Abstract
BackgroundSexual reproduction involving the fusion of egg and sperm is prevailing among eukaryotes. In contrast, the nematode Diploscapter coronatus, a close relative of the model Caenorhabditis elegans, reproduces parthenogenetically. Neither males nor sperm have been observed and some steps of meiosis are apparently skipped in this species. To uncover the genomic changes associated with the evolution of parthenogenesis in this nematode, we carried out a genome analysis.ResultsWe obtained a 170 Mbp draft genome in only 511 scaffolds with a N50 length of 1 Mbp. Nearly 90% of these scaffolds constitute homologous pairs with a 5.7% heterozygosity on average and inversions and translocations, meaning that the 170 Mbp sequences correspond to the diploid genome. Fluorescent staining shows that the D. coronatus genome consists of two chromosomes (2n = 2). In our genome annotation, we found orthologs of 59% of the C. elegans genes. However, a number of genes were missing or very divergent. These include genes involved in sex determination (e.g. xol-1, tra-2) and meiosis (e.g. the kleisins rec-8 and coh-3/4) giving a possible explanation for the absence of males and the second meiotic division. The high degree of heterozygosity allowed us to analyze the expression level of individual alleles. Most of the homologous pairs show very similar expression levels but others exhibit a 2–5-fold difference.ConclusionsOur high-quality draft genome of D. coronatus reveals the peculiarities of the genome of parthenogenesis and provides some clues to the genetic basis for parthenogenetic reproduction. This draft genome should be the basis to elucidate fundamental questions related to parthenogenesis such as its origin and mechanisms through comparative analyses with other nematodes. Furthermore, being the closest outgroup to the genus Caenorhabditis, the draft genome will help to disclose many idiosyncrasies of the model C. elegans and its congeners in future studies.
Highlights
Sexual reproduction involving the fusion of egg and sperm is prevailing among eukaryotes
Genome assembly revealed the paired structure of the D. coronatus genome Initially, we obtained more than 270,000 Expressed sequence tag (EST)
These were classified into about 13,000 groups based on the 3′ end sequence comparison, 48% of which showed strong homology with C. elegans proteins, and interestingly most of the ESTs showed clear heterozygosity within the groups
Summary
Sexual reproduction involving the fusion of egg and sperm is prevailing among eukaryotes. Selected representatives of different animal phyla can reproduce parthenogenetically, in most cases this is a facultative feature, depending on environmental conditions. If these are favorable, only parthenogenetic females are found, while under stress they switch to bisexual reproduction [2]. Bdelloid rotifers are thought to have followed parthenogenetic reproduction for millions of years without males and meiosis. Similar cases have been reported for certain ostracods, mites and root-knot nematodes [6]. A series of unusual features were observed in the genome, which are probably linked to plant-parasitic lifestyle, absence of meiosis and parthenogenetic reproduction [15]
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