VARIATION IN MALE AND FEMALE REPRODUCTIVE SUCCESS AMONG FLORAL MORPHS IN THE TRISTYLOUS PLANT LYTHRUM SALICARIA (LYTHRACEAE)

The purpose of this study was to assess variation in male and female reproductive success among the three morphs of the tristylous plant, Lythrum salicaria. Fluorescent dyes were used as pollen analogs to determine whether morphs differ in their abilities to donate and receive pollen, and actual and potential seed set was measured with a hand pollination experiment. Dye transfer among morphs was highly asymmetric, with more frequent transfer from the short-styled morph to the long- and mid-styled morphs. This suggests that shorts are performing better at pollen donation and longs and mids are performing better at pollen receipt. All flowers on 95 plants were hand pollinated to test whether female reproductive success is more pollen-limited in the short-styled morph than in other morphs. Hand-pollinated short-styled plants had significantly higher total seed mass and more seeds per capsule than short controls, whereas hand pollination failed to increase seed set in long and mid morphs. As predicted, short-styled morphs showed significant pollen limitation, whereas seed set in long- and midstyled morphs was not pollen-limited. Thus, in Lythrum salicaria asymmetrical pollen flow generates morph-specific differences in male and female fitness.     

FACTORS INFLUENCING FREQUENCY OF THE MID-STYLED MORPH IN TRISTYLOUS POPULATIONS OF OXALIS ALPINA

Genetic models and computer simulations suggest that modifications of incompatibility in tristylous populations of Oxalis alpina from southeastern Arizona should result in evolution of distyly, due to the inability of the mid floral morph to compete effectively with the short and long floral morphs. To evaluate this hypothesis, progeny testing of naturally pollinated individuals was carried out over a three-year period for populations with differing mid frequencies. In contrast to expectations, mids appeared in greater than expected proportions in many cases. Detailed analysis of results from two trimorphic populations where the mid form is relatively uncommon revealed that mid morphs produced far more mid progeny among their offspring than expected. Assuming that all compatible pollen has the same probability of deposition on appropriate stigmas, mid excesses were found among progeny of shorts and longs in these populations as well. In a third population where the mid form is abundant, mid excesses were not found among the progeny of mids, nor were mid excesses typical among the progeny of shorts and longs. Mid excesses among progeny of populations where the mid form is uncommon may explain retention of this form, despite incompatibility modifications that should favor evolution of distyly. Over-representation of mids may result from gametophytic selection favoring mid alleles. If this is the case, tristylous populations may be in equilibrium, with the frequencies of the three morphs determined by the balance of the opposing selective forces resulting from incompatibility modifications and gametophytic selection.     

Trimorphic incompatibility in Eichhornia azurea (Pontederiaceae)

Eichhornia azurea (Pontederiaceae) is a mat-forming, clonal aquatic that inhabits lakes, marshes and river systems in many parts of the Neotropics. The species is tristylous with long-, mid-, and short-styled morphs commonly represented in natural populations. To investigate whether E. azurea possesses a trimorphic incompatibility system typical of tristylous species, we conducted a controlled pollination experiment on 15 clones representing the three style morphs from a natural population near Rosario, Argentina. Comparisons of fruit and seed set following self-, illegitimate, and legitimate pollinations clearly demonstrated the presence of trimorphic incompatibility in E. azurea. Self- and illegitimate pollinations produced significantly less fruit and seed than legitimate pollinations in all three style morphs. Pollen from the two anther levels within a flower exhibited contrasting compatibility relations in self-pollinations. In common with several other tristylous species in Pontederiaceae, the expression of self-incompatibility was weakest in the mid-styled morph and strongest in the short-styled morph. We discuss the ecological and evolutionary significance of the partial expression of trimorphic incompatibility in E. azurea. Received: 3 May 1999 / Revision accepted: 27 July 1999     

Heterostyly in the Lamiaceae: The case ofSalvia brandegeei

Heterostyly rarely occurs in families with strongly zygomorphic flowers. For this reason Darwin (1877) doubted whether heterostyly would occur in the Lamiaceae and recent reviews have not reported the floral polymorphism in this family. Here we describe distyly in a rare species ofSalvia restricted to bluffs and seaward canyons on Santa Rosa Island (Santa Barbara Co., California) and northwestern Baja California (Mexico).Salvia brandegeei is morphologically distylous with populations composed of equal frequencies of long-and short-styled morphs differing reciprocally in stigma and anther position. Controlled hand pollinations demonstrated no significant differences in the seed set of self, intramorph or intermorph pollinations. Unlike most heterostylous species investigated,S. brandegeei does not possess diallelic incompatibility or ancillary polymorphisms of pollen and stigmas. We propose that the evolution of distyly inS. brandegeei may have been associated with an ecological shift to a new environment in which protandry failed to prevent increased levels of geitonogamy. Heterostyly was then selected because it increased the proficiency of cross-pollination. The origin of distyly in self-compatibleS. brandegeei is consistent with Lloyd and Webb's theoretical model for the evolution of distyly.     

Genetic incompatibility and offspring quality in the tristylous plant Lythrum salicaria (Lythraceae)

The purpose of this study was to determine the incompatibility relationships among the three different morphs of the tristylous plant Lythrum salicaria, and to assess whether there are fitness consequences to breakdown in incompatibility. Twenty-four different types of pollinations were performed using all possible combinations of anthers and stigmas. These can be grouped as legitimate pollinations (pollinations from the appropriate anther level of a compatible morph), illegitimate intermorph pollinations (pollinations from the inappropriate anther level of a compatible morph), intramorph pollinations (pollinations between individuals of the same morph), and self pollinations. Legitimate pollinations produced significantly more seed than illegitimate intermorph pollinations, intramorph pollinations, and self pollinations. The difference between legitimate and illegitimate intermorph pollination success is particularly interesting because pollen for these crosses came from the same genetic individual. Pollination types also differed in offspring quality. In the rare examples where progeny were produced by illegitimate intermorph pollinations, the progeny did not have significantly lower values for seedling growth traits compared to legitimate progeny. Seedlings produced by self pollinations had significantly lower values for four out of six seedling growth traits measured. Comparisons of legitimate and self progeny traits indicate that inbreeding depression for most traits is close to or above 0.5. The finding that some seed were produced from illegitimate pollinations suggests that there is variation among individuals in incompatibility. There was a significant effect of parental morph type on the probability of breakdown in incompatibility with the midstyled maternal parents setting more seed from illegitimate pollinations.     

Distyly and variation in heteromorphic incompatibility in Gaertnera vaginata (Rubiaceae) endemic to La Reunion Island

Documenting the floral biology of species throughout the Rubiaceae family is of particular interest since heterostyly and dioecy may have evolved more than once in this large family. Unfortunately many species in several tropical regions remain unstudied. The purpose of this paper is to describe the floral biology, the nature of self-incompatibility, morph ratios, and fecundity in natural populations of Gaertnera vaginata, a small tree endemic to the island of La Reunion in the Indian Ocean. Measurements of floral characters in populations across the entire distribution of this species showed that G. vaginata exhibits a reciprocal stigma height and anther height dimorphism characteristic of a distylous species. Pollen grain size and corolla tube length are consistently greater in short-styled plants and long-styled plants produce more pollen per flower. Controlled pollinations in a natural population showed that 25% of the short-styled plants gave at least one fruit on intramorph (illegitimate) pollination, whereas no long-styled plants set fruit on illegitimate pollination. In total, 19.4% of illegitimate pollinations produced fruit on short-styled plants. No self-pollination gave fruit on either morph and between-morph pollinations produced 92.2 and 92.8% for short and long-styled plants, respectively. Overall, short-styled plants were significantly more abundant than long-styled plants. Short-styled plants outnumbered long-styled plants in 16 of the 19 populations. In three of these populations the morph ratio was significantly different from 1:1. In two natural populations, fruit set was significantly higher on long-styled plants, although the number of seeds per fruit was not significantly different between the two morphs. The possible effect of variation in the strength of heteromorphic incompatibility on observed variation in morph abundance and the possible causes for the variation in fruit set are discussed.     

Partial self-incompatibility and inbreeding depression in a native tree species of La Réunion (Indian Ocean)

We investigated the reproductive system of the threatened taxon Dombeya acutangula ssp. acutangula Cav. (Sterculiaceae), an endemic tree of the Mascarene archipelago (Indian Ocean). A controlled crossing experiment was performed in two natural populations located in the remnants of the low-elevation dry forest on the island of La Réunion. Active pollination, probably mainly by insects, was necessary for reproduction in this species. Individuals varied in their degree of self-sterility from 0 to 100%. Outcrossing between nearby individuals produced lower seed set than did crosses between more distant individuals within one of the two tested populations. The variation in reproductive success on selfing and in the different types of crosses could result from inbreeding depression causing embryo death, and we provide evidence that progenies from selfing have lower seed size and quality. However, for inbreeding depression to account for the dramatic variation in seed set found in our crossing experiment, the distribution of genetic load and number of lethal factors required appear unrealistic. We favour an alternative interpretation, that D. acutangula possesses an incompatibility system similar to that found in other Sterculiaceae species such as Theobroma cacao L. Such an incompatibility system allows a certain amount of selfing, and different individuals vary in their degree of self-incompatibility. The low success of crosses among close neighbours in one population suggests that there was spatial structure for incompatibility alleles in that population. This could partly explain the decline of the species in fragmented and disturbed habitats, since relatedness at incompatibility loci may increase in small or isolated population and thus reduce mate availability. Received: 2 March 1998 / Accepted: 3 August 1998     

Dimorphisms and self-incompatibility in the distylous species <Emphasis Type="Italic">Palicourea demissa</Emphasis> (Rubiaceae): possible implications for its reproductive output

Distyly has been interpreted as a mechanism that favors cross-fertilization. In this research we describe floral attributes and ancillary floral polymorphisms typically associated to heterostylous plants in Palicourea demissa (Rubiaceae), a distylous shrub of the Venezuelan Andes cloud forests. A hand-pollination experiment was done to evaluate self- and intramorph incompatibility and female reproductive output in both floral morphs. The studied population was morphologically distylous but morph differences in most ancillary floral polymorphisms and reciprocity of the sexual organ heights were found. The floral morphs were self-incompatible and did not differ in fruit set under controlled cross-pollination conditions, but at the population level they exhibited imperfect reciprocal herkogamy. Fruits and seeds of short-styled plants were larger than those of long-styled plants and fruit set was higher in short-styled plants under natural conditions, suggesting a higher reproductive potential among short-styled plants. Given the 1:1 morph ratio within the studied population, further evidence is needed to determine the influence of floral visitors and seed dispersers in the expression of heterostyly in P. demissa under natural conditions.     

Flexible mating system in distylous populations of <Emphasis Type="Italic">Psychotria carthagenensis</Emphasis> Jacq. (Rubiaceae) in Brazilian Cerrado

In family Rubiaceae distyly is very common, and large variation in heterostyly characteristics has been previously documented. Analysis of these variations, even within a species, is very useful to our understanding of the evolutionary process that caused this polymorphism. For this reason, the goal of this study is to investigate the floral morphology and diallelic incompatibility system of three populations of Psychotria carthagenensis. The three studied populations of P. carthagenensis occur in forest fragments in protected areas in an urban matrix in the Municipality of Campo Grande, Mato Grosso do Sul State, Brazil. Reciprocal position of style length and stamen height was found in populations, and in general short-styled flowers were larger than long-styled flowers. All populations presented 1:1 morph ratio between short-styled and long-styled flowers. Regarding breeding system, flowers of P. carthagenensis were self-compatible and compatible within plants of the same morph, and there was no pollen limitation in the populations in any case. In only one of the populations were there differences in the extent of compatibility between morphs, with the long-styled morph being more self-compatible than the short-styled morph. The reproductive strategy of these populations can be advantageous in case of fluctuation of pollinator activity.     

The Effect of Partial Self-fertilization on Type Frequencies in Heterostylous Plants

A population of heterostylous plants is studied mathematicallyunder the assumption that a small amount of self-fertilizationis possible. The frequencies of the incompatibility types aredetermined as expressions involving the probabilities of self-fertilization.A comparison is made with observed frequencies of the Long,Mid and Short types in Lythrum salicaria, and it is concludedthat the differences between the type frequencies seem to betoo large to be accounted for by unequal probabilities of self-fertilization. Lythrum salicaria L, heterostyly, self fertilization, incompatibility systems     

Functional fragments of a relictual gametophytic self-incompatibility system are associated with the loci determining flower type of the heterostylous outcrosser Fagopyrum esculentum Moench. and the homostylous selfer F. homotropicum Ohnishi

Functional fragments of presumably a relictual gametophytic self-incompatibility system (GSI) linked with the loci determining flower type were discovered by genetic analysis of an unilateral pre-zygotic barrier between the short-styled (thrum) morph of a heterostylous cross-pollinated species, Fagopyrum esculentum Moench., and a self-pollinator with homostylous flowers, F. homotropicum Ohnishi (asseccion C9139). The relic genes of GSI were revealed only in interspecific crosses. However, this is a direct experimental confirmation of a hypothesis proposed by Lewis (1954) which combined the heterostyly supergene components (G, P and A) with “pistil” and “pollen” parts of the S-locus of homomorphic self-incompatibility systems (I ₁ and I ₂). Also, this result provides strong evidence for the evolution of heterostyly upon the ruins of a gametophytic self-incompatibility system.     

Fertility differences among floral morphs following selfing in tristylous Eichhornia paniculata (Pontederiaceae): inbreeding depression or partial incompatibility?

Reduction in seed set following self- vs. cross-pollination in flowering plants can result from abortion of selfed offspring owing to inbreeding depression and/or partial self-incompatibility. Previous studies on tristylous Eichhornia paniculata (Pontederiaceae) indicate that reduced seed set following self-pollination generally occurs in the short- (S), but not the long-(L) or mid-styled (M) morphs. To determine whether this pattern results from morph-specific differences in inbreeding depression owing to the sheltering of deleterious alleles at the S locus and/or partial self-incompatibility, we conducted controlled hand-pollinations of the floral morphs and measured seed set and levels of seed abortion. There were no significant differences in fertilization success and seed set following self-, illegitimate, and legitimate pollinations in the L and M morphs. In contrast, in the S morph self-, intramorph and intermorph illegitimate pollinations resulted in significant reduction in seed set in comparison with legitimate pollination. This indicates that the reduced seed set observed in self-pollination is the result of partial incompatibility rather than inbreeding depression. Significantly reduced fertilization success and low levels of ovule abortion in illegitimate pollinations of S plants also supported this conclusion. Reduced fertility in the S morph may have implications for the observed loss of this morph from natural populations and the evolutionary breakdown of tristyly.     

Density-dependent variation in reproductive success in a terrestrial orchid

 Within population variation in plant density can affect reproductive success and breeding systems. We examined such effects in Listera cordata R. Br., a north temperate terrestrial orchid, in a subalpine population in Colorado, USA. Hand pollinations showed that L. cordata was self-compatible as all pollinations produced fruits. Cross-pollinations differed significantly from self-pollinations, and had a higher number of seeds per capsule and higher % of fertilized ovules. Together they could compensate for the transmission advantage of self-pollinations. Average number of flowers per inflorescence was 4.2–4.4 and average fruit set was 20.7–23.7%. By establishing 36 plots with varying plant densities (1, 4, 8 and 16) and emasculating all flowers in target plants of a set of 20 plots, we estimated density effects on fruit and seed production. Plant density had no effect on the amount of reproduction except in reduced seed production of highly dispersed plants. Our results suggest that inbreeding depression may be an important selective factor in L. cordata. Further studies are needed to define its role in the evolution of mating systems in this species. Received February 21, 2000 Accepted December 26, 2000     

An unusual heteromorphic incompatibility system

Summary By means of intra- and intermorph crosses it has been shown that the distyly in Anchusa officinalis L. is governed by a single diallelic locus with dominance. Presence of the dominant allele is reflected in the short-styled morph, while homozygous recessives become long-styled. Most of the short-styled plants are heterozygous, but homozygotes have been found among the raised progeny resulting from crossings. Diallel crosses have revealed that at least two incompatibility loci must exist. These and the morphological locus segregate independently. The type of incompatibility system is as yet unknown, though the great frequency of non-reciprocal incompatibility points to a sporophytic system.     

ON THE DARWINIAN HYPOTHESIS OF THE ADAPTIVE SIGNIFICANCE OF TRISTYLY

Darwin proposed that the function of the stamen-style polymorphism in heterostylous plants is to increase the probability of legitimate (compatible) pollinations among the floral morphs. Conspicuous pollen trimorphism in tristylous Pontederia cordata enables a test of the hypothesis. Comparison of the composition of pollen loads in naturally pollinated stigmas of intact and emasculated flowers were made at a population in Paugh Lake, Ontario, which was visited primarily by bumblebees. The magnitude of legitimate pollination was analyzed by ANOVA. In intact flowers, significant legitimate pollination was detected in the long-styled morph only. Following emasculation legitimate pollination was evident in the long- and short-styled morphs, with the mid-styled morph just short of displaying significant legitimate pollination. Similar results were obtained by chi-square analysis. It has been suggested that heterostyly may reduce mutual interference between maternal and paternal reproductive function. Two aspects of pollen-stigma interference were investigated in P. cordata. The potential importance of stigmatic or stylar clogging by incompatible pollen was examined by controlled field pollinations and measurements of seed set. The results indicate that prior application of large amounts of incompatible pollen has no significant effect on the seed set of open-pollinated inflorescences. Comparison of legitimate pollen capture in intact and emasculated flowers provided no evidence that the presence of stamens within flowers of the floral morphs interferes with the receipt of legitimate pollen. Pollen-stigma interference remains to be demonstrated in heterostylous plants.     

BREEDING SYSTEMS IN ASCLEPIAS INCARNATA L., A. SYRIACA L., AND A. VERTICILLATA L.

A series of experimental pollinations involving three sympatric species provided strong evidence for physiological and/or genetic barriers to hybridization in Asclepias. Pollen tubes were observed to penetrate the style in some interspecific crosses of all species pairs. Aniline blue fluorescence microscopy also demonstrated pollen tube penetration of foreign ovules following pollinations between A. incarnata and A. verticillata. However, none of the 279 total pollinations attempted between species yielded mature seed, indicating the presence of late pre-fertilization or early post-fertilization incompatibility. Intraspecific pollinations in greenhouse and field populations revealed greater crossability between populations than within populations of a single species. Self-pollinations of A. verticillata were unsuccessful, while 29% and 4% of those of A. incarnata and A. syriaca, respectively, yielded mature follicles. It is suggested that the potential for autogamy, combined with floral mechanisms requiring pollination by insects, insure the advantages of both the genetic variability promoted by outcrossing and the reproductive assurance of uniparental reproduction. Strong reproductive barriers between species reduce the change of intergradation where species occur sympatrically.     

The Breeding System of Oxalis squamata,a Tristylous South American Species

Oxalis squamata is a perennial endemic herb of the Chilean-Argentine Andes previously reported as a distylous species. In this paper, analysis of floral morphology, incompatibility relations and population structure show that O. squamata possesses morphological and functional tristyly. The three floral morphs differ in calyx and corolla length, style and stamen length, pollen grain size and stylar pubescence. Results of hand pollinations showed the presence of a strong incompatibility system. Legitimate pollinations with the L and M morphs as seed parents produced approximately equal numbers of seeds per pollination. The S morph produced less than half the number of seeds per legitimate pollination compared to the other two morphs. Intra- and intermorph illegitimate pollinations produce almost no seeds. Morph representation in the population was isoplethic. Fruit and seed production did not differ significantly for the three morphs in the population. Comparison of experimental and natural production of fruits and seeds suggests that seed set in the natural population may be pollen-limited for the L and M morphs.     

Pollination and reproduction enhance the invasive potential of an early invader: the case of Lythrum salicaria (purple loosetrife) in South Africa

The potential of an alien plant to spread rapidly and colonize new habitat may be related to the mode of reproduction and the ability to attract pollinators. Most studies focus on widespread invasive plants, in which pollinators are rarely limiting. Here, we assess the ability of a recent invader in South Africa, the tristylous Lythrum salicaria to self-reproduce and whether this can explain the delay between introduction and spread. This study was conducted in one of the largest known populations (a total of 7 populations in South Africa) of L. salicaria in the Liesbeek river in the fynbos biome. We assessed the importance of pollinators and autonomous selfing in L. salicaria by comparing seed set between pollinator excluded and naturally pollinated flowers. Overall, 5 pollinators (4 native and 1 alien) were recorded with Cape honeybees and Africa Monarch butterflies the most prominent. Seed and fruit set were significantly higher in open pollinated flowers compared to pollinator excluded flowers. Also, seed and fruit set in pollinator excluded flowers were higher in long and medium morphs compared to short morphs. Germination was high for seeds from pollinator, but also from pollinator excluded treatments. This shows that L. salicaria in South Africa is self-compatible to some extent, but it is frequented by pollinators, significantly increasing seed production. Despite L. salicaria being tristylous, all 3 morphs are present in South Africa and with a huge seed production, this species has the potential to become a major invader of rivers and wetlands in South Africa.

     

Self-incompatibility reactions and compatible pollen-tube growth are retained with in vitro pollinations of sugar beet,Beta vulgaris L.

Summary It is shown that in vitro pollination can be used in future studies of the time course of pollen-tube development and analysis of self-incompatibility in sugar beet, Beta vulgaris L. Upon selfng a self-incompatible genotype showed the same incompatibility response after both in vitro and in vivo pollinations. No differences between cross-compatible and self-compatible pollentube growth were observed. The pollen-tube rejection occurred whether or not the pollen was prehydrated. RNA staining with Acridine Orange showed that there was less cellular RNA in the pistil tissue from in vitro-pollinated flowers. Nevertheless, pollen-tube growth and the self-incompatibility response were similar after in vivo and in vitro pollinations.     

THE BREEDING SYSTEM OF OXALIS SUKSDORFII

The Pacific Coast species Oxalis suksdorfii (section Corniculatae) is morphologically tristylous. Flowers of the 3 kinds of plants present in natural populations differ in style length, stamen length, pollen size, stigma size and orientation, and stylar pubescence. Experimental studies and field observations indicate that plants with long-styled flowers are weakly self-compatible and show little differentiation in size or compatibility relationships of pollen from the 2 sets of stamens. Plants with mid-styled flowers have strongly differentiated pollen, but produce few capsules or seed after any artificial or natural pollinations. Plants with short-styled flowers are self-incompatible and show slight pollen differentiation. The data presented suggest that O. suksdorfii approaches functional dimorphism since the mid-styled form contributes little pollen and few seeds in sexual reproduction. The species may be in the process of losing the mid-styled form completely and provides a model of certain physiological and morphological changes which may have accompanied elimination of the mid-styled form in other Oxalis species. The abundance of plants with mid-styled flowers in natural populations suggests, however, that this form may be retained because it contributes to general population fitness, or because the genetic control of heterostyly in this species prevents its elimination.