Opus: University of Bath Online Publication Store Triploid Plover Female Provides Support for a Role of the W Chromosome in Avian Sex Determination 2 3 Introduction

20 Two models, Z Dosage and Dominant W, have been proposed to explain sex determination in birds 21 where males are characterized by the presence of two Z chromosomes and females are hemizygous 22 with a Z and a W chromosome. According to the Z Dosage model, high dosage of a Z-linked gene is 23 needed to trigger male development, whereas the Dominant W model postulates that female 24 development is triggered by a still unknown W-linked gene. Using 33 polymorphic microsatellite 25 markers we describe a female triploid Kentish plover Charadrius alexandrinus identified by 26 characteristic ‘three allele’ genotypes at 14 autosomal markers that produced viable diploid offspring. 27 Peak ratio analysis showed that the sex chromosome composition of this female was ZZW. Our 28 results suggest a prominent role for a female determining gene on the W chromosome. In light of this 29 result, we propose that avian sex determination is more dynamic and complex than currently 30 envisioned. 31