THE EFFECTS OF POPULATION SIZE AND PLANT DENSITY ON OUTCROSSING RATES IN LOCALLY ENDANGERED SALVIA PRATENSIS

Multilocus outcrossing rates were estimated in natural and experimental populations of Salvia pratensis, an entomophilous, gynodioecious, protandrous perennial. Male steriles were used to check the estimation procedure of outcrossing rates in hermaphrodites. Estimates of outcrossing rates in hermaphroditic plants ranged from 38.2% to 81.8% in natural populations and from 71.5% to 95.5% in experimental populations. No correlations were found between outcrossing rates and population size. However, outcrossing in hermaphrodites was promoted by high plant densities and low frequencies of male steriles. It is argued that effective management to preserve genetic variation in populations of S. pratensis should provide for the maintenance of high plant densities.     

INBREEDING EFFECTS IN CLARKIA TEMBLORIENSIS (ONAGRACEAE) POPULATIONS WITH DIFFERENT NATURAL OUTCROSSING RATES

Inbreeding depression is commonly observed in natural populations. The deleterious effects of forced inbreeding are often thought to be less pronounced in populations with self-pollinating mating systems than in primarily outcrossing populations. We tested this hypothesis by comparing the performance of plants produced by artificial self- and cross-pollination from three populations whose outcrossing rate estimates were 0.03, 0.26, and 0.58. Outcrossing rates and inbreeding coefficients were estimated using isozyme polymorphisms as genetic markers. Analysis of F statistics suggests that biparental inbreeding as well as self-fertilization contribute to the level of homozygosity in the seed crop. Biparental inbreeding will reduce the heterozygosity of progeny produced by outcrossing, relative to random outcrossing expectations, and hence will reduce the effects of outcrossing versus self-fertilization. Heterotic selection may increase the average heterozygosity during the life history. Selfed and outcrossed seeds from all three populations were equally likely to germinate and survive to reproduce. However, inbreeding depression was observed in fecundity traits of plants surviving to reproduction in all three populations. Even in the population whose natural self-fertilization rate was 97%, plants grown from seed produced by self-pollination produced fewer fruits and less total seed weight than plants grown from outcrossed seed. There was no detectable inbreeding depression in estimated lifetime fitness. Inbreeding effects for all reproductive yield characters were most severe in the accession from the most outcrossing population and least severe in the accession from the most self-fertilizing population.     

DETERMINANTS OF OUTCROSSING RATE IN A PREDOMINANTLY SELF-FERTILIZING WEED,DATURA STRAMONIUM (SOLANACEAE)

We measured outcrossing rates of several North Carolina populations of the annual weed Datura stramonium including both natural populations and experimental populations in which we manipulated plant spatial arrangement. Because capsules of D. stramonium typically produce hundreds of seeds and we used an easily scored genetic marker for flower and hypocotyl color, we could measure outcrossing rates accurately for both individual plants and single flowers. The population-wide estimates of outcrossing rates were surprisingly low for a species with showy, entomophilous flowers and ranged from 1.9% in an experimental population with a “clumped” spatial arrangement to 8.5% in an experimental population with a “dispersed” arrangement. These low values were not produced by pollinator discrimination among flower color morphs, as determined by outcrossing measurements on test plants of different colors and by direct observations of pollinator behavior. For individual plants and single flowers in the experimental populations, variation in outcrossing rates was significantly affected by such population-wide characteristics as plant spatial arrangement and nightly fluctuations in total floral abundance. However, by far the most important factor was stigma position. Flowers with stigmas above the anthers had significantly higher outcrossing rates than did flowers with overlapping stigma and anthers. The strong effect of floral morphology suggests that the very low population-wide levels of outcrossing in D. stramonium may represent a persistent mixing mating system rather than a transition to complete selfing.     

Two's company, three's a crowd: experimental evaluation of the evolutionary maintenance of trioecy in Mercurialis annua (Euphorbiaceae)

Trioecy is an uncommon sexual system in which males, females, and hermaphrodites co-occur as three clearly different gender classes. The evolutionary stability of trioecy is unclear, but would depend on factors such as hermaphroditic sex allocation and rates of outcrossing vs. selfing. Here, trioecious populations of Mercurialis annua are described for the first time. We examined the frequencies of females, males and hermaphrodites across ten natural populations and evaluated the association between the frequency of females and plant densities. Previous studies have shown that selfing rates in this species are density-dependent and are reduced in the presence of males, which produce substantially more pollen than hermaphrodites. Accordingly, we examined the evolutionary stability of trioecy using an experiment in which we (a) indirectly manipulated selfing rates by altering plant densities and the frequency of males in a fully factorial manner across 20 experimental plots and (b) examined the effect of these manipulations on the frequency of the three sex phenotypes in the next generation of plants. In the parental generation, we measured the seed and pollen allocations of hermaphrodites and compared them with allocations by unisexual plants. In natural populations, females occurred at higher frequencies in denser patches, a finding consistent with our expectations. Under our experimental conditions, however, no combination of plant densities and male frequencies was associated with increased frequencies of females. Our results suggest that the factors that regulate female frequencies in trioecious populations of M. annua are independent of those regulating male frequencies (density), and that the stable co-existence of all three sex phenotypes within populations is unlikely.     

Outcrossing rates and male sterility in natural populations of Plantago coronopus

Summary Outcrossing rates were estimated in three populations of the gynodioecious species Plantago coronopus by means of electrophoresis of adult plants and their natural progenies. A multilocus estimation procedure was used. Heterogeneity among the pollen-pool allele frequencies did not exist either in space of in time. Differences between populations in mean outcrossing rates were large (range: 0.34–0.93), probably caused by differences in densities of flowering plants. In addition, there was considerable variability between individuals, which was at least partly caused by the presence of male sterility. Population density may, via its influence on outcrossing rates, be a factor influencing the maintenance of male sterile plants in the population. The level of outcrossing in hermaphrodites was not low enough to explain the maintenance of male steriles. Outcrossing rates in two populations, established via progeny analysis, were much lower than calculated with the fixation index, possibly indicating heterozygote advantage in these natural populations.Grassland Species Research Group Publication no. 134     

Consequences of inbreeding for offspring fitness and gender in Silene vulgaris,a gynodioecious plant

Abstract In gynodioecious plants, hermaphrodite and female plants co‐occur in the same population. In these systems gender typically depends on whether a maternally inherited cytoplasmic male sterility factor (CMS) is counteracted by nuclear restorer alleles. These restorer alleles are often genetically dominant. Although plants of the female morph are obligatorily outcrossing, hermaphrodites may self. This selfing increases homozygosity and may thus have two effects: (1) it may decrease fitness (i.e. result in inbreeding depression) and (ii) it may increase homozygosity of the nuclear restorer alleles and therefore increase the production of females. This, in turn, enhances outcrossing in the following generation. In order to test the latter hypothesis, experimental crosses were conducted using individuals derived from four natural populations of Silene vulgaris, a gynodioecious plant. Treatments included self‐fertilization of hermaphrodites, outcrossing of hermaphrodites and females using pollen derived from the same source population as the pollen recipients, and outcrossing hermaphrodites and females using pollen derived from different source populations. Offspring were scored for seed germination, survivorship to flowering and gender. The products of self‐fertilization had reduced survivorship at both life stages when compared with the offspring of outcrossed hermaphrodites or females. In one population the fitness of offspring produced by within‐population outcrossing of females was significantly less than the fitness of offspring produced by crossing females with hermaphrodites from other populations. Self‐fertilization of hermaphrodites produced a smaller proportion of hermaphroditic offspring than did outcrossing hermaphrodites. Outcrossing females within populations produced a smaller proportion of hermaphrodite offspring than did crossing females with hermaphrodites from other populations. These results are consistent with a cytonuclear system of sex determination with dominant nuclear restorers, and are discussed with regard to how the mating system and the genetics of sex determination interact to influence the evolution of inbreeding depression.     

Predominance of outcrossing in Lymnaea stagnalis despite low apparent fitness costs of self-fertilization

We have quantified the natural mating system in eight populations of the simultaneously hermaphroditic aquatic snail Lymnaea stagnalis, and studied the ecological and genetic forces that may be directing mating system evolution in this species. We investigated whether the natural mating system can be explained by the availability of mates, by the differential survival of self- and cross-fertilized snails in nature, and by the effects of mating system on parental fecundity and early survival. The natural mating system of L. stagnalis was found to be predominantly cross-fertilizing. Density of snails in the populations had no relationship with the mating system, suggesting that outcrossing rates are not limited by mate availability at the population densities observed. Contrary to expectations for outcrossing species, we detected no evidence for inbreeding depression in survival in nature with inferential population genetic methods. Further, experimental manipulations of mating system in the laboratory revealed that self-fertilization had no effect on parental fecundity, and only minor effects on offspring survival. Predominance of cross-fertilization despite low apparent fitness costs of self-fertilization is at odds with the paradigm that high self-fertilization depression is necessary for maintenance of cross-fertilization in self-compatible hermaphrodites.     

Differences in floral traits and flower visitation rates in mating systems in Prunella vulgaris (Lamiaceae)

夏枯草交配系统对花特征和访花频率差异的影响 植物花特征和传粉者的访问次数与交配系统类型密切相关。唇形科植物夏枯草(Prunella vulgaris)存 在两种植株类型,分别为柱头伸出花冠和柱头在花冠内部的植株,而且两种植株的比例在不同种群中存在差异。本研究选择柱头伸出花冠外植株占绝大多数、柱头伸出花冠外植株占多数和柱头在花冠内部植株占多数的3个种群,通过比较每个种群中两种植株类型的开花物候、花形态特征、昆虫访问频率、自交能力、传粉者对结实的贡献以及近交衰退的水平,以检验花特征和传粉者访问次数与交配系统类型的关系。研究结果表明,与柱头在花冠内部的植株相比,柱头伸出花冠外的植株具有更大和更多的花,产生更多的花粉和花蜜,具有更高的访花频率,并主要通过异交产生种子。在种群水平,柱头伸出花冠外的植株占多数种群的访花频率显著高于柱头在花冠内部植株占多数的访花频率。柱头在花冠内部的植株比柱头伸出花冠外的植株具有更强的自动自交能力,在传粉者缺乏时为其提供了繁殖保障,但繁殖保障和异交率在不同种群中差异不显著,表明较低的昆虫访问能够满足夏枯草的授粉需求以产生种子,这可能与夏枯草较少的胚珠数量(每朵花仅有4个胚珠)有关。柱头在花冠内部植株的近交衰退水平低于柱头伸出花冠外植株的近交衰退水平,但两种植株类型的近交衰退水平均低于0.5,说明近交衰退不足以阻止该物种中自交的进化。综上所述,柱头在花冠内部的植株能够通过自交为夏枯草提供繁殖保障,而柱头伸出花冠外的植株能够利用昆虫传粉确保异交,表明混合交配系统在该物种中是一个稳定的状态。     

The mating system of Hydrophyllum appendiculatum,a protandrous species

Summary The mating system of Hydrophyllum appendiculatum (Hydrophyllaceae), a protandrous, self-compatible, monocarpic plant was examined using progeny arrays assayed at three polymorphic allozyme loci. We were particularly interested in the effect of ecological factors on spatial and temporal variation in outcrossing rates. Multilocus estimates of outcrossing rates in three populations ranged from 0.62–0.81 indicating that the majority of seeds are produced via outcrossing. The population estimates did not differ significantly from each other indicating that there is little or no spatial variation in the mating system of this species. The estimates were, however, significantly less than unity, which demonstrates that although mainly outcrossing, a significant fraction of seeds are produced by selfing. Estimates suggested that biparental inbreeding occurred, although it was statistically significant in only one population. Individuals of H. appendiculatum may remain in anthesis 3–4 weeks and produce up to 30 inflorescences. As a result, the possibility exists for the mating system to vary through the flowering season. Although the mean outcrossing rate was highest in the middle of the flowering phenology than at the beginning or end, there was no evidence for statistically significant temporal heterogeneity. We were also interested in determining if the size of the floral display (number of inflorescences produced by a plant) influenced the outcrossing rate. The results obtained by two different statistical analyses were contradictory; there was a significant positive correlation between size of floral display and outcrossing rate, but the outcrossing rates of large plants (\s> 8 inflorescences) did not differ significantly from small plants (> 8 inflorescences).     

Effects of self-pollination and outcrossing with cultivated plants in small natural populations of American ginseng, Panax quinquefolius (Araliaceae)

For rare plants, self-pollination and inbreeding can increase in small populations, while unusual levels of outcrossing can occur through restoration efforts. To study both inbreeding and outcrossing, we performed experimental pollinations using Panax quinquefolius (American ginseng), a wild-harvested plant with a mixed mating system. For inbreeding, plants were either cross-pollinated within the population or self-pollinated, which resulted in a higher proportion of seeds from self-pollinated flowers. For outcrossing, wild plants were either cross-pollinated within the population or with cultivated plants from West Virginia or Wisconsin. Offspring of all crosses were followed for 4 yr. Two-yr-old seedlings from self-pollination had 45% smaller leaf areas and 33% smaller heights relative to those from cross-pollination. Leaf area is a positive predictor of longer-term survival in wild populations. Our results suggest inbreeding depression, which is unexpected in this self-fertile species. Seedlings from crosses with cultivated plants had 127% greater leaf area and 165% greater root biomass relative to outcrosses within the population. The accelerated growth suggests genetic differences between wild and cultivated populations, but outbreeding depression may not appear until later generations. Assessment of the ultimate fitness consequences of introducing cultivated genotypes requires monitoring over longer time periods.     

The mating system and genetic significance of polycarpy in the neem tree (Azadirachta indica)

The objectives of this study were to estimate the outcrossing rate and to explain genetic consequences of the development of seed in the endocarp in a natural population of neem in Bangladesh. Cotyledons of germinated open-pollinated seeds of individual trees were analyzed by starch-gel electrophoresis to examine allozymes. Three loci with clear Mendelian segregation were used to estimate outcrossing rate. A multilocus mixed mating model was used to evaluate the mating system. The population exhibited high outcrossing rates both for multilocus (t_m=0.90±0.024) and mean single-locus (t_s=0.92±0.020) estimates. The difference between these two parameters (t_m–t_s=0±SE 0.038) was insignificant, indicating that there was no ’biparental inbreeding’ in the population. The degree of variance of the estimates of multilocus outcrossing rates decreased when two or more loci were included. In order to elucidate the significance of polycarpy a total of 471 seeds were counted out of 440 endocarps. This mechanism appears to be a possible way of avoiding inbreeding. The results indicated that the studied neem population was predominantly allogamous. Received: 10 January 1999 / Accepted: 10 April 1999     

Breeding systems of hermaphroditic and gynodioecious populations of the colonizing species Trifolium hirtum All. in California

Summary A multilocus procedure was used to estimate outcrossing rates in ten roadside populations of Trifolium hirtum in California. Three groups of populations were studied: cultivars, hermaphroditic, and gynodioecious (sexually dimorphic) populations. The multilocus outcrossing rate (t_m) varied from 0.05 to 0.43 among populations. Population level t_m estimates were significantly correlated with the observed heterozygosity in gynodioecious populations but not in hermaphroditic populations. The outcrossing rate of hermaphrodites and females was estimated in three gynodioecious populations; the estimates of t_m varied from 0.09 to 0.23 for hermaphrodites and from 0.73 to 0.80 for females. The distribution of outcrossing rates in gynodioecious populations is bimodal. Our results indicate that for the levels of selfing observed among hermaphrodites, inbreeding depression is likely to be a major factor in the maintenance of females in gynodioecious populations.     

EVOLUTION OF GYNODIOECY AND MAINTENANCE OF FEMALES: THE ROLE OF INBREEDING DEPRESSION,OUTCROSSING RATES,AND RESOURCE ALLOCATION IN SCHIEDEA ADAMANTIS (CARYOPHYLLACEAE)

Levels of inbreeding depression, outcrossing rates, and phenotypic patterns of resource allocation were studied to examine their relative importance in the maintenance of high numbers of females in gynodioecious Schiedea adamantis (Caryophyllaceae), an endemic Hawaiian shrub found in a single population on Diamond Head Crater, Oahu. In studies of inbreeding depression in two greenhouse environments, families of hermaphrodites exhibited significant inbreeding depression (δ = 0.60), based on a multiplicative fitness function using seeds per capsule, germination, survival, and the inflorescence biomass of progeny. Differences between inbred and outcrossed progeny were smallest at the early stage of seeds per capsule and more pronounced at the later stages of survival and inflorescence production. These results are consistent with inbreeding depression caused by many mutations of small effect. Using allozyme analyses, the inbreeding coefficient of adult plants in the field was not significantly different from zero, implying that δ in nature may be equal to one. The single locus estimate of the outcrossing rate for hermaphrodites was 0.50 based on progeny that survived to flowering; corrected for the disproportionate loss before flowering of progeny from selfing, the adjusted outcrossing rate at the zygote stage was 0.32, suggesting that considerable selfing occurs in hermaphrodites. Females were totally outcrossed. When females and hermaphrodites were compared for reproductive output in the field, females produced over twice as many seeds per plant as hermaphrodites, primarily because females had far more capsules per inflorescence than hermaphrodites. Females had greater mass per seed than hermaphrodites in the field, either because of greater provisioning or reduced inbreeding depression. There was no significant differential mortality with respect to sex over a seven year period. The higher number of seeds per plant of females, combined with substantial inbreeding depression and relatively high selfing rates for hermaphrodites, are probably responsible for the maintenance of females in this population. The predicted frequency of females based on data for seed production, the adjusted selfing rate, and inbreeding depression is 42%, remarkably close to the observed frequency of 39%. High levels of inbreeding depression suggest that considerable quantitative genetic variation is present for traits affecting fitness in this population, despite low allozyme variability and a presumed founder effect.     

Outcrossing rates in the gynomonoecious-gynodioecious species Dianthus sylvestris (Caryophyllaceae)

Some species described as gynodioecious are truly gynomonoecious-gynodioecious. Three distinct phenotypes may be found in their natural populations-female and hermaphrodite pure-sexed plants bearing either only pistillate or perfect flowers, respectively, and mixed plants bearing both types of flowers. In one such species, Dianthus sylvestris, we investigated mating system parameters using allozyme data. Outcrossing rates and correlations of outcrossed paternity were calculated for the three types of plants and separately for pistillate and perfect flowers on mixed plants. The mean outcrossing rate for the population was t(m) ± SD = 0.885 ± 0.032. Females were more outcrossed than hermaphrodites (0.987 ± 0.112 and 0.790 ± 0.076, respectively), whereas mixed plants were not significantly more or less outcrossed than hermaphrodites (0.840 ± 0.060). Within mixed plants, perfect flowers showed an intermediate outcrossing rate (0.898 ± 0.057), whereas pistillate flowers were as selfed as perfect flowers on hermaphrodite plants (0.782 ± 0.111). Family estimates of outcrossing rates were highly variable. Globally, no biparental inbreeding was detected in this species, and there was a mean of 61.5 ± 19.9% of full-sibs within families. Floral dimorphism between small pistillate and large perfect flowers together with pollinator preference for larger flowers could explain the observed patterns for both mating parameters. The advantages of gynomonoecy-gynodioecy are discussed. We conclude that mixed plants do not reduce selfing for all flowers on a plant, but perfect flowers on these plants seem to have an outcrossing advantage.     

Bayesian quantification of ecological determinants of outcrossing in natural plant populations: Computer simulations and the case study of biparental inbreeding in English yew

The mating system is a central parameter of plant biology because it shapes their ecological and evolutionary properties. Therefore, determining ecological variables that influence the mating system is important for a deeper understanding of the functioning of plant populations. Here, using old concepts and recent statistical developments, we propose a new statistical tool to make inferences about ecological determinants of outcrossing in natural plant populations. The method requires codominant genotypes of seeds collected from maternal plants within different locations. Using extensive computer simulations, we demonstrated that the method is robust to the issues expected for real‐world data, including the Wahlund effect, inbreeding and genotyping errors such as allele dropout and allele misclassification. Furthermore, we showed that the estimates of ecological effects and outcrossing rates can be severely biased if genotyping errors and genetic differentiation are not treated explicitly. Application of the new method to the case study of a dioecious tree (Taxus baccata) allowed revealing that female trees that grow in lower local densities have a greater tendency towards mating with relatives. Moreover, we also demonstrated that biparental inbreeding is higher in populations that are characterized by a longer mean distance between trees and a smaller mean trunk perimeter. We found these results to agree with both the theoretical predictions and the history of English yew.     

Estimation of outcrossing rate in a breeding population of Eucalyptus urophylla with dominant RAPD and AFLP markers

 Eucalyptus breeding is typically conducted by selection in open-pollinated progenies. As mating is controlled only on the female side of the cross, knowledge of outcrossing versus selfing rates is essential for maintaining adequate levels of genetic variability for continuous gains. Outcrossing rate in an open-pollinated breeding population of Eucalyptus urophylla was estimated by two PCR-based dominant marker technologies, RAPD and AFLP, using 11 open-pollinated progeny arrays of 24 individuals. Estimated outcrossing rates indicate predominant outcrossing and suggest maintenance of adequate genetic variability within families. The multilcous outcrossing rate (t_m) estimated from RAPD markers (0.93±0.027), although in the same range, was higher (α>0.01) than the estimate based on AFLP (0.89±0.033). Both estimates were of similar magnitude to those estimated for natural populations using isozymes. The estimated Wright’s fixation index was lower than expected based on t_m possibly resulting from selection against selfed seedlings when sampling plants for the study. An empirical analysis suggests that 18 is the minimum number of dominant marker loci necessary to achieve robust estimates of t_m. This study demonstrates the usefulness of dominant markers, both RAPD and AFLP, for estimating the outcrossing rate in breeding and natural populations of forest trees. We anticipate an increasing use of such PCR-based technologies in mating-system studies, in view of their high throughput and universality of the reagents, particularly for species where isozyme systems have not yet been optimized. Received: 25 March 1997 / Accepted: 13 May 1997     

小慈姑的开花状态、传粉机制与交配系统

在自然种群中对沼生植物小慈姑（Sagittaria potamogetifolia Merr.）的开花状态和传粉过程作了观测,并用同工酶遗传标记法定量估测了其3个种群中5个样本的异交率。该种为中心媒传粉为主的虫／风 媒兼性传粉机制,访花昆虫的飞行距离多在0-2m范围内,并与花序密度呈不显著的正相关。异交率（－／^／t）为50.0％－92.8％,表明该种为异交为主的混合交配系统（ixed mating system）。对雌雄同株的小慈姑而言,异交率与各种群花期植株密度无关,异交水平取决于种群中开花花序的两个或两个以上的植株所占比例。     

GENDER VARIATION AND SEXUAL DIFFERENCES IN REPRODUCTIVE CHARACTERS AND SEED PRODUCTION IN GYNODIOECIOUS GERANIUM MACULATUM

Variation in sex expression, flowering pattern, and seed production was studied in the self-compatible perennial herb Geranium maculatum in Illinois and Indiana. In a survey of eight populations, female (male-sterile) plants were found in seven (frequencies ranging from 0.5% to 24.3% [median 4.2%]), and intermediate plants (with partly reduced male function) were found in all populations. Gender variation and sexual differences in reproductive characters were studied in detail in two populations. One population consisted of 5% female, 27% intermediate, and 68% hermaphrodite plants; the other consisted of 1% female, 20% intermediate, and 79% hermaphrodite plants. Females produced smaller flowers and began flowering earlier than hermaphrodites. Intermediates produced flowers of an intermediate size and began flowering as early as females. Females and hermaphrodites did not differ in flower number, vegetative size, flowering frequency, survival, or seed size. However, females produced 1.6 times more seeds than hermaphrodites. Intermediates produced 1.3–1.6 times more seeds than hermaphrodites. Some between-year variation in sex expression was observed. Hand-pollination with outcross pollen produced two to four times as many seeds as hand-pollination with self-pollen. A lower outcrossing rate in hermaphrodites than in females may at least partly explain the lower seed set in hermaphrodites. The higher seed production of females, and possibly the high fecundity of the intermediates, should contribute to the maintenance of this sexual polymorphism.     

MUTATIONAL MELTDOWN IN SELFING ARABIDOPSIS LYRATA

The majority of plant species and many animals are hermaphrodites, with individuals expressing both female and male function. Although hermaphrodites can potentially reproduce by self‐fertilization, they have a high prevalence of outcrossing. The genetic advantages of outcrossing are described by two hypotheses: avoidance of inbreeding depression because selfing leads to immediate expression of recessive deleterious mutations, and release from drift load because self‐fertilization leads to long‐term accumulation of deleterious mutations due to genetic drift and, eventually, to extinction. I tested both hypotheses by experimentally crossing Arabidopsis lyrata plants (self‐pollinated, cross‐pollinated within the population, or cross‐pollinated between populations) and measuring offspring performance over 3 years. There were 18 source populations, each of which was either predominantly outcrossing, mixed mating, or predominantly selfing. Contrary to predictions, outcrossing populations had low inbreeding depression, which equaled that of selfing populations, challenging the central role of inbreeding depression in mating system shifts. However, plants from selfing populations showed the greatest increase in fitness when crossed with plants from other populations, reflecting higher drift load. The results support the hypothesis that extinction by mutational meltdown is why selfing hermaphroditic taxa are rare, despite their frequent appearance over evolutionary time.     

Constructing a linkage–linkage disequilibrium map using dominant-segregating markers

The relationship between linkage disequilibrium (LD) and recombination fraction can be used to infer the pattern of genetic variation and evolutionary process in humans and other systems. We described a computational framework to construct a linkage–LD map from commonly used biallelic, single-nucleotide polymorphism (SNP) markers for outcrossing plants by which the decline of LD is visualized with genetic distance. The framework was derived from an open-pollinated (OP) design composed of plants randomly sampled from a natural population and seeds from each sampled plant, enabling simultaneous estimation of the LD in the natural population and recombination fraction due to allelic co-segregation during meiosis. We modified the framework to infer evolutionary pasts of natural populations using those marker types that are segregating in a dominant manner, given their role in creating and maintaining population genetic diversity. A sophisticated two-level EM algorithm was implemented to estimate and retrieve the missing information of segregation characterized by dominant-segregating markers such as single methylation polymorphisms. The model was applied to study the relationship between linkage and LD for a non-model outcrossing species, a gymnosperm species, Torreya grandis, naturally distributed in mountains of the southeastern China. The linkage–LD map constructed from various types of molecular markers opens a powerful gateway for studying the history of plant evolution.