Cephalopod Sex Determination and its Ancient Evolutionary Origin

Abstract

AbstractSex chromosomes are critical elements of sexual reproduction in many animal and plant taxa, however they show incredible diversity and rapid turnover even within clades. Until now, the mechanism of sex determination in cephlaopods has been a mystery. Using a chromosome-level genome assembly generated with long read sequencing, we report the first evidence for genetic sex determination in cephalopods. We have uncovered a sex chromosome in California two-spot octopus (Octopus bimaculoides) in which males/females have ZZ/ZO karyotypes respectively. We show that the octopus Z chromosome is an evolutionary outlier with respect to divergence and repetitive element content as compared to autosomes and that it is present in all cephalopods that we have examined including Nautilus, the outgroup to squids and octopuses. Our results suggest that the cephalopod Z chromosome system originated before the split of all extant cephalopod lineages, over 480 million years ago and has been conserved to the present, making it the among the oldest conserved animal sex chromosome systems known.