On the repeated evolution of parthenogenesis in stick insects

Female-producing parthenogenesis is widespread in stick insects. It ranges from rare in sexual species to facultative or obligate, the latter sometimes in hybrids. This review synthesizes current knowledge on its origins, mechanisms, and evolutionary consequences, highlighting the distinction between hybrid and intra-specific origins. Hybrid-derived parthenogens are rare, obligate, frequently polyploid, and produce heterozygous eggs via endoduplication. Intra-specific parthenogens are more frequent, typically diploid, and often homozygous due to gamete duplication. It can be facultative, allowing both reproductive modes. However, natural populations usually exhibit only one strategy, and intermediate sex ratios are rare. The mosaic distribution of mixed-sex and female-only populations without clear ecological differences suggests other factors drive the observed patterns. Sexual conflict has been proposed, but empirical data is not yet conclusive. In the Timema genus, multiple obligate parthenogens evolved independently from rare spontaneous parthenogenesis in sexual species. This suggests repeated selection for increased parthenogenesis frequencies in different genomic backgrounds. However, parthenogenesis is linked to reduced selection efficiency and slower adaptation. Overall, by providing an update on the current understanding of the phylogenetic distribution, mechanistic diversity, and transitions to parthenogenesis in stick insects, this review establishes Phasmatodea as a model to study the evolutionary significance of parthenogenesis.