Journal of Heredity 1997:88:325-329; 0022-1503/97/S5.00 Enzyme electrophoresis was used to Investigate aiiozyme diversity and outcrossing rates In a population of Trillium erectum and T. grandiflorum from central New York. Enzyme polymorphisms were observed for 11 of 14 and 7 of 13 loci examined in T. erectum and T. grandiflorum, respectively. Percent polymorphic loci (P), mean number of alleles per locus (A), and mean expected heterozygosities («.) were greater In T. erectum {P = 78.6%; A = 2.48; H. = 0.220) than in T. grandiflorum (P = 43.6%; A = 2.18; H. = 0.130). Neither species showed a net excess nor deficiency of heterozygotes as measured by fixation Indices. These levels of aiiozyme diversity are more typical of species with outcrossing breeding systems than species with selflng breeding systems. A mating system analysis revealed that outcrossing is high In T. grandiflorum (^ = 0.761 ± 0.076; x ± SD). In contrast, the selflng rate In T. erectum was nearly 57% (^ = 0.423 ± 0.093) more typical of a species with a mixed-mating system. Nevertheless, these data strongly challenge the view that selflng breeding systems are typical of all North American species of Trillium.
Genetic diversity in three species of Rhus was investigated using starch gel electro- phoresis. Rhus michauxii is a rare species endemic to North Carolina, whereas R. glabra and R. copallina are widespread. All three species are clonal, typically dioecious, woody shrubs. Forty-seven percent of the loci examined for R. michauxii were polymorphic, compared to 88% of the loci examined for both R. glabra and R. copallina. Mean genetic diversity (He) within populations of R. michauxii (0.05) was lower than the genetic diversity within populations of R. glabra and R. copallina (0.15 for both species). Mean number of multilocus genotypes per population was lower for R. michauxii, 4.1, versus the other two species, R. glabra, 27.5 and R. copallina, 26.6. Genotypic diversity indices (DG) were 0.41, 0.95 and 0.95 for R. michauxii, R. glabra and R. copallina, respectively. The proportion of genetic diversity among populations (GST) was higher in R. michauxii (0.335) and R. copallina (0.301) than in R. glabra (0.198). Genetic identity values among the species ranged from 0.52-0.96, and grouped R. michauxii closely with R. glabra. Rhus glabra and R. copallina have levels of genetic diversity similar to wide ranging plant species with the same suite of life history characteristics. The lower level of genetic diversity in R. michauxii is characteristic of species with limited geographic distributions. The relatively high levels of genetic diversity seen among populations indicates that clonal repro- duction may play an important role in the genetic structuring of these species.
Unlike most of its close relatives, Arabidopsis thaliana is capable of self-pollination. In other members of the mustard family, outcrossing is ensured by the complex self-incompatibility ( S ) locus,which harbors multiple diverged specificity haplotypes that effectively prevent selfing. We investigated the role of the S locus in the evolution of and transition to selfing in A. thaliana . We found that the S locus of A. thaliana harbored considerable diversity, which is an apparent remnant of polymorphism in the outcrossing ancestor. Thus, the fixation of a single inactivated S -locus allele cannot have been a key step in the transition to selfing. An analysis of the genome-wide pattern of linkage disequilibrium suggests that selfing most likely evolved roughly a million years ago or more.
Enzyme electrophoresis of pollinia and seeds was used to identify pollinia on pollinators, interspecific pollinations, and hybrid seed production in sympatric populations of Asclepias exaltata and A. syriaca. The frequency of mixed pollinia loads on pollinators was low. Only 4.1% (N= 169 pollinia) and 8% (N= 244 pollinia) of the pollinia collected from pollinators of A. syriaca and A. exaltata , respectively, were identified as pollinia from the other species. Natural levels of interspecific pollination, 3.6% ( N = 166 pollinia) on A. syriaca and 3.1% (N = 228 pollinia) on A. exaltata , were typically lower than the number of foreign pollinia carried by pollinators. Hybrid seeds were identified in only two of 208 A. syriaca fruits and one of 178 A. exaltata fruits. Hybrid seeds were largely underdeveloped in the single A. exaltata fruit. High fruit set (34.7%) and near normal seed set following hand‐pollination of A. syriaca with A. exaltata pollinia suggest that hybridization is more likely between these taxa when A. syriaca is the maternal parent. Our findings indicate that the likelihood of hybridization between A. exaltata and A. syriaca is remote. Nevertheless, introgressive hybridization has been documented in these species, which suggests that hybridization remains a strong evolutionary force even for species that infrequently interbreed.
As a major agent of rapid speciation, interspecific hybridization has played an important role in plant evolution. When hybridization involves species that exhibit self-incompatibility (SI), this prezygotic barrier to self-fertilization must be overcome or lost to allow selfing. How SI, a normally dominant trait, is lost in nascent hybrids is not known, however. Here we demonstrate that hybrid self-fertility can result from epigenetic changes in expression of the S-locus genes that determine specificity in the SI response. We analyzed loss of SI in synthetic hybrids produced by crossing self-fertile and self-incompatible species in each of two crucifer genera. We show that SI is lost in the stigmas of A. thaliana-lyrata hybrids and their neo-allotetraploid derivatives and in the pollen of C. rubella-grandiflora hybrids and their homoploid progenies. Aberrant processing of S-locus receptor kinase gene transcripts as detected in Arabidopsis hybrids and suppression of the S-locus cysteine-rich protein gene as observed in Capsella hybrids are two reversible mechanisms by which SI might break down upon interspecific hybridization to generate self-fertile hybrids in nature.
We investigated the distribution of genetic variation in cedar elm, Ulmus crassifolia. This species has an unusual geographic distribution with a primary range in the south central United States (central Texas east to Mississippi) and a few isolated populations located in central Florida. Cedar elm was polymorphic at 86% of the 15 loci examined and maintained high levels of genetic diversity within populations (H, = 0.22). Over 94% of the total genetic variation was partitioned within populations (GST = 0.062), even though significant levels of variation at individual loci were detected among populations. Genetic identity between populationrs located in the primary population was high (I = 0.990), while populations located in central Florida were more divergent from each other (I = 0.923) than from populations found within the primary range (I = 0.952). The Florida populations had reduced allelic variation per locus (A = 2.07) in comparison to the primary populations (A = 2.32), even though both sets of populations had equivalent levels of genetic diversity (H, = 0.20 and 0.19, respectively). This reduction in alleles suggests that the Florida populations of U. crassifolia may have experienced a mild genetic bottleneck during their evolu- tionary history.
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