Simulation analyses of the evolution of intra-inflorescence flowering patterns assuming selection on anthesis interval among individual flowers

What conditions select flowering patterns within inflorescences, or variation in the anthesis interval within inflorescences among plants? Under what conditions are gradual blooming and simultaneous blooming, both traits related to floral display size, advantageous? We constructed a simulation model in which the opening times and longevities of individual flowers within inflorescences, the sizes of attractive structures of individual flowers, and the numbers of ovules and pollen grains produced by individual flowers evolve. Individual plants in the population compete for pollinators, and plants are selected by pollinators according to their floral display sizes and amounts of resources allocated to attractive structures. We found that, if the proportion of pollen on a pollinator deposited on a stigma was low, gradual blooming did not evolve even if inbreeding depression was greater than 0.5. This is because the amount of outcross-pollen on pollinators decreased at a low rate during flower visits within a single inflorescence, and the selfing rate was suppressed to a low level even if the floral display size was large. On the other hand, if the proportion of pollen deposition was high, gradual blooming evolved even if inbreeding depression was smaller than 0.5. This may be because gradual blooming can enhance pollen delivery to other plants by reducing the loss of self-pollen by geitonogamy. On the other hand, allocation ratios among floral organs (female and male organs and attractive structures) were independent of the degree of simultaneous and gradual blooming within inflorescences. We concluded that the evolution of gradual blooming is more strongly affected by the proportion of pollen on a pollinator deposited on a stigma than by inbreeding depression.

Parallel evolution of morphological and genomic selfing syndromes accompany the breakdown of heterostyly

Evolutionary transitions from outcrossing to selfing in flowering plants have convergent morphological and genomic signatures and can involve parallel evolution within related lineages. Adaptive evolution of morphological traits is often assumed to evolve faster than nonadaptive features of the genomic selfing syndrome. We investigated phenotypic and genomic changes associated with transitions from distyly to homostyly in the Primula oreodoxa complex. We determined whether the transition to selfing occurred more than once and investigated stages in the evolution of morphological and genomic selfing syndromes using 22 floral traits and both nuclear and plastid genomic data from 25 populations. Two independent transitions were detected representing an earlier and a more recently derived selfing lineage. The older lineage exhibited classic features of the morphological and genomic selfing syndrome. Although features of both selfing syndromes were less developed in the younger selfing lineage, they exhibited parallel development with the older selfing lineage. This finding contrasts with the prediction that some genomic changes should lag behind adaptive changes to morphological traits. Our findings highlight the value of comparative studies on the timing and extent of transitions from outcrossing to selfing between related lineages for investigating the tempo of morphological and molecular evolution.

Inheritance of distyly and homostyly in self-incompatible Primula forbesii

The evolutionary transition from self-incompatible distyly to self-compatible homostyly frequently occurs in heterostylous taxa. Although the inheritance of distyly and homostyly has been deeply studied, our understanding on modifications of the classical simple Mendelian model is still lacking. Primula forbesii, a biennial herb native to southwest China, is a typical distylous species, but after about 20 years of cultivation with open pollination, self-compatible homostyly appeared, providing ideal material for the study of the inheritance of distyly and homostyly. In this study, exogenous homobrassinolide was used to break the heteromorphic incompatibility of P. forbesii. Furthermore, we performed artificial pollination and open-pollination experiments to observe the distribution of floral morphs in progeny produced by different crosses. The viability of seeds from self-pollination was always the lowest among all crosses, and the homozygous S-morph plants (S/S) occurred in artificial pollination experiments but may experience viability selection. The distyly of P. forbesii is governed by a single S-locus, with S-morph dominant hemizygotes (S/-) and L-morph recessive homozygotes (-/-). Homostylous plants have a genotype similar to L-morph plants, and homostyly may be caused by one or more unlinked modifier genes outside the S-locus. Open pollinations confirm that autonomous self-pollination occurs frequently in L-morphs and homostylous plants. This study deepens the understanding of the inheritance of distyly and details a case of homostyly that likely originated from one or more modifier genes.

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

BACKGROUND AND AIMS

Theory 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.

METHODS

We 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 RESULTS

As 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.

CONCLUSIONS

Genetically 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.

Phylogenomic analysis reveals multiple evolutionary origins of selfing from outcrossing in a lineage of heterostylous plants

Evolutionary transitions from outcrossing to selfing often occur in heterostylous plants. Selfing homostyles originate within distylous populations and frequently evolve to become reproductively isolated species. We investigated this process in 10 species of Primula section Obconicolisteri using phylogenomic approaches and inferred how often homostyly originated from distyly and its consequences for population genetic diversity and floral trait evolution. We estimated phylogenetic relationships and reconstructed character evolution using the whole plastome comprised of 76 protein-coding genes. To investigate mating patterns and genetic diversity we screened 15 microsatellite loci in 40 populations. We compared floral traits among distylous and homostylous populations to determine how phenotypically differentiated homostyles were from their distylous ancestors. Section Obconicolisteri was monophyletic and we estimated multiple independent transitions from distyly to homostyly. High selfing rates characterised homostylous populations and this was associated with reduced genetic diversity. Flower size and pollen production were reduced in homostylous populations, but pollen size was significantly larger in some homostyles than in distylous morphs. Repeated transitions to selfing in section Obconicolisteri are likely to have been fostered by the complex montane environments that species occupy. Unsatisfactory pollinator service is likely to have promoted reproductive assurance in homostyles leading to subsequent population divergence through isolation.

Geographic variation of reproductive traits and competition for pollinators in a bird-pollinated plant

Geographic variation in the reproductive traits of animal-pollinated plants can be shaped by spatially variable selection imposed by differences in the local pollination environment. We investigated this process in Babiana ringens (Iridaceae), an enigmatic species from the Western Cape region of South Africa. B. ringens has evolved a specialized perch facilitating cross-pollination by sunbirds and displays striking geographic variation in perch size and floral traits. Here, we investigate whether this variation can be explained by geographic differences in the pollinator communities. We measured floral and inflorescence traits, and abiotic variables (N, P, C, and rainfall) and made observations of sunbirds in populations spanning the range of B. ringens. In each population, we recorded sunbird species identity and measured visitation rates, interfloral pollen transfer, and whether the seed set of flowers was pollen limited. To evaluate whether competition from co-occurring sunbird-pollinated species might reduce visitation, we quantified nectar rewards in B. ringens and of other co-flowering bird-pollinated species in local communities in which populations occurred. Variation in abiotic variables was not associated with geographical variation of traits in B. ringens. Malachite sunbirds were the dominant visitor (97% of visits) and populations with larger-sized traits exhibited higher visitation rates, more between-flower pollen transfer and set more seed. No sunbirds were observed in four populations, all with smaller-sized traits. Sunbird visitation to B. ringens was not associated with local sunbird activity in communities, but sunbird visitation was negatively associated with the amount of B. ringens sugar relative to the availability of alternative nectar sources. Our study provides evidence that B. ringens populations with larger floral traits are visited more frequently by sunbirds, and we propose that visitation rates to B. ringens may be influenced, in part, by competition with other sunbird-pollinated species.

Genetics of distyly and homostyly in a self-compatible Primula

The transition from outcrossing to selfing through the breakdown of distyly to homostyly has occurred repeatedly among families of flowering plants. Homostyles can originate by major gene changes at the S-locus linkage group, or by unlinked polygenic modifiers. Here, we investigate the inheritance of distyly and homostyly in Primula oreodoxa, a subalpine herb endemic to Sichuan, China. Controlled self- and cross-pollinations confirmed that P. oreodoxa unlike most heterostylous species is fully self-compatible. Segregation patterns indicated that the inheritance of distyly is governed by a single Mendelian locus with the short-styled morph carrying at least one dominant S-allele (S-) and long-styled plants homozygous recessive (ss). Crossing data were consistent with a model in which homostyly results from genetic changes at the distylous linkage group, with the homostylous allele (S^h) dominant to the long-styled allele (s), but recessive to the short-styled allele (S). Progeny tests of open-pollinated seed families revealed high rates of intermorph mating in the L-morph but considerable selfing and possibly intramorph mating in the S-morph and in homostyles. S-morph plants homozygous at the S-locus (SS) occurred in several populations but may experience viability selection. The crossing data from distylous and homostylous plants are consistent with either recombination at the S-locus governing distyly, or mutation at gene(s) controlling sex-organ height; both models predict the same patterns of segregation. Recent studies on the molecular genetics of distyly in Primula demonstrating the hemizygous nature of genes at the S-locus make it more likely that homostyles have resulted from mutation rather than recombination.

NEUTRAL GENETIC DIVERSITY IN A METAPOPULATION WITH RECURRENT LOCAL EXTINCTION AND RECOLONIZATION

Many species exist as metapopulations in balance between local population extinction and recolonization, processes that may strongly affect the distribution of neutral genetic diversity within demes and in the metapopulation as a whole. In this paper we use both the infinite-alleles and the infinite-sites models to reframe Slatkin's propagulepool and migrant-pool models in terms of mean within-deme and among-deme genetic diversity; the infinite-sites model is particularly relevant to DNA sequence data. Population turnover causes a major reduction in neutral genetic diversity within demes, π_S , and in the metapopulation as a whole, π_t . This effect is particularly strong for propagulepool colonization, in which colonists are drawn from a single extant deme. Because metapopulation dynamics affect both within-deme and total metapopulation diversity similarly, comparisons between species with different ecologies on the basis of ratios such as F_ST are difficult to interpret and absolute measures of divergence between populations should be used as well. Although the value of F_ST in a metapopulation with local extinction depends strongly on the mode of colonization, this has almost no effect on the numerator of the F_ST ratio, π_t - π_S , so that F_ST is influenced mainly by the effect of the colonization mode on the denominator (π_t ). Our results also indicate that it is inappropriate to use measures of average within-deme diversity in species with population turnover to estimate the scaled mutation rate, θ, because extinction can greatly reduce π_S . Finally, we discuss the effect of population turnover on the effective size of a metapopulation.

POLLEN DISCOUNTING AND THE SPREAD OF A SELFING VARIANT IN TRISTYLOUS EICHHORNIA PANICULATA: EVIDENCE FROM EXPERIMENTAL POPULATIONS

Floral traits that increase self-fertilization are expected to spread unless countered by the effects of inbreeding depression, pollen discounting (reduced outcross pollen success by individuals with increased rates of self-fertilization), or both. Few studies have attempted to measure pollen discounting because to do so requires estimating the male outcrossing success of plants that differ in selfing rate. In natural populations of tristylous Eichhornia paniculata, selfing variants of the mid-styled morph are usually absent from populations containing all three style morphs but often predominate in nontrimorphic populations. We used experimental garden populations of genetically marked plants to investigate whether the effects of population morph structure on relative gamete transmission by unmodified (M) and selfing variants (M') of the mid-styled morph could explain their observed distribution. Transmission through ovules and self and outcross pollen by plants of the M and M' morphs were compared under trimorphic, dimorphic (S morph absent), and monomorphic (L and S morphs absent) population structures. Neither population structure nor floral morphology affected female reproductive success, but both had strong effects on the relative transmission of male gametes. The frequency of self-fertilization in the M' morph was consistently higher than that of the M morph under all morph structures, and the frequency of self-fertilization by both morphs increased as morph diversity of experimental populations declined. In trimorphic populations, total transmission by the M and M' morphs did not differ. The small, nonsignificant increase in selfing by the M' relative to the M morph was balanced by decreased outcross siring success, particularly on the S morph. In populations lacking the S morph, male gamete transmission by the M' morph was approximately 1.5 times greater than that by the M morph because of both increased selfing and increased success through outcross pollen donation. Therefore, gamete transmission strongly favored the M' morph only in the absence of the S morph, a result consistent with the distribution of the M' morph in nature. This study indicates that floral traits that alter the selfing rate can have large and context-dependent influences on outcross pollen donation.

EXPERIMENTAL STUDIES ON THE FUNCTIONAL SIGNIFICANCE OF HETEROSTYLY

Heterostyly has been viewed as both an antiselfing device and a mechanism that increases the proficiency of pollen transfer between plants. We used experimental manipulation of the morph structure of garden populations of self-compatible, tristylous Eichhornia paniculata to investigate the function of floral polymorphism. Outcrossing rates (t), levels of intermorph mating (d), and morph-specific male and female reproductive success were compared in replicate trimorphic and monomorphic populations. In trimorphic populations, t and d averaged 0.81 (2 SE = 0.03) and 0.77 (2 SE = 0.03) respectively, with no difference in either parameter among morphs. Ninety-five percent of outcrossed seeds were therefore the result of intermorph fertilizations. Male reproductive success of the long-styled morph was low, especially in comparison with plants of the short-styled morph. Outcrossing rates for each morph were higher in trimorphic than monomorphic populations where t averaged 0.71 (2 SE = 0.01), 0.30 (2 SE = 0.04) and 0.43 (2 SE = 0.1) for the long-, mid-, and short-styled morphs, respectively. Seed set was lower in monomorphic populations, particularly those composed of the L morph, reflecting reduced pollen deposition. Floral polymorphism therefore increased both outcrossing rate and fecundity but the magnitude of the differences varied among morphs. If the ancestral condition in heterostylous groups resembled the L morph, as has been suggested, data from this study suggests that the selective basis for the establishment of floral polymorphism could have been increased pollen transfer rather than higher levels of outcrossing.

Recent mating-system evolution in Eichhornia is accompanied by cis-regulatory divergence

The evolution of predominant self-fertilization from cross-fertilization in plants is accompanied by diverse changes to morphology, ecology and genetics, some of which likely result from regulatory changes in gene expression. We examined changes in gene expression during early stages in the transition to selfing in populations of animal-pollinated Eichhornia paniculata with contrasting mating patterns. We crossed plants from outcrossing and selfing populations and tested for the presence of allele-specific expression (ASE) in floral buds and leaf tissue of F1 offspring, indicative of cis-regulatory changes. We identified 1365 genes exhibiting ASE in floral buds and leaf tissue. These genes preferentially expressed alleles from outcrossing parents. Moreover, we found evidence that genes exhibiting ASE had a greater nonsynonymous diversity compared to synonymous diversity in the selfing parents. Our results suggest that the transition from outcrossing to high rates of self-fertilization may have the potential to shape the cis-regulatory genomic landscape of angiosperm species, but that the changes in ASE may be moderate, particularly during the early stages of this transition.

Postpollination discrimination between self and outcross pollen covaries with the mating system of a self-compatible flowering plant

PREMISE OF THE STUDY

Variation in the mating system of hermaphroditic plant populations is determined by interactions between genetic and environmental factors operating via both pre- and postmating processes. Models predicting the maintenance of intermediate outcrossing rates in animal-pollinated plants often assume that the mating system is primarily controlled by floral morphology and pollinator availability, but rarely has the influence of postpollination processes on variation in outcrossing been examined.

METHODS

We investigated the influence of stylar discrimination between illegitimate and legitimate pollen-tube growth and the pollen-load capacity of stigmas on mating-system variation in the annual, tristylous species Eichhornia paniculata using controlled crosses and genetic markers. This species exhibits an exceptionally broad range of outcrossing rates in natural populations.

KEY RESULTS

There was significant variation among populations in the pollen-load capacity of stigmas and the ability of styles to discriminate between illegitimate vs. legitimate pollen. There was strong correspondence between stylar-discrimination ability and variation in outcrossing rate among populations and style morphs. The combination of stigmatic pollen-load capacity and stylar discrimination explained more than 80% of the variation in outcrossing rates among populations.

CONCLUSIONS

The finding that stigmatic pollen-load capacity and stylar-discrimination ability contributed significantly to explaining the wide range of outcrossing rates in E. paniculata suggests that postpollination mechanisms play an important role in governing mating patterns in this species. The difference in levels of stylar discrimination between outcrossing and selfing populations may reflect a trade-off between selection for increased outcrossing and greater reproductive assurance.

Dramatic vestigialization of floral fragrance across a transition from outcrossing to selfing in Abronia umbellata (Nyctaginaceae)

PREMISE OF THE STUDY

Vestigialization of traits that no longer enhance fitness is a common theme in evolution. Plants often use colorful, scented flowers to attract pollinators that mediate outcross pollination. After an evolutionary shift from outcrossing to self-fertilization, where cross-pollination is no longer necessary, attractive traits may be reduced, especially because these traits may also attract herbivores. Selection may be particularly strong in moth-pollinated lineages where pollinators are also herbivores.

METHODS

We used field surveys and common garden glasshouse experiments to test this hypothesis by quantifying components of flower size and display and floral volatiles in outcrossing vs. selfing populations of the moth-pollinated Pacific coastal dune endemic Abronia umbellata.

KEY RESULTS

Floral face diameter and flower tube length were 43% and 54% smaller in selfing than outcrossing populations, and selfers displayed 15% fewer flowers per umbel. Selfers emitted 99% less total floral volatiles per flower per hour than outcrossers; a much stronger reduction. The chemical composition of volatiles also differed between outcrossers and selfers. Similar differences were observed in a common glasshouse environment, suggesting genetic differentiation in these floral attractive traits among populations. Contrary to expectations, there were no differences in leaf or flower herbivory between selfing and outcrossing populations.

CONCLUSIONS

Floral fragrance is much more dramatically reduced in selfing compared to outcrossing populations than other floral attractive traits, but probably not due to selection exerted by moth herbivory. Reduction in aspects of flower size may be constrained by developmental linkages with fruit and seed size.

Broad geographic covariation between floral traits and the mating system in Camissoniopsis cheiranthifolia (Onagraceae): multiple stable mixed mating systems across the species' range?

BACKGROUND AND AIMS

Plants vary widely in the extent to which seeds are produced via self-fertilization vs. outcrossing, and evolutionary change in the mating system is thought to be accompanied by genetic differentiation in a syndrome of floral traits. We quantified the pattern of variation and covariation in floral traits and the proportion of seeds outcrossed (t) to better understand the evolutionary processes involved in mating system differentiation among and within populations of the short-lived Pacific coastal dune endemic Camissoniopsis cheiranthifolia across its geographic range in western North America.

METHODS

We quantified corolla width and herkogamy, two traits expected to influence the mating system, for 48 populations sampled in the field and for a sub-sample of 29 populations grown from seed in a glasshouse. We also measured several other floral traits for 9-19 populations, estimated t for 16 populations using seven allozyme polymorphisms, and measured the strength of self-incompatibility for nine populations.

KEY RESULTS

Floral morphology and self-incompatibility varied widely but non-randomly, such that populations could be assigned to three phenotypically and geographically divergent groups. Populations spanned the full range of outcrossing (t = 0·001-0·992), which covaried with corolla width, herkogamy and floral life span. Outcrossing also correlated with floral morphology within two populations that exhibited exceptional floral variation.

CONCLUSIONS

Populations of C. cheiranthifolia seem to have differentiated into three modal mating systems: (1) predominant outcrossing associated with self-incompatibility and large flowers; (2) moderate selfing associated with large but self-compatible flowers; and (3) higher but not complete selfing associated with small, autogamous, self-compatible flowers. The transition to complete selfing has not occurred even though the species appears to possess the required genetic capacity. We hypothesize that outcrossing populations in this species have evolved to different stable states of mixed mating.

De novo sequence assembly and characterization of the floral transcriptome in cross- and self-fertilizing plants

Background

The shift from cross-fertilization to predominant self-fertilization is among the most common evolutionary transitions in the reproductive biology of flowering plants. Increased inbreeding has important consequences for floral morphology, population genetic structure and genome evolution. The transition to selfing is usually characterized by a marked reduction in flower size and the loss of traits involved in pollinator attraction and the avoidance of self-fertilization. Here, we use short-read sequencing to assemble, de novo, the floral transcriptomes of three genotypes of Eichhornia paniculata, including an outcrosser and two genotypes from independently derived selfers, and a single genotype of the sister species E. paradoxa. By sequencing mRNA from tissues sampled at various stages of flower development, our goal was to sequence and assemble the floral transcriptome and identify differential patterns of gene expression.

Results

Our 24 Mbp assembly resulted in ~27,000 contigs that averaged ~900 bp in length. All four genotypes had highly correlated gene expression, but the three E. paniculata genotypes were more correlated with one another than each was to E. paradoxa. Our analysis identified 269 genes associated with floral development, 22 of which were differentially expressed in selfing lineages relative to the outcrosser. Many of the differentially expressed genes affect floral traits commonly altered in selfing plants and these represent a set of potential candidate genes for investigating the evolution of the selfing syndrome.

Conclusions

Our study is among the first to demonstrate the use of Illumina short read sequencing for de novo transcriptome assembly in non-model species, and the first to implement this technology for comparing floral transcriptomes in outcrossing and selfing plants.

Modification of flower architecture during early stages in the evolution of self-fertilization

BACKGROUND AND AIMS

The evolution of selfing from outcrossing is characterized by a series of morphological changes to flowers culminating in the selfing syndrome. However, which morphological traits initiate increased self-pollination and which are accumulated after self-fertilization establishes is poorly understood. Because the expression of floral traits may depend on the conditions experienced by an individual during flower development, investigation of changes in mating system should also account for environmental and developmental factors. Here, early stages in the evolution of self-pollination are investigated by comparing floral traits among Brazilian populations of Eichhornia paniculata (Pontederiaceae), an annual aquatic that displays variation in selfing rates associated with the breakdown of tristyly to semi-homostyly.

METHODS

Thirty-one Brazilian populations under uniform glasshouse conditions were compared to investigate genetic and environmental influences on flower size and stigma-anther separation (herkogamy), two traits that commonly vary in association with transitions to selfing. Within-plant variation in herkogamy was also examined and plants grown under contrasting environmental conditions were compared to examine to what extent this trait exhibits phenotypic plasticity.

KEY RESULTS

In E. paniculata a reduction in herkogamy is the principal modification initiating the evolution of selfing. Significantly, reduced herkogamy was restricted to the mid-styled morph and occurred independently of flower size. Significant genetic variation for herkogamy was detected among populations and families, including genotypes exhibiting developmental instability of stamen position with bimodal distributions of herkogamy values. Cloned genets exposed to contrasting growth conditions demonstrated environmental control of herkogamy and genotypic differences in plasticity of this trait.

CONCLUSIONS

The ability to modify herkogamy independently of other floral traits, genetic variation in the environmental sensitivity of herkogamy, and the production of modified and unmodified flowers within some individuals, reveal the potential for dynamic control of the mating system in a species that commonly confronts heterogeneous aquatic environments.

Evidence for reductions in floral attractants with increased selfing rates in two heterandrous species

Although theoretical models predict low allocation to attractive structures with increased selfing in animal-pollinated plants, empirical measurement of the reproductive costs and benefits is complicated. Here, floral sex allocation was compared in two nectarless heterandrous species with different mating systems: Monochoria korsakowii (Pontederiaceae), which has moderate outcrossing rates, and Monochoria vaginalis, a predominant selfer. In both species, mirror-image flowers have one large dark-purple anther and five small yellow anthers. Experimental evidence is provided for functional differences between the two sets of anthers using data on pollinator visitation, pollen removal and deposition, and seed set after hand pollinations. Flower manipulations in bee-pollinated M. korsakowii demonstrated different functions of the two sets of anthers: the yellow (feeding) anthers function to attract pollinators, but have similar pollen performance to the purple (pollinating) anthers. Furthermore, a disproportional reduction in pollen production of the feeding anthers in the selfing species was found. This differential allocation between feeding and pollinating anthers in heterandrous species has not been recognized before. The finding of reduced allocation to attractive structures with an increase in the rate of self-fertilization supports the theory of sex allocation.

Evolution of floral display in Eichhornia paniculata (Pontederiaceae): direct and correlated responses to selection on flower size and number

Trade-offs between flower size and number seem likely to influence the evolution of floral display and are an important assumption of several theoretical models. We assessed floral trade-offs by imposing two generations of selection on flower size and number in a greenhouse population of bee-pollinated Eichhornia paniculata. We established a control line and two replicate selection lines of 100 plants each for large flowers (S+), small flowers (S-), and many flowers per inflorescence (N+). We compared realized heritabilities and genetic correlations with estimates based on restricted-maximum-likelihood (REML) analysis of pedigrees. Responses to selection confirmed REML heritability estimates (flower size, h2 = 0.48; daily flower number, h2 = 0.10; total flower number, h2 = 0.23). Differences in nectar, pollen, and ovule production between S+ and S- lines supported an overall divergence in investment per flower. Both realized and REML estimates of the genetic correlation between daily and total flower number were r = 1.0. However, correlated responses to selection were inconsistent in their support of a trade-off. In both S- lines, correlated increases in flower number indicated a genetic correlation of r = -0.6 between flower size and number. In contrast, correlated responses in N+ and S+ lines were not significant, although flower size decreased in one N+ line. In addition, REML estimates of genetic correlations between flower size and number were positive, and did not differ from zero when variation in leaf area and age at first flowering were taken into account. These results likely reflect the combined effects of variation in genes controlling the resources available for flowering and genes with opposing effects on flower size and number. Our results suggest that the short-term evolution of floral display is not necessarily constrained by trade-offs between flower size and number, as is often assumed.

Evolution of the self-pollinating flower in Clarkia xantiana (Onagraceae). I. Size and development of floral organs

Clarkia xantiana has two subspecies that differ in breeding system: ssp. xantiana, which is outcrossing, and ssp. parviflora, which is self-fertilizing. Outcrossing is the ancestral breeding system for the genus Clarkia. Flowers of ssp. parviflora have characteristics commonly associated with selfing taxa: they are smaller and have little temporal and spatial separation between mature anthers and stigma (dichogamy and herkogamy, respectively). Flower morphology and development were studied in four populations of each subspecies to establish the developmental changes that occurred in the evolution of selfing. In particular, we sought to evaluate the hypothesis that the selfing flower may have arisen as a byproduct of selection for rapid maturation in the arid environment occupied by ssp. parviflora. This hypothesis predicts that development time should be reduced in spp. parviflora relative to ssp. xantiana. We also sought to compare the pattern of covariation of flower morphology and development between subspecies to that within subspecies. Similar within vs. between patterns of covariation could be indicative of developmental or functional constraints on the independent evolution of floral parts. In spite of significant variation among populations within subspecies, the subspecies clearly differ in flower morphology and development. All floral organs, except ovaries, are smaller in ssp. parviflora than in ssp. xantiana. The flower plastochron, the duration of flower development from bud initiation to anthesis, and the duration of protandry are all shorter in ssp. parviflora than in ssp. xantiana. Maximum relative growth rates are higher for all organs in ssp. parviflora than in ssp. xantiana. Thus, progenesis (i.e., via a reduction in development time) is combined with growth acceleration in the evolution of the selfing flower. Since reduced development time and growth acceleration both allow selfing flowers to mature earlier than outcrossing ones, selection for early maturation may have contributed to the evolution of the selfing flower form. The pattern of trait covariation differs within spp. parviflora relative to the patterns within spp. xantiana and between the two subspecies, suggesting that floral parts can and have evolved independently of one another.

The mating consequences of sexual segregation within inflorescences of flowering plants

Many co-sexual plants segregate female and male function among flowers on an inflorescence through dichogamy or the production of unisexual flowers. Sexual segregation may reduce self-pollination among flowers within inflorescences (geitonogamy), thereby increasing the pollen available for export to other plants. To assess these complementary roles we manipulated the simultaneously hermaphroditic (adichogamous) flowers of Eichhornia paniculata to produce ten-flowered inflorescences with either female above male flowers (female/male inflorescences) or male/female inflorescences, which competed for mating opportunities with five-flowered adichogamous inflorescences. Because of the upward movement of bumble-bees, selfing increased upward in adichogamous inflorescences (overall female selfing rate s+/-s.e.=0.320+/-0.026). Female flowers of male/female inflorescences selfed less than flowers in corresponding positions in adichogamous inflorescences so s fell to 0.135+/-0.027. In contrast, all-female flowers of female/male inflorescences selfed similarly to upper flowers on adichogamous inflorescences, elevating s (0.437+/-0.043). During 1997, male/female inflorescences sired more outcrossed seeds than female/male or adichogamous inflorescences, whereas during 1994 flowers on male/female inflorescences received fewer visits than those of adichogamous inflorescences, reducing their outcross siring success. Hence, sexual segregation limits geitonogamy and enhances outcross siring success when it does not affect pollinator behaviour, illustrating the importance of both female and male function in inflorescence design.

Negative correlation between male allocation and rate of self-fertilization in a hermaphroditic animal

Sex-allocation theory predicts that the evolution of increased rates of self-fertilization should be accompanied by decreased allocation to male reproduction (sperm production and broadcast). This prediction has found support in plants but has not previously been tested in animals, which, in contrast to biotically pollinated plants, are free of complications associated with incorporating the costs of attractive structures such as petals. Here we report rates of self-fertilization as well as proportional allocation to male reproductive tissues within populations of the simultaneous hermaphrodite Utterbackia imbecillis, a freshwater mussel. Individuals from populations with higher selfing rates devoted a lower proportion of reproductive tissue to sperm production (correlation = -0.99), in support of theory.