Cryptic asexual reproduction in Caenorhabditis nematodes

Despite potential advantages of asexuality, most animal species reproduce by sex. This suggests that there are barriers that prevent asexuality arising from an obligately sexual ancestor. One such barrier is the transition from haploid to diploid maternal inheritance. Here we report that females of the normally sexual species Caenorhabditis nouraguensis can produce offspring asexually by sperm-dependent parthenogenesis (also called gynogenesis or pseudogamy) in two contexts: (1) when crossed to males of their sister species C. becei, or (2) when crossed to UV-irradiated males of their own species. We found that crosses between C. nouraguensis females and C. becei males result in two classes of viable offspring. The first class consists of fertile offspring, which are produced asexually by gynogenesis; these progeny inherit a diploid maternal genome but fail to inherit a paternal genome. The second class consists of sterile hybrid offspring, which inherit both a diploid maternal genome and a haploid paternal genome. Using whole-genome sequencing of individual viable worms, we show that diploid maternal inheritance results from an altered meiosis in C. nouraguensis oocytes and the inheritance of two randomly selected homologous chromatids. This genetic mechanism of diploid maternal inheritance is indistinguishable from that of many obligately asexual species. Thus, C. nouraguensis may provide a genetically tractable model to study the evolutionary origins of asexuality from obligately sexual species.