Infraspecific variation in Cerastium alpinum s. lat. (Caryophyllaceae) in Central Norway

Within Cerastium alpinum s. lat., several taxa have been circumscribed. Three infraspecific taxa are currently recognized in Nordic literature, but the opinions have varied about their taxonomic levels and relations to each other. Populations of these three taxa — alpinum, glabratum and lanatum — were investigated in Central Norway to elucidate taxonomic relationships on a regional scale. Morphometric analyses (PCA ordination) based on two data sets; one with 61 characters, and one without the 35 indumentum characters, gave different results. The former partly support the use of a higher rank for glabratum , but this analysis may put too much weight on indumentum characters. The reduced data set gave a more continuous transition between the three taxa, with no special status for glabratum. Crossing experiments between the taxa indicated full cross compatibility as far as relative seed set was concerned. Finally, vegetation analysis (DCA ordination) indicated that the three taxa have somewhat different ecological demands. This suggests that they, in spite of hybridization abilities, normally are maintained as separate entities due to habitat differences. A delimitation of three infraspecific taxa at the same level within C. alpinum s. lat. is thus supported by this study. Whether these taxa should be treated as subspecies or varieties strongly depends upon the definitions of these categories. Viewed in a wide geographical context, however, the arguments for subspecific rank are strengthened.     

Ovule pre-emption and pollen limitation in a self-fertile perennial herb (Blandfordia grandiflora,Liliaceae)

The extent, frequency and causes of pollenlimited seed production were examined in partially selffertile populations of Blandfordia grandiflora for 2 years. Percentage seed set of open-pollinated plants (50–57%) did not differ within or between years, and was about 19% less than experimentally cross-pollinated plants (70–75%). Floral visits by honeybees did not differ through the flowering season and the number of pollen grains deposited on stigmas within 12 h of flowers opening exceeded the number of ovules per flower, indicating that the quality rather than the quantity of pollination limited seed set. Pollen limitation was caused by concurrent self- and cross-pollination and the subsequent abortion of some selfed ovules due to inbreeding depression. Natural seed set (55%) was intermediate between selfed (43%) and crossed (75%) flowers and was not increased when flowers that had been available to pollinators for 24 h were hand cross-pollinated, suggesting that ovules were already fertilized. Similarly, experimental pollination with both cross and self pollen within 24 h of flowers opening did not increase seed set relative to natural seed set, indicating that both cross- and self-fertilizations had occurred. In contrast, when selfing followed crossing by 48 h, or vice versa, seed set did not differ from crossed-only or selfed-only flowers, respectively, indicating that ovules were pre-empted by the first pollination. Collectively, these results indicate that under natural conditions self pollen pre-empts ovules, rendering them unavailable for cross-fertilization. This selfing reduces fecundity by 50%, as estimated from the natural production of cross seeds when selfing was prevented. Consequently, selection should favour floral traits, such as increased stigma-anther separation or protandry, that reduce interference between male and female functions that leads to selfing.     

Sources of variation in pollinator contribution within a guild: the effects of plant and pollinator factors

Among plants visited by many pollinator species, the relative contribution of each pollinator to plant reproduction is determined by variation in both pollinator and plant traits. Here we evaluate how pollinator movement among plants, apparent pollen carryover, ovule number, resource limitation of seed set, and pollen output affect variation in contribution of individual pollinator species to seed set in Lithophragma parviflorum (Saxifragaceae), a species visited by a broad spectrum of visitors, including beeflies, bees and a moth species. A previous study demonstrated differences among visitor species in their single-visit pollination efficacy but did not evaluate how differences in visitation patterns and pollen carryover affect pollinator efficacy. Incorporation of differential visitation patterns and pollen carryover effects —commonly cited as potentially important in evaluating pollinator guilds — had minor effects (0–0.6% change) on the estimates of relative contribution based on visit frequency and single-visit efficacy alone. Beeflies visited significantly more flowers per inflorescence than the bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating that apparent pollen carryover did not reduce per-visit efficacy of these taxa. In contrast, Greya moth visits showed a decrease in seed set by 55.4% and the smaller bees by 45.4% from first to second flower. The larger carryover effects in smaller bees and Greya were diminished in importance by their small overall contribution to seed set. Three variable plant traits may affect seed set: ovule number, resource limitation on seed maturation, and pollen output. Ovule number per flower declined strongly with later position within inflorescences. Numbers were much higher in first-year greenhouse-grown plants than in field populations, and differences increased during 3 years of study. Mean pollen count by position varied 7-fold among flowers; it paralleled ovule number variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowering, with seed set in hand-pollinated flowers ranging from 66% in early flowers to 0% in the last two flowers of all plants. Variation in ovule number and resource limitation of seed maturation jointly had a strong effect on the number of seeds per flower. Visitation to early flowers had the potential to cause more seed set than visitation to later flowers. Overall, the most important sources of variation to seed production contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potentially differential phenology among visitor species. These effects are likely to vary among populations and years.     

Maintenance of gynodioecy in Wurmbea biglandulosa (Colchicaceae): gender differences in seed production and progeny success

 In gynodioecious species, females contribute genes to future generations only through ovules, and to persist in populations they must have a compensatory advantage compared with hermaphrodites that reproduce via ovules and pollen. This compensation can result from greater fecundity and/or superior success of progeny from females. We examined differences in seed production and progeny success between females and hermaphrodites in the geophyte Wurmbea biglandulosa to explain the maintenance of females. Females produced more ovuliferous flowers and had more ovules per flower than did hermaphrodites but this did not necessarily result in greater fecundity, in part because seed production of females was pollen-limited. Over four years in one population, open-pollinated females produced 1.32 more seeds than open-pollinated hermaphrodites (range 1.09–1.63). In two other populations examined for one year only females produced 1.07 and 0.79 as many seeds as hermaphrodites. Seed production of open-pollinated females and hermaphrodites was only 55% and 73% that of cross-pollinated plants, respectively, indicating that both genders were pollen-limited but females more so than hermaphrodites. Open-pollinated seeds from females were 1.18–1.27 times more likely to germinate than seeds from hermaphrodites. No gender differences existed in seedling growth or survival. Hermaphrodites were self-compatible, but selfed seed set was only 80% that of crossed seed set. Crossed seed set of females and hermaphrodites did not differ. Assuming nuclear control of male sterility, relative female fitness is insufficient to maintain females at their current frequencies of 17%, and substantial female fitness advantages at later life-cycle stages are required. Received May 4, 2001 Accepted February 25, 2002     

Mating systems and limits to seed production in two Dicerandra mints endemic to Florida scrub

We used hand-pollination experiments to test the mating systems of and evaluate limits to seed production in two federally listed endangered plants endemic to the Lake Wales Ridge in Florida, USA: Dicerandra frutescens Shinners ssp. frutescens Huck and D. christmanii Huck and Judd (Lamiaceae). Both are nonclonal, short-lived perennials found in gaps created by disturbance (e.g., fire, roads) in Florida scrub. We found that both species require pollen and insect visitation to produce seeds. We detected pollinator limitation of seed production in D. christmanii but not D. frutescens ssp. frutescens, which we suggest is a function of time-since-disturbance or gap size rather than intrinsic differences between the two species. Both species are self-compatible. Inbreeding depression reduced seed set by 60% in D. frutescens ssp. frutescens but did not occur in D. christmanii. We conclude that pollinator limitation (in fire-suppressed populations of both species) and inbreeding depression (in D. frutescens ssp. frutescens) have the potential to limit seed production in these seed-dependent, rare species. Appropriate fire management should mitigate both of these risks, by maintaining large populations and conditions attractive to pollinators. Although these two species are very similar in reproductive biology, comparisons with other Florida scrub endemics and with rare plants in general suggest that potential threats to conservation via reproductive biology are difficult to predict, depending on combinations of ecology, life-history, and phylogenetic history.     

Variation of pollination and resource limitation in a low seed-set tree, Liriodendron chinense (Magnoliaceae)

Pollen limitation and resource limitation have been documented as the major factors responsible for plants commonly producing more ovules than seeds, but few studies have examined pollen deposition directly in natural populations at different sites and times. We investigated the causes of low seed set in four populations of Liriodendron chinense (Magnoliaceae), an insect‐pollinated endangered tree endemic to southern China, over 2–3 years. One pistil potentially produces two ovules. The number of pistils per flower varies among populations, but in three of the four populations the variation in a given population was not significantly different among years. Overall, populations with higher pistil numbers tend to set more seeds per flower, but a positive correlation between pistil numbers and seed production per flower was observed in only one of the four populations. The numbers of pollen grains deposited per stigma varied from 0 to 60. The proportion of pollinated stigmas per flower ranged from 44% to 88% among populations and years. The numbers of pollen grains deposited per stigma and the percentages of pollinated stigmas were significantly different between populations, and two populations showed significant differences between years. A positive correlation between stigmatic pollen load and seed set was sought in ten population‐by‐year combinations but, in a given population, high stigmatic pollen loads did not always result in high seed set. Examination of pollen deposition, pistil and seed production over several sites and years showed that in addition to pollination, other factors such as resource or genetic loads were likely to limit the (lower than 10%) seed set in L. chinense. It appears that small, isolated populations experience severe pollination limitation; one population studied had seed/ovule ratios of 0.84% and 1.88% in 1995 and 1996. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 2002, 140 , 31–38.     

Intra-inflorescence variation in floral traits and reproductive success of the hermaphrodite Silene acutifolia

BACKGROUND AND AIMS: Intraspecific variation in floral components and reproductive success is often located at the intra-individual level. The arrangement of flowers within inflorescences may explain a great deal of this variation. The variation in number of ovules, fruit set, number of seeds per fruit, seed set, seed weight and seed germination is investigated at different positions within the inflorescence of Silene acutifolia. METHODS: Data were obtained in natural populations, and germination experiments were conducted in a germination chamber. KEY RESULTS: The number of ovules, fruit set, number of seeds, seed set and seed weight, showed a significant decline from early (primary) position to later (tertiary) position. The patterns of intra-inflorescence variation were consistent in different populations and years of study. Seed germination showed an opposite pattern, seeds from primary position showed the lowest germination percentages and seeds from tertiary position the highest, although the effect of position on germination was only marginally significant. There was significant among-population variation in number of ovules per flower. Fruit set also varied significantly among populations, with lower fruit set in the smaller and more isolated population. No significant among-population differences were detected in number of seeds per fruit and seed set. Seeds from the smallest and more isolated population (Arnado) were the lightest. Seed germination showed strong differences between populations, seeds from Arnado started to germinate later, and showed the lowest final germination percentages. CONCLUSIONS: Architectural effects or resource competition are the most commonly proposed hypothesis to explain these patterns. Data suggest that there is less pollen available to pollinate tertiary flowers, and that there is not enough outcross pollen in Arnado. The germination percentages suggest that there is variation in the source of pollen within inflorescences, with high probability of receiving outcross pollen in flowers from primary position, and higher probability of geitonogamous crosses in tertiary flowers.     

Experimental dissection of inbreeding and its adaptive significance in a flowering plant, Aquilegia canadensis (Ranunculaceae)

Inbreeding is a major component of the mating system in populations of many plants and animals, particularly hermaphroditic species. In flowering plants, inbreeding can occur through self-pollination within flowers (autogamy), self-pollination between flowers on the same plant (geitonogamy), or cross-pollination between closely related individuals (biparental inbreeding). We performed a floral emasculation experiment in 10 populations of Aquilegia canadensis (Ranunculaceae) and used allozyme markers to estimate the relative contribution of each mode of inbreeding to the mating system. We also examined how these modes of inbreeding were influenced by aspects of population structure and floral morphology and display predicted to affect the mating system. All populations engaged in substantial inbreeding. On average, only 25% of seed was produced by outcrossing (range among populations = 9-37%), which correlated positively with both population size (r = +0.61) and density (r = +0.64). Inbreeding occurred through autogamy and biparental inbreeding, and the relative contribution of each was highly variable among populations. Estimates of geitonogamy were not significantly greater than zero in any population. We detected substantial biparental inbreeding (mean = 14% of seeds, range = 4-24%) by estimating apparent selfing in emasculated plants with no opportunity for true selfing. This mode of inbreeding correlated negatively with population size (r = -0.87) and positively with canopy cover (r = +0.90), suggesting that population characteristics that increase outcross pollen transfer reduce biparental inbreeding. Autogamy was the largest component of the mating system in all populations (mean = 58%, range = 37-84%) and, as expected, was lowest in populations with the most herkogamous flowers (r = -0.59). Although autogamy provides reproductive assurance in natural populations of A. canadensis, it discounts ovules from making superior outcrossed seed. Hence, high autogamy in these populations seems disadvantageous, and therefore it is difficult to explain the extensive variation in herkogamy observed both among and especially within populations.     

Clonal diversity in the rare Calamagrostis porteri ssp. insperata (Poaceae): comparative results for allozymes and random amplified polymorphic DNA (RAPD) and intersimple sequence repeat (ISSR) markers

Four populations of the rare, highly clonal grass Calamagrostis porteri ssp. insperata were examined using allozymes and the two polymerase chain reaction (PCR)-based markers, random amplified polymorphic DNA (RAPD) and intersimple sequence repeat (ISSR) bands. Only one of the 15 allozyme loci was variable and two alleles were detected, both of which were found in two populations, while only one genotype was detected in the other two populations. ISSR and RAPD markers detected more genotypes within populations than did allozymes. ISSR markers detected more diversity than RAPD markers in three of the four populations examined. In one population, no RAPD diversity was found whereas eight different genotypes were found among the 10 plants with ISSR markers. This diversity is present despite rare flowering, no documented occurrence of seed set in natural populations and very low seed set with experimental pollinations, all of which suggest that sexual reproduction rarely occurs. The subspecies is self-compatible, but seed initiation is lower in selfed ovules; also, there is high embryo abortion regardless of pollen source. Variation detected by RAPD and ISSR primers may reflect higher levels of sexual reproduction in the past, very rare sexual reproduction in extant populations, somatic mutations, or a combination of the three. Although the PCR-based markers identify several multilocus genotypes within populations, it is not known whether these all represent distinct genets generated by sexual reproduction or result from somatic mutations in the old, perennial and highly clonal plants.     

The roles of female and hermaphroditic flowers in the gynodioecious–gynomonoecious Silene littorea: insights into the phenology of sex expression

Some gynodioecious species have intermediate individuals that bear both female and hermaphroditic flowers. This phenomenon is known as a gynodioecious–gynomonoecious sexual system. Gender expression in such species has received little attention in the past, and the phenologies of male and female functions have also yet to be explored. In this study, we examined variations in gender patterns, their effects on female reproductive success and sex expression in depth throughout the flowering period in two populations. The studied populations of Silene littorea contained mostly gynomonoecious plants and the number of pure females was very low. The gynomonoecious plants showed high variability in the total proportion of female flowers. In addition, the proportion of female flowers in each plant varied widely across the flowering season. Although there was a trend towards maleness, our measures of functional gender suggested that most plants transmit their genes via both pollen and ovules. Fruit set and seed set were not significantly different among populations; in contrast, flower production significantly varied between the two populations – and among plants – with consequent variation in total seed production. Conversely, gender and sex expression were similar in both populations. Plants with higher phenotypic femaleness did not have higher fruit set, seed set or total female fecundity. The mating environment fluctuated little across the flowering period, but fluctuations were higher in the population with low flower production. We therefore conclude that the high proportion of gynomonoecious individuals in our studied populations of S. littorea may be advantageous for the species, providing the benefits of both hermaphroditic and female flowers.     

Do annual and perennial populations of an insect-pollinated plant species differ in mating system?

Background and AimsTheory predicts that outcrossing should be more prevalent among perennials than annuals, a pattern confirmed by comparative evidence from diverse angiosperm families. However, intraspecific comparisons between annual and perennial populations are few because such variation is uncommon among flowering plants. Here, we test the hypothesis that perennial populations outcross more than annual populations by investigating Incarvillea sinensis, a wide-ranging insect-pollinated herb native to China. The occurrence of both allopatric and sympatric populations allows us to examine the stability of mating system differences between life histories under varying ecological conditions.MethodsWe estimated outcrossing rates and biparental inbreeding in 16 allopatric and five sympatric populations in which both life histories coexisted using 20 microsatellite loci. In each population we measured height, branch number, corolla size, tube length and herkogamy for ~30 individuals. In a sympatric population, we recorded daily flower number, pollinator visitation and the fruit and seed set of annual and perennial plants.Key ResultsAs predicted, outcrossing rates (t) were considerably higher in perennial (mean = 0.76) than annual (mean = 0.09) populations. This difference in mating system was also maintained at sympatric sites where plants grew intermixed. In both allopatric and sympatric populations the degree of herkogamy was consistently larger in outcrossing than selfing plants. Perennials were more branched, with more and larger flowers than in annuals. In a sympatric population, annuals had a significantly higher fruit and seed set than perennials.ConclusionsGenetically based differences in herkogamy between annuals and perennials appear to play a key role in governing outcrossing rates in populations, regardless of variation in local ecological conditions. The maintenance of mating system and life history trait differentiation between perennial and annual populations of I. sinensis probably results from correlated evolution in response to local environmental conditions.     

Pollinators and Reproductive Success of the Wild Cucurbit Cucurbita maxima ssp. andreana (Cucurbitaceae)

Abstract: We studied the reproductive success and pollinators of Cucurbita maxima ssp. andreana in different disturbed habitats where it grows naturally. Data were obtained from three populations. One grew within a soybean crop, the other within a corn crop, and the third in an abandoned crop field. Cucurbita maxima ssp. andreana is an annual vine with a flowering period from December to April. Male flowers appear first, thereafter female and male flowers appear together. Flower lifetime (9 h) was similar in male and female flowers. The pollinator guild was comparable for the three populations but some differences in the frequency of the insect species were observed. Native bees were the main pollinators in the population in the abandoned field, while beetles pollinated the populations in crop fields. These differences were not linked with the pre-emergent reproductive success, fruit and seed set, or fruit quality. This is a self-compatible plant. Fruit and seed set and fruit traits (total mass, width and length of fruits, number of seeds per fruit, and seed mass) did not show significant differences between hand-cross and hand-self pollinated flowers. This wild cucurbit is a generalist with respect to pollinator guild, and flower visitors seem to be highly efficient in pollen transference. Cucurbita maxima ssp. andreana is well adapted to disturbed habitats because plants ripened fruits successfully, regardless of the group of insects visiting flowers.     

Pollen and Resource Limitation in Veratrum nigrum L. （Liliaceae）, an Andromonoecious Herb

Pollen limitation and resource limitation were invoked to account for the pattern that flowering plants produce more flowers and ovules than fruits and seeds. This study aimed to determine their relative importance in Veratrum nigrum, a self-compatible, perennial, andromonoecious herb. In order to determine whether female production was limited by pollen grains on stigmas or by available resources, we performed supplemental hand pollination in three populations, male-flower-bud removal in three other populations, and emasculation of hermaphroditic flowers in still another population, resulting in a total of seven populations experimentally manipulated. Across the three populations, supplemental hand pollination did not significantly increase fruit set, seed number per fruit, and total seed production per individual, nor did emasculation of hermaphroditic flowers. Taken together, our results suggest that pollen grains deposited on stigmas were abundant enough to fertilize all the ovules. Male-flower-bud removal significantly increased the mean size of hermaphroditic flowers in all three populations. Female reproductive success was increased in one population, but not in the other two populations possibly due to heavy flower/seed predation. We concluded that the female reproductive success of V. nigrum was not limited by pollen grains but by available resources, which is consistent with Bateman＇s principle. Furthermore, the female reproduction increase of male-flower-bud removal individuals might suggest a trade-off between male and female sexual functions.     

Differences in Seed Production Between Stellaria media Populations from Different Habitat Types

Seed production in several populations of Stellaria media fromdifferent types of habitat was examined. Two populations, onefrom a flower garden and the other from a herring gull colonywere compared in detail by growing them at three soil fertilitylevels under similar conditions in a greenhouse. Three important differences between the two populations independentof soil fertility emerged: the garden plants flowered on averageeleven weeks earlier, they produced much smaller seeds, andthey produced more seeds per capsule. Other differences were influenced by soil fertility: at highfertility the garden plants produced about 50 per cent moreseeds than the gull colony plants, but on a seed weight basis,the gull colony plants were the more productive. However forboth populations at high fertility the proportion of shoot biomassallocated to seed production was similar. Some of these differences can be accounted for in terms of conditionsoccurring in the two habitats, in particular the relative importanceof disturbance and competition at the sites. Chickweed, Stellaria media (L.) Vill., ecotypic differentiation, time to flowering, seed production, resource allocation     

Heavy metals in soil affect reproductive processes more than morphological characters in Viola tricolor

Viola tricolor (Violaceae), a species very differentiated morphologically and showing intra- and interpopulation variability, occupies metalliferous (Zn, Pb, Cd, Cu) and nonmetalliferous sites through its geographic range. Here we analyzed morphological and anatomical features and also sexual reproduction in metallicolous and nonmetallicolous populations to determine whether and how they differ.Two-dimensional correspondence analysis based on selected morphological characters of vegetative and generative organs showed that plants from metalliferous soils did not form a compact group separated from those growing on nonmetalliferous soils. SEM of leaf and stipule anatomy showed differences in leaf hair ornamentation. There were significant differences in embryological processes in ovules and anthers: disturbed microsporogenesis (metallicolous 33% vs. nonmetallicolous 18%), lower pollen stainability (75% and 78% vs. 84% and 93%, depending on test), and higher frequency of degeneration in ovules (23% vs. 4%). These ultimately did not impede sexual reproduction of metallicolous populations but they do provide evidence that reproductive processes are sensitive to elevated heavy metals in soil and therefore can be viewed as a cost of metal tolerance.     

Microgametophyte population sizes and plant reproductive output in the insect-pollinated Prunella grandiflora (Lamiaceae)

Intraspecific variation in pollen deposition and number of pollen tubes per style is rarely quantified, but is essential for assessing the occurrence of pollen limitation and pollen competition and their evolutionary implications. Moreover, pollen deposition, pollen tube growth, and the fate of fertilized ovules are rarely distinguished in field studies. Here we present such a study in eight natural populations of Prunella grandiflora. We quantified microgametophyte population sizes and inferred pollen limitation when the number of fertilizable ovules exceeded pollen tubes, and assessed seed set and fate after open pollination. Two and three populations had on average significantly fewer pollen grains and pollen tubes per flower, respectively, than the fixed number of fertilizable ovules per fruit, while one population experienced significant pollen competition. Style length was positively correlated with the number of pollen tubes. While pollen availability was very variable, seed abortion was significantly less frequent in denser populations, and in one population the proportion of well-developed seeds was significantly, positively correlated with the number of pollen tubes in the style. Less pollen deposition, lower numbers of pollen tubes reaching the base of the style, lower pollen quality and therefore increased abortion of fertilized ovules can all reduce seed set in natural P. grandiflora stands. Substantial intraspecific variability implies that microgametophyte competition also occurs in this species. Finally, style morphology may affect pollen receipt.     

Pollination and reproduction of an invasive plant inside and outside its ancestral range

Comparing traits of invasive species within and beyond their ancestral range may improve our understanding of processes that promote aggressive spread. Solanum elaeagnifolium (silverleaf nightshade) is a noxious weed in its ancestral range in North America and is invasive on other continents. We compared investment in flowers and ovules, pollination success, and fruit and seed set in populations from Arizona, USA (“AZ”) and Greece (“GR”). In both countries, the populations we sampled varied in size and types of present-day disturbance. Stature of plants increased with population size in AZ samples whereas GR plants were uniformly tall. Taller plants produced more flowers, and GR plants produced more flowers for a given stature and allocated more ovules per flower. Similar functional groups of native bees pollinated in AZ and GR populations, but visits to flowers decreased with population size and we observed no visits in the largest GR populations. As a result, plants in large GR populations were pollen-limited, and estimates of fecundity were lower on average in GR populations despite the larger allocation to flowers and ovules. These differences between plants in our AZ and GR populations suggest promising directions for further study. It would be useful to sample S. elaeagnifolium in Mediterranean climates within the ancestral range (e.g., in California, USA), to study asexual spread via rhizomes, and to use common gardens and genetic studies to explore the basis of variation in allocation patterns and of relationships between visitation and fruit set.     

Patterns of Differentiation in Wiregrass (Aristida beyrichiana): Implications for Restoration Efforts

Aristida beyrichiana (wiregrass) is increasingly being planted in restoration projects across the southeastern coastal plain, with little focus on genetic differences among populations across the region. Local and regional population differentiation for establishment and growth traits were examined in common garden and reciprocal transplant experiments. Seeds from up to 20 plants from each of seven populations were collected in northern and central Florida sites that encompassed gradients of soils, hydrology, and temperature. Reciprocal seed transplants using three of the common garden populations were conducted in two consecutive years. In the common garden, significant population differences were seen in seed weight, seedling emergence and survival, tiller height, number of tillers, the relationship between tiller number and tiller height, and flowering. Variation among maternal families was seen in tiller number and in the relationship between tiller number and tiller height. The reciprocal transplant study did not detect either local adaptation to sites of origin or consistent superiority of one source population or planting site in seedling establishment. These results suggest that the probability of seedling establishment is primarily dependent on environmental conditions rather than genetic differences. Genetic variation for traits related to fitness (e.g., tiller number) may be retained within populations because phenotypically plastic growth responses of seedlings to environmental variation buffer genetic variation against the action of selection. But despite the lack of evidence for genetic influences on initial establishment in wiregrass, our common garden study suggests genetic differences among populations. This result, when combined with previous results indicating local adaptation in later life stages of wiregrass, suggests that restoration efforts involving this species should use local seed sources from sites with similar soil and hydrological conditions.     

Divergent mating systems and parental conflict as a barrier to hybridization in flowering plants

Parental conflicts can lead to antagonistic coevolution of the sexes and of parental genomes. Within a population, the resulting antagonistic effects should balance, but crosses between populations can reveal conflict. Parental conflict is less intense in self-pollinating plants than in outcrossers because outcrossing plants are pollinated by multiple pollen donors unrelated to the seed parent, while a self-pollinating plant is primarily pollinated by one individual (itself). Therefore, in crosses between plants with differing mating systems, outcrossing parents are expected to "overpower" selfing parents. We call this the weak inbreeder/strong outbreeder (WISO) hypothesis. Prezygotically, such overpowering can alter pollination success, and we argue that our hypothesis explains a common pattern of unilateral incompatibility, in which pollen from self-incompatible populations fertilizes ovules of self-compatible individuals but the reciprocal cross fails. A postzygotic manifestation of overpowering is aberrant seed development due to parent-of-origin effects such as genomic imprinting. We evaluate evidence for the WISO hypothesis by reviewing published accounts of crosses between plants of different mating systems. Many, but not all, of such reports support our hypothesis. Since parental conflicts can perturb fertilization and development, such conflicts may strengthen reproductive barriers between populations, contributing to speciation.     

Quantifying the effects of nutrient addition on the taxonomic distinctness of serpentine vegetation

Traditional diversity indices summarize the information about the relative abundances of species within a community without regard to differences between species. However, intuitively, a community composed of dissimilar taxa is more diverse than a community composed of more similar taxa. Therefore, useful indices of diversity should account for taxonomic relations among species. In this paper, a new parametric diversity index that combines species relative abundances and their taxonomic distinctiveness is used to quantify the way in which soil fertilization affects the diversity of a garigue community on ultramafic soils of Tuscany (central Italy). Results show that, while ultramafic soils generally host plant communities of limited taxonomic diversity with respect to similar communities on other substrates, fertilization significantly enhances the biomass production of species that are not exclusive to ultramafic soils. As a consequence, if diversity is measured combining species relative abundances with their taxonomic distinctiveness, nutrient addition tends to increase the diversity of ultramafic communities.