Self-fertilization in the white-lipped land snail Triodopsis Albolabris

The breeding system of the polygyrid land snail Triodopsis albolabris was studied in laboratory colonies through the use of allozyme-genetic markers. Isolated virgin individuals self-fertilized only after several months of apparent self-sterility. Isolated pairs appeared to reproduce solely by outcrossing. Overall, the normalized reproductive success of paired individuals was about 86 times greater than that of isolates.
Nine natural populations of this snail were surveyed at eight allozyme loci. All populations were highly polymorphic, and heterozygosity was high and agreed with Hardy-Weinberg expectations. This indicates that selfing is not common in established populations. We conclude that T. albolabris avoid inbreeding by selfing if at all possible. However, if the probability of finding a mate is low (assessed by several months without finding one), lone individuals self-fertilize.     

Effects of Interference Between Selected Loci on the Mutation Load,Inbreeding Depression,and Heterosis

A classical prediction from single-locus models is that inbreeding increases the efficiency of selection against partially recessive deleterious alleles (purging), thereby decreasing the mutation load and level of inbreeding depression. However, previous multilocus simulation studies found that increasing the rate of self-fertilization of individuals may not lead to purging and argued that selective interference among loci causes this effect. In this article, I derive simple analytical approximations for the mutation load and inbreeding depression, taking into account the effects of interference between pairs of loci. I consider two classical scenarios of nonrandomly mating populations: a single population undergoing partial selfing and a subdivided population with limited dispersal. In the first case, correlations in homozygosity between loci tend to reduce mean fitness and increase inbreeding depression. These effects are stronger when deleterious alleles are more recessive, but only weakly depend on the strength of selection against deleterious alleles and on recombination rates. In subdivided populations, interference increases inbreeding depression within demes, but decreases heterosis between demes. Comparisons with multilocus, individual-based simulations show that these analytical approximations are accurate as long as the effects of interference stay moderate, but fail for high deleterious mutation rates and low dominance coefficients of deleterious alleles.     

Why do plants abort so many developing seeds: bad offspring or bad maternal genotypes?

It has been suggested that abortion of ovules in perennials is caused partly by early acting genetic load (abortions due to ‘bad offspring’). However, it is still unclear what proportion of abortions of naturally pollinated seeds are due to early genetic load. Here we suggest that variation between maternal genotypes (abortions due to ‘bad maternal genotypes’) may be an even more important factor causing genetic abortions than early load, based on results from Scots pine. The early load is severe in Scots pine: in experimental self-pollinations on average 76% of the seeds were aborted. Comparison of naturally pollinated and experimentally cross-pollinated seeds showed that the abortion rate of naturally pollinated seeds was only slightly, and not statistically significantly, higher than that of experimentally cross-pollinated seeds (30% vs. 26.5%, respectively). Thus, although early load can be high under self-pollination in Scots pine, it does not account for a high share of abortions of naturally pollinated seeds. Instead, maternal genotype determined the seed abortion rate: in a separate experiment using an experimental population (clonal stand), 29% of the total variance in seed abortion was due to variation between maternal genotypes. We studied further whether ‘bad maternal genotypes’ could be explained by trade-offs between seed abortion and other fitness functions. Only one statistically significant genetic correlation was found, a positive association between cone production and successful seed development. Thus ‘bad maternal genotypes’ aborted a higher proportion of their seed and produced less cones than the ‘good maternal genotypes’. This revised version was published online in July 2006 with corrections to the Cover Date.     

The demography and population genomics of evolutionary transitions to self-fertilization in plants

The evolution of self-fertilization from outcrossing has occurred on numerous occasions in flowering plants. This shift in mating system profoundly influences the morphology, ecology, genetics and evolution of selfing lineages. As a result, there has been sustained interest in understanding the mechanisms driving the evolution of selfing and its environmental context. Recently, patterns of molecular variation have been used to make inferences about the selective mechanisms associated with mating system transitions. However, these inferences can be complicated by the action of linked selection following the transition. Here, using multilocus simulations and comparative molecular data from related selfers and outcrossers, we demonstrate that there is little evidence for strong bottlenecks associated with initial transitions to selfing, and our simulation results cast doubt on whether it is possible to infer the role of bottlenecks associated with reproductive assurance in the evolution of selfing. They indicate that the effects of background selection on the loss of diversity and efficacy of selection occur rapidly following the shift to high selfing. Future comparative studies that integrate explicit ecological and genomic details are necessary for quantifying the independent and joint effects of selection and demography on transitions to selfing and the loss of genetic diversity.     

NONADAPTIVE EVOLUTION OF DISEASE RESISTANCE IN AN ANNUAL LEGUME

Local populations of the plant Amphicarpaea bracteata often contain genetically divergent lineages that differ strongly in disease resistance toward the specialist pathogen Synchytrium decipiens. In one population, lineages with disease resistance were observed to significantly decrease in frequency over a two-year period, despite the continued presence of pathogens. Extensive self-pollination in A. bracteata restricts the opportunity for recombination of alleles affecting separate traits, resulting in strong correlations between disease resistance and other ecologically important characters, including plant morphology, phenology, and patterns of reproductive allocation. Natural selection on these correlated characters may thus cause nonadaptive changes in disease resistance. These results imply that A. bracteata's mating system is a basic constraint interfering with its adaptation to pathogen attack.