Parental care is associated with cell cycle arrest in a terrestrial isopod

Parental care is a diverse phenotype present in many lineages that is both an important social adaptation and a precursor to other elaborate social systems. Transcriptomics has been widely used to determine the origins of care but has been less often deployed to understand the mechanisms that parenting species use to respond to the costs imposed by care. Here, we perform RNA-sequencing to understand the behavioural and physiological consequences of maternal care in the terrestrial isopod Armadillidium nasatum, a species that broods and provisions both eggs and hatched offspring (mancae) in a marsupium. We sequenced heads of individuals prior to reproduction, during egg-brooding, during mancae-brooding and post-reproduction. As predicted, we found modest but clear gene expression differences between the non-parenting and parenting stages, with few differences differentiating mothers brooding eggs versus mancae. However, we were surprised to find that the bulk of gene expression differences represented downregulation of cell-cycle genes during both parenting stages. This indicates that A. nasatum mothers enter a period of cell-cycle arrest resembling diapause, which may facilitate the diversion of resources towards the developing brood. This finding broadens our understanding of the diverse developmental pathways organisms have used in the evolution of social phenotypes.