Dosage compensation and meiotic sex chromosome inactivation are maintained in the absence of selection

Dosage compensation (DC) and meiotic sex chromosome inactivation (MSCI) are key mechanisms regulating gene expression from the X chromosome in male-heterogametic species. While the convergent evolution of these mechanisms is well-documented, their evolutionary fate under relaxed selection remains poorly understood. Here, we test whether DC and MSCI persist in the absence of selection by investigating three independently derived parthenogenetic species of Timema stick insects that occasionally produce rare males. Using chromosome-level genome assemblies, RNA-seq from multiple tissues, and immunocytochemistry, we find that dosage compensation in somatic tissues and meiotic X inactivation are conserved despite the absence of selective pressure in all-female lineages. Surprisingly, expression data and cytological markers indicate that MSCI signatures are even stronger in parthenogenetic males, a pattern driven by prolonged autosomal transcription during meiosis. These results indicate that X-targeting DC and MSCI mechanisms are either weakly selected against or evolutionarily constrained, while autosomal mis-expressing during meiosis occurs rapidly following the relaxation of selection, shedding light on the stability of X chromosome regulation.