Elevation-related variation in the population characteristics of distylous Primula nivalis affects female fitness and inbreeding depression

The population characteristics of distylous species are highly sensitive to stochastic natural selection pressure.Therefore,populations growing under different environmental conditions may vary in floral morph ratios,potentially affecting female fitness and leading to inbreeding depression.However,the variation in offspring quality among populations as a result of inbreeding depression is poorly understood in distylous species.This study investigates variations in plant density,seed mass,seed viabilityfemale fitness,and post-dispersal inbreeding depression in both sexual morphs(long-styled and shortstyled plants)of the distylous Primula nivalis that were subjected to different pollination treatments along an elevational gradient from 1657 to 2704 m a.s.l.Population characteristics(morph plant density and ratio)and fruit set were significantly affected by sexual morph and elevation.Plant density and fruitset frequencies were lower for short-styled than for long-styled plants at 2704 m a.s.l.The seeds from the cross-pollinated flowers of both morphs were higher in quality than those of self-pollinated flowers.The female fitness of seeds from cross-pollinated flowers of both morphs was higher than that of seeds from open-pollinated and self-pollinated flowers.The female fitness of seeds from long-styled flowers was higher than that of seeds from short-styled flowers at all elevations.Inbreeding depression increased with elevation among plants with short-styled flowers but not among those with long-styled flowers.Variation in the elevation-dependent mating system might influence female fitness and affect inbreeding depression in both floral morphs.In conclusion,the low quality of seeds from short-styled flowers at high elevations might decrease short-styled flower frequency,affecting population characteristics.     

Between-year changes in functional gender expression of Palicourea padifolia (Rubiaceae), a distylous, hummingbird-pollinated shrub

BACKGROUND AND AIMS: Because distylous species have two hermaphroditic style-length floral morphs, they face two sex allocation problems: the equilibrium morph ratio and the optimal allocation to pollen and seed production in each floral morph. Gender specialization is expected among distylous species when floral morphs differ in reproductive output. However, spatio-temporal variability in female reproductive output between morphs needs to be investigated to fully understand patterns of sexual expression and gender specialization in distylous plants. Between-year variation in flower and fruit production of hummingbird-pollinated Palicourea padifolia (Rubiaceae) was examined, focusing on functional gender expression of long- and short-styled morphs and comparing their reproductive performance in five consecutive years (1998-2002). METHODS: Between-year variation in inflorescence, floral bud and fruit production was monitored and quantified. These traits were then used as parameters to determine functional gender differences between floral morphs through time. KEY RESULTS: Inflorescence production varied among years but no significant differences were found between floral morphs. Long-styled plants initiated more floral buds per inflorescence every year than short-styled plants, suggesting higher allocation to pollinator attraction and, potentially, an increase in male fitness through pollen donation. Although fruit production was similar between morphs, their functional gender shifted across years. CONCLUSIONS: The gender expression inconsistency across years is surprising because a number of floral characters and attributes that contribute to differently attracting and rewarding effective pollinators in P. padifolia suggest gender specialization. The evidence that morphs of distylous species might specialize in functional gender mostly comes from differences among populations in seed production and non-equilibrium morph ratios based on 1-year field population surveys. The results suggest that more sampling through time is needed to detect gender specialization among distylous species with a perennial habit.     

Distyly and incompatibility in Psychotria homalosperma (Rubiaceae), an endemic plant of the oceanic Bonin (Ogasawara) Islands

Distyly rarely occurs on oceanic islands. Indeed, this form of polymorphism is absent on the Hawaiian Islands and in New Zealand. However, Psychotria homalosperma (Rubiaceae), which is endemic to the oceanic Bonin Islands of Japan, was presumed to maintain a distylous breeding system and floral morphology. We investigated the floral and reproductive characteristics of this species and examined incompatibility in self- and intramorph pollination. Psychotria homalosperma is morphologically and functionally distylous, based on findings with the examined populations, but its stigma and anther heights are not exactly reciprocal between the two morphs. This is the second example of distyly among the flora from the Bonin Islands, the other being the endemic P. boninensis. Both of these distylous species contrast with the Hawaiian Psychotria species, which have already become dioecious. The flowers of the long (L)-styled (pin) morph of the Bonin Islands plants are completely self- and intramorph incompatible, while the flowers of the short (S)-styled (thrum) morphs are partially intramorph compatible. The S-styled morph was more abundant in the examined populations from three islands of the archipelago, probably due to the existence of partial S–S intramorph compatibility.     

Morph-specific patterns of variation in stigma height in natural populations of distylous Jasminum fruticans

The purpose of this study was to describe morph-specific patterns of stigma–anther separation and style curling in the distylous Jasminum fruticans . We also examined whether variation in floral traits is correlated with variation in seed production. Stigma–anther separation is more variable in short-styled plants than in long-styled plants. In all populations studied, some plants bear flowers that lack any stigma–anther separation. These plants have the pollen characteristics (size and number) and compatibility relations of short-styled plants. Comparison with other distylous species illustrates that the variability of stigma–anther separation in short-styled plants is a novel finding for a distylous species. Long-styled plants have greater stigma–anther separation than short-styled plants, styles are often curled and protrude from the corolla, and anthers are placed well within the corolla tube. The frequency of long-styled plants with curled styles and the mean degree of style curling were significantly correlated with style length. Short-styled plants have larger corollas than long-styled plants and never have curled styles. Morph ratios are always 50:50 in natural populations. Mean values of each floral trait in the two morphs were significantly correlated among populations. There were no consistent differences in fecundity of the two morphs nor any correlation between floral traits and seed set for each morph in natural populations. We discuss the potential causes and significance of the two morph-specific patterns we describe; reduced stigma–anther separation in short-styled plants and the presence of curled styles in long-styled plants.     

Impact of mate availability, population size, and spatial aggregation of morphs on sexual reproduction in a distylous, aquatic plant

In distylous, self-incompatible plants, clonal propagation, unbalanced floral morph frequencies, and reduced population size can interfere with the functioning of distyly by compromising legitimate intermorph pollinations, resulting in reduced reproductive output. Here, we examined the mating system and the impact of mate availability, population size, and spatial aggregation of morphs on reproductive output in the distylous, clonal, aquatic plant Hottonia palustris. Controlled pollinations under greenhouse conditions detected no spontaneous selfing without the action of a pollen vector (autonomous autogamy) and demonstrated very low fruit and seed development after self-pollination. Intermorph (legitimate) crossings resulted in high reproductive output in both floral morphs (long- and short-styled individuals), whereas intramorph (illegitimate) crossings decreased fruit and seed development by more than 50%, indicating that the species has partial intramorph-incompatibility. In natural populations, small population size and increasing deviation of floral morph frequencies negatively affected reproductive outcome. Individuals of the majority morph type developed significantly fewer fruit and seeds than individuals of the minority morph type. This rapid decline in fecundity was symmetrical, indicating that regardless of which morph was in the majority, the same patterns of negative frequency-dependent mating occurred. Increasing spatial isolation between compatible morphs significantly reduced fruit and seed set in both morphs similarly. This study provides clear indications of frequency- and context-dependent mating in natural populations of a distylous plant species.     

Asymmetry in pollen flow promotes gender specialization in morphs of the distylous neotropical herb <Emphasis Type="Italic">Arcytophyllum lavarum</Emphasis> (Rubiaceae)

Distyly is a floral polymorphism, characterized by a reciprocal positioning between stigmas and anthers in different flowers, where two floral morphs, long-styled (pin) and short-styled (thrum) occur within the population. Distyly is suggested as one of the routes leading to the evolution of separate sexes in plants. In this evolutionary pathway, pollinators may disrupt the complementarity of pollen transfer between morphs. Consequently, the floral morphs gradually specialize as either male or female. A key process required for gender specialization in distylous plants is a deviation of the realized functional gender (i.e. the proportion of genes transmitted to the next generation via pollen donation and seed production) from the potential functional gender (i.e. the expected contribution of male and female function to reproductive success from the number of ovules or pollen grains produced by each morph). I selected the distylous herb Arcytophyllum lavarum (Rubiaceae) to determine if asymmetry in pollen flow promotes differences in seed production, pollen donation and a discrepancy between the potential and the realized functional genders in pin and thrum floral morphs. Pollen flow in A. lavarum is asymmetric and the pin morph is more efficient at performing cross-pollination than the thrum morph. Conversely, the thrum morph produced two times more seeds than the pin morph. Male and female contributions to the potential functional gender were equivalent in both morphs. However, the pin morph transmitted more genes through pollen donation and the thrum morph more through seed production than expected from their potential functional genders. These results support the hypothesis that if pollinators consistently promote asymmetric pollen flow between morphs over generations, it is possible that gender specialization may evolve to the extreme of dioecism from an original distylous condition.     

异质生境对二型花柱植物雪地报春花性状分化的影响

该研究以生长在新疆北部的二型花柱植物雪地报春(Primula nivalis)为研究材料,对7个低海拔(1 657~2 013m)森林居群和7个高海拔(2 400~2 700m)草原居群的花型比例进行调查,对花部特征进行检测,并对长、短花柱植株在不同海拔居群的花性状分化进行了比较研究,以揭示该类群植物的演化与环境的关系。结果表明:(1)在高海拔草原居群中长花柱植株比例高于短花柱植株。(2)比较草原和森林生态系统中的雪地报春居群,长花柱花形态特征间存在显著差异,但短花柱花形态特征间的差异不显著。(3)高海拔草原居群中长花柱花的花冠筒长度、雄蕊长度、雌雄蕊距离、雌雄异位程度以及雌雄蕊空间位置是导致花性状分化的主要因素。(4)在草原居群中,长花柱花的雄蕊、雌雄异位程度及花冠口至雄蕊距离的变化减少了长短花柱花间的互补式对应性。研究认为,在异质生境中长花柱花性状的变化是导致该物种长短花柱花性状分化的主要因素。     

FLORAL VARIATION AND DISTYLY IN GUETTARDA SCABRA (RUBIACEAE)

Controversy exists as to whether the tropical shrub Guettarda scabra (Rubiaceae) is distylous. Variations in stigma and anther position and floral morphology of G. scabra were studied in a population in south Florida. Stigma and anther height have unimodal distributions, but stigma-anther separation is bimodally distributed and can be used to identify a long-styled and a short-styled morph. Stigma width varies between morphs, but anther length, pollen diameter, and stigma papillae length do not. The morphs occur in a 1:1 ratio in the two populations studied. G. scabra is self-compatible and can pollinate itself. Styles of the two morphs have similar relative growth rates in early development. Stylar growth is inhibited in the short-styled morph when buds are approximately 12 mm long. Anther height differs between morphs because of different relative growth rates and because the long-styled morph corolla tube, where the anthers are attached, stops growth before the tube of the short-styled morph. Reciprocity between morphs for average stigma and anther height falls within the range of reciprocity found in other distylous Rubiaceae. Thus G. scabra is morphologically distylous but unusual among distylous species in the variation within morphs and overlap between morphs in stigma and anther heights.     

COMPARATIVE FECUNDITY AND POPULATION COMPOSITION OF HETEROSTYLOUS AND NON-HETEROSTYLOUS SPECIES OF VILLARSIA (MENYANTHACEAE) IN WESTERN AUSTRALIA

Several species of Villarsia in Western Australia are distylous, but their floral morphology is variable and mostly differs from the expected positional relationships of stigmas and anthers. Villarsia albiflora is non-heterostylous but self-incompatible. In the distylous V. calthifolia, V. capitata, and V. lasiosperma, morph ratios were 1:1, but in some populations of V. parnassiifolia the short morph was present in excess. Ovule production per flower of the two morphs of the distylous species varied between populations, but within most populations was equal. In nine populations of the distylous species, percentage seed-set of Longs and Shorts was equal. In the five populations of the distylous species in which percentage seed-set differed between the morphs, seed-set of Longs exceeded that of Shorts. In four populations of V. parnassiifolia with Shorts in excess, percentage seed-set of the two morphs was equal; in a fifth, that of Longs exceeded that of Shorts. Percent seed-set of V. albiflora did not differ from that of the distylous species and remained constant during the flowering season despite a substantial decrease in ovule production during this period. This study demonstrated no reproductive advantage of distyly over monomorphism in the species studied.     

Relationship between herkogamy,incompatibility and reciprocity with pollen–ovule ratios in Melochia (Malvaceae)

Heterostyly is a floral polymorphism that increases pollination efficiency by promoting cross-pollination and reducing pollen wastage. Efficiency in pollination has been related to plant investment in gamete production and to the pollen to ovule ratio (P/O), which has been proposed as an indication of the likelihood of enough pollen grains reaching the stigmas to result in maximum reproductive success. In heterostylous species, cross-pollination is promoted by the reciprocal position of sexual organs between morphs and a heteromorphic incompatibility system, which precludes selfing and fertilizations among plants of the same morph. Morphological features like reciprocity (between morphs) and herkogamy (within morph) together with the breeding system are thought to influence pollination quality. Therefore, a close relationship between the pollination efficiency, morphological characteristics, and incompatibility would be expected. Pollination treatments and morphological measurements were carried out to describe the breeding system, herkogamy, and reciprocity of six Melochia species. Afterward, the relation between the P/O (as a surrogate of the efficiency in pollination), and reciprocity, herkogamy and incompatibility was evaluated. Monomorphic M. nodiflora and distylous M. pyramidata are self-compatible species, whereas the rest of the species are self- and morph-incompatible. There was a positive relationship between the P/O value and the degree of herkogamy and incompatibility. However, P/O values appear to increase when higher reciprocity is found in the populations. As expected, the lower values of P/O are associated with lower levels of herkogamy and compatibility in the Melochia species studied. The relationship between the factors is discussed under different scenarios of the pollinators’ predictability.     

Heterostyly in Erythroxylum coca (Erythroxylaceae)

Erythroxylum coca is a distylous species with a strong self-incompatibility system linked with the floral dimorphism. The two sets of stamens in the flowers are usually unequal in length, but between individuals of both morphs there is considerable variation in the relative qengths of the two sets of stamens, which is unrelated to the incompatibility system. Pin flowers produce more pollen grains than thrum flowers, but thrum pollen is larger than pin pollen. Within each morph the two sets of stamens produce pollen grains of slightly different diameter. Erythroxylum novogranatewe is also distylous. Pin flowers of E. novogranatense var. novogranatense are partially self-compatible, while thrum flowers of E. novogranatense var. truxillense are self-incompatible. Reports of tristyly and of four different morphs in species of Erythroxylum are probably misinterpretations, resulting from limited sampling, of the continuous variation in the relative lengths of the two sets of stamens.     

濒危植物安徽羽叶报春两种花型的繁育特性及其适应进化

二型花柱的维持机制和自然选择压力多年来一直是生态学和进化学研究领域的热点之一。通过实验室栽培和野外观察统计相结合的方法,对安徽羽叶报春两种花型(长柱花和短柱花)的形态特征、花粉活力、柱头可授性、花粉/胚珠比、自然授粉及结籽能力、自交亲和性等繁育特性进行了比较研究。结果表明:长、短柱花的花冠直径和裂片宽无明显差异,而花冠筒、雌蕊和雄蕊高、花粉数目及大小、P/O比均有显著差异。在自然条件下,长柱花所接受的总花粉数要明显高于短柱花的总花粉数,但所接受的异型花花粉数和平均每果结籽数两者无显著差异。长柱花和短柱花的花粉和柱头活力相似,均能在较长时间内维持较高活力,仅在开花末期显著下降。两种花型的花在自花授粉、同型异花授粉、异型花授粉条件下均能结籽,但异型花授粉的结籽数均明显高于自花授粉和同型异花授粉结籽数。在长柱花各种授粉方式中,花粉萌发率无明显差异,但异型花花粉管的生长速度明显比同型异花花粉和自花花粉的快,而在短柱花柱头上表现为异型花授粉的萌发率最高,但只要萌发后在花柱中的生长速度无显明差异。此外,综合上述结果,对二种花型花部综合征的维持机制及自然选择压力进行了探讨。     

Morphological plasticity and adaptation level of distylous Primula nivalis in a heterogeneous alpine environment

Plant populations at high elevation face extreme climatic conditions and resource limitations. The existence of distylous species at different elevations can help us investigate their adaptation to high altitudes, the evolution of their morphological characteristics, as well as their responses to limited resources. Here, 17 populations of Primula nivalis at different elevations were evaluated regarding variations in plant morphological characteristics, biomass allocation, and morphological plasticity in a heterogeneous environment. Our results demonstrate that heterogeneous environments can affect plant morphological characteristics and resource allocation in each sexual morph of these plants. Moreover, environmental variations reduced morphological plasticity in the two plant morphs, and the plasticity of long style (LS) plants was greater than that of short style (SS) plants. There were significant negative correlations between morphological characteristics and elevation, rainfall, temperature, and sunshine, and these are the main variables that affect morphological characteristics and resource allocation of both morphs of P. nivalis plants in heterogeneous environments. The morphological characteristics of P. nivalis plants transplanted from high to lower elevations were not significantly different in either population. LS plants had greater morphological plasticity and adaptability in heterogeneous environments than SS plants. Elevational gradients and heterogeneous environments differentiated both morphs of P. nivalis plants with regards to morphology as well as adaptations. LS plants showed a higher level of adaptability than SS plants.     

Skewed morph ratios lead to lower genetic diversity of the heterostylous Primula veris in fragmented grasslands

• Populations of heterostylous plant species are ideally composed of equal frequencies of two (distylous) or three (tristylous) morphologically different floral morphs. Intra-morph incompatibility helps to avoid inbreeding and to maintain genetic diversity, supporting plant fitness and long-term viability. Habitat fragmentation can lead to skewed morph ratios and thereby reduce the abundance of compatible mates. This, in turn, can result in a loss of genetic diversity. We tested whether the genetic diversity of heterostylous plants is affected by morph ratio bias using populations of the distylous grassland plant Primula veris in recently fragmented grasslands.
• We recorded morph frequencies and population sizes in 30 study populations of P. veris on two Estonian islands characterised by different degrees of habitat fragmentation. Examining variation of thousands of single nucleotide polymorphisms (SNPs) and heterostyly-specific genetic markers, we quantified overall and morph-specific genetic diversity and differentiation in these populations.
• Morph frequencies deviated more in smaller populations. Skewed morph ratios had a negative effect on the genetic diversity of P. veris in more fragmented grasslands. In the populations of better-connected grassland systems, genetic differentiation among S-morphs was higher than among L-morphs.
• Our study shows that deviations from morph balance are stronger in small populations and have a negative impact on the genetic diversity of the distylous plant P. veris. Together with the direct negative effects of habitat loss and decreased population size on the genetic diversity of plants, morph ratio bias may intensify the process of genetic erosion, thus exacerbating the local extinction of heterostylous species.

     

茜草科二型花柱植物滇丁香传粉精确性研究

异型花柱是一种受遗传因素控制的花型多态性现象,其适应意义在于提高不同花型间传粉精确性,从而促进异交。为检验二型花柱植物滇丁香（Luculia pinceana）是如何促进异交并确定有效传粉者的类型,实验统计自然种群中长、短柱型的植株数量,测量花部特征,统计自然种群植株柱头上的型间花粉落置的数量和比例,比较型内、型间花粉的萌发率以及花粉管长度,比较不同人工授粉处理结实情况,观察传粉昆虫类型并测量虫体特征。结果表明（1）滇丁香自然居群中长、短柱型植株的数量没有显著性差异。长柱型的筒深、筒直径、开口直径、雄蕊长和柱头长均显著小于短柱型,而长柱型的叶片长和宽、雌蕊长、花药体积、花粉粒体积、柱头厚均显著大于短柱型。（2）自然种群植株柱头上型间花粉所占比例显著低于型内花粉的比例。（3）型间授粉的花粉萌发率以及花粉管长度均大于型内授粉和混合授粉。（4）型间授粉的结实率显著高于型内授粉的结实率,型内授粉具有一定的结实。（5）夜行性长喙条背天蛾（Cechenena lineosa）是滇丁香的有效传粉者,其吻长和滇丁香的花筒深相适应。本研究表明二型花柱植物滇丁香主要是通过与其花部特征相匹配的长喙天蛾实现有效的传粉,通过促进型间花粉的萌发和花粉管的伸长而有利于型间授粉结实,提高传粉精确性。     

Trends in the reproductive biology of Venezuelan Melochia (Malvaceae) species

The breeding system, floral morphology, morph frequency, biomass allocation to floral whorls, pollination and reproductive efficiency were examined in four distylous and three monomorphic Melochia species. Monomorphic species were self-compatible and distylous species were self-incompatible. Flowers of homomorphic species were longer than those of distylous species. Herkogamy was significantly higher in pin than in thrum morphs of the two distylous species, and monomorphic species exhibited the lowest values of herkogamy. Pollen/ovule ratios were similar between monomorphic and dimorphic species, irrespective of the self-incompatibility level. Biomass allocation to flowers was biased toward non-sexual structures, attraction and support. Androecium biomass and androecium/gynoecium biomass ratio were larger in thrum than in pin flowers of distylous species indicating maleness in the thrum morph and femaleness in the pin morph. There was no clear difference between fruit set of monomorphic and dimorphic species; however, the greatest fruit set was found in the monomorphic species, M. pyramidata, which is a self-compatible species. Fruit set was significantly higher in pin than in thrum morphs in three out four distylous species and fecundity was only significantly higher in pin morphs of M. caracasana and M. parvifolia. Melochia species have generalist pollination systems. According to the taxonomic classes and number of pollinator species, M. caracasana and M. parvifolia have similar generalist pollination systems. Our comparative analyses of the characters between floral morphs of distylous species and the relationship with these characters in monomorphic species allow divergences and similarities to be established and different evolutionary trends to be postulated in the breeding systems of Melochia species. Specifically, M. parvifolia and M. caracasana are apparently biased toward monomorphy and dioecy, respectively.     

The evolution of distyly from tristyly in populations of Oxalis alpina (Oxalidaceae) in the Sky Islands of the Sonoran Desert

The evolution of distyly from tristyly was investigated in populations of Oxalis alpina at high elevations throughout the Sky Islands of the Sonoran Desert. Incompatibility systems in tristylous populations, where self-incompatible short-, mid-, and long-styled morphs occur in populations, vary from those typical of tristylous species in which each morph is equally capable of fertilizing ovules of the other two morphs, to breeding systems in which incompatibility relationships are asymmetric. In these populations, selection against the allele controlling expression of the mid-styled morph is likely. The degree of modification of incompatibility in the short- and long-styled morphs in 10 populations was strongly associated with fewer mid-styled morphs, supporting models predicting the effect of these modifications of incompatibility on frequency of the mid-styled morph. Self-compatibility of the mid-styled morph may be important for maintaining the frequency of this morph, depending on the level of self-pollination, self-fertilization, and the extent of inbreeding depression. Modifications of incompatibility in tristylous populations and the distribution of distylous populations of O. alpina in the Sky Island region have similar geographic components, indicating the potential importance of historical factors in the evolution of distyly from tristyly.     

Unravelling the stylar polymorphism in Melochia (Malvaceae): reciprocity and ancillary characters

Heterostyly is a genetic polymorphism in which plant populations possess two (distyly) or three (tristyly) morphs with flowers differing reciprocally in stigma and anther height. Sex organ deployment has been described as being highly variable among and within species of several distylous taxa belonging to different taxonomic groups. However, the number of studies considering within‐species disparities is still limited. For a better overview of the existing amount of variation that can occur within and between heterostylous species, we sampled 46 populations of six Melochia spp., a style‐polymorphic genus in Cuba. We characterized the floral morphology in all populations and described a set of ancillary characters per species. All of these Melochia spp. are distylous, except for the monomorphic M. nodiflora. The S‐morph produces fewer, larger pollen than L‐morphs, and has verrucose ornamentation. The L‐morph produces reticulate pollen and has larger stigmatic papillae than the S‐morph. The monomorphic M. nodiflora shows ancillary characteristics that are similar to the L‐morph individuals in the related species. As expected, there are differences in ancillary characters among species and also dissimilarities in reciprocity among and within species of Melochia. Our results highlight the importance of considering intraspecific variation in the morphometric characterization of heterostylous taxa. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 147–158.     

Patterns of male sterility within and among populations of the distylous shrub Erythroxylum havanense (erythroxylaceae)

Distyly, a reproductive system characterized by the presence of long-styled (thrum). and short-styled (pin) individuals within a population, has been repeatedly used as a model for the study of the evolution of the reproductive systems in plants. Erythroxylum havanense is a distylous species in which most thrum plants fail to develop a fertile androecium, thus behaving as male-sterile or partially male-sterile plants. Short-styled (thrum) individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. Previous studies of populations within close proximity to each other suggest that E. havanense may be involved in a process of gender specialization in which, unlike other heterostylous species, thrum plants are specializing as females and pins (long-styled) as males. In this paper we describe more general patterns of male sterility, one of the first steps in the evolution of gender specialization, among populations of the distylous shrub Erythroxylum havanense. Pollen germination differed among populations (range 0.52 ± 0.03 to 0.06 ± 0.04), and between morphs. Pollen from pin plants was almost two times (1.89) as fertile as that from thrums (0.36 ± 0.03 and 0.19 ± 0.03, pin and thrums respectively). Thrums were significantly more male sterile in four out of five populations. The population where differences between the floral morphs were not apparent showed the lowest levels of pollen fertility. Accordingly, our results indicate that populations of E. havanense show marked differences in pollen fertility and higher male sterility associated with the thrum morph. We hypothesize that differences between morphs could be explained if restorers of male sterility are linked to the distyly haplotype, while differences in genes associated with male sterility could explain the variation among populations. Overall, the prevalence of thrum-biased male sterility across populations suggests that E. havanense is subject to a process of gender specialization.     

Gender specialization in <Emphasis Type="Italic">Palicourea demissa</Emphasis> (Rubiaceae), a distylous,hummingbird-pollinated treelet

Distyly has been interpreted as a mechanism that promotes cross-pollination among conspecific plants and as one of the routes leading to the evolution of dioecy. In one of the possible evolutionary pathways, pollinators may disrupt intermorph pollen flow, and, as a consequence, floral morphs may gradually specialize as either male or female (functional dioecy). Natural patterns of pollen deposition and fruit and seed production were estimated in Palicourea demissa (Rubiaceae) and used as parameters to assess functional gender differences between floral morphs. Pollen flow was asymmetrical in P. demissa. Long-styled flowers were more effective than short-styled flowers in pollen deposition towards compatible stigmas, whereas short-styled flowers were more effective in legitimate pollen receipt. Accordingly, short-styled plants produced more fruits and viable seeds than long-styled plants. The contributions of male and female function to the potential functional gender were equivalent in both morphs. However, the realized functional gender deviated significantly from the potential functional gender in both morphs, in which short-styled plants were more successful through their female function, but long-styled plants through their male function. If pollinators disrupt the complementarities of pollen transfer between the two morphs (asymmetric pollen flow), the expression of a more profitable gender is expected in each morph. Thus, our results support the hypothesis that dioecy may evolve in distylous populations through the gradual specialization of each morph as either male or female.