference in female compensation in seed mass between the two populations (10% in OT vs. 6% in MP) and the large difference in seedling performance (50% in OT vs. 0% in MP) if seed mass were the only factor accountable for the seedling performance. Other factors, such as nutrient provisioning (Avila-Sakar and Dominguez, 2000
) or inbreeding coefficients (e.g., Kohn and Biardi, 1995
; Sakai et al., 1997
), that cannot be measured by biomass, are also likely to have contributed to the difference in seedling performance between genders.
One caveat for the results of seedling performance is that it was evaluated in the greenhouse, a rather benign environment compared to the plants' natural habitat. The degree of difference between genders may, therefore, be an underestimate in this study. Future studies that evaluate seedling performance derived from hand pollination and in the field conditions would be most valuable.
Variation in female compensation
Though both genders had little yearly variation on the seed production and individual seed mass (notice the similarity between 2003 and 2004 for a given gender in Figs. 1a and 2), female compensation (F/H – 1) fluctuated both between years and between populations. Female compensation for seed number was about twice as high in 2003 than in 2004 and higher in MP for 2003 but higher in OT for 2004. This temporal and spatial variation in female compensation is not unusual in gynodioecious species, including one example in the same genus Geranium. Asikainen and Mutikainen (2003)
found that FC ranged from 0.2 to 0.7 in G. sylvaticum across populations.
In addition to the three factors described previously, pollen limitation has also been suggested to explain at least a portion of the spatial variation seen in female compensation. Female compensation through seed production observed in the current study (averaged 0.3 for both MP and OT) is about half of what Ågren and Willson found (0.6). In another mixed population in GA with only 2% of female plants (Bear Hollow, not included in the results reported previously), females produced 75% more seed relative to the hermaphrodites (FC = 0.75). When these populations are examined together, FC appeared to correlate negatively with female frequencies in these populations (Fig. 3). This pattern is consistent with the prediction that females might have reduced seed compensation when they are present in higher frequencies within a population as a result of greater pollen limitation. The small number of populations used in the current analysis prohibits any conclusions from being drawn regarding the effect of sex ratios on female compensation. However, the preliminary pattern found here, along with similar results from other gynodioecious plants (McCauley and Brock, 1998
; Graff, 1999
; Asikainen and Mutikainen, 2003
), suggests that pollen limitation is a potentially important factor for the reproductive success of G. maculatum and deserves further study. Additional populations with a wider range of sex ratios are needed to directly test the effect of pollen limitation on female compensation.
|
; Lloyd, 1975
; Charlesworth and Charlesworth, 1978
; Charlesworth, 1981
). Male sterility can be caused by either nuclear or cyto-nuclear mutations, and the requirement for female compensation is different for these two systems. Nuclear control of gender would result if the gene(s) causing male sterility were located in the nucleus and would be transmitted through both pollen and seeds. Because females can only produce seeds, transmission of the male sterile allele would be at a 50% disadvantage relative to the allele allowing male fertility. To remain in the population, females would have to produce at least twice as many seeds as hermaphrodites in order to make up the fitness loss from the inability to produce pollen grains, which account for half of nuclear transmission to next generation. Cyto-nuclear control of gender would result if the gene(s) causing male sterility were located in one of the organelles located in cytoplasm (most likely the mitochondria). Because these organelles are only transmitted through seeds in most angiosperms, in the absence of a nuclear restorer gene, virtually all seeds produced by female plants would be expected to be female. As a result, any level of seed production in females that is above the level shown by hermaphrodites would be sufficient for females to remain in a population with hermaphrodites. Based on these theoretical criteria, the amount of female compensation in G. maculatum, by itself, would be sufficient for a cyto-nuclear system but not for a nuclear control system. Because female compensation in this and another species in the same genus, G. sylvaticum (FC = 0.2–0.7 for seed number), are both below 100%, we can reasonably predict that the genetic system for male sterility is more likely to be of the cyto-nuclear nature. A direct test for this hypothesis using crosses between hermaphrodites from different populations would be most informative and is currently underway.
In summary, female plants in G. maculatum did produce more seeds and, at least in one population, also produced seedlings with better performance than their hermaphroditic counterparts within the same population. The amount of compensation through increased seed fitness did contribute to the maintenance of female plants in natural populations; however, the magnitude of compensation seemed to vary both spatially and temporally. Other factors such as pollen limitation, inbreeding depression, and resource limitation could also influence the evolution of this sexual system in G. maculatum and warrant further studies.
FOOTNOTES
The author thanks N. Armstrong, C. Caruso, L. Donovan, M. Olson and the Donovan lab group for their help to improve an earlier version of the manuscript and L. Chaffin, A. Good, H. Hamilton, S. Held, E. Lafevre and J. Patel for their technical help in the greenhouse and in the field. Funding for this study was provided by the University of Georgia Research Foundation. 
2 Author for correspondence (e-mail: chang{at}plantbio.uga.edu
)
LITERATURE CITED
Ågren J Willson M. F 1991 Gender variation and sexual differences in reproductive characters and seed production in gynodioecious Geranium maculatum. American Journal of Botany 78: 470-480[CrossRef][Web of Science]
Aizen M. A 2001 Flower sex ratio, pollinator abundance, and the seasonal pollination dynamics of a protandrous plant. Ecology 82: 127-144[Web of Science]
Alonso C Herrera C. M 2001 Neither vegetative nor reproductive advantages account for high frequency of male-steriles in southern Spanish gynodioecious Daphne laureola (Thymelaeaceae). American Journal of Botany 88: 1016-1024
Ashman T. L 1992 The relative importance of inbreeding and maternal sex in determining progeny fitness in Sidalcea oregana ssp. spicata, a gynodioecious plant. Evolution 46: 1862-1874[CrossRef][Web of Science]
Ashman T. L 1994 Reproductive allocation in hermaphrodite and female plants of Sidalcea oregana ssp. spicata (Malvaceae) using 4 currencies. American Journal of Botany 81: 433-438[CrossRef][Web of Science]
Ashman T. L Cole D. H Bradburn M 2004 Sex-differential resistance and tolerance to herbivory in a gynodioecious wild strawberry. Ecology 85: 2550-2559[CrossRef][Web of Science]
Ashman T. L Diefenderfer C 2001 Sex ratio represents a unique context for selection on attractive traits: consequences for the evolution of sexual dimorphism. American Naturalist 157: 334-347[CrossRef][Web of Science]
Ashman T. L Schoen D. J 1997 The cost of floral longevity in Clarkia tembloriensis: an experimental investigation. Evolutionary Ecology 11: 289-300
Ashman T. L Stanton M 1991 Seasonal variation in pollination dynamics of sexually dimorphic Sidalcea oregana ssp. spicata (Malvaceae). Ecology 72: 993-1003[CrossRef][Web of Science]
Asikainen E Mutikainen P 2003 Female frequency and relative fitness of females and hermaphrodites in gynodioecious Geranium sylvaticum (Geraniaceae). American Journal of Botany 90: 226-234
Asikainen E Mutikainen P 2005a Pollen and resource limitation in a gynodioecious species. American Journal of Botany 92: 487-494
Asikainen E Mutikainen P 2005b Preferences of pollinators and herbivores in gynodioecious Geranium sylvaticum. Annals of Botany 95: 879-886
Avila-Sakar G Dominguez C. A 2000 Parental effects and gender specialization in a tropical heterostylous shrub. Evolution 54: 866-877[Web of Science][Medline]
Bacha M. A Ali M. A Farahat F. A 1997 Chemical composition of pollen grains of some date palm males grown in Riyadh, Saudi Arabia. Arab Gulf Journal of Scientific Research 15: 783-803[Web of Science]
Barrett S. C. H 2002 The evolution of plant sexual diversity. Nature Reviews Genetics 3: 274-284[CrossRef][Web of Science][Medline]
Budar F Touzet P De Paepe R 2003 The nucleo-mitochondrial conflict in cytoplasmic male sterilities revisited. Genetica 117: 3-16[CrossRef][Web of Science][Medline]
Charlesworth B 1980 The cost of sex in relation to mating system. Journal of Theoretical Biology 84: 655-671[Web of Science][Medline]
Charlesworth D 1981 A further study of the problem of the maintenance of females in gynodioecious species. Heredity 46: 27-40
Charlesworth B 1992 Evolutionary rates in partially self-fertilizing species. American Naturalist 140: 126-148[CrossRef][Web of Science]
Charlesworth D 2002 Plant sex determination and sex chromosomes. Heredity 88: 94-101[CrossRef][Web of Science][Medline]
Charlesworth B Charlesworth D 1978 Model for evolution of dioecy and gynodioecy. American Naturalist 112: 975-997[CrossRef][Web of Science]
Charlesworth D Ganders F. R 1979 The population genetics of gynodioecy with cytoplasmic-genic male-sterility. Heredity 43: 213-218[Web of Science]
Darwin C 1877 The different forms of flowers on plants of the same species D. Appleton New York, New York, USA
De Hann A. A Koelewijn H. P Hundscheid M. P. J van Damme J. M. M 1997 The dynamics of gynodioecy in Plantago lanceolata L. II. Mode of action and frequencies of restorer genets. Genetics 147: 1317-1328[Abstract]
Del Castillo R. F 1993 Consequences of male-sterility in Phacelia dubia. Evolutionary Trends in Plants 7: 15-22
Delph L. F Bailey M. F Marr D. L 1999 Seed provisioning in gynodioecious Silene acaulis (Caryophyllaceae). American Journal of Botany 86: 140-144
Delph L. F Carroll S. B 2001 Factors affecting relative seed fitness and female frequency in a gynodioecious species, Silene acaulis. Evolutionary Ecology Research 3: 487-505[Web of Science]
Delph L. F Mutikainen P 2003 Testing why the sex of the maternal parent affects seedling survival in a gynodioecious species. Evolution 57: 231-239[Web of Science][Medline]
Dinnetz P Jerling L 1997 Gynodioecy in Plantago maritima L: no compensation for loss of male function. Acta Botanica Neerlandica 46: 193-206[Web of Science]
Eckhart V. M 1992 Resource compensation and the evolution of gynodioecy in Phacelia linearis (Hydrophyllaceae). Evolution 46: 1313-1328[CrossRef][Web of Science]
Futuyma D. J 1998 Evolutionary biology Sinauer Sunderland, Massachusetts, USA
Galen C Sherry R. A Carroll A. B 1999 Are flowers physiological sinks or faucets? Costs and correlates of water use by flowers of Polemonium viscosum. Oecologia 118: 461-470[CrossRef][Web of Science]
Gouyon P. H Couvet D 1987 A conflict between the two sexes, females and hermaphrodites. In S. C. Sterns [ed.] The evolution of sex and its consequences 245-261 Birkhauser Basel, Switzerland
Graff A 1999 Population sex structure and reproductive fitness in gynodioecious Sidalcea malviflora malviflora (Malvaceae). Evolution 53: 1714-1722[CrossRef][Web of Science]
Hanson M. R 1991 Plant mitochondrial mutations and male-sterility. Annual Review of Genetics 25: 461-486[CrossRef][Web of Science][Medline]
Harder L. D Barrett S. C. H 1992 The energy cost of bee pollination for Pontederia cordata (Pontederiaceae). Functional Ecology 6: 226-233[CrossRef][Web of Science]
Jacobs M. S Wade M. J 2003 A synthetic review of the theory of gynodioecy. American Naturalist 161: 837-851[CrossRef][Medline]
Jordano P 1993 Pollination biology of Prunus mahaleb L: deferred consequences of gender variation for fecundity and seed size. Biological Journal of the Linnean Society 50: 65-84
Kohn J. R 1988 Why be female. Nature 335: 431-433[CrossRef]
Kohn J. R 1989 Sex-ratio, seed production, biomass allocation, and the cost of male function in Cucurbita foetidissima HBK (Cucurbitaceae). Evolution 43: 1424-1434[CrossRef][Web of Science]
Kohn J. R Biardi J. E 1995 Outcrossing rates and inferred levels of inbreeding depression in gynodioecious Cucurbita foetidissima (Cucurbitaceae). Heredity 75: 77-83[Web of Science]
Lewis D 1941 Male-sterility in natural populations of hermaphrodite plants: the equilibirum between females and hermaphrodites to be expected with different types of inheritance. New Phytologist 40: 56-63[CrossRef]
Lloyd D. G 1974 Theoretical sex-ratios of dioecious and gynodioecious angiosperms. Heredity 32: 11-34[Web of Science]
Lloyd D. G 1975 Maintenance of gynodioecy and androdioecy in angiosperms. Genetica 45: 325-339[CrossRef][Web of Science]
Lloyd D. G 1976 Transmission of genes via pollen and ovules in gynodioecious angiosperms. Theoretical Population Biology 9: 299-316[CrossRef][Web of Science][Medline]
Marshall M Ganders F. R 2001 Sex-biased seed predation and the maintenance of females in a gynodioecious plant. American Journal of Botany 88: 1437-1443
Martin M. C 1965 An ecological life history of Geranium maculatum. American Midland Naturalist 73: 111-149[CrossRef][Web of Science]
McCauley D. E Brock M. T 1998 Frequency-dependent fitness in Silene vulgaris, a gynodioecious plant. Evolution 52: 30-36
Medrano M Alonso C Herrera C. M 2005 Mating system, sex ratio, and persistence of females in the gynodioecious shrub Daphne laureola L. (Thymelaeaceae). Heredity 94: 37-43[CrossRef][Web of Science][Medline]
Nobel P. S 1977 Water relations of flowering of Agave deserti. Botanical Gazette 138: 1-6[CrossRef]
Olson M. S 2001 Patterns of fruit production in the subdioecious plant Astilbe biternata (Saxifragaceae). Journal of Ecology 89: 600-607[CrossRef]
Puterbaugh M. N Wied A Galen C 1997 The functional ecology of gynodioecy in Eritrichum aretoides (Boraginaceae), the alpine forget-me-not. American Journal of Botany 84: 393-400[Abstract]
Pyke G. H 1991 What does it cost a plant to produce floral nectar. Nature 350: 58-59[CrossRef]
Rabie A. L Wells J. D Dent L. K 1983 The nitrogen-content of pollen protein. Journal of Apicultural Research 22: 119-123[Web of Science]
Radford A. E Ahles H. E Bell C. R 1968 Manual of the vascular flora of the Carolinas University of North Carolina Press Chapel Hill, North Carolina, USA
Ramsey M Vaughton G 2002 Maintenance of gynodioecy in Wurmbea biglandulosa (Colchicaceae): gender differences in seed production and progeny success. Plant Systematics and Evolution 232: 189-200[CrossRef][Web of Science]
Ramula S Mutikainen A. P 2003 Sex allocation of females and hermaphrodites in the gynodioecious Geranium sylvaticum. Annals of Botany 92: 207-213
Richards A. J 1997 Plant breeding systems G. Allen & Unwin London, UK
Rosas L. F Perez-Alquicira J. P Dominguez C. A 2005 Environmentally induced variation in fecundity compensation in the morph-biased male-sterile distylous shrub Erythroxylum havanense (Erythroxylaceae). American Journal of Botany 92: 116-122
Sakai A. K Weller S. G Chen M. L Chou S. Y Tasanont C 1997 Evolution of gynodioecy and maintenance of females: the role of inbreeding depression, outcrossing rates, and resource allocation in Schiedea adamantis (Caryophyllaceae). Evolution 51: 724-736[CrossRef][Web of Science]
SAS. 1999 SAS user's guide: statistics SAS Institute Cary, North Carolina, USA
Saumitou-Laprade P Cuguen J Vernet P 1994 Cytoplasmic male-sterility in plants: molecular evidence and the nucleocytoplasmic conflict. Trends in Ecology & Evolution 9: 431-435[CrossRef][Web of Science]
Schemske D. W 1978 Evolution of reproductive characteristics in Impatiens (Balsaminaceae): significance of cleistogamy and chasmogamy. Ecology 59: 596-613[CrossRef][Web of Science]
Schnable P. S Wise R. P 1998 The molecular basis of cytoplasmic male sterility and fertility restoration. Trends in Plant Science 3: 175-180
Schultz S. T 1994 Nucleocytoplasmic male-sterility and alternative routes to dioecy. Evolution 48: 1933-1945[CrossRef][Web of Science]
Schultz S. T 2003 Sexual dimorphism in gynodioecious Sidalcea hirtipes (Malvaceae). I. Seed, fruit, and ecophysiology. International Journal of Plant Sciences 164: 165-173[CrossRef][Web of Science]
Southwick E. E 1984 Photosynthate allocation to floral nectar: a neglected energy investment. Ecology 65: 1775-1779[CrossRef][Web of Science]
Stamp N. E Lucas J. R 1983 Ecological correlates of explosive seed dispersal. Oecologia 59: 272-278[CrossRef][Web of Science]
Webb C. J 1999 Empirical studies: evolution and maintenance of dimorphic breeding systems. In M. A. Geber, T. E. Dawson, and L. F. Delph [eds.] Gender and sexual dimorphism in flowering plants 61-96 Springer, New York, New York, USA
Weller S. G Sakai A. K 2005 Selfing and resource allocation in Schiedea salicaria (Caryophyllaceae), a gynodioecious species. Journal of Evolutionary Biology 18: 301-308[CrossRef][Web of Science][Medline]
Williams C. F Kuchenreuther M. A Drew A 2000 Floral dimorphism, pollination, and self-fertilization in gynodioecious Geranium richardsonii (Geraniaceae). American Journal of Botany 87: 661-669
Wolfe L. M Shmida A 1997 The ecology of sex expression in a gynodioecious Israeli desert shrub (Ochradenus baccatus). Ecology 78: 101-110[CrossRef][Web of Science]
CiteULike Complore Connotea Del.icio.us Digg Facebook Reddit Technorati Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  M. Hayashi, K. L. Feilich, and D. J. Ellerby The mechanics of explosive seed dispersal in orange jewelweed (Impatiens capensis) J. Exp. Bot., May 1, 2009; 60(7): 2045 - 2053. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S.-M. Chang Gender-specific inbreeding depression in a gynodioecious plant, Geranium maculatum (Geraniaceae) Am. J. Botany, July 1, 2007; 94(7): 1193 - 1204. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
