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Department of Ecology and Evolutionary Biology, University of California, Irvine, California, 92697
Received for publication June 4, 1998. Accepted for publication November 23, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Caryophyllaceae • Hawaiian Islands • hermaphroditism • inbreeding depression • phylogenetic analysis • Schiedea; • selfing • outcrossing
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, 1976
; Charlesworth and Charlesworth, 1978
; Lande and Schemske, 1985
; Uyenoyama, 1986
; Holsinger, 1988
; Sakai, Karoly, and Weller, 1989
; Charlesworth and Charlesworth, 1990
; Charlesworth, Morgan, and Charlesworth, 1990
; Barrett and Charlesworth, 1991
; Uyenoyama and Waller, 1991a–c
; Couvet and Ronfort, 1996
; Norman et al., 1995
; Muirhead and Lande, 1997
; Norman, Weller, and Sakai, 1997
; Sakai et al., 1997a
). Inbreeding depression, defined as the reduction in fitness in the progeny of closely related individuals, may be caused either by the homozygous expression of deleterious alleles after inbreeding (partial dominance or mutation-selection balance) or a decrease in heterozygotes that exhibit a fitness advantage over homozygotes (overdominance). Population bottlenecks or loss of pollinators may result in higher rates of selfing and the expression of inbreeding depression. Continued inbreeding may purge deleterious alleles from populations and reduce the level of inbreeding depression (Charlesworth and Charlesworth, 1987). In many hermaphroditic plant species, mechanisms that separate reproductive structures in time (dichogamy) or space (herkogamy) may have evolved to avoid the negative effects of inbreeding depression. The evolution of a dioecious breeding system prevents the expression of inbreeding depression through selfing, the most severe form of inbreeding.
The classic argument of Lloyd (1975
, 1976)
and Charlesworth and Charlesworth (1978)
suggests that pistillate individuals (females) may increase in frequency in a hermaphroditic population in which male sterility is under nuclear control when k >1 - 2s
, where k refers to the increase in fecundity of females over that of hermaphrodites, s is the selfing rate, and is a measure of inbreeding depression. Assuming that females and hermaphrodites have equal seed production (k = 0), females will increase in frequency in a population if the product of the selfing rate and the level of inbreeding depression is greater than one-half (s > 0.5). According to this result, females will be favored in highly selfing populations that exhibit high levels of inbreeding depression. Hermaphroditic species with a history of outcrossing would be expected to have high levels of inbreeding depression. In contrast, hermaphroditic species with a past history of selfing would have lower levels of inbreeding depression because deleterious alleles would have been purged from the population.
Even when s < 0.5, modification of breeding systems may still occur in a population if there is sufficient variation in family levels of inbreeding depression and/or selfing rates (Uyenoyama, Holsinger, and Waller, 1993
). Variation in levels of inbreeding depression among families is likely in part because maternal parents will differ in their levels of selfing and numbers of mutant deleterious alleles (Karron, 1989
; Ågren and Schemske, 1993
; Johnston and Schoen, 1994
). For example, individuals near the edge of a population may be more inbred and have higher selfing rates because they are more isolated than individuals near the center of the population.
The Hawaiian monophyletic lineage comprising Schiedea and Alsinidendron contains both hermaphroditic and dimorphic species (dimorphic species have gynodioecious, subdioecious, or dioecious breeding systems). Schiedea is paraphyletic, with the four species of Alsinidendron nested within the basal clade of the lineage (Wagner, Weller, and Sakai, 1995
; Weller, Wagner, and Sakai, 1995
). Nuclear genes control inheritance of male sterility in dimorphic species (Weller and Sakai, 1991
). Among hermaphroditic species, selfing rates vary widely, ranging from outcrossing (Norman, Weller, and Sakai, 1997
) to facultative or obligate selfing (Weller, Wagner, and Sakai, 1995
).
Character mapping using phylogenetic trees based on morphological and molecular data suggests that dimorphism evolved subsequent to colonization of the Hawaiian Islands by a hermaphroditic ancestor (Fig. 1; Wagner, Weller, and Sakai, 1995
; Weller, Wagner, and Sakai, 1995
; Soltis et al., 1996
; Sakai et al., 1997b
). The evolution of dimorphism may have been favored by increases in selfing rates and expression of inbreeding depression, which have resulted in increases in the frequency of male-sterile individuals (Weller and Sakai, 1990
). Hermaphroditic, often autogamous species are found in two of the four clades in Schiedea and Alsinidendron identified using morphological and molecular phylogenetic approaches (the S. membranacea and S. nuttallii clades, Fig. 1; Weller, Wagner, and Sakai, 1995
). Historically high levels of inbreeding among species in these clades may have purged deleterious alleles from populations and favored hermaphroditism and evolution of autogamy (Lande and Schemske, 1985
; Barrett and Charlesworth, 1991
). In contrast, outcrossing may have been maintained in the remaining clades through the appearance of unisexual individuals. These hypotheses are difficult to test through analysis of current levels of inbreeding depression; if the level of inbreeding depression changes during mating system evolution, it will be difficult to determine the original level of inbreeding depression that led to this change (Pray and Goodnight, 1995
; Norman, Weller, and Sakai, 1997
). Phylogenetic approaches may be used to elucidate historical levels of inbreeding depression and changes in the magnitude of inbreeding depression that have occurred during the course of breeding system evolution.
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; Weller, Wagner, and Sakai, 1995
). To test this prediction, levels of inbreeding depression and selfing rates were investigated in S. membranacea. Selfing rates were measured over several years to determine the extent of temporal variation in the mating system. |
MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). During the dry summer, plants become dormant and die back to thick, fleshy, roots. Schiedea membranacea is federally listed as an endangered species and is restricted to the Mahanaloa-Kuia, Pa'aiki, Kalalau, Nualolo, Wainiha, and Waialae Valleys on the western side of the island (USDI Fish and Wildlife, 1996
). The plants used in this study were located near the intersection of Mahanaloa and Kuia Valleys. The population consists of 
100 plants growing on steep cliffs.
The apetalous flowers of S. membranacea contain ten stamens and 3–5 (rarely 6–7) stigmas. Flowers are strongly protandrous, and the species is self-compatible as progeny are produced following controlled self-pollinations. Geitonogamy is likely in the field during peak flowering when the large inflorescences produce hundreds of flowers. Although pollen is easily dislodged by shaking, high winds are rare in the native habitat (Weller, personal observation), and there is no indication that S. membranacea is wind pollinated (Weller et al., 1998
). Nectar is produced, suggesting the possibility of insect pollination (Weller et al., 1998
).
Level of inbreeding depression
The level of inbreeding depression in S. membranacea was determined by comparing the fitness of selfed vs. outcrossed progeny. Hand-pollinations in a pollinator-free greenhouse were begun in October 1992, at the beginning of the flowering season, using 16 plants originally collected from Kaua'i. All flowers were emasculated prior to anther dehiscence to prevent any self-pollination. Two to three days after emasculation, pollinations were carried out using fine forceps to brush groups of newly dehisced anthers from a single pollen donor across the receptive stigmas of a single flower. Stigmas were coated with either pollen from flowers on the same plant (self) or pollen from flowers on different plants (outcross). Each of 16 maternal parents was crossed with an average of seven pollen donors, and each maternal family consisted of progeny with the same maternal parent. As a control, some flowers were emasculated and left unpollinated; these flowers failed to set fruit, showing that geitonogamy or accidental cross-pollination had not occurred.
Capsules were collected just before they dehisced, 25 d after pollination. A total of 1193 capsules was collected (an average of 99 capsules per maternal parent), and the number of viable seeds per capsule was recorded. The seeds were separated into groups of 25 according to cross type, weighed using a Cahn 25 electrobalance, and stored at room temperature.
In October 1993, selfed and outcrossed seeds were planted in lots of 25 in 5-cm pots in the greenhouse. All pots were randomized and placed in flats, which were rotated every 3 d. Germination and percentage seedling survival were scored approximately every 3 d, and percentage germination was calculated using the total number of seeds germinated after 8 wk. Percentage germination per pot was correlated with the total mass of seeds in each pot to determine whether large seed mass provided an advantage during germination. In December 1993, the seedlings were transplanted from the community pots into individual 5-cm pots. Three months later, the plants were moved into 10-cm pots and scored approximately every 3 d for the date of first flower. All plants were harvested in June 1994, after the majority (97%) had flowered. Each plant was cut off at the soil line, and the inflorescence and vegetative parts were collected separately. Plants were dried at room temperature for several weeks, then dried further at 40°C for 6 d, and weighed.
To calculate the level of inbreeding depression in S. membranacea, relative fitness for each character within each maternal family was calculated as the ratio of mean fitness values for selfed progeny to those of outcrossed progeny. Cumulative relative fitness was calculated for each maternal family by multiplying all relative fitness values for percentage viable seeds per capsule, percentage germination, percentage seedling survival in community and individual pots, and inflorescence biomass. These characters were chosen because of their importance to overall fitness and their presumed independence from one another. Due to the large number of flowers per inflorescence, inflorescence biomass was substituted for the number of flowers. Other studies have found a significant correlation between inflorescence biomass and flower number in various Schiedea species (Sakai, Karoly, and Weller, 1989
; Norman et al., 1995
). Inflorescence biomass was used in fitness calculations for comparison to other studies of inbreeding depression in Schiedea, although total aboveground biomass was also analyzed. Seed mass was not used in the cumulative fitness calculations because it was significantly correlated with percentage germination (r = 0.30, P < 0.0001, N = 384). Inbreeding depression was measured as (1 - the cumulative relative fitness). Ågren and Schemske's (1993)
relative performance (RP) was calculated in addition to inbreeding depression.
A mixed-model ANOVA (PROC GLM; SAS, 1990
) using Type IV sums of squares was used to analyze the effect of cross type (fixed) and maternal family (random) on all characters, except percentage survival in individual pots, which was analyzed with a G test of heterogeneity. Percentage viable seeds per capsule, percentage germination, and percentage seedling survival in community pots were arcsine transformed and mean seed mass was log transformed to improve normality. Total biomass and inflorescence biomass were untransformed because variances were homogeneous, and data were normally distributed. Five maternal families with sample sizes less than five at any of the stages measured were excluded from statistical analyses and from calculations of the level of inbreeding depression. Measures of inbreeding depression were obtained from the remaining 11 maternal families.
Selfing rate
The selfing rate was determined using starch gel electrophoresis of buds obtained from plants grown from seed. Seeds were collected from the Mahanaloa-Kuia Valley population during the 1993, 1994, 1995, and 1996 field seasons. Seeds were planted and grown to flowering in the greenhouse.
Floral buds were collected and ground using previously described methods (Weller, Sakai, and Straub, 1996
). Three different isozymes with one locus each were used to determine selfing rates. Five alleles (or four in years when a rare allele was absent) of acid phosphatase (ACP) were resolved using a morpholine-citrate gel-buffer system at pH 6.4 (Clayton and Tretiak, 1972
). Two alleles of glutamate oxaloacetate transaminase (GOT) and four alleles of shikimate dehydrogenase (SkDH) were resolved using a tris-citrate gel-buffer system (Soltis System 4) at pH 7.8 (Soltis et al., 1983
). Staining recipes were those of Soltis et al. (1983)
, modified from Shaw and Prasad (1970).
Ritland's (1990
, unpublished data) multilocus outcrossing estimation program was used to calculate outcrossing rates. The total number of maternal families (N = 11–38) and progeny (N = 165–883) scored for each locus varied in the four years because of seed availability. Standard errors for all outcrossing rates were based on 500 bootstrap samples. Pairs of outcrossing rates from different years were analyzed with a z test, and the critical value for the multiple comparison was adjusted according to the Dunn-
idák method (Sokal and Rohlf, 1995
). The selfing rate was calculated as (1 - the outcrossing rate). Because inbred progeny tend to have higher mortality rates than outcrossed progeny before the selfing rate can be measured, the selfing rates were corrected to take into account the loss of selfed progeny (Maki, 1993
; Husband and Schemske, 1996
).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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= -0.12), levels of inbreeding depression in the remaining families ranged from 0.43 to 0.97 (Table 1). A mean inbreeding depression value of 0.70 was calculated for the 11 maternal families. A similar value of 0.68 was obtained using the relative performance (RP) measure of Ågren and Schemske (1993)
. Strong family effects were evident for all traits, and for several traits there were strong interactions with maternal family (Table 2).
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Differences between selfed and outcrossed progeny were evident during germination (Table 2, Fig. 2b) with selfed seeds in general having lower germination than outcrossed seeds. Variation in inbreeding depression among maternal families caused a significant interaction of maternal family and cross type (Table 2).
Seedling survival in the community pots was similar for outcrossed and selfed progeny, with survival of both cross types averaging >50% (Fig. 2c). Differences between cross types became apparent once the seedlings were transplanted into individual pots (Fig. 2d). In all but one maternal family (family 9), selfed seedlings had lower survival rates than outcrossed seedlings (G test of heterogeneity; G = 50.36, P = 0.001).
Inbreeding depression was most apparent in inflorescence biomass and total aboveground biomass, the last stages measured. In all maternal families, selfed progeny had significantly smaller inflorescence biomass than outcrossed progeny (Table 2, Fig. 2e). The same was true for total aboveground biomass (Table 2, Fig. 2f). Total aboveground biomass and inflorescence biomass were strongly correlated (r = 0.89, P = < 0.0001).
Selfing rate
The mating system in S. membranacea varied slightly in the four years in which it was measured (Table 3). The multilocus estimates of the outcrossing rates ranged from 0.75 to 0.93. The outcrossing rates were significantly different from one another in only one comparison (1993 vs. 1994; z test, P < 0.05). Both multilocus (tm) and single-locus (ts) estimates were similar in all years, suggesting that there was no substantial biparental inbreeding in the population. When corrected for early mortality of selfed progeny, the selfing rate was low in all four years and ranged from 0.13 to 0.38.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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and is likely to be the result of expression of many, mildly deleterious alleles rather than a few major, lethal alleles. Inbreeding depression detected in percentage viable seeds per capsule for S. membranacea was lower than predicted based on Husband and Schemske's review of habitually outcrossed species and may have occurred if some inviable seeds were scored as viable and if inbred seeds were more likely to be inviable. This would also explain the relatively higher level of inbreeding depression (Table 1) detected during germination compared to Husband and Schemske's results (1996).
The average level of inbreeding depression for all life history stages in S. membranacea (0.70) was higher than the average level found in predominantly outcrossing species ( = 0.53; Husband and Schemske, 1996
). Because only a small period of the perennial life cycle of S. membranacea was studied and because inbreeding depression may also change or increase with age (Schmitt an Ehrhardt, 1990
; Wolfe, 1993
), it is likely that inbreeding depression in S. membranacea was underestimated in our study. Under field conditions, the level of inbreeding depression would also be expected to be much higher (Dudash, 1990
; Jarne and Charlesworth, 1993
; Eckert and Barrett, 1994
). In natural populations of S. membranacea, most adult individuals probably result from outcrossing because of the low likelihood that individuals derived from selfing survive to the adult stage. Despite the prevalence of high inbreeding depression, the absence of inbreeding depression in family 112 suggests a mechanism for the spread of a selfing variant in a population where the average inbreeding is high.
The selfing rate in Schiedea membranacea was low and showed little variation in the four years in which it was measured. The slight variance in selfing rates might result from changes in pollinator availability, the number of reproductive individuals in a population, and other factors affecting pollination and fertilization. In S. membranacea, it seems likely that the most important factor contributing to the variation in selfing rates is pollinator availability. Schiedea membranacea is probably biotically pollinated, based on low potential for wind pollination and the production of nectar (Weller et al., 1998
). Pollinators of S. membranacea may be native moths in the Pyralid family, similar to those observed on S. lydgatei (Norman, Weller, and Sakai, 1997
). Although no moths or other insects have been observed on S. membranacea, only a single set of nighttime observations has been made (Cabin and Weller, unpublished observations).
There are few studies that have addressed the extent of variation in selfing rates. Dole and Ritland (1993)
concluded that annual variation in pollinator visitation rates was responsible for fluctuations in selfing rates in two species of Mimulus. In contrast, Kesseli and Jain (1984)
found little variation in selfing rates from year to year in Limnanthes douglasii. Rates may also vary between populations in any one year. Selfing rates significantly differed in populations of Collinsia heterophylla (Mayer, Charlesworth, and Meyers, 1996
), Clarkia tembloriensis (Holtsford and Ellstrand, 1990
), Ardisia escallonioides (Pascarella, 1997
), Gilia achilleifolia (Schoen, 1982
), and Lupinus (Harding, Mankinen, and Elliott, 1974
; Harding and Barnes, 1977
). The extent of variation in selfing rates among populations of S. membranacea is unknown.
The combination of high outcrossing rates and high inbreeding depression found in S. membranacea was also found in hermaphroditic Schiedea lydgatei (Norman et al., 1995
; Norman, Weller, and Sakai, 1997
). In both species, selection appears to have favored outcrossing hermaphrodites. In contrast, hermaphrodites in two gynodioecious species, Schiedea adamantis with 39% females and S. salicaria with 12% females, had considerably lower outcrossing rates (0.50 and 0.60, respectively; Sakai et al., 1997a
; Sakai and Weller, unpublished data). Schiedea adamantis has nearly the same range of variation in breeding depression as S. membranacea ( = -0.29–0.94; Sakai et al., 1997a
); inbreeding depression averaged over five families of S. salicaria was 0.89 (Sakai, Karoly, and Weller, 1989
). In the gynodioecious species, the combination of high selfing rates and high inbreeding depression appears to favor the presence of females in populations. In S. adamantis, the presence of females is also favored by shifts in resource allocation (Sakai et al., 1997a
).
Using Charlesworth and Charlesworth's (1978)
model, the product of the level of inbreeding depression and the selfing rate (s) in S. membranacea ranged from 0.09 to 0.27 over four years. Both values are much lower than 0.5, the average threshold value that must be exceeded in order for a male sterility mutation to spread in a hermaphroditic population. Even though the average level of inbreeding depression is very high in S. membranacea, the selfing rate is not high enough to result in the expression of significant inbreeding depression. Consequently, any male sterility genes that may be present in the population will not be favored and hermaphroditism is stable.
Although high inbreeding depression should favor retention of outcrossing in S. membranacea, variation in levels of inbreeding depression and selfing rates among individuals of this species could result in invasion of the population by selfing variants. For example, an individual of S. membranacea isolated from the main population by 25 m was completely selfed (s = 1.0) in both years for which seeds were collected. Continued production of selfed progeny in subsequent generations might lead to purging of deleterious alleles and loss of adaptations favoring outcrossing in the lineage derived from this individual. Selfing could evolve in this lineage, even though average conditions in the population might continue to favor maintenance of outcrossing. Better estimates of family variation in selfing rates and information on variation in inbreeding depression for those same families would be useful for assessing the potential significance of this variation in breeding system evolution (e.g., Dudash, Carr, and Fenster, 1997
).
Similar differences in inbreeding depression levels among maternal plants have been found in Astragalus (Karron, 1989
), Sabatia angularis (Dudash, 1990
), Begonia hirsuta and B. semiovata (Ågren and Schemske, 1993
), Mimulus (Latta and Ritland, 1994
; Dudash, Carr, and Fenster, 1997
), Epilobium angustifolium (Husband and Schemske, 1995
), Silene douglasii (Kephart, Brown, and Hall, 1999
), and Clarkia tembloriensis (Holtsford, 1996
). In contrast to these studies, Heywood (1993)
found little variation in the magnitude of inbreeding depression across families in Gaillardia pulchella. Johnston (1992)
also found little evidence for variation in inbreeding depression among families of Lobelia cardinalis and L. siphilitica measured both in the greenhouse and in the field.
In S. membranacea, population estimates of selfing rates and inbreeding depression provide a reasonable indication that current selection regimes favor breeding system stability. Predictions of levels of inbreeding depression and selfing rates using phylogenetic approaches may be less reliable, however, when considerable extinction has occurred in clades. Schiedea membranacea belongs to a portion of the Schiedea-Alsinidendron lineage, which is probably compressed due to the extinction of species and perhaps entire lineages (Wagner, Weller, and Sakai, 1995
). Extinction may result in sister taxa with widely varying breeding systems and morphological attributes. For this reason, predicting the basal condition for traits of interest would be difficult. Schiedea membranacea and its sister taxon S. verticillata may be the remnants of an extensive clade of outcrossing species, making the highly selfing Alsinidendron species less closely related to S. membranacea than the phylogenetic analysis of extant species suggests (Fig. 1; Wagner, Weller, and Sakai, 1995
). Information on the mating system of S. verticillata, the sister taxon of S. membranacea, might suggest whether outcrossing was widespread among basal taxa of hermaphroditic Schiedea species that are now largely extinct.
Paradoxically, the level of confidence in older portions of phylogenies may be greatest because shared, derived morphological and molecular characters (synapomorphies) are more common and permit ready identification of clades. This is the case for the S. membranacea clade, which possesses more characters useful for cladistic analysis than the clades found on younger islands. On the younger islands extinction is less likely, but relationships in Schiedea are more difficult to resolve due to a scarcity of synapomorphies (Weller, Wagner, and Sakai, 1995
; Soltis et al., 1996
; Sakai et al., 1997b
). Well-supported clades on younger islands offer the most promise for using macroevolutionary approaches to the analysis of breeding system evolution. Despite some inherent limitations to phylogenetic approaches, these methods are useful for identifying the assumptions underlying the comparative approach and indicating which comparisons are most likely to be informative.
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FOOTNOTES |
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2 Current address: Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, 1735 Neil Ave., Columbus, OH 43210.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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