Multiple direct transitions from sexual reproduction to apomictic parthenogenesis in Timema stick insects

Transitions from sexual reproduction to parthenogenesis may occur along multiple evolutionary pathways and involve various cytological mechanisms to produce diploid eggs. Here, we investigate routes to parthenogenesis in Timema stick insects, a genus comprising five obligate parthenogens. By combining information from microsatellites and karyotypes with a previously published mitochondrial phylogeny, we show that all five parthenogens likely evolved spontaneously from sexually reproducing species, and that the sexual ancestor of one of the five parthenogens was probably of hybrid origin. The complete maintenance of heterozygosity between generations in the five parthenogens strongly suggests that eggs are produced by apomixis. Virgin females of the sexual species were also able to produce parthenogenetic offspring, but these females produced eggs by automixis. High heterozygosity levels stemming from conserved ancestral alleles in the parthenogens suggest, however, that automixis has not generated the current parthenogenetic Timema lineages but that apomixis appeared abruptly in several sexual species. A direct transition from sexual reproduction to (at least functional) apomixis results in a relatively high level of allelic diversity and high efficiency for parthenogenesis. Because both of these traits should positively affect the demographic success of asexual lineages, spontaneous apomixis may have contributed to the origin and maintenance of asexuality in Timema.