Abstract
The ability to reproduce is one of the most fundamental traits that distinguishes living organisms from inorganic matter, yet, organisms use a panoply of strategies for reproduction. The evolution of these strategies, especially sexual and asexual reproduction, has been the focus of intensive study. By contrast, the molecular underpinnings of sexual and asexual reproduction remain relatively unknown. We investigated convergent gene expression changes and patterns of molecular evolution across five independent transitions to asexuality in stick insects. We compared gene expression of asexual females to those of females from close sexual relatives in whole-bodies and two tissues: the reproductive tract and legs. We identified a striking amount of convergent gene expression change, ranging from 5 to 8% of genes examined. Convergent changes were also tissue-specific, with most convergent genes changing in only one tissue type. Functional enrichment tests found that genes showing convergent changes in the reproductive tract were associated with meiotic spindle formation and centrosome organization. These genes are particularly interesting as they can influence the production of unreduced eggs, a key barrier to asexual reproduction. Changes in legs and whole-bodies were likely involved in female sexual trait decay, with enrichment in terms such as sperm-storage and pigmentation. By identifying changes occurring across multiple independent transitions to asexuality, our results provide a rare insight into the molecular basis of asexual phenotypes and suggest that the evolutionary path to asexuality is highly constrained, requiring repeated changes to the same key genes.