A fundamental puzzle in the evolution of sex determination is how and why species switch between XY and ZW sex chromosomes.Swordtails and platyfish (genusXiphophorus) offer the ideal system to study this problem. Among the 25 closely related species are some with XY, some with ZW, and some with polymorphic XY/ZW systems.We have already obtained phased sex chromosome sequencing data for the majority of these species, and have developed a strategy for testing competing hypotheses.We are searching for a motivated postdoc to develop a fellowship proposal that would provide them funding to carry out these analyses. The postdoc will be based in the Peichel lab (University of Bern, Switzerland) and work in close collaboration with the Kirkpatrick lab (University of Texas, USA). The ideal candidate will have skills in bioinformatics and statistics, and familiarity with genomic evolution and sex chromosomes. Two opportunities for funding are the ENBO Fellowships (deadline 23 January 2026) and Marie Curie Fellowships.Interested candidates should email Catherine Peichel (catherine.peichel@unibe.ch) and Mark Kirkpatrick (kirkp@mail.utexas.edu) with their C.V.s and names of two references. Mark Kirkpatrick (to subscribe/unsubscribe the EvolDir send mail to golding@mcmaster.ca)