Mating system of wild Phaseolus lunatus L. and its relationship to population size

Using isozyme variation in a naturally pollinated seed family for 10 wild Phaseolus lunatus L. (Lima bean) populations, ranging in sizes from 10 to 60 reproductive individuals, we estimated levels of outcrossing (t) and parental inbreeding coefficient (F). We also examined the relationship between outcrossing rate and population size. Average estimates of the single-locus outcrossing rate (ts) ranged from 0.024 to 0.246 (mean=0.091+/-0.065). Estimates of the multilocus outcrossing rate (tm) ranged from 0.027 to 0.268, and averaged 0.096+/-0.071. Inbreeding coefficients based on genotypic frequencies of maternal plants were positive and significantly greater than zero (F=0.504), suggesting an excess of homozygotes in all the populations studied. There was indirect evidence of nonrandom mating for outcrosses and this was mainly attributed to self-fertilisation since the averaged difference between tm and ts, which provides a measure of biparental inbreeding, represents only 1% of the autogamy rate. No significant correlation was observed between outcrossing rate and population size. Estimates of t showed significant heterogeneity among populations and factors explaining this tendency are suggested.     

Fine-scale estimation of outcrossing in western redcedar with microsatellite assay of bulked DNA

Western redcedar (Thuja plicata, Cupressaceae) is a self-fertile conifer with a mixed mating system and significant variation for outcrossing among populations. In this paper, we conducted a fine-scale study of mating system variation to identify correlates of outcrossing in natural populations. We examined variation for outcrossing within and among individual trees, and describe a new method to estimate outcrossing using bulked DNA samples. Bulking (assaying DNA tissues from several individuals simultaneously) increases the experimental power without increasing the experimental effort. We sampled 80 trees from four natural populations in southwestern British Columbia. From each tree, we sampled from up to six crown positions (three heights and inner vs outer branches). From each position, two samples of three seedlings each were bulked before DNA extractions. Using four microsatellite loci, we obtained outcrossing rates for each tree and for each of the six crown positions. We found individual tree selfing rates to increase with tree height in all four populations, but selfing rates did not differ among crown positions. The higher selfing rate of larger trees is probably due to their greater proportional contribution to local pollen clouds. Individual tree outcrossing rates ranged from 22 to 100% and the population outcrossing rates from 66 to 78%. Missed alleles due to bulking and the estimation method used both cause a downward bias in outcrossing rates, so that these estimates are probably lower than the actual outcrossing rates. Nevertheless, the trends we observed are not affected by systematic biases of estimation.     

Flexible mating system in a logged population of <Emphasis Type="Italic">Swietenia macrophylla</Emphasis> King (Meliaceae): implications for the management of a threatened neotropical tree species

Microsatellites were used to evaluate the mating system of the remaining trees in a logged population of Swietenia macrophylla, a highly valuable and threatened hardwood species, in the Brazilian Amazon. A total of 25 open pollinated progeny arrays of 16 individuals, with their mother trees, were genotyped using eight highly polymorphic microsatellite loci. Genotypic data analysis from the progeny arrays showed that 373 out of the 400 seedlings (93.25%) were unambiguously the result of outcrossed matings and that the remaining 6.75% had genotypes consistent with self-fertilisation. Apomixis could be ruled out, since none of the 400 seedlings analysed had a multi-locus genotype identical to its mother tree. The high estimate of the multi-locus outcrossing rate (t _m = 0.938 ± 0.009) using the mixed mating model also indicated that the population in this remnant stand of S. macrophylla was predominantly allogamous. The relatively large difference between the multi-locus and single-locus outcrossing estimates (t _m −t _s = 0.117 ± 0.011) provides evidence that, in spite of the high outcrossing rate, a considerable degree of biparental inbreeding has contributed to the genetic structure of this population. Levels of outcrossing were not evenly distributed among maternal trees (t _m ranging from 0.39 to 1.00), suggesting the occurrence of a variable degree of self-incompatibility and/or dichogamy among individual trees of this monoecious species. Due to its generalist pollination system and some level of tolerance for selfing, S. macrophylla seems to be resilient to environmental disturbances such as those caused by logging, since it sets fruits with predominantly outcrossed seeds even at low stand densities. Therefore, the remaining individuals in logged areas or in relict fragments may be very important for long-term population recovery and genetic conservation programmes.     

MATING SYSTEM OF A TROPICAL RAIN FOREST TREE SPECIES

Canopy tree species are the dominant elements of the species-rich, fragile and endangered tropical rain forest ecosystems, yet little is known about the genetics of these species. We provide an estimate of the outcrossing rate in a population of Pithecellobium pedicellare, a large canopy tree in the tropical rain forests of Costa Rica. The outcrossing rate was high (t = 951 ± 0.021) and the pollen pool contributing to the progeny arrays used to measure outcrossing rate showed departure from homogeneity. The high outcrossing rate indicates that individuals scattered over a large area in this low density population could be bound with each other via outcrossing. However, despite a high outcrossing rate, the potential for inbreeding in this population is not negligible. We found a relatively large number of albino and chlorotic seedlings among the progeny arrays of several seed parents. Heterogeneity of pollen pool further indicates that the population, though widely outcrossed, may be structured. A more detailed study of mating systems in tropical rain forest trees may provide additional insights into the mating patterns of these trees. Such studies will be useful not only in understanding the dynamics of micro-evolutionary processes, but also in the conservation and management of tropical forest trees.     

用遗传标记监测不同环境和经营措施下的加勒比松森林群体遗传动态

对古巴加勒比松的6个群体（包括天然林、采伐林、母树林和种子园）进行了同功酶分析,根据5个酶系统8个位点的同功酶数据,对各群 交配系统以及群体遗传变异和结构进行了分析,天然林、种子园和母树林的多位点异交率和绝大多数单位点异交率都和完全异交无显著差异,过渡采伐的松树岛群体多位点异交率显著小于完全异交,而只有一半单位点异交率显著小于完全异交,而且该群体单位点平均异交率和多位点异交率均低于其它3个群体的估计值。采伐群体中同功酶变异和基因多样性与天然林群体JAG的相似,但低于其它群体,其近交系数较大,但小于天然林MAN和中国栽培群体的近交系数。中国引种栽培群体无论是同功酶变异还是基因多样性都显著高于古巴群体,与所有古巴群体的遗传距离都显著大于古巴群体之间的遗传距离。结果表明过度采伐导致群体自交程度增加,营建种子园可有效减少近交。自然分布区以外的引种栽培群体遗传变化量大,无论遗传变异和基因多样性都比参试其它群体大。     

Effects of logging rotation in a lowland dipterocarp forest on mating system and gene flow in <Emphasis Type="Italic">Shorea parvifolia</Emphasis>

Selective logging is one of several silvicultural practices used in sustainable forest management in the lowland dipterocarp forest in Indonesia. Selecting only trees with diameters >50 cm for logging can reduce the density of reproductive trees, thereby affecting pollen dispersal and influencing the mating system among remaining trees. We evaluated the effect of logging rotations on the mating system, gene flow and genetic diversity in populations of Shorea parvifolia in primary forest, and in first and second rotation forest. Our results revealed that multiple (or at least two) selective logging events with a 30-year logging rotation had a significant impact on the genetic diversity of pollen clouds. However, the average pollen dispersal distance did not differ significantly among the multiple selective logging rotations. The multiple rotations reduced the outcrossing rate and the number effective of pollen donors in the logged forest. Moreover, the number of pollen donors in a plot was affected by the basal area of reproductive trees present. These results suggest that reducing the number of reproductive trees by logging with multiple rotations might increase the bi-parental inbreeding rate due to the reduction in density of reproductive trees in a selectively logged forest. We conclude that multiple rotations with a 30-year cycle of selective logging as currently practiced would reduce the density of reproductive trees, and would not be sustainable in terms of maintaining genetic diversity in tropical forests of Southeast Asia.     

Mating patterns, pollen dispersal, and the ecological maternal neighbourhood in a Prunus mahaleb L. population

Gender polymorphism, plant-animal interactions, and environmental heterogeneity are the three important sources of variation in mating system and pollen dispersal patterns. We used progeny arrays and paternity analysis to assess the effects of gender type and density level on variation in mating patterns within a highly isolated population of Prunus mahaleb, a gynodioecious species. All the adult trees in the population were sampled and located. The direct estimate of long-distance insect-mediated pollination events was low (< 10%). Gender expression deeply influenced the mating system, decreasing the outcrossing rates (t(m)) and the pollen pool diversity in hermaphrodite trees. Long intermate distances (> 250 m) were significantly more frequent among female mother trees. Variation in local tree density also affected pollen pool diversity and intermate distance, with a higher effective number of fathers (k(e)) and longer intermate distances for female trees in low-density patches. A canonical correlation analysis showed significant correlations between mating variables and the maternal ecological neighbourhood. Only the first canonical variable was significant and explained 78% of variation. Outcrossing rates tended to decrease, and the relatedness among the fathers tended to increase, when mother trees grew in dense patches with high cover of other woody species and taller vegetation away from the pine forest edge. We highlight the relevance of considering maternal ecological neighbourhood effects on mating system and gene flow studies as maternal trees act simultaneously as receptors of pollen and as sources of the seeds to be dispersed.     

Low selfing in a mass-flowering, endangered perennial, Eryngium alpinum L. (Apiaceae)

We investigated the reproductive ecology of an endangered alpine species, Eryngium alpinum L., to determine its selfing rate and to propose possible mechanisms that may shape its breeding system. Whereas pollinators' foraging behavior suggested a high potential for geitonogamy (70% of the flights occur within plants), microsatellite analyses of seed progenies demonstrated that plants are primarily outcrossing (outcrossing rate [tm] = 0.65, 0.96, and 1 in three populations). Given the relatively long pollen viability (at least 4-5 d) and the high number of simultaneously opened flowers on each plant, protandry is not sufficient to eliminate selfing. Second, controlled crosses demonstrated not only auto-fertility, but also partial self-incompatibility. Partial self-incompatibility is probably due to the competitive advantage of cross vs. self-pollen, and, together with protandry, could lead the species to selfing as a reproductive assurance. These results are encouraging for the maintenance of large populations. However, higher selfing was observed in a small population that could suffer inbreeding depression, as observed on experimentally selfed seeds. Thus, these populations should be carefully monitored. Finally, this study shows how molecular markers and field experiments may complement each other in our reaching a global understanding of mating patterns.     

Genetic structure and mating system of Manilkara huberi (Ducke) A. Chev., a heavily logged Amazonian timber species

In this work, we report on the population genetic structure of the endangered tree species Manilkara huberi, an Amazonian tree species intensely exploited due to the high density and resistance of its wood. We investigated the patterns of spatial distribution, genetic structure, and mating system using 7 microsatellite loci and here discuss the consequences for conservation and management of the species. To examine the population genetic structure, 481 adult trees and 810 seedlings were sampled from an area of 200 ha from a natural population in FLONA Tapajós, PA, Brazil. We found relatively high and consistent inbreeding levels (intrapopulation fixation index [f] 0.175 and 0.240) and a significant spatial genetic structure up to a radius of approximately 300 m, most likely due to a limited seed and pollen flow. The multilocus (tm) population outcrossing rate was high (0.995), suggesting that the species is predominantly allogamous with a pollen flow restricted to 47 m. These results suggest that M. huberi is spatially structured, consistent with a model of isolation by distance. Fragmentation may therefore cause the loss of subpopulations, suggesting that management programs for production and conservation should include large areas. The genetic data also revealed that for ex situ conservation, seeds should be collected from more than 175 maternal trees, in order to keep an effective population size of 500. Furthermore, as the species is widely distributed across the Amazon Forest, samples should include several populations in order to represent the highest genetic diversity possible. These results provide a blueprint to guide the production and conservation management policies of this valuable timber species.     

Long-distance pollen flow and tolerance to selfing in a neotropical tree species

Outcrossing rates, pollen dispersal and male mating success were assessed in Dicorynia guianensis Amshoff, a neotropical tree endemic to the Guiana shield. All adult trees within a continuous area of 40 ha (n = 157) were mapped, and were genotyped with six microsatellite loci. In addition, progenies were genotyped from 22 mature trees. At the population level, the species was mostly outcrossing (tm = 0.89) but there was marked variation among individuals. One tree exhibited mixed mating, confirming earlier results obtained with isozymes that D. guianensis can tolerate selfing. A Bayesian extension of the fractional paternity method was used for paternity analysis, and was compared with the neighbourhood method used widely for forest trees. Both methods indicated that pollen dispersal was only weakly related to distance between trees within the study area, and that the majority (62%) of pollen came from outside the study stand. Using maximum likelihood, male potential population size was estimated to be 1119, corresponding to a neighbourhood size of 560 hectares. Male mating success was, however, related to the diameter of the stem and to flowering intensity assessed visually. The mating behaviour of D. guianensis is a combination of long-distance pollen flow and occasional selfing. The species can still reproduce when it is extremely rare, either by selfing or by dispersing pollen at long distances. These results, together with the observation that male mating success was correlated with the size of the trees, could be implemented in management procedures aiming at regenerating the species.     

Reproductive biology and genetic diversity of a cryptoviviparous mangrove aegiceras corniculatum (Myrsinaceae) using allozyme and intersimple sequence repeat (ISSR) analysis

Mangroves consist of a group of taxonomically diverse species representing about 20 families of angiosperms. However, little is known about their reproductive biology, genetic structure, and the ecological and genetic factors affecting this structure. Comparative studies of various mangrove species are needed to fill such gaps in our knowledge. The pollination biology, outcrossing rate, and genetic diversity of Aegiceras corniculatum were investigated in this study. Pollination experiments suggested that the species is predominantly pollinator-dependent in fruit setting. A quantitative analysis of the mating system was performed using progeny arrays assayed for intersimple sequence repeat (ISSR) markers. The multilocus outcrossing rate (tm) was estimated to be 0.653 in a wild population. Both allozyme and ISSR were used to investigate genetic variation within and among populations. The combined effects of founder events and enhanced local gene flow through seedling dispersal by ocean currents apparently played an important role in shaping the population genetic structure in this mangrove species. Both allozyme variation (P = 4.76%, A = 1.05, HE = 0.024) and ISSR diversity (P = 16.18%, A = 1.061, HE = 0.039) were very low at the species level, in comparison with other woody plants with mixed-mating or outcrossing systems. Gene differentiation among populations was also low: allozyme GST = 0.106 and ISSR GST = 0.178. The unusually high genetic identities (0.997 for allozyme and 0.992 for ISSR loci), however, suggest that these populations are probably all descended from a common ancestral population with low polymorphism.     

Reproductive biology of the dry forest tree Enterolobium cyclocarpum (guanacaste) in Costa Rica: a comparison between trees left in pastures and trees in continuous forest

We compared the rate of pollen deposition, the likelihood of fruit production, the number of seeds per fruit, the outcrossing rate, and the progeny vigor of the tropical dry forest tree Enterolobium cyclocarpum for individuals in pastures vs. individuals in continuous forest. We found that flowers from trees growing in continuous forests were more likely to have pollen deposited on their stigmas than flowers from trees in pastures (52.1 vs. 32.3%, respectively). We also found that trees from continuous forests were almost six times more likely to set fruits and produce more seeds per fruit than trees in pastures. Morever, progeny from trees in continuous forests were, on average, more vigorous than the progeny from trees in pastures, as indicated by 12 of 16 indicators of plant vigor. However, there was no significant difference in the multilocus estimate of the outcrossing rate between the two groups of trees (tm = 1.00 and 0.99 for trees from continuous forest and trees from pastures, respectively). But there are differences in the correlation of paternity between the progeny of the two groups, where the progeny from trees in pastures showed a lower correlation of paternity than progeny of trees from continuous forests (r(p) = 0.104 and r(p) = 0.189, respectively). We argue that the mechanisms that regulate progeny vigor are disrupted in trees from pastures. We discuss the implications of these findings for the conservation of E. cyclocarpum.     

Limited pollen dispersal and biparental inbreeding in Symphonia globulifera in French Guiana

In this paper, we report a study of the mating system and gene flow of Symphonia globulifera, a hermaphroditic, mainly bird-pollinated tree species with a large geographic distribution in the tropical Americas and Africa. Using three microsatellites, we analysed 534 seeds of 28 open pollinated families and 164 adults at the experimental site 'Paracou' in French Guiana. We observed, compared to other tropical tree species, relatively high values for the effective number of alleles. Significant spatial genetic structure was detected, with trees at distances up to 150 m more genetically similar than expected at random. We estimated parameters of the mating system and gene flow by using the mixed mating model and the TwoGener approach. The estimated multilocus outcrossing rate, tm, was 0.920. A significant level of biparental inbreeding and a high proportion of full-sibs were estimated for the 28 seed arrays. We estimated mean pollen dispersal distances between 27 and 53 m according to the dispersal models used. Although the adult population density of S. globulifera in Paracou was relatively high, the joint estimation of pollen dispersal and density of reproductive trees gave effective density estimates of 1.6 and 1.3 trees/ha. The parameters of the mating system and gene flow are discussed in the context of spatial genetic and demographic structures, flowering phenology and pollinator composition and behaviour.     

MATING SYSTEM AND GENETIC STRUCTURE OF THE DISTYLOUS TROPICAL TREE PSYCHOTRIA FAXLUCENS (RUBIACEAE)

The population genetics and mating system of the understory tropical rain forest tree Psychotria faxlucens were studied at two plots in Los Tuxtlas, Veracruz, México, on the Gulf of México coast. This species is distylous and is pollinated by moths, mainly small hawkmoths. The seeds are dispersed by gravity and by frugivorous birds. Controlled pollinations indicate that the trees are self-incompatible and that intramorph pollinations have lower compatibilities than intermorph pollinations. The pollen: ovule ratio is high, suggesting obligate xenogamy. Using electrophoretic allozyme methods we found that eight out of 20 loci were polymorphic (P = 0.400), the observed heterozygosity (H) was 0.198, and the mean expected heterozygosity (H) was 0.495, both relatively high values compared with that reported for tropical trees. The genetic differentiation between the two plots is low, as shown by the heterogeneity in allele frequencies and the F_st (mean F_st seedlings = 0.031, mean F_st adults = 0.026), although for some loci the plot differentiation is statistically significant. The studied populations are near Hardy-Weinberg proportions, both for seedlings (mean F = 0.128) and adults (mean F = 0.075). From the fixation rate, an indirect estimate of the outcrossing rate at equilibrium gave a mean of t = 0.898 for plot 1 and 0.685 for plot 2. Direct single loci and multiloci outcrossing rate estimates were generally not statistically different from 1.0.     

Genetic diversity and mating system in a tropical tree,Carapa guianensis (Meliaceae)

Carapa guianensis Aublet. (Meliaceae) is an abundant, canopy tree species often exported as fine wood products. The populations in the Atlantic coastal plain of Costa Rica have undergone a wide variety of human-induced changes in distribution and abundance, including isolation due to deforestation. Using protein electrophoretic analysis of seeds, we investigated the genetic variability and population differentiation of nine populations and the outcrossing rate in two populations that differed in density by a factor of 50%. Average polymorphism (35%) and heterozygosity (0.12) across populations were relatively low in comparison with other tropical tree species. Only 4.6% of the total genetic variability could be attributed to population differentiation even though populations were separated by as much as 70 km. However, genetic differentiation did increase with distance between pairs of compared populations. There were no apparent differences in genetic variation between logged and unlogged forests. The outcrossing rate for each population was not significantly different from 1.0, but there was evidence of nonrandom mating in the lower density population. High levels of gene flow are probably maintained by seed dispersal as seeds travel well throughout flooded forests and rivers. High population density and synchronous flowering also contribute to high outcrossing rates. These genetic characteristics suggest that Carapa guianensis is a good candidate for natural forest management where it occurs at high density. However, current management practices need to account for the rapid rate of deforestation occurring around managed natural forest sites that will result in the elimination of intervening populations.     

To self, or not to self... a review of outcrossing and pollen-mediated gene flow in neotropical trees

Despite the typically low population densities and animal-mediated pollination of tropical forest trees, outcrossing and long-distance pollen dispersal are the norm. We reviewed the genetic literature on mating systems and pollen dispersal for neotropical trees to identify the ecological and phylogenetic correlates. The 36 studies surveyed found >90% outcrossed mating for 45 hermaphroditic or monoecious species. Self-fertilization rates varied inversely with population density and showed phylogenetic and geographic trends. The few direct measures of pollen flow (N=11 studies) suggest that pollen dispersal is widespread among low-density tropical trees, ranging from a mean of 200 m to over 19 km for species pollinated by small insects or bats. Future research needs to examine (1) the effect of inbreeding depression on observed outcrossing rates, (2) pollen dispersal in a wide range of pollination syndromes and ecological classes, (3) and the range of variation of mating system expression at different hierarchical levels, including individual, seasonal, population, ecological, landscape and range wide.     

Among- and within-population variation in outcrossing rate of a mixed-mating freshwater snail

Mixed-mating animals self-fertilize a proportion of their offspring. Outcrossing rate may covary with the ecological and historical factors affecting the population. Theory predicts that outcrossing is favored when inbreeding depression is high and when individual heterozygosity is important. Self-fertilization is predicted to be favored when costs of male function, or mate finding are high, for example, when empty patches are colonized by few individuals. In this study, we assessed primary (after hatching) and secondary (after juvenile mortality) outcrossing rates of two mixed-mating snail populations. Our purpose was to assess the variation in mating-system parameters and estimate significance of inbreeding depression for secondary outcrossing rate (the realized outcrossing rate of parents that produce the next generation). Secondary outcrossing rate was higher than the primary outcrossing rate in one of the two populations, suggesting considerable inbreeding depression. In the other study population, secondary outcrossing rates were found to increase when initially low, or decrease when initially high, depending on the family. Moderate outcrossing rates were found to be more stable. Parental inbreeding coefficients were close to zero in both populations. Outcrossing rate was much more variable among families in the population with the lower average outcrossing rate, suggesting that individuals differed considerably in their mating system. Our results add to recent studies suggesting that populations of mixed-mating animals may differ in their mating system parameters and expression of inbreeding depression.     

Increased selfing and correlated paternity in a small population of a predominantly outcrossing conifer, Pinus sylvestris

Outcrossing rate, the rates of ovule and seed abortion, and levels of correlated paternity were estimated in a small population of Pinus sylvestris, a predominantly outcrossing conifer, and were compared with estimates from two widely dispersed woodlands of the same species, showing a range of densities. On average, seed trees of the small population showed an eight-fold higher selfing rate (25 vs. 3%) and a 100-fold greater incidence of correlated paternity (19.6 vs. 0.2%) than did trees from the large populations. No evidence was found of pollen limitation within the remnant stand, as suggested by ovule abortion rates. Investigation of the mating patterns in the small population, based on the unambiguous genealogy of 778 open-pollinated seeds, showed a large departure from random mating. Only 8% of the possible mating pairs within the stand were observed. Correlated paternity rate within a maternal sibship was negatively associated (rs = -0.398, P < 0.050) with the distance to the nearest neighbour, and shared paternity among maternal sibships was negatively correlated (rs = -0.704, P < 0.001) with the distance between seed trees. Numerical simulations, based on the estimated individual pollen dispersal kernel, suggest that restricted dispersal might have been the key factor affecting mating patterns in the small population and, together with low population density, may account for the observed mating system variation between the small and the large populations. The results of this study show that a severe size reduction may substantially affect the mating system of a wind-pollinated, typically outcrossed plant species.     

Pollen pool heterogeneity in jack pine (Pinus banksiana Lamb.): a problem for estimating outcrossing rates?

Summary Pollen pool heterogeneity, which violates an assumption of the mixed-mating model, is a major potential problem in measuring plant mating systems. In this study, isozyme markers were used to examine pollen pool heterogeneity in two natural populations of jack pine, Pinus banksiana Lamb., in northwestern Ontario, Canada. Population multilocus estimates of outcrossing rate ranged from 0.83 to 0.95 and differed significantly between populations. Single-tree multilocus outcrossing rates were found to be homogeneous among trees in both populations. Computer simulation studies indicated that a consanguineous pollen pool (pollen gametes related to the mother tree) was capable of biasing population outcrossing estimates downward. Random pollen pool heterogeneity (uncorrelated with maternal genotypes) did not appear to affect population outcrossing estimates in the simulations. Heterogeneity G-tests and Spearman rank tests showed that pollen pool heterogeneity existed in the two natural populations examined; however, it did not have a major effect on population outcrossing estimates, since the consanguineous pollen pool detected was probably a relatively minor component of the outcross pollen pool in both populations. In addition, heterogeneity G-tests were found to be not sensitive in detecting pollen pool heterogeneity caused by consanguineous pollen pool.     

Mating system parameters in species of genus Prosopis (Leguminosae)

The section Algarobia of genus Prosopis involves important natural resources in arid and semiarid regions of the world. Their rationale use requires a better knowledge of their biology, genetics and mating system. There are contradictory information about their mating system. Some authors claim they are protogynous and obligate outcrosser. However, some evidence have been shown indicating that they might not be protogynous and that they might be somewhat self-fertile. The current paper analyses genetic structure and mating system parameters in populations of seven species of this section from South and North America based on isozyme data. In all species a significant homozygote excess was found in the offspring population but not in mother plant genotypes. Multilocus and mean single locus outcrossing rates (tm, ts) indicated that about 15% selfing can occur in the studied populations. The heterogeneity between pollen and ovule allele frequencies was low suggesting population structuration, in agreement with the estimates of correlation of tm within progeny (rt) and correlation of outcrossed paternity (rp). The difference of FIS estimates between offspring and mother plants suggest some selection favouring heterozygotes between seedling and adult stages.