AN ELECTROPHORETIC INVESTIGATION OF INTRAGAMETOPHYTIC SELFING IN EQUISETUM ARVENSE

Because homosporous pteridophytes (Psilotophyta, Arthrophyta, most Microphyllophyta and Pteridophyta) produce bisexual gametophytes, it was maintained that high levels of inbreeding would characterize these plants. Electrophoretic evidence was used to estimate the frequency of intragametophytic selfing in Equisetum arvense (Arthrophyta). A total of 669 samples from 17 populations was examined from western North America. Although some populations exhibited as many as seven or eight genotypes, 10 populations were each characterized by only a single genotype; eight of these populations were heterozygous for one or more loci. For most populations, estimates of intragametophytic self-fertilization are 0.000, indicating that virtually all matings involve different gametophytes. Genetic data corroborate predictions based on earlier field and laboratory investigations of Equisetum gametophytes. These detailed studies demonstrated that in many species, including E. arvense, gametophytes are initially either male or female; only later and in the absence of fertilization do some gametophytes become bisexual. Our findings join a growing electrophoretic data base which demonstrates that homosporous pteridophytes are not highly inbreeding as previously suggested.     

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.     

Temporal genetic variation in a population of the pocket gopher,Geomys bursarius

Genetic variation at eight polymorphic loci was examined in a population of a subterranean rodent, the plains pocket gopher (Geomys bursarius), sampled over a 10-yr period. Two loci exhibited relatively minor changes in gene frequencies, while the remaining loci demonstrated major shifts in predominant alleles, loss of minor alleles, and addition of alleles due to migration. Significant deviations from Hardy-Weinberg expectations, concomitant to heterozygote deficiencies, were observed for several loci. Temporal heterogeneity, as measured by F_ST, was high and comparable to that exhibited by local populations sampled over relatively short periods of time. The high degree of temporal genetic variation is consistent with observations that fossorial rodents occur in locally isolated populations with small effective population sizes that are subject to genetic drift, bottlenecking, and inbreeding.     

BREEDING SYSTEM OF THE FERN DRYOPTERIS EXPANSA: EVIDENCE FOR MIXED MATING

The levels and distribution of genetic variation within and among populations of the homosporous fern Dryopteris expansa were assessed using enzyme electrophoresis. Twelve loci representing six enzymes were examined in 502 sporophytes from nine populations. Values of P, H, and mean number of alleles per locus are much lower in D. expansa compared to values for other fern species. Values of F varied from –0.014 to 0.745 with an average F of 0.335. There was also considerable population to population variation in F. In three populations the observed heterozygosity closely approximated that expected at Hardy-Weinberg equilibrium, whereas in the remaining populations observed heterozygosity was significantly lower than the expected heterozygosity. In D. expansa the major component of F_IT is F_IS; the relatively high F_ST value suggests a high degree of interpopulational differentiation. Electrophoretic data suggest that this species possesses a mixed mating system. This study, in conjunction with other investigations, indicates that just as in angiosperms, a variety of breeding systems operates in homosporous ferns, ranging from extreme inbreeding, through mixed mating to outcrossing.     

POPULATION STRUCTURE AND ESTIMATES OF GENE FLOW IN THE HOMOSPOROUS FERN POLYSTICHUM MUNITUM

Levels and distribution of genetic variation were investigated in the homosporous fern, Polystichum munitum. Homosporous ferns differ from higher vascular plants in that they possess potentially bisexual gametophytes which can produce a completely homozygous sporophyte in a single generation. Because of this, it has long been maintained that ferns possess an inbreeding mating system, resulting in low levels of genetic variation and high levels of homozygosity within populations. The four populations sampled maintain high levels of genetic variation (P? = 0.542; H? = 0.111; ā = 2.23), comparable to that maintained by populations of outcrossing seed plants. The mean fixation index, F, for the four populations was 0.052, indicating no significant deviations from Hardy-Weinberg genotypic expectations. Polystichum munitum distributes most of its genetic variation within rather than among populations. Population-genetic structure was assessed by subdividing each of two large populations into 10 × 10-m subpopulations. Comparisons of genetic variation within and among subpopulations indicated little genetic substructure within either of the artificially subdivided populations. Estimates of interpopulational gene flow (Nm) are extremely high, comparable to those reported for gymnosperms. Statistical estimates of intragametophytic selling are very low, ranging from 0 to 3%. This study suggests that Polystichum munitum is an outcrossing species. Evidence from this and other investigations indicates that fern species do not typically self-fertilize and that mating systems in ferns vary as they do among species of seed plants.     

CORRELATED EVOLUTION OF SELF-FERTILIZATION AND INBREEDING DEPRESSION: AN EXPERIMENTAL STUDY OF NINE POPULATIONS OF AMSINCKIA (BORAGINACEAE)

The relation between inbreeding depression and rate of self-fertilization was studied in nine natural populations of the annual genus Amsinckia. The study included two clades (phylogenetic lineages) in which small-flowered, homostylous populations or species are believed to have evolved from large-flowered, heterostylous, self-compatible ones. In one lineage the small-flowered species is tetraploid with disomic inheritance. Rates of self-fertilization were 25% to 55% in the four large-flowered, heterostylous populations; 72% in a large-flowered but homostylous population; and greater than 99.5% in the four small-flowered, homostylous populations, which produce seed autonomously. When present, inbreeding depression occurred in the fertility but not the survival components of fitness. Using a cumulative fitness measure incorporating both survival and fertility (flower number), we found inbreeding depression to be lower in the four very highly self-fertilizing populations than in the five intermediate ones. The Spearman rank correlation between inbreeding depression and selfing rate for the nine populations was –0.50, but was not statistically significant (P = 0.12). Inbreeding depression was greater in the two tetraploid populations than in the very highly self-fertilizing, diploid ones. Phenotypic stability of progeny from self-fertilization tended to be higher in populations with lower inbreeding depression. We conclude that levels of self-fertilization and inbreeding depression in Amsinckia are determined more by other factors than by each other. Estimates of mutation rates and dominance coefficients of deleterious alleles, obtained from a companion study of the four highly self-fertilizing populations, suggest that a strong relationship may not be expected. We discuss the relationship of the present results to current theory of the coevolution of self-fertilization and inbreeding depression.     

Inbreeding depression for various traits in two cultured populations of the American bay scallop, Argopecten irradians irradians Lamarck (1819) introduced into China

This paper examines the effect of inbreeding level of population on the magnitude of inbreeding depression expressed by comparing them between two cultured populations (A and B) in the hermaphroditic animal of the bay scallop Argopecten irradians irradians. Population A is expected to have less genetic variations and higher inbreeding level due to longer cultured history (20 generations) and less “ancestral” individuals (26 individuals) than population B due to shorter cultured history (4 generations) and more “ancestral” individuals (406 individuals). Two groups within each population were produced, one using self-fertilization and one using mass-mating within the same population. Selfed offspring (AS and BS) from two populations both had lower fitness components than their mass-mated counterparts (AM and BM) and exhibited inbreeding depression for all examined traits, e.g. lower hatching, less viability and slower growth, indicating that inbreeding depression is a common feature in this animal. Fitness components in all traits of offspring from population A significantly differed those from population B and the magnitude of inbreeding depression for all traits in population A with higher inbreeding level was significantly smaller than that in population B with lower inbreeding level, indicating that both fitness components and magnitude of inbreeding depression were significantly affected by inbreeding level of populations and genetic load harbored in population A may be partially purged through inbreeding. Moreover, the magnitude of inbreeding depression in the two populations both varied among traits and life history stages. The present results support the partial-dominance hypothesis of inbreeding depression.     

ESTIMATED RATES OF INTRAGAMETOPHYTIC SELFING IN LYCOPODS

Homosporous pteridophytes are characterized by the production of free-living, potentially bisexual gametophytes. Because of the close proximity of archegonia and antheridia on the same thallus, it has been assumed that high rates of intragametophytic self-fertilization would predominate in natural populations of homosporous pteridophytes. Using enzyme electrophoresis we determined sporophytic genotype frequencies for natural populations of three lycopod species, Lycopodium clavatum, L. annotinum, and Huperzia miyoshiana. Based on these genotype frequencies and the estimation procedures of Holsinger (1987), the estimated rates of intragametophytic selfing in these species are extremely low. Estimated selfing rates were greater than 0.000 in only two of 13 populations of L. clavatum, one of six populations of L. annotinum, and one of four populations of H. miyoshiana. Despite the potential for intragametophytic self-fertilization, the gametophytes of these three lycopod species predominantly cross-fertilize, although the mechanism(s) promoting intergametophytic matings are unknown. These results are similar to those obtained for homosporous ferns and Equisetum arvense. It is therefore clear that most homosporous pteridophyte species investigated do not exhibit high rates of intragametophytic self-fertilization; in contrast, intergametophytic matings predominate.     

EFFECTS OF CROSS AND SELF-FERTILIZATION ON PROGENY FITNESS IN LOBELIA CARDINALIS AND L. SIPHILITICA

Inbreeding depression, or the decreased fitness of progeny derived from self-fertilization as compared to outcrossing, is thought to be the most general factor affecting the evolution of self-fertilization in plants. Nevertheless, data on inbreeding depression in fitness characters are almost nonexistent for perennials observed in their natural environments. In this study I measured inbreeding depression in both survival and fertility in two sympatric, short-lived, perennial herbs: hummingbird-pollinated Lobelia cardinalis (two populations) and bumblebee-pollinated L. siphilitica (one population). Crosses were performed by hand in the field, and seedlings germinated in the greenhouse. Levels of inbreeding depression were determined for one year in the greenhouse and for two to three years for seedlings transplanted back to the natural environment. Fertility was measured as flower number, which is highly correlated with seed production under natural conditions in these populations. Inbreeding depression was assessed in three ways: 1) survival and fertility within the different age intervals; 2) cumulative survival from the seed stage through each age interval; and 3) net fertility, or the expected fertility of a seed at different ages. Net fertility is a comprehensive measure of fitness combining survival and flower number. In all three populations, selfing had nonsignificant effects on the number and size of seeds. Lobelia siphilitica and one population of L. cardinalis exhibited significant levels of inbreeding depression between seed maturation and germination, excluding the consideration of possible differences in dormancy or longterm viability in the soil. There was no inbreeding depression in subsequent survival in the greenhouse in any population. In the field, significant survival differences between selfed and outcrossed progeny occurred only in two years and in only one population of L. cardinalis. For both survival and fertility there was little evidence for the expected differences among families in inbreeding depression. Compared to survival, inbreeding depression in fertility (flower number) tended to be much higher. By first-year flower production, the combined effects on survival and flower number caused inbreeding depression in net fertility to reach 54%, 34% and 71% for L. siphilitica and the two populations of L. cardinalis. By the end of the second year of flowering in the field, inbreeding depression in net fertility was 53% for L. siphilitica and 54% for one population of L. cardinalis. For the other population of L. cardinalis, these values were 76% through the second year of flowering and 83% through the third year. Such high levels of inbreeding depression should strongly influence selection on those characters affecting self-fertilization rates in these two species.     

Effects of crossing distance on fitness components in the carnivorous plant Pinguicula moranensis (Lentibulariaceae)

Plant species vary in the pollination distance at which negative fitness effects are expressed, and it has been proposed that optimal mating should occur at a distance large enough to diminish the risks of inbreeding, but short enough to prevent outbreeding depression. In a natural population of Pinguicula moranensis we assessed the fitness of plants (seeds per fruit) and their progeny (germination and seedling survival) as a function of pollination distance by hand-pollinating with donors from five distances: 0 m (self-fertilization), 1, 4 and 460 m (within-population) and 9,900 m (between-population) under in situ and ex situ conditions. We found that average values for fitness components were consistently lower in the in situ experiment than in the ex situ experiment. Under both conditions, the self-fertilization treatment had the lowest values. Inbreeding depression values were high (0.94 to 0.98), corresponding with the values expected for an outcrossing species subjected to self-fertilization. Overall, no evidence of outbreeding depression was detected. Considering that our experiment was restricted to one single donor population and one recipient population, our results indicate that despite the within-population patchy distribution of individuals, biparental inbreeding could occur at low rates, due in part to short seed dispersal distance. However, pollen movement of at least 1 m is apparently sufficient to reverse negative effects.     

THE CYTOLOGY AND DERIVATION OF A TEMPERATURE-SENSITIVE MEIOTIC MUTANT IN THE FERN CERATOPTERIS

Meiotic mutants were obtained from an inbreeding program following a hybridization between two diploid species of the homosporous fern Ceratopteris. The mutants are characterized by high levels of asynapsis at high temperatures (27 C, 33 C) and by aberrant spindle function at low temperatures (18 C). Spore viabilities vary with temperature and are highest at 23 C. Occasional restitution during the first division results in the production of dyads of diploid spores, some of which are viable. The inbreeding program was characterized by the expression of high levels of apparent heterozygosity in spite of intragametophytic selfing. It appears that segregation within the inbreeding program was responsible for the production of the mutants. The apparent heterozgosity and segregation can be explained by a previously documented genetic system within the polyploid homosporous ferns.     

A Measure of the Various Modes of Inbreeding in Kalmia latifolia

Inbreeding can occur in several ways. For a given species, analysingoverall inbreeding down to its component parts can be usefulin explaining how and why inbreeding is maintained in a population.Kalmia latifolia, a long-lived evergreen shrub, was chosen forthis study for two reasons: its unique floral morphology suggestedthat it was specially adapted to assure reproduction in thedelayed mode of self-fertilization, and previous studies hadsuggested that populations that were more limited by pollinatorsshowed higher rates of within-flower self-fertilization whenpollinators were excluded. Floral manipulations, controlledpollinations, and paternity analysis were performed to determinethe most common form of inbreeding within a flower, among flowersof one individual, or among related individuals. Although plantsare capable of setting seed within a flower, the overall rateof inbreeding was low. Among the modes of inbreeding withinone individual, a negligible selfing rate among emasculatedflowers suggests that selfing is more prevalent within a flowerthan among flowers on the same plant. In addition, as the multi-locusestimate of inbreeding was less than the single-locus estimate,inbreeding among related individuals also contributes to thenatural level of inbreeding. Copyright 2000 Annals of BotanyCompany Kalmia latifolia; mountain laurel; pollination; geitonogamy; autogamy; self-fertilization; inbreeding     

GENETIC VARIATION AND POPULATION STRUCTURE IN THE FERN BLECHNUM SPICANT (BLECHNACEAE) FROM WESTERN NORTH AMERICA

Population genetic structure in the homosporous fern Blechnum spicant was analyzed in six populations from western North America. Each population was divided into approximately 10 m by 10 m subpopulations, and genetic variation within and among subpopulations was compared using enzyme electrophoresis and F statistics. These analyses indicated that there was no evidence of genetic structure in four of the six populations examined. However, significant genetic heterogeneity among subpopulations was observed for the other two populations. The genetic structure of these populations may be attributable, in part, to family structure resulting from high rates of intragametophytic selling and/or spatial patchiness in the distribution of individuals due to limited habitat availability in these areas. Outcrossing populations of B. spicant generally lack genetic structure, whereas the most highly inbreeding population maintains significant genetic structure. The information obtained in this investigation of population genetic structure in Blechnum spicant is consistent with data for angiosperms and gymnosperms. It appears that the outcrossing mating system and effective mechanism of spore dispersal in B. spicant may account for the general lack of genetic structure within populations of this species.     

INBREEDING DEPRESSION IN PARTIALLY SELF-FERTILIZING DECODON VERTICILLATUS (LYTHRACEAE): POPULATION-GENETIC AND EXPERIMENTAL ANALYSES

Inbreeding depression is a major selective force favoring outcrossing in flowering plants. Some self-fertilization, however, should weaken the harmful effects of inbreeding by exposing genetic load to selection. This study examines the maintenance of inbreeding depression in partially self-fertilizing populations of the long-lived, herbaceous wetland plant, Decodon verticillatus (L.) Ell. (Lythraceae). Estimates from ten populations indicate that 30% of offspring are produced through self-fertilization. Population-genetic estimates of inbreeding depression (δ = 1 – relative mean fitness of selfed progeny) based on changes in the inbreeding coefficient for the same ten populations were uniformly high, ranging from 0.49 to 1.79 and averaging 1.11 ± 0.29 SE. Although confidence intervals of individual population estimates were large, estimates were significantly greater than 0 in six populations and greater than 0.5 in four. Inbreeding depression was also estimated by comparing growth, survival, and flowering of experimentally selfed and outcrossed offspring from two of these populations in a 1-yr glasshouse experiment involving three density regimes; after which offspring were transplanted into garden arrays and two field sites and monitored for two consecutive growing seasons. Overall for survival averaged 0.27 ± 0.01 in the glasshouse, 0.33 ± 0.04 in the garden, and 0.46 ± 0.04 in the field. The glasshouse experiment also revealed strong inbreeding depression for growth variables, especially above-soil dry weight ( = 0.42 ± 0.03). The fitness consequences of inbreeding depression for these growth variables approximately doubles if survival to maturity is determined by severe truncation selection. Despite substantial selfing, inbreeding depression appears to be a major selective force favoring the maintenance of outcrossing in D. verticillatus.     

THE DISTRIBUTION OF SELFING RATES IN HOMOSPOROUS FERNS

The models of Lande and Schemske predict that among species in which the selfing rate is largely under genetic control and not subject to tremendous environmental variation, the distribution of selfing rates should be bimodal. When this prediction was tested empirically using data from the literature for species of angiosperms and gymnosperms, the distribution of outcrossing rates for all species was clearly bimodal. To provide another empirical test of the prediction, we analyzed mating-system data for 20 species of Pteridophyta (ferns). Homosporous ferns and their allies are unique among vascular plants because three types of mating are possible: intragametophytic selfing (selfing of an individual gametophyte); intergametophytic selfing (analogous to selfing in seed plants); and intergametophytic crossing (analogous to outcrossing in seed plants). The distribution of intragametophytic selfing rates among species of homosporous ferns is clearly uneven. Most species of homosporous ferns would be classified as extreme outcrossers. In contrast, a few species are nearly exclusively inbreeding. In only a few populations of Dryopteris expansa and Hemionitis palmata and a single population of Blechnum spicant do we see convincing evidence of a mixed mating system. The uneven distribution of selfing rates we observed for homosporous ferns, coupled with a corresponding bimodality of the magnitude of genetic load, strongly supports the model.     

INBREEDING DEPRESSION,NEUTRAL POLYMORPHISM,AND COPULATORY BEHAVIOR IN FRESHWATER SNAILS: A SELF-FERTILIZATION SYNDROME

This paper examines the consequences of self-fertilization on life-history traits and neutral genetic polymorphism in natural populations of three species of hermaphrodite freshwater snails: Biomphalaria straminea, Bulinus globosus, and the aphallic species Bulinus truncatus. Life-history traits (fecundity, growth, hatching rate, and survival of offspring) are compared under laboratory conditions between isolated (obligatory selfing) and paired (outcrossing possible) snails in one population of B. straminea and B. globosus and two populations of B. truncatus. The genetic polymorphism of the same four populations is analyzed using electrophoretic markers in B. straminea and B. globosus and microsatellite markers in B. truncatus. In B. truncatus and B. straminea, isolated snails have a higher fecundity than paired snails, whereas the contrary is observed in B. globosus. For all populations, no difference in hatching rate and offspring survival is detected between the two treatments. Genetic analyses using microsatellite markers conducted in B. truncatus on progeny of paired snails reveal a high selfing rate in spite of high copulation rates, highlighting the difficulties of obtaining outcrossing in highly selfing snails. The high survival of selfed offspring in B. truncatus and B. straminea indicates that inbreeding depression is limited. The extent of inbreeding depression in B. globosus is less clear. Overall, fitness decrease in this species is limited to fecundity. The extent of allozyme polymorphism is very limited whereas a much higher variability is observed with microsatellites. Biomphalaria straminea and B. truncatus populations are also characterized by very low observed heterozygosities and large heterozygote deficiencies, whereas the B. globosus population does not exhibit such a deficiency. Overall these results allow the definition of a self-fertilization syndrome in hermaphrodite freshwater snails: selfing populations (such as those of B. straminea and B. truncatus studied here) are characterized by high selfing rates in spite of copulations, limited deleterious effects of selfing, limited neutral genetic polymorphism, and large heterozygote deficiencies.     

GAMETOPHYTIC SELF-FERTILIZATION IN HOMOSPOROUS PLANTS: DEVELOPMENT,EVALUATION, AND APPLICATION OF A STATISTICAL METHOD FOR EVALUATING ITS IMPORTANCE

The methods described here make it possible to use data on sporophytic genotype frequencies to estimate the frequency of gametophytic self-fertilization in populations of homosporous plants. Bootstrap bias reduction is effective in reducing or eliminating the bias of the maximum likelihood estimate of the gametophytic selfing rate. The bias-corrected percentile method provides the most reliable confidence intervals for allele frequencies. The percentile method gives the most reliable confidence intervals for the gametophytic selfing rate when selfing is common. The maximum likelihood intervals, the percentile intervals, the bias-corrected percentile intervals, and the bootstrap t intervals are all overly conservative in their construction of confidence intervals for the gametophytic selfing rate when self-fertilization is rare. Application of the recommended methods indicates that gametophytic self-fertilization is quite rare in two sexually reproducing populations of Pellaea andromedifolia studied by Gastony and Gottlieb (1985).     

BIPARENTAL INBREEDING DEPRESSION IN THE SELF-INCOMPATIBLE ANNUAL PLANT GAILLARDIA PULCHELLA (ASTERACEAE)

Biparental inbreeding is thought to be a common feature of plant populations with restricted pollen dispersal. It is generally assumed that the inbreeding depression frequently observed to accompany self-fertilization can be extrapolated to the lesser degrees of consanguinity involved in biparental inbreeding, but this is virtually untested. To test this assumption, seeds collected from a single natural population of the self-incompatible annual Gaillardia pulchella were used to generate full-sib families derived by crossing either noninbred full-sibs (inbred families) or noninbred nonrelatives (outbred families). Members of each family were divided between high-stress and low-stress treatments that differed in soil volume and nutrient level. Inbred seedlings had a lower chance of survival, were more likely to be morphologically abnormal, and grew more slowly than outbred seedlings, indicating the presence of biparental inbreeding depression. Stress treatment had no significant effect on inbreeding depression, and no family stress-environment interactions were detected. Inbreeding did not increase the among-family variance in growth rate, suggesting that inbreeding depression of growth rate is caused by many genes with small individual effects. Relative to direct estimates of inbreeding depression, observed levels of near-neighbor outcrossing depression, presumed to be biparental inbreeding depression, are surprisingly high in many plant species.     

CONTRIBUTIONS OF SEXUAL AND ASEXUAL REPRODUCTION TO POPULATION STRUCTURE IN THE CLONAL SOFT CORAL,ALCYONIUM RUDYI

Numerous studies of population structure in sessile clonal marine invertebrates have demonstrated low genotypic diversity and nonequilibrium genotype frequencies within local populations that are monopolized by relatively few, highly replicated genets. All of the species studied to date produce planktonic sexual propagules capable of dispersing long distances; despite local genotypic disequilibria, populations are often panmictic over large geographic areas. The population structure paradigm these species represent may not be typical of the majority of clonal invertebrate groups, however, which are believed to produce highly philopatric sexual propagules. I used allozyme variation to examine the population structure of the temperate soft coral, Alcyonium rudyi, a typical clonal species whose sexually produced larvae and asexually produced ramets both have very low dispersal capabilities. Like other clonal plants and invertebrates, the local population dynamics of A. rudyi are dominated by asexual reproduction, and recruitment of new sexually produced genets occurs infrequently. As expected from its philopatric larval stage, estimates of genetic differentiation among populations of A. rudyi were highly significant at all spatial scales examined (mean θ = 0.300 among 20 populations spanning a 1100-km range), suggesting that genetic exchange seldom occurs among populations separated by as little as a few hundred meters. Mapping of multilocus allozyme genotypes within a dense aggregation of A. rudyi ramets confirmed that dispersal of asexual propagules is also very limited: members of the same genet usually remain within < 50 cm of one another on the same rock surface. Unlike most previously studied clonal invertebrates, populations of A. rudyi do not appear to be dominated by a few widespread genets: estimates of genotypic diversity (G_o) within 20 geographically distinct populations did not differ from expectations for outcrossing, sexual populations. Despite theoretical suggestions that philopatric dispersal combined with typically small effective population sizes should promote inbreeding in clonal species, inbreeding does not appear to contribute significantly to the population structure of A. rudyi. Genet genotype frequencies conformed to Hardy-Weinberg expectations in all populations, and inbreeding coefficients (f) were close to zero. In general, the population structure of A. rudyi did not differ significantly from that observed among outcrossing sexual species with philopatric larval dispersal. Age estimates suggest, however, that genets of A. rudyi live for many decades. Genet longevity may promote high genotypic diversity within A. rudyi populations and may be the most important evolutionary consequence of clonal reproduction in this species and the many others that share its dispersal characteristics.     

Evidence for Inbreeding and Genetic Differentiation among Geographic Populations of the Saprophytic Mushroom Trogia venenata from Southwestern China

During the past 40 years, more than 400 Sudden Unexplained Deaths (SUDs) have occurred in Yunnan, southwestern China. Epidemiological and toxicological analyses suggested that a newly discovered mushroom called Trogia venenata was the leading culprit for SUDs. At present, relatively little is known about the genetics and natural history of this mushroom. In this study, we analyzed the sequence variation at four DNA fragments among 232 fruiting bodies of T. venenata collected from seven locations. Our ITS sequence analyses confirmed that all the isolates belonged to the same species. The widespread presence of sequence heterozygosity within many strains at each of three protein-coding genes suggested that the fruiting bodies were diploid, dikaryotic or heterokaryotic. Within individual geographic populations, we found significant deviations of genotype frequencies from Hardy-Weinberg expectations, with the overall observed heterozygosity lower than that expected under random mating, consistent with prevalent inbreeding within local populations. The geographic populations were overall genetically differentiated. Interestingly, while a positive correlation was found between population genetic distance and geographic distance, there was little correlation between genetic distance and barium concentration difference for the geographic populations. Our results suggest frequent inbreeding, geographic structuring, and limited gene flow among geographic populations of T. venenata from southwestern China.