The Bateman gradient and the cause of sexual selection in a sex-role-reversed pipefish

As a conspicuous evolutionary mechanism, sexual selection has received much attention from theorists and empiricists. Although the importance of the m ating system to sexual selection has long been appreciated, the precise rel ationship remains obscure. In a classic experimental study based on parenta ge assessment using visible genetic markers, more than 50 years ago A. J. B ateman proposed that the cause of sexual selection in Drosophila is 'the st ronger correlation, in males (relative to females), between number of mates and fertility (number of progeny)'. Half a century later, molecular geneti c techniques for assigning parentage now permit mirror-image experimental t ests of the 'Bateman gradient' using sex-role-reversed species. Here we sho w that, in the male-pregnant pipefish Syngnathus typhie, females exhibit a stronger positive association between number of mates and fertility than do males and that this relationship responds in the predicted fashion to chan ges in the adult sex ratio. These findings give empirical support to the id ea that the relationship between mating success and number of progeny, as c haracterized by the Bateman gradient, is a central feature of the genetic m ating system affecting the strength and direction of sexual selection.