Female Preference for Male Sword Edge Pigmentation in Hybrid Xiphophorus birchmanni-type Swordtails

Abstract

Female preference for male traits can drive the asymmetric introgression of male secondary sexual characteristics. In the Rio Calnali, Xiphophorus birchmanni and Xiphophorus malinche have hybridized to create two distinct genetic clusters within the same area. The X. malinche cluster shows a small amount of introgression from X. birchmanni, but the X. birchmanni cluster is genetically 75% X. birchmanni and 25% X. malinche. Females of an allopatric population of X. birchmanni demonstrate a preference against the male sword ornament on the caudal fin while females from an allopatric population of X. malinche show no preference for this trait. The X. birchmanni cluster from the Rio Calnali shows introgression of the pigmentation on the lower caudal fin but not of the sword extension. Using dichotomous choice tests and computer animations made with the Anyfish software platform, female hybrids were tested for preference for parental species, X. birchmanni versus X. birchmanni with an X. malinche sword, and hybrid X. birchmanni versus X. birchmanni with an added sword edge pigmentation. Low sample size of X. birchmanni-type females prevented accurate conclusions, but X. malinche-type females showed no preference for the three stimuli. Further research must be conducted to increase sample size, confirm lack of X. malinche-type preference, and ascertain differences in personality between clusters.