S-allele diversity suggests no mate limitation in small populations of a self-incompatible plant

Small populations of self-incompatible plants are assumed to be threatened by a limitation of compatible mating partners due to low genetic diversity at the self-incompatibility (S) locus. In contrast, we show by using a PCR-RFLP approach for S-genotype identification that 15 small populations (N = 8-88) of the rare wild pear (Pyrus pyraster) displayed no mate limitation. S-allele diversity within populations was high (N = 9-21) as was mate availability (92.9-100%). Although population size and S-allele diversity were strongly related, no relationship was found between population size and mate availability, gene diversity (He), or fixation index (F(IS)), based on five neutral microsatellite loci. As we determined the principal mate availability within populations based on the S-genotypes observed, the realized mate availability under natural conditions may differ from our estimates, for example, due to spatially limited pollen dispersal. We therefore urge studies on self-incompatible plants to proceed from the simple assessment of principal mate availability to the determination of realized mate availability in natural populations.     

Fecundity Effects of Dichogamy in an Asynchronically Flowering Population: a Genetic Model

Timing of pollen presentation and ovule maturation at the levelof the flower, individual and population may exert a profoundinfluence on individual fitness and population structure. Weexplored the evolutionary consequences of asynchrony in floweringin a population of dichogamous plants, making use of a geneticmodel to represent pollen and ovule availability over time.When the model was run for 18 generations, starting from a mixedpopulation of both early- and late flowering, protandric andprotoginous genotypes, the system evolved differently accordingto whether genetic dominance for precocity (or lateness) waspresent or absent. Under the assumption of dominance, the populationevolved towards a structure dominated by protandrous, late floweringgenotypes. In the absence of dominance, the system seemed toattain steady state equilibrium in which both dichogamous modescoexisted in predominantly intermediate-flowering genotypes.Copyright1998 Annals of Botany Company Dichogamy, protandry, protogyny, asynchronic flowering, genetic model.     

Inheritance and dominance of self-incompatibility alleles in polyploid Arabidopsis lyrata

Natural populations of diploid Arabidopsis lyrata exhibit the sporophytic type of self-incompatibility system characteristic of Brassicaceae, in which complicated dominance interactions among alleles in the diploid parent determine self-recognition phenotypes of both pollen and stigma. The purpose of this study was to investigate how polyploidy affects this already complex system. One tetraploid population (Arabidopsis lyrata ssp kawasakiana from Japan) showed complete self-compatibility and produced viable selfed progeny for at least three generations subsequent to field collection. In contrast, individuals from a second tetraploid population (A. lyrata ssp petraea from Austria) were strongly self-incompatible (SI). Segregation of SI genotypes in this population followed Mendelian patterns based on a tetrasomic model of inheritance, with two to four alleles per individual, independent segregation of alleles, and little evidence of dosage effects of alleles found in multiple copies. Similar to results from diploids, anomalous compatibility patterns involving particular combinations of individuals occurred at a low frequency in the tetraploids, suggesting altered dominance in certain genetic backgrounds that could be due to the influence of a modifier locus. Overall, dominance relationships among S-alleles in self-incompatible tetraploid families were remarkably similar to those in related diploids, suggesting that this very important and complicated locus has not undergone extensive modification subsequent to polyploidization.     

MATE AVAILABILITY AND FECUNDITY SELECTION IN MULTI-ALLELIC SELF-INCOMPATIBILITY SYSTEMS IN PLANTS

We investigate mate availability in different models of multiallelic self-incompatibility systems in mutation-selection-drift balance in finite populations. Substantial differences among self-incompatibility systems occur in average mate availability, and in variances of mate availability among individual plants. These differences are most pronounced in small populations in which low mate availability may reduce seed set in some types of sporophytic self-incompatibility. In cases where the pollination system causes a restriction in the number of pollen genotypes available to an individual plant, the fecundity of that plant depends on the availability of compatible pollen, which is determined by its genotype at the incompatibility locus. This leads to an additional component of selection acting on self-incompatibility systems, which we term “fecundity selection.” Fecundity selection increases the number of alleles maintained in finite populations and increases mate availability in small populations. The strength of fecundity selection is dependent on the type of self-incompatibility. In some cases, fecundity selection markedly alters the equilibrium dynamics of self-incompatibility alleles. We discuss the population genetic consequences of mate availability and fecundity selection in the contexts of conservation management of self-incompatible plant species and experimental investigations on self-incompatibility in natural populations.     

High paternal diversity in the self-incompatible herb Arabidopsis halleri despite clonal reproduction and spatially restricted pollen dispersal

The number of sires fertilizing a given dam is a key parameter of the mating system in species with spatially restricted offspring dispersal, since genetic relatedness among maternal sibs determines the intensity of sib competition. In flowering plants, the extent of multiple paternity is determined by factors such as floral biology, properties of the pollen vector, selfing rate, spatial organization of the population, and genetic compatibility between neighbours. To assess the extent of multiple paternity and identify ecological factors involved, we performed a detailed study of mating patterns in a small population of a self-incompatible clonal herb, Arabidopsis halleri . We mapped and genotyped 364 individuals and 256 of their offspring at 12 microsatellite loci and jointly analysed the level of multiple paternity, pollen and seed dispersal, and spatial genetic structure. We found very low levels of correlated paternity among sibs ( P _full-sib = 3.8%) indicating high multiple paternity. Our estimate of the outcrossing rate was 98.7%, suggesting functional self-incompatibility. The pollen dispersal distribution was significantly restricted (mean effective pollen dispersal distance: 4.42 m) but long-distance successful pollination occurred and immigrating pollen was at most 10% of all pollination events. Patterns of genetic structure indicated little extent of clonal reproduction, and a low but significant spatial genetic structure typical for a self-incompatible species. Overall, in spite of restricted pollen dispersal, the multiple paternity in this self-incompatible species was very high, a result that we interpret as a consequence of high plant density and high pollinator service in this population.     

The genetics of self-incompatibility inLeavenworthia crassa rollins (Cruciferae)

Six plants of a self-incompatible population ofLeavenworthia çrassa were grown from seed collected in nature and cross-pollinated in all combinations. The incompatibility relationships between sibs were determined in eleven of the F₁ families. A one-locus sporophytic incompatibility system was established. None of the parents was homozygous at the S locus. At least five, and possibly all six, of the parents did not share an S allele. Only one pair of alleles was shown to have different interactions in the pollen and stigmata. The identity and expression of the S alleles were determined in six families. Eight pairs of alleles were independently expressed in both the pollen grains and the stigmata. Sixteen pairs of alleles showed dominance of one allele over the other in the pollen grains or the stigmata or both.F₁ plants of two crosses between different self-incompatible races were self-incompatible. F₁ plants of six crosses between self-incompatible and self-compatible races were self-incompatible; in five of the families, the frequency of pseudo-compatibility was higher than in the self-incompatible parent. Self-incompatible hybrids from a cross between a self-incompatible and a self-compatible population provide a method for rapidly determining allelic interactions in plants with a sporophytic incompatibility system.The research was carried out at the Biological Laboratories, Harvard University, Cambridge, Massachusetts, U.S.A.     

Loss of gametophytic self-incompatibility with evolution of inbreeding depression

Gametophytic self-incompatibility (SI) in plants is a widespread mechanism preventing self-fertilization and the ensuing inbreeding depression, but it often evolves to self-compatibility. We analyze genetic mechanisms for the breakdown of gametophytic SI, incorporating a dynamic model for the evolution of inbreeding depression allowing for partial purging of nearly recessive lethal mutations by selfing, and accounting for pollen limitation and sheltered load linked to the S-locus. We consider two mechanisms for the breakdown of gametophytic SI: a nonfunctional S-allele and an unlinked modifier locus that inactivates the S-locus. We show that, under a wide range of conditions, self-compatible alleles can invade a self-incompatible population. Conditions for invasion are always less stringent for a nonfunctional S-allele than for a modifier locus. The spread of self-compatible genotypes is favored by extremely high or low selfing rates, a small number of S-alleles, and pollen limitation. Observed parameter values suggest that the maintenance of gametophytic SI is caused by a combination of high inbreeding depression in self-incompatible populations coupled with intermediate selfing rates of the self-compatible genotypes and sheltered load linked to the S-locus.     

Rapid-cycling Brassica Species: Inbreeding and Selection of B. campestris for Anther Culture Ability

A study was made to determine the feasibility of producing,by inbreeding and selection, lines of rapid-cycling Brassicacampestris with high or low potential for anther culture. Sincethe base population of rapid-cycling B. campestris is self-incompatible,inbreeding was achieved by a combination of bud-pollinationand the application of pollen to the cut surfaces of decapitatedstigmas. Three inbred generations were raised, and in each generationplants were selected for high or low potential for anther embryogenesis.The proportion of viable pollen present in anthers, as indicatedby a fluorochromatic reaction and a germination test, was alsodetermined at each stage. Lines of rapid-cycling B. campestriswith clearly defined high or low potential for anther embryogenesiswere isolated in these experiments. Within each line, however,continuous variation was always observed. Pollen viability andanther efficiency were not correlated. Although inbreeding depressioncaused a significant decrease in pollen viability over the threegenerations, there were no obvious deleterious effects on antherefficiency. In general, over the three generations of inbreeding,no segregation in plant morphological characters was observed,although many developmental abnormalities were seen in the 3rdinbred generation and there was a marked reduction in the numberof seeds set. No association between plant vigour and high orlow anther efficiency was noted. All plants regenerated fromanther embryoids of rapid cycling B. campestris were haploid.By treating anther-derived embryoids, axillary buds and wholeplants with colchicine, dihaploid plants were produced, butthese failed to set seed after self-pollination. The diploidnature of the plants was confirmed, however, when they producednormal seeds after cross-pollination with plants of the basepopulation. Brassica campestris, anther culture, inbreeding, selection     

Overcoming Self-incompatibility through the Use of Lectins and Sugars in Petunia and Eruca

Treatment of stigma with a lectin (Con A/PHA) before pollinationwas effective in overcoming self-incompatibility in Petuniahybrida, a gametophytic self-incompatible system, and Erucasativa, a sporophytic self-incompatible system. Treatment ofpollen with glucose/N-acetyl-D-galactosamine (tested only withPetunia) was also effective. These results suggest the involvementof pollen lectins and specific sugar components of the pistilin self-incompatibility recognition. Petunia hybrida, Eruca sativa, self-incompatibility, pollen recognition, lectins     

Souroubea guianensis Aubl.: Quest for its Legitimate Pollinator and the First Record of Tapetal Oil in the Marcgraviaceae

The floral biology of Souroubea guianensis Aubl. was studiedat Dois Irmãos forest, a remnant of Atlantic forest inRecife, Pernambuco state, northeastern Brazil. The flowers ofS. guianensis have attributes which are not easily related toa particular pollination syndrome. They also have some uncommoncharacteristics, among them the presentation of pollen immersedin an oily fluid. The occurrence of this kind of pollen presentationin species of Marcgraviaceae is described here for the firsttime. The term tapetal oil is proposed for this unusual fluid.The flowers are protandrous, self-incompatible, last for 6 to7 d, and fruit-set under natural conditions is low. Sugar concentrationsin the nectar vary from 27.6 to 58.8%. After 105 h of observationfor pollinators in 3 different years, no efficient pollinatorwas found. Fragmentation of natural habitats could be one ofthe causes for the local loss of the legitimate pollinator ofSouroubeaguianensis . Copyright 2000 Annals of Botany Company Marcgraviaceae, Souroubea, pollination, tapetal oil, pollen presentation, protandry, psychophily, ornithophily, Atlantic forest, fragmentation, northeastern Brazil     

Microevolution of S-allele frequencies in wild cherry populations: respective impacts of negative frequency dependent selection and genetic drift

Negative frequency dependent selection (NFDS) is supposed to be the main force controlling allele evolution at the gametophytic self-incompatibility locus (S-locus) in strictly outcrossing species. Genetic drift also influences S-allele evolution. In perennial sessile organisms, evolution of allelic frequencies over two generations is mainly shaped by individual fecundities and spatial processes. Using wild cherry populations between two successive generations, we tested whether S-alleles evolved following NFDS qualitative and quantitative predictions. We showed that allelic variation was negatively correlated with parental allelic frequency as expected under NFDS. However, NFDS predictions in finite population failed to predict more than half S-allele quantitative evolution. We developed a spatially explicit mating model that included the S-locus. We studied the effects of self-incompatibility and local drift within populations due to pollen dispersal in spatially distributed individuals, and variation in male fecundity on male mating success and allelic frequency evolution. Male mating success was negatively related to male allelic frequency as expected under NFDS. Spatial genetic structure combined with self-incompatibility resulted in higher effective pollen dispersal. Limited pollen dispersal in structured distributions of individuals and genotypes and unequal pollen production significantly contributed to S-allele frequency evolution by creating local drift effects strong enough to counteract the NFDS effect on some alleles.     

Are there genetically controlled habitat-specific differences in spatial aggregation of drosophilids?

Variation in spatial patterns of competing organisms is of fundamental importance for community and population processes. Yet the mechanisms controlling subjects like the degree of spatial aggregation in competing insect larvae across fragmented resources have rarely been addressed. In the present study, we tested for systematic differences in the spatial distribution patterns of Drosophila subobscura in natural fly communities, and found significant differences in two habitats that differ in the availability of breeding substrate types (decaying fruits). Assuming that the spatial egg-laying behaviour of drosophilids is under genetic control, and that different breeding substrates mediate different density-dependent larval fitness consequences, we tested whether adaptive genotypic variability is involved in the local variation of egg distribution patterns. We extracted isofemale lines from both habitats and analysed the spatial distribution of eggs achieved by single female flies under controlled laboratory conditions. This is a reasonable first test, because spatial patterns at the fly population level can be attributed to individual egg-laying behaviour. The degree of individual egg aggregation significantly depended on fly line identity, which indicates the existence of behavioural variants in natural populations. Based on habitat-dependent differences in the degree of spatial aggregation, we discuss to what extent our findings may reflect a behavioural adaptation to local breeding conditions.     

Mate limitation in populations of the endangered Convolvulus lineatus L.: A case for genetic rescue?

In self-incompatible clonal plants, the spread of individual plants can exacerbate mate limitation to the point that it becomes a serious constraint on long-term population persistence, especially in small, isolated populations. In such species, it may be necessary to introduce new genetic material from other populations to restore seed production, a strategy termed “genetic rescue”. In this study we assess the potential pertinence of such genetic rescue in the clonal perennial plant Convolvulus lineatus L., whose populations are often highly reduced in spatial extent and are currently being fragmented by land development projects in Mediterranean France. To do so, we quantify fruit production in a range of populations of different size over four years and perform a series of hand-pollination experiments in natural populations to assess whether fruit set is limited by mate availability. We found that C. lineatus is a self-incompatible species that shows extremely low values of fruit set in natural populations and that a principal cause of this low fruit set is a lack of compatible pollen. This may be primarily due to clonal spread that causes individual populations to be comprised of patches containing one or very few incompatibility types. In small populations fragmented by human activities and which show an absence of fruit production, we thus argue that genetic rescue represents a promising conservation management strategy to avoid inevitable long-term future population decline. We discuss how best to introduce new genetic material into the study populations.     

Small populations are mate-poor but pollinator-rich in a rare, self-incompatible plant, Hymenoxys herbacea (Asteraceae)

If pollinators or compatible mates are scarce, plants in small populations may be pollen-limited and consequently produce fewer offspring. However, determining the relative importance of the genetic vs ecological mechanisms limiting successful pollination is challenging. We explored the relationships among population size, population connectivity, pollinator visitation, allozyme diversity, mate availability (measured as percent compatible crosses among plants within a population), and pollen limitation in 12 populations (N = 39-885,274) of the self-incompatible plant Hymenoxys herbacea in Ontario, Canada. Unexpectedly, small populations had more insect flower visitors per capitulum than large populations. Consistent with the effects of genetic drift, both allozyme polymorphism and mate availability decreased with decreasing population size. Pollen limitation was low and significant in only one population and could not be predicted based on knowledge of population size, connectivity, compatible mate availability, or pollinator visitation. Population size had detectable effects on both pollinator activity and mate availability. However, because the effect of population size was complementary on these two potentially limiting mechanisms, this threatened plant was rarely pollen-limited.     

Population structure in Arabidopsis lyrata: evidence for divergent selection on trichome production

Leaf trichomes may serve several biological functions including protection against herbivores, drought, and UV radiation; and their adaptive value can be expected to vary among environments. The perennial, self-incompatible herb Arabidopsis lyrata is polymorphic for trichome production, and occurs in a glabrous and a trichome-producing form. Controlled crosses indicate that the polymorphism is governed by a single gene, with trichome production being dominant. We examined the hypothesis that trichome production is subject to divergent selection (i.e., directional selection favoring different phenotypes in different populations) by comparing patterns of variation at the locus coding for glabrousness and at eight putatively neutral isozyme loci in Swedish populations of A. lyrata. The genetic diversity (He) and allele number at isozyme loci tended to increase with population size and decreased with latitude of origin, whereas genetic diversity at the locus coding for glabrousness did not vary with population size and increased with latitude of origin. The degree of genetic differentiation at the glabrousness locus was much higher than that at isozyme loci. Genetic identity at isozyme loci was negatively related to geographic distance, suggesting isolation by distance. In contrast, there was no significant correlation between genetic identity at the glabrousness locus and at isozyme loci. The results are consistent with the hypothesis that divergent selection contributes to population differentiation in trichome production in A. lyrata.     

An experimental study of the S-Allee effect in the self-incompatible plant <Emphasis Type="Italic">Biscutella neustriaca</Emphasis>

Homomorphic self-incompatibility (SI) evolved in many plant families to enforce selfing avoidance, and is controlled by a single multiallelic locus (the S-locus). In a fragmented landscape, strong variation in population size and in local density is expected to cause strong variation in allelic diversity at the S-locus, which could generate an Allee effect on female reproductive success by constraining compatible pollen availability. In this experimental study, we aimed at detecting this SI-specific Allee effect (or S-Allee effect) in the endangered species Biscutella neustriaca. We demonstrated the occurrence of a SI mating system in the species and determined compatibility relationships among genotypes through a large set of controlled pollinations. For the experiment, we chose three different pollen receptor genotypes, each compatible with respectively 100, 75 and 25% of four other genotypes, which constituted the pollen sources. We placed different ramets of each receptor at different distances from the pollen sources to control for pollen limitation due to low local density, and we measured the seed set on each receptor plant three times consecutively. Analyses performed with generalized linear mixed models showed that both the distance to the pollen sources and the mate availability due to SI had a significant effect on seed set, with a strong reduction observed when mate availability was limited to 25%. Our results suggest that pollen limitation due to a restriction in compatible mate availability could occur in small or scattered populations exhibiting low allelic diversity at the S-locus.     

Effects of population size and pollen diversity on reproductive success and offspring size in the narrow endemic Cochlearia bavarica (Brassicaceae)

In small, fragmented populations of self-incompatible plant species, genetic drift and increasingly close relationships between plants may restrict the number of genetically different pollen donors, the availability of compatible mates, and the opportunity for pollen competition and selection. These restrictions may reduce the siring success or increase the probability of inbreeding depression in the offspring. To test if this was the case, we hand-pollinated maternal plants in small and large populations of the rare, endemic plant Cochlearia bavarica (Brassicaceae) with pollen from one, three, or nine donors from the same population or with nine donors from a different population. In one additional population of intermediate size, maternal plants were hand-pollinated with ten donors located at a distance of 1, 10, 100, or 1000 m. We then recorded seed and offspring characters. On average, offspring from small populations were smaller than normal and fewer survived to maturity. Increasing the number of pollen donors had a positive effect on reproductive success in small and large populations, but at the highest pollen diversity this occurred at the expense of slightly reduced offspring fitness. Because the total amount of transferred pollen was held constant, these effects could not be attributed to increasing pollen load. Rather, the increasing pollen diversity may have increased the chances of selecting a particularly "good" donor for fertilization-an example of a sampling effect of diversity. Pollen from outside a population or from 10-100 m away resulted in higher reproductive success and greater offspring size. Effects of population size and pollination treatments on reproductive success and offspring fitness were additive. Apparently, there is no obvious size threshold above which the potential of inbreeding depression can be ignored in C. bavarica.     

Adaptive significance of self-fertilization in a hermaphroditic perennial, Trillium camschatcense (Melanthiaceae)

The evolution of self-fertilization from primarily outcrossing ancestors is one of the most common evolutionary transitions in plants; however, the ecological mechanisms that maintain self-fertilization have remained controversial. Theoretical studies suggest that selfing is advantageous over outcrossing in terms of genetic transmission and assurance of seed production under pollen-limited circumstances. Trillium camschatcense is a herbaceous perennial distributed in Hokkaido and northern Honshu, Japan. Geographical variation in the breeding system (self-compatible, SC; or self-incompatible, SI) has been reported in populations in Hokkaido. Here, we used several SC and SI populations of T. camschatcense to investigate the adaptive significance and the evolutionary basis of self-fertilization. Pollination experiments and genetic analyses demonstrated that the potential availability of outcross pollen in SC populations was sufficient and that the number of pollen donors was equal to that of SI populations. However, despite the high availability of outcross pollen, the SC populations produced seeds predominantly by selfing and so underwent severe inbreeding depression. Although none of the suggested advantages for self-fertilization were supported by our analyses, we propose two possible scenarios for the evolution of self-fertilization in T. camschatcense.     

Diversity of S-alleles and mate availability in 3 populations of self-incompatible wild pear (Pyrus pyraster)

Small populations of self-incompatible plants may be expected to be threatened by the limitation of compatible mating partners (i.e., S-Allee effect). However, few empirical studies have explicitly tested the hypothesis of mate limitation in small populations of self-incompatible plants. To do so, we studied wild pear (Pyrus pyraster), which possesses a gametophytic self-incompatibility system. We determined the S-genotypes in complete samplings of all adult trees from 3 populations using a PCR-RFLP approach. We identified a total of 26 different S-alleles, homologous to S-alleles of other woody Rosaceae. The functionality of S-alleles and their Mendelian inheritance were verified in artificial pollination experiments and investigations of pollen tube growth. The smallest population (N = 8) harbored 9 different S-alleles and showed a mate availability of 92.9%, whereas the 2 larger populations harbored 18 and 25 S-alleles and exhibited mate availabilities of 98.4% and 99.2%, respectively. Therefore, we conclude that even small populations of gametophytic self-incompatible plants may exhibit high diversity at the S-locus and are not immediately threatened owing to reduced mate availability.     

Partially Fertile Line with Apospory Obtained from Tissue Culture of Male Sterile Plant of Sorghum (Sorghum bicolor L. Moench)

A partial restoration of male fertility has been revealed inthree among 20 regenerants from callus cultures obtained fromthe panicle fragments of the sorghum plant with cytoplasmicmale sterility. Induced fertility is retained under self-pollinationfor eight generations. The pollen fertility level in every generationvaries in different plants (0-70%) and depends on the environmentalfactors, rather than on plant genotype. A high positive correlation(r = 0·90; P < 0·01) has been revealed betweenthe pollen fertility level and the total rain precipitationduring the microgametophytogenesis. Cytoembryological investigationof this line has shown the presence of structures similar toaposporous embryo sacs (ESs), which have developed in the ovulesalong with the sexual ESs. The ovules with aposporous formationshave been observed almost in all of the studied plants in severalgenerations, however, their frequency varied (2-53%). Parthenogeneticproembryos and endosperm nuclei without signs of pollen tubepenetration have been found in some of such ESs. The natureof the instability observed in a number of generations in maleand female generative structures is discussed.Copyright 1995,1999 Academic Press Cytoplasmic male sterility, fertile revertants, apomixis, somaclonal variability, Sorghum bicolor L. (Moench)