On the Repeated Evolution of Parthenogenesis in Stick Insects

A striking aspect of the biology of stick insects is the widespread occurrence of parthenogenesis, including rare, spontaneous events in sexual species, facultative parthenogenesis as well as obligately parthenogenetic species. This review synthesizes current knowledge on the origins, mechanisms, and evolutionary consequences of parthenogenesis in stick insects, with a particular focus on its hybrid and intra-specific origins. Hybrid-derived parthenogens are relatively rare, but invariably obligate. They are often polyploid and produce eggs via endoduplication, a parthenogenesis mechanism maintaining genome-wide heterozygosity. Intra-specific cases of parthenogenesis (facultative and obligate) are much more frequent, are associated with diploidy, and frequently involve genome-wide homozygosity as a consequence of gamete duplication. Facultative parthenogenesis allows females to reproduce both sexually and parthenogenetically, yet natural populations tend to either comprise equal numbers of males and females or consist entirely of females, with intermediate sex ratios being rare. In multiple species, mixed-sex and female-only populations are distributed in a mosaic pattern without clear ecological differences, suggesting that factors beyond environment shape their maintenance. Sexual conflict, including male harassment and female resistance, has been proposed as a potential driver of these patterns, but empirical evidence does not point to male-inflicted harm on females and instead indicates fitness benefits associated with mating and sexual reproduction. Several negative evolutionary consequences of parthenogenesis, including reduced selection efficiency and slower adaptation, have been identified in the Timema genus. Multiple, mostly obligately parthenogenetic Timema species have independently evolved from different sexual ancestors, with a shared mechanism of gamete duplication. Because rare, spontaneous parthenogenesis in sexual species involves the same mechanism, the repeated evolution of parthenogenetic species likely occurred via recurrent selection for increased frequencies of parthenogenesis in different genomic backgrounds. Overall, this review highlights the diversity and distribution of parthenogenesis in stick insects and their broader implications for understanding the evolution of asexual reproduction.