Genomic features of parthenogenetic animals

Abstract

Evolution under asexuality is predicted to impact genomes in numerous ways, but empirical evidence remains unclear. Case studies of individual asexual animals have reported peculiar genomic features which were suggested to be caused by asexuality, including high heterozygosity, a high abundance of horizontally acquired genes, a low transposable element load, or the presence of palindromes. We systematically characterized these genomic features in published genomes of 26 asexual animals representing at least 18 independent transitions to asexuality. Surprisingly, not a single feature is systematically replicated across a majority of these transitions, suggesting that no genomic feature is characteristic of asexuality and that previously reported patterns were lineage specific rather than caused by asexuality. We found that only asexuals of hybrid origin were characterized by high heterozygosity levels. Asexuals that were not of hybrid origin appeared to be largely homozygous, independently of the cellular mechanism underlying asexuality. Overall, despite the importance of recombination rate variation for the evolution of sexual animal genomes, the genome-wide absence of recombination does not appear to have had the dramatic effects which are expected from classical theoretical models. The reasons for this are probably a combination of lineage-specific patterns, impact of the origin of asexuality, and a survivorship bias of asexual lineages. Box 1: Transitions to asexualityMeiotic sex and recombination evolved once in the common ancestor of eukaryotes [41].Asexual animals therefore derive from a sexual ancestor, but how transitions from sexual to asexual reproduction occur can vary and have different expected consequences for the genome [13]. Hybrid origin:Hybridization between sexual species can generate hybrid females that reproduce asexually [13,42]. Asexuality caused by hybridization can generate a highly heterozygous genome, depending on the divergence between the parental sexual species prior to hybridization. Hybridization can also result in a burst of transposable element activity [12]. Intraspecific origins: Endosymbiont infection. Infection with intracellular endosymbionts (such asWolbachia, Cardinium or Rickettsia) can cause asexuality, a pattern that is frequent in species with haplodiploid sex determination [43]. This type of transition often (but not always) results in fully homozygous lineages because induction of asexuality frequently occurs via gamete duplication (see Box 2). Spontaneous mutations/Contagious asexuality. Spontaneous mutations can alsounderlie transitions from sexual to asexual reproduction. In addition, asexual females of some species produce males that mate with females of sexual lineages, and thereby generate new asexual strains (contagious asexuality). In both cases, the genomes of incipient asexual lineages are expected to be very similar to those of their sexual relatives and subsequent changes should be largely driven by the cellular mechanism underlying asexuality (Box 2). an...

The genomic determinants of adaptive evolution in a fungal pathogen

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Genome Analysis of the Fatal Tapeworm Sparganum Proliferum Uncovers the Cryptic Life Cycle and Mechanisms Underlying Aberrant Larval Proliferation

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