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a Center for Population Biology, University of California Davis, Davis, California 95616
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: cross-pollination • embryo abortion • founder population • inbreeding depression • plant invasion • Poaceae • population evolution • self-fertility, Spartina alterniflora
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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A population where all maternal plants exhibit high, persistent inbreeding depression is expected to experience little purging even if selfing does occur (Lande, Schemske, and Schultz, 1994), and such a population would be unlikely to evolve towards increased self-fertility. In contrast, large variation among maternal plants in their magnitude of inbreeding depression could open a pathway for a rapid shift towards selfing, particularly during an invasion where recruitment is by seed, and outcross seed production is limited by pollen availability (e.g., Fenster and Ritland, 1994; Parker, 1997; Daehler, 1998). A population shift towards self-fertility could be further facilitated if especially benign growing conditions in the non-native habitat increased survival and reproduction among inbred progeny.
Several studies have identified peripheral populations that exhibit higher selfing rates than main populations (Moore and Lewis, 1965; Rick, Forbes, and Tanksley, 1979; Wyatt, 1986; Wyatt, 1990; Karkkainen, Koski, and Savolaines, 1996), although, with the exception of Karkkainen, Koski, and Savolaines (1996), no attempt was made to correlate observed differences in the selfing rates of peripheral and main populations with differences in the magnitude of inbreeding depression. The general observation of lower inbreeding depression in populations with higher selfing rates (Holtsford and Ellstrand, 1990; Barrett and Charlesworth, 1991; Latta and Ritland, 1994; Karkkainen, Koski, and Savolaines, 1996) suggests that the magnitude of inbreeding depression in a population can change quickly, in concert with changes in selfing rates. In several cases, introduced plant populations have reportedly evolved towards higher selfing rates, although a shift towards higher outcrossing rates has also apparently occurred in a few introduced populations (reviewed in Brown and Marshall, 1981; Molina-Freaner and Jain, 1993).
This study quantifies variation in the magnitude of inbreeding depression among maternal plants sampled from a recently founded, pollen-limited (Daehler, 1998) population of smooth cordgrass (Spartina alterniflora Loisel). The study aims to answer three questions regarding inbreeding depression in an invading plant population: (1) is inbreeding depression during the invasion sufficient in magnitude to prevent or reduce recruitment from selfed seeds? (2) is there significant variation in the magnitude of inbreeding depression among maternal plants such that some maternal plants can likely produce successful offspring by selfing? and (3) among maternal plants, is the magnitude of inbreeding depression correlated with self-fertility rate, where self-fertility is defined as the proportion of ovules fertilized following self-pollination? Such a correlation has been reported among populations differing in self-fertility, but studies are needed to address whether such a correlation might also be found among maternal plants differing in self-fertility within a single population (Uyenoyama, Holsinger, and Waller, 1993). The results of this study are used to make predictions about the evolution of selfing and inbreeding depression in this invading plant population.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Experimental pollinations
Pollen manipulations were performed in order to obtain selfed and outcrossed progeny from a set of genetic clones. In early May 1994, clonal fragments were dug from ten discrete smooth cordgrass clones established in San Francisco. Clones were shown to be genetically different using RAPD (random amplified polymorphic DNA) (Daehler, 1998). Eight of the clones were chosen randomly, while two were chosen because they had high rates of viable seed set in the field (high self-fertility). All clones were shown to have similar, high rates of seed set following outcrossing (Daehler, 1998). The fragments were transferred into 2.2-L pots containing a mixture of 25% Bodega Bay intertidal mud and 75% vermiculite (by volume) and grown in a common greenhouse environment over the summer and fall. The plants were fertilized biweekly with Plantex 20–20–20. When the plants flowered in fall, for each of the clones, some inflorescences were selfed and others were outcrossed using a camel hair brush. A mixture of pollen from at least three different clones was used for the outcrossing treatment, and outcross pollen came from clones not being used as maternal plants. Self-pollinations were performed using freshly dehisced self-pollen. Each stigma was pollinated at least three times over the ~1 wk period of stigma receptivity. Although florets in the outcross treatment were not emasculated, individual spikelets of smooth cordgrass are protogynous, making it likely that stigmas received outcross pollen before being exposed to self-pollen. Nevertheless, because some selfing may have occurred in the outcross treatment, outcrossing was later confirmed in the progeny using polymorphic DNA markers.
Embryo abortion and self-fertilization rates
As the earliest measure of inbreeding depression, embryo abortion rates were estimated following self- and cross-pollination. Mature inflorescences from the greenhouse pollinations were harvested in fall 1994. Three spikes were haphazardly chosen, one from near the top, middle, and bottom of each inflorescence. Each spike contained 10–30 spikelets, each of which contained a single ovule and could potentially mature a single seed. The spikelets were dissected under a dissecting microscope at 7.5x magnification, and the proportion of ovules that had developed into aborted embryos was determined. Aborted embryos were defined as any embryo lacking a mature, green scutellum. Embryos in this condition are always nonviable, as determined by germination tests (Daehler, 1996a). Aborted embryos as small as 0.1 mm in length were visible under the dissecting microscope (mature embryos are 2–5 mm long). Among the ten maternal plants used for this experiment, rates of embryo abortion were scored from 72 self-pollinated and 144 outcrossed inflorescences in 1994. Embryo abortion rates were also determined from the same clones following self-pollination in 1995 by scoring a total of 55 additional self-pollinated inflorescences. Self-fertility rates were estimated as the proportion of ovules showing evidence of fertilization following self-pollination, calculated as the sum of aborted embryos and mature seeds divided by the total number of ovules scored. A previous study had determined that the proportion of a maternal plant's ovules developing into aborted embryos following selfing did not differ among clonal replicates grown under high- and low- nutrient conditions (paired t test on means, by clone, t = 0.939, df = 11, P = 0.391; Daehler, unpublished data), indicating that nutrient availability within the range that the plants are likely to experience in the field did not have an important effect on embryo abortion rate.
Greenhouse inbreeding depression experiment
Estimates of inbreeding depression from later life stages were obtained by comparing seedling survival and end-of-season biomass among selfed and outcrossed progeny in three environments. In April 1995, selfed and outcrossed seeds (containing mature embryos) from the ten maternal plants were germinated on flats of vermiculite. All seeds with mature embryos were found to be at least 90% germinable for all crosses, based on earlier germination tests (Daehler, 1996a). These seeds had been stored overwinter in 50% seawater at 4°C to remove dormancy (Woodhouse, 1979) and promote synchronous germination. Four days after germination, seedlings were transferred to 1-L pots containing 25% Bodega Bay intertidal mud and 75% vermiculite (by volume). For most maternal plants, between 24 and 28 each of selfed and outcrossed progeny were potted. Two maternal plants with very low self-fertility rates (coy513 and coy721) produced very few selfed seeds, so only 12 and 24 replicate seedlings per cross could be potted from those clones, respectively. These potted plants were grown in three treatments in the greenhouse: low nutrient, high nutrient, and competition.
For the competition treatment, an additional 12 pots per maternal plant were planted with two seedlings, one selfed and one outcrossed, 2 cm apart, following a frequently implemented experimental design (e.g., Schmitt and Ehrhardt, 1990; Parker, Nakamura, and Schemske, 1995; Dudash, Carr, and Fenster, 1997). All pots were placed on a cluster of adjacent greenhouse benches in shallow (2 cm) plastic trays, allowing the pots to stand in ~1 cm of 50% seawater. The initial positioning of the pots was randomized, and positions of individual pots were rerandomized weekly. Water levels in the trays were maintained by the addition of freshwater over the summer. All pots were initially fertilized with 0.5 g Plantex 20–20–20 dissolved in water.
Two weeks after transplanting to the pots, initial seedling mortality for all families was recorded. If fewer than 36 selfed or outcrossed potted progeny remained from any maternal plant (12 per treatment), dead seedlings were replaced with spare seedlings from the flats. The replacement seedlings from the flats were of similar size to the seedling in the pots. The 24 one-seedling pots were then randomly divided between low- and high-nutrient treatments. For maternal plant coy513, the high-nutrient treatment was omitted due to an insufficient number of selfed progeny, while for maternal clone coy721, there were only sufficient selfed progeny for the competition treatment. Beginning in May, differential fertilizer treatments were imposed to establish high- and low-nutrient environments. Pots in the high-nutrient and competition treatments were fertilized at 3-wk intervals from May through September, while the low-nutrient treatment pots were fertilized only twice during the summer (once in late July and once in late August).
In mid-November (near the end of the growing season), the aboveground biomass of all plants was harvested, dried to a constant mass at 50°C, and weighed. Overall, only 11% of the progeny flowered by the end of their first season of growth, at which time all aboveground biomass was harvested. Because flowering was so variable and rare even among outcrossed progeny and because the plants are long-lived perennials with many opportunities to flower, I did not attempt to use flowering in the first season as a measure of fitness in this study. In contrast, final biomass is probably a very good measure of fitness for these plants because small plants usually do not survive their first winter under field conditions (Daehler, 1996a, b).
Confirmation of outcrossed progeny using RAPDs
Because emasculation was not practical, I used random amplified polymorphic DNA markers (RAPDs) to confirm outcrossing. Limited time and funds prevented me from screening all of the >300 outcross progeny; therefore, I focused on "outcrossed" plants having a final biomass within 2 SD of the mean biomass of the selfed progeny from the same maternal plant and growth environment. This screening method of identifying selfed plants makes no assumption about the magnitude of inbreeding depression but assumes only that hidden selfed plants in the "outcross" treatment will have similar biomass to the known selfed plants. Polymorphic RAPD markers had been previously identified using random primers (Daehler and Strong, 1997; Daehler, unpublished data; see also Stiller and Denton, 1995). The specific RAPD protocols used are described in Daehler and Strong (1997). In previous trials, selfed progeny reliably contained markers from the maternal plant. In contrast, assays of outcrossed progeny from a male-sterile clone showed the presence of nonmaternal DNA markers in eight out of ten progeny when screening with three primers, each resolving 3–5 polymorphic markers.
Progeny were scored as outcrossed if they contained one or more of the polymorphic markers not present in the maternal plant. Two selfed progeny from each maternal clone were also compared alongside the putative outcrossed progeny as controls. "Outcross" progeny containing only markers from the maternal plant, after screening with three different primers, were considered likely to be selfed progeny and were excluded from the analyses.
Statistical analyses
Statistical analyses were conducted using SYSTAT version 5 for Windows (SYSTAT Inc., Evanston, Illinois). To test whether maternal clones varied in their magnitude of inbreeding depression, I used analysis of variance of the logarithmically transformed final biomasses of individual plants (Johnston and Schoen, 1994). Maternal clone was treated as a random factor, while environment (high nutrient, low nutrient, competition) and pollination (selfed or outcrossed) were treated as fixed factors. A significant maternal plant-by-pollination interaction indicates variation in the magnitude of inbreeding depression among maternal plants (Johnston and Schoen, 1994). The results of the analysis do not differ qualitatively if the two nonrandomly chosen clones are excluded from the analysis.
Estimates of inbreeding depression were calculated using the formula 
= 1 - wi/wo, where is inbreeding depression, wi is the average survival rate or growth for the inbred progeny of a maternal plant, and wo is the average survival rate or growth for the outcrossed progeny of the same maternal plant. Because no estimates of were <0, is identical to relative performance (RP), which has been used to examine inbreeding effects in some other studies (Ågren and Schemske, 1983). Estimates of inbreeding depression were calculated at three life stages: survival from embryo to mature seed, initial (first 2 wk) seedling survival, and survival and growth in each of the three greenhouse environments. Comparisons of inbreeding depression among environments were based on the product of survival rate after 2 wk and average final biomass for progeny of each maternal plant in each environment. Estimates of total inbreeding depression from embryo survival to end-of-season biomass were obtained by multiplying ratios of fitness estimates for selfed and outcrossed progeny across life stages. Bootstrapped 95% confidence intervals were calculated for inbreeding depression estimates of each maternal plant in the greenhouse environments by resampling with replacement from the observed biomasses of the selfed and outcrossed progeny and recalculating 4000 estimates of inbreeding depression (Crowley, 1992). In the competitive environment where two plants (one selfed, one outcrossed) where grown in each pot, only one plant per pot was used in the analysis in order to eliminate the possibility of nonindependence of plants sharing pots. In order to assess significant differences in inbreeding depression between each environment, a random block effects ANOVA was used (Neter, Wasserman, and Kutner, 1990, p. 954), treating mean inbreeding depression as the dependent variable, clones as blocks, and environment as a factor. Contrasts between environments were evaluated using F tests (Wilkinson, 1997). Pearson correlation coefficients were calculated among maternal plants for estimates of inbreeding depression at different life stage, and in different environments. Nonparametric tests were applied when data departed substantially from normality, as determined from normal probability plots.
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RESULTS |
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Abortion rates for self-fertilized embryos varied by maternal plant, ranging from 8 to 63% of fertilized ovules, and a maternal plant's rate of embryo abortion following selfing was significantly correlated across years (r = 0.82, P = 0.007). The proportion of ovules maturing to viable seeds following selfing of the different maternal plants was also significantly correlated among years (r = 0.89, P < 0.001; Daehler, 1998). Abortion rates for outcrossed progeny ranged from 4 to 38% of fertilized ovules and were significantly lower than those for selfed progeny (paired t test, by clone, t = 4.25, df = 9, P = 0.002). Abortion rates for outcrossed progeny were correlated with abortion rates for selfed progeny (r = 0.93, P < 0.01), but not with viable seed set rates following outcrossing (r = 0.01, P = 0.98).
Screening of putative outcrossed progeny using RAPDs
Polymorphic RAPD markers were compared between the maternal plants and the 72 putative outcrossed progeny that had biomasses within 2 SD of the mean biomass of the selfed progeny. None of the 20 known selfed progeny (controls) contained new markers not present in the maternal parent. Forty of the putative outcrossed progeny were confirmed as outcrossed, as they carried one or more of the polymorphic RAPD markers not carried by the maternal parent. The remaining 32 putative outcrossed progeny contained only RAPD markers found in the maternal parent. These progeny likely resulted from self-pollination and were excluded from analyses. Assuming the previously determined 80% correct detection rate for outcrossed plants, no more than six plants would have been erroneously identified as selfed in the RAPD screenings, amounting to <1% of plants used in the analysis. This low error rate is unlikely to significantly influence inbreeding depression estimates in this study.
Inbreeding depression in different environments
Both outcrossed and selfed progeny in the competitive environment averaged lower biomass than outcrossed and selfed progeny in the high-nutrient (noncompetitive) environment, indicating that competition significantly affected plant growth (post hoc contrasts from Table 1 ANOVA, F = 11.5, P < 0.001, and F = 54.4, P < 0.001, for comparisons among outcrossed and selfed progeny, respectively). Population-level estimates for cumulative inbreeding depression (from embryo to final biomass) were 0.61, 0.71, and 0.81 for low-nutrient, high-nutrient, and competition environments, respectively. A significant difference in inbreeding depression among environments is indicated by the pollination-by-environment interaction in the ANOVA (Table 1, this analysis considers final biomass only). Within a given environment, maternal plants also varied significantly in their magnitude of inbreeding depression, as indicated by the significant maternal plant-by-pollination interaction in the ANOVA (Table 1). The ANOVA results are based only on the final biomasses of the surviving plants because zeros could not be log transformed. Estimates of inbreeding depression were also calculated based on the product of survival rate and final biomass for the progeny of each maternal plant, and bootstrapped 95% confidence intervals were applied to these estimates to allow comparisons (Fig. 1).
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Inbreeding depression at different life stages
Inbreeding depression estimates from the high-nutrient environment were used for comparing life stages because the seedlings were initially grown under high nutrient-conditions, and the maternal plants maturing the embryos were also grown under high-nutrient conditions. The magnitude of inbreeding depression for juvenile growth and survival was significantly higher (mean = 0.62) than inbreeding depression for embryo survival (mean = 0.13) and inbreeding depression for seedling survival (mean = 0.08) (Fig. 2, Wilcoxon signed rank test, Bonferroni-corrected P = 0.024 for both comparisons). The relative magnitude of inbreeding depression for each maternal plant was not significantly correlated among life stages (Spearman rank correlations among life stages, all P > 0.41).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The low-nutrient environment reduced the fitness advantage of outcrossed plants. Although plants in the low-nutrient environment often had etiolated leaves, relative to those in the high nutrient treatment, survival was high under the low-nutrient conditions. Fitness comparisons were therefore based primarily on biomass differences. Relatively slow growth among all plants in the low-nutrient conditions may have prevented the development of large differences in biomass between selfed and outcrossed progeny. Norman et al. (1995) also compared inbreeding depression in low- and high-nutrient environments, finding lower average inbreeding depression in a low-nutrient environment, although the difference was not statistically significant. If resource limitation had been low enough to cause substantial mortality, then inbreeding depression under low-resource conditions might have been higher than under high resource conditions (e.g., Hauser and Loeschcke, 1996).
In a small, accompanying field study (Daehler, 1996a), maternal smooth cordgrass plants with the highest inbreeding depression in the greenhouse environments had similarly high inbreeding depression in the field; however, among the maternal plants with lower inbreeding depression in the greenhouse, there was wide variance in survival in the field, probably as a result of stochastic mortality superimposed on inbreeding depression (e.g., Ashman, 1992).
In contrast to the native range of smooth cordgrass where competition between established plants and seedlings is often intense (Metcalfe, Ellison, and Bertness, 1986), during the invasion of San Francisco Bay, smooth cordgrass seedlings are establishing on large expanses of nutrient-rich, open intertidal mud. Seedlings only rarely establish in close proximity to other plants. Under such growing conditions, inbreeding depression is probably not as severe as would be expected in its native range, and this should allow higher establishment rates for inbred progeny in the invaded habitat.
Variation in inbreeding effects across life stages
The results of this study are consistent with other studies of outcrossing species, where inbreeding depression estimates averaged high in the juvenile growth/reproduction stage, but were notably lower in the seed germination stage (reviewed in Husband and Schemske [1996]). Since data on the ovule fertilization rates are rarely available, Husband and Schemske (1996) assumed that differences in seed set following selfing and outcrossing were entirely due to inbreeding depression (i.e., the same proportion of ovules was fertilized both after selfing and after outcrossing). This assumption will lead to an overestimate of inbreeding depression in the seed production stage if there is an incompatibility mechanism that limits self-fertilization. On the other hand, the estimates of inbreeding depression in the embryo maturation stage determined here for smooth cordgrass may be underestimates if some ovules classified as unfertilized were in fact embryos that had aborted before any visible ovule expansion. Furthermore, some aborted embryos in the outrcross treatment were likely to have been selfed embryos, leading to further underestimation of inbreeding depression in the embryo maturation stage. The observed lack of correlation in inbreeding depression estimates across life stages has also been reported in other species and may reflect different sets of deleterious alleles that act at different life stages (Husband and Schemske, 1995; Parker, Nakamura, and Schemske, 1995; Husband and Schemske, 1996).
Variation among maternal plants
Maternal plants varied significantly in their magnitude of inbreeding depression in all environments, with cumulative estimates (embryo to final biomass). Among the four maternal plants with the lowest self-fertility rates, cumulative estimates among these maternal plants were > 0.85 in the competitive environment and > 0.75 in the low-nutrient environment. Based on their very weak growth even under high-nutrient, noncompetitive greenhouse conditions, it is unlikely that any of these selfed progeny would survive in the field, even if they happened to be dispersed to "safe sites." In a small field planting, all five selfed replicates from each of these four maternal plants died, while several outcrossed progeny survived (Daehler, 1996a). Because most seed set in San Francisco Bay is the result of selfing (Daehler, 1998), these maternal plants probably contribute very few offspring to the growing population in San Francisco Bay.
In contrast, the maternal plants with higher self fertility rates suffered from less inbreeding depression. In some cases, the growth of selfed progeny was not visibly different from than that of outcrossed progeny; indeed, the 95% confidence interval for the magnitude of inbreeding depression sometimes included zero. Selfed progeny of the maternal plants with lower levels of inbreeding depression are likely to survive at "safe sites" in the field, as evidenced from a small field planting (Daehler, 1996a). The low genetic load of some clones with high self-fertility, combined with an amelioration of inbreeding effects due to relaxed competition in San Francisco Bay, may provide an avenue for the spread of selfing clones (Lloyd, 1980; Dudash, Carr, and Fenster, 1997). Pollen discounting (a decline in male fitness) may also influence the evolution of self-fertility (Holsinger, 1992) but is probably not a strong selective factor in this system because of low cross-fertilization rates.
Although fitness estimates for long-lived smooth cordgrass in this study were based only on survival and growth in their first year, these fitness estimates are probably correlated with long-term field fitness because only large plants survive their first winter in the field (Daehler, 1996a, b). Furthermore, a plant's growth rate in its first year appears to be a good predictor of its growth rate (lateral expansion) in subsequent years (Daehler, unpublished data).
Correlation between inbreeding depression and self-fertilization rate
Despite the small number of maternal clones used in this study, significant negative correlations were observed between self-fertility (proportion of ovules known to be self-fertilized after self-pollination) and both cumulative inbreeding depression and inbreeding depression for juvenile growth/survival. These correlations are predicted by some partial dominance models of inbreeding depression due to purging of recessive deleterious alleles and developing genetic associations between selfing loci and loci subject to deleterious mutations (Holsinger, 1988; Uyenoyama and Waller, 1991). Purging should be especially effective if the recessive alleles are lethal or highly deleterious (Lande and Schemske, 1985; Charlesworth and Charlesworth, 1987). Differences among individuals in the magnitude of inbreeding depression due to inbreeding history would most likely be detectable in populations where little cross-pollination among fertility genotypes occurs and individual differences in self-fertility are extreme (Schultz and Willis, 1995). Both conditions apply to smooth cordgrass invading San Francisco Bay. The observed correlation between self-fertility and juvenile growth/survival was not expected based on Schultz and Willis' (1995) models that predicts a correlation will be extremely difficult to detect for nonlethal alleles. An alternative explanation for the observed variation in magnitude of inbreeding depression would be random variation in genetic load among plants (Schultz and Willis, 1995). Unfortunately, definitive interpretation of the observed correlation is not possible because the inbreeding history of each clone is not known. Allozyme analyses would not resolve the issue because smooth cordgrass exhibits fixed heterozygosity (it is hexaploid), and there are also other allozyme visualization problems with smooth cordgrass (Campton, 1992). Carr, Fenster, and Dudash (1997) failed to find a significant correlation between degree of autogamy and magnitude of inbreeding among maternal lines of Mimulus guttatus; however, all maternal plants used were derived from artificial outcrosses, which may have obscured any correlation that may have developed due to differences in selfing history among plants. In addition, as with the current study, the history of selfing among maternal plants was not known (Carr, Fenster, and Dudash, 1997). Additional studies on the relationship between self-fertility and inbreeding depression are needed to resolve the issue. Especially valuable would be studies using plants with known inbreeding history from invading populations, where pollen limitation imposes strong selection on mating systems.
Possible explanations for self-fertility variation
Extremely heavy genetic load might lead to self-sterility or near-self-sterility in some plant populations (Weins et al., 1989; Burbidge and James, 1991; Krebs and Hancock, 1991; Seavey and Carter, 1994). Several lines of reasoning suggest that extremely heavy genetic load probably is not the primary factor responsible for low self-fertility rates observed in many smooth cordgrass plants. First, in most cases where self-sterility has been attributed to genetic load, self-pollen and outcross pollen germinated and grew equally well on self-stigmas. In smooth cordgrass, by contrast, viable self-pollen rarely penetrated the self-stigmas of some maternal plants that had low self-fertilization rates (Daehler, 1996a). Second, self-fertility was not correlated with outcross fertility, as has been observed in individuals of some other plant species where low self fertility was attributed entirely to genetic load (Krebs and Hancock, 1991; Helenurm and Schaal, 1996). Finally, progeny obtained from outcrossing the maternal plant coy738 (low genetic load, self seed set rate not significantly different from outcross seed set rate) were distributed bimodally with respect to self-fertility, suggesting a genetic modifier that influences self-fertility (Daehler, unpublished data).
Individual plants within some grass species can vary greatly in the strength of their (gametophytic) incompatibility response (Heslop-Harrison, 1982), and this may be the case with smooth cordgrass. Among the maternal plants used for this study, protogyny was not pronounced enough to prevent contamination of receptive stigmas with self-pollen, but variation in self-pollen tube growth rates in conjunction with protogyny could lead to high variation in selfing ability because ovules have limited lifespans (Cooper and Brink, 1940; Bubar, 1958). Smooth cordgrass in San Francisco Bay seems to exhibit less pronounced protogyny than in its native range (Somers and Grant, 1981), as would be predicted from selection for selfing ability during this invasion. In smooth cordgrass's native range, available evidence suggests that plants are highly outcrossing, although some plants may be capable of low rates (3–9%) of selfed seed production (Somers and Grant, 1981; D. Hammer, Cape May Plant Materials Center, Cape May, New Jersey). At present, <20% of clones in San Francisco Bay have high viable self seed set rates (45–63% seed set following selfing), while most of the clones have well below 20% viable seed set (Daehler and Strong, 1994; Daehler, 1998).
Implications for the invading population
Selfed seeds of highly self-fertile clones appear to make up most of the seed crop each year in San Francisco Bay (Daehler, 1996; Daehler, 1998), and effective cross-pollination may not occur until a mature stand with intermingling genets has formed locally. By that time, opportunities for local seed recruitment may be limited to sites with rare, patchy disturbances (Metcalfe, Ellison, and Bertness, 1986). The magnitude of inbreeding depression can vary substantially with environment, and the lack of interspecific or intraspecific competition associated with this invasion may allow survival of selfed progeny that would not survive in the native range. Simultaneously, limited cross-pollination opportunities observed during the early stages of this invasion will select for self-fertility (Fenster and Ritland, 1994). The observed association between self-fertility and viability loci should reinforce selection for high self-seed set during the invasion in San Francisco Bay. If similar selective processes occur in other introduced plant populations, a shift towards increased self-fertility over time could serve as one explanation for the "lag time" phenomenon that is frequently associated with plant invasions (Kowarik, 1995). Rates of spread by seed may increase if self-fertility rates increase in an invading population.
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FOOTNOTES |
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2 Current address: Department of Botany, University of Hawai`i M
noa, 3190 Maile Way, Honolulu, HI 96822 (FAX 808-956-3923; Tel. 808-956-3929; e-mail:daehler{at}hawaii.edu
).
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