The Spotted Caudal Project: Examining the Genetic Basis of a Sexually Selected Trait in Xiphophorus birchmanni

Abstract

The project aims to identify which genes play a role in the expression of a sexually selected trait. The spotted caudal dorsal fin is both selected for in X. birchmanni males during mate courtship, yet detrimental in the possibility of melanomas developing from over-expression of the pigmentation, especially in hybrids. This trait can therefore be under natural and sexual selection. The xmrk gene has been previously thought to be involved in spotted caudal expression, but also the relatively unstudied gene, epha3. This experiment used X. birchmanni tissue samples from spotted caudal fins, unpigmented regions adjacent to the spotted caudal, and non-spotted caudal individuals to determine whether the presence of these two genes is more abundant in the spotted caudal samples. If these two genes play a role in spotted caudal expression, we expect their mRNA transcription products to exist in significant quantities within the cell population, and less so in the “transparent” and non-spotted caudal samples. By designing primers to isolate these genes in the X. birchmanni genome, PCR and subsequent statistical analysis show that epha3 contributes significantly to spotted caudal expression, and xmrk less so. Knowing the contributing genes for this trait will facilitate the number of experimental manipulations that can alter the progression to cancerous melanomas and also test how changing expression of the spotted caudal influences X. birchmanni mate choice.