%0 Journal Article %A Céline M. O. Reisser %A Dominique Fasel %A Evelin Hürlimann %A Marinelad Dukic %A Cathy Haag-Liautard %A Virginie Thuillier %A Yann Galimov %A Christoph Haag %T Transition from environmental to partial genetic sex determination in Daphnia through the evolution of a female-determining incipient W-chromosome %D 2016 %R 10.1101/064311 %J bioRxiv %P 064311 %X Sex chromosomes can appear through the evolution of genetic sex determination (GSD) from hermaphroditism or environmental sex determination (ESD). However, despite their extensive theoretical description, the early mechanisms involved in the transition from ESD to GSD have yet to be observed in nature, as no mixed ESD-GSD species has been reported in the literatureand studied on the molecular level. Here, we focused on Daphnia magna, a small freshwater crustacean in which sex is determined solely by the environment, but for which a dominant female sex-determining locus segregates in multiple populations. We found that the sex determining genomic region shares a common location in all populations studied, in theperi-centromeric region of linkage group 3, in a region with reduced but non-zero recombination. The region also harbors numerous genes known to be involved in female and male sex determination/differentiation in other taxa, including transformer 2 and sox9, as well as genes involved in chromatin remodeling. Overall, our results suggest that D. magna has evolved an incipient W chromosome. Inaddition, the occurrence of the sex-determining mutation in an area of pre-existing low recombination contributes to the debate on the degree of involvement of sexually antagonistic selection in early stages of recombination suppression in sex chromosomes. As such, D. magna represents the first animal species for which transition from ESD to GSD is evidenced at the genetic level in multiple populations, and could serve as amodel to empirically study the role of selective forces in the early stages of sex chromosome evolution. %U https://www.biorxiv.org/content/biorxiv/early/2016/07/18/064311.full.pdf