Dynamics of recombination, X inactivation and centromere proteins during stick insect spermatogenesis

In eukaryotes, the cellular processes contributing to gamete formation form the building blocks of genetic inheritance across generations. While traditionally viewed as conserved across model organisms, emerging studies reveal significant variation among eukaryotes in meiotic and post-meiotic processes. Extending our knowledge to non-model organisms is therefore critical to improve our understanding of the evolutionary origin, significance and subsequent modifications associated with gamete formation. We describe the mechanisms underlying recombination, chromosome segregation, and meiotic sex chromosome inactivation in the stick insect group Timema. Our results reveal that in males, 1) recombination initiates before synapsis, 2) the mechanisms of X inactivation are variable among species and the X remains silenced despite two waves of transcriptional activation in autosomes during spermatogenesis, 3) chromosome segregation is preceded by the recruitment of centromere proteins that either bind locally or longitudinally along the chromosomes. Together, our observations help understand the evolutionary significance of key cellular processes related to spermatogenesis and shed light on the diversity of their underlying mechanisms among species, including Timema.