Abstract
It is commonly assumed that asexual lineages are short-lived evolutionarily, yet many asexual organisms can generate genetic and phenotypic variation, providing an avenue for further evolution. Previous work on the asexual plant pathogen Phytophthora ramorum NA1 revealed considerable genetic variation in the form of Structural Variants (SVs). To better understand how SVs arise and their significance to the California NA1 population, we studied the evolutionary histories of SVs and the forest conditions associated with their emergence. Ancestral state reconstruction suggests that SVs arose by somatic mutations among multiple independent lineages, rather than by recombination. We asked if this unusual phenomenon of parallel evolution between isolated populations is transmitted to extant lineages and found that SVs persist longer in a population if their genetic background had a lower mutation load. Genetic parallelism was also found in geographically distant demes where forest conditions such as host density, solar radiation, and temperature, were similar. Parallel SVs overlap with genes involved in pathogenicity such as RXLRs and have the potential to change the course of an epidemic. By combining genomics and environmental data, we identified an unexpected pattern of repeated evolution in an asexual population and identified environmental factors potentially driving this phenomenon.
Highlights
Asexuality in eukaryotes presents a challenge to the diversification of genotypes, yet many asexual eukaryotic lineages persist, showing considerable genetic diversity, and have the ability to adapt to novel and changing environments [1,2,3,4,5,6,7]
In order to determine the mode by which Structural Variants (SVs) develop in parallel evolution, accurate reconstruction of genetic relationships is required between demes as well as between individuals
Since other conditions associated with population expansion were not explanatory variables of parallel SVs, our results suggest that conditions imposing stress on survival drive genetic parallelism
Summary
Asexuality in eukaryotes presents a challenge to the diversification of genotypes, yet many asexual eukaryotic lineages persist, showing considerable genetic diversity, and have the ability to adapt to novel and changing environments [1,2,3,4,5,6,7]. Phytophthora ramorum (Stramenopiles Peronosporales), an oomycete plant pathogen, is an asexually reproducing organism able to colonize and persist in new ecosystems. Only a single mating type, A2, has been identified in California forests and its sexual reproduction has never been observed in nature [18]. Previous studies of the California NA1 lineage employing microsatellite markers have shown higher than expected observed heterozygosity under the Hardy–Weinberg equilibrium and have revealed an accumulation of identical multilocus genotypes (MLGs). These findings are consistent with asexual reproduction and the absence of recombination [20]
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