Why Are Some Male Pygmy Swordtails Large?

Abstract

Abstract Swordtails in the genus Xiphophorus exhibit substantial variation in male body size influenced by genetic variation at the pituitary (P) locus on the Y-chromosome. However, males of Xiphophorus pygmaeus have historically been classified as uniformly small; they were thought to possess one of two P-alleles, both for small size. In 1988, large male X. pygmaeus were discovered at two sites in the Rio Huichihuayan, Mexico. Surprisingly, females from these sites have lost the ancestral preference for large males although females from adjacent sites have not. This study investigates the heritability of large body size in male X. pygmaeus to understand its role in the evolution of male mating strategies and female preference in this species. We conducted breeding experiments with X. pygmaeus from two populations to determine whether large size is influenced by the same P-allele system found in other swordtails. Small fathers had a significant effect on the size at maturity of their sons but large father...