One Fish, Two Fish, Old Fish, New Fish: The Evolution of the Sword Edge Trait in Xiphophorus birchmanni - X. malinche Hybrid Populations

Abstract

Hybridization is now recognized to be a common part of many processes of speciation. Such an occurrence has been documented between two species of Xiphophorus swordtails: X. birchmanni and X. malinche. These species independently form hybrid zones in several rivers that run along the eastern slopes of the Sierra Madre Oriental in Mexico; areas such as these provide useful natural laboratories for researchers studying introgression. Traits of particular interest in this genus are the sword trait, which is hypothesized to be used in sexual selection in these fish, and the sword edge trait, which outlines the sword in black pigmentation. These traits differ between species; male X. birchmanni have no sword, while male X. malinche do. Hybrids of the two display intermediate phenotypes. This study phenotyped the lower and upper sword edge widths of 719 male X. malinche, X. birchmanni, and X.malinche-X. birchmanni hybrid individuals from seven populations in order to explore how the sword edge trait varies in hybrid populations and if that variation is predicted by ancestry proportions. In addition, admixture mapping was used to identify genomic regions of interest, and BLAST was used to suggest possible genes associated with the sword edge trait. It was found that hybrids at Tlatemaco resembled the X. malinche phenotype and hybrids at Totonicapa resembled the X. birchmanni phenotype, which matches with previous genome-wide ancestry studies of individuals from these hybrid populations. However, Calnali hybrids posed a bit of a puzzle; despite deriving more of their genetic material from X. birchmanni, these hybrids displayed the widest lower sword edges of all hybrid populations. This unexpected finding could potentially be attributed to adaptive introgression of the lower sword edge trait in the Calnali population, though further tests should be conducted to verify this conclusion. In addition, admixture mapping revealed Xiphophorus linkage groups 5 and 13 to be connected with the sword lower edge trait and Xiphophorus linkage group 11 to be connected with the sword upper edge trait. BLAST analysis revealed that the gene XPNPEP1 might be involved with expression of the lower sword edge trait; additionally, the gene NCoR1 might be involved with expression of the upper sword edge trait.