Abstract
Asexual reproduction has evolved repeatedly from sexual ancestors across a wide range of taxa. While the costs and benefits associated with the proliferation of asexual lineages has received considerable attention, the molecular underpinnings of the changes required for asexual reproduction remain relatively unexplored. In particular, it is completely unknown whether the repeated evolution of asexual phenotypes involves similar molecular changes, as previous studies have focused on changes occurring in single lineages. Here we investigate the extent of 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, greatly exceeding that expected by chance. Convergent changes were also tissue-specific, with most convergent genes changing in only one tissue type, as expected from selection for functional changes. Conversely, lineage-specific changes were not tissue-specific, as expected from drift. 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.