Evolution of Gonopodial Asymmetry and Behavioral Laterality in the Livebearing Fish Xenophallus umbratilis

Authors: Mary-Elise Johnson, Erik S. Johnson, Jerald B. Johnson

Year: 2018

Abstract

The livebearing fish Xenophallus umbratilis shows an unusual form of handedness, where the male intromittent organ (gonopodium) terminates with either a dextral or sinistral twist. We tested the hypothesis that these two morphs are maintained in the wild by negative frequency-dependent selection, wherein males with the rare morph are favored in a mating context when they are uncommon. We sampled males from multiple populations and found that some populations show a clear dextral bias in the gonopodium, where others show a sinistral bias. However, the ratio of left- to right-handed males appears to be dynamic as evidenced by our sampling of a single population over time wherein we found an extreme shift in the ratio of left- to right-handed males over a relative short period of time, from March to June. In addition to presenting these field results, we will also present our findings from two laboratory experiments exploring the mechanism by which rare males morphs have a mating advantage when they are rare. First, we show that mature females actively avoid male mating attempts by positioning their bodies to the side of males in such a way that the most common male morph is not able to successfully mate with females, but the rare morph is. Second, to determine if different male morphs vary in how they approach potential mates, we used a detour test where males are forced to approach from either the left or right side. This is a key first step in determining if this species exhibits behavioral laterality that is linked to gonopodial morphology. We present our results to date on these questions, and outline several additional experiments that are in progress that we anticipate will throw light on the evolution of brain lateralization in fishes, and potentially help explain why handedness has evolved in bilaterally symmetrical organisms.