Date of Award

12-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Biological Sciences

Advisor

Ptacek, Margaret B

Committee Member

Childress , Michael J

Committee Member

Bridges, Jr. , William C

Abstract

Changes in mating signals often result in the development of new species, thus, understanding the genetic basis of traits that confer pre-mating reproductive isolation can shed light on the speciation process. This study used interspecific hybridization between poeciliid fish, a sailfin molly (Poecilia velifera) and a shortfin molly (P. mexicana), to generate reciprocal F1 and backcross hybrids to investigate patterns of inheritance of traits that contribute to mating signal differences between these two groups of mollies. The first part of my study focused on behavioral differences in the mating system of sailfin and shortfin mollies. I observed mating behaviors of males from the two parental species and males of, F1 and backcross generations from two Y-chromosome lines, the sailfin Y-line and the shortfin Y-line. For courtship displays, the mating behavior that separates sailfin from shortfin mollies, the pattern of inheritance showed both strong Y-linked effects as well as autosomal influences. There were no Y-linked effects on gonopodial thrusting behavior, a behavior common to both sailfin and shortfin mollies, and both species shows a similar pattern of autosomal inheritance.
The second part of my study focused on morphological differences between sailfin and shortfin mollies and their relationship to mating behavior differences. Traits that best defined the two species were those associated with the dorsal fin (length of the dorsal fin, length of first fin ray, depth at mid-body, fin ray number), the gonopodium, and the caudal fin. A joint-scaling analysis showed that an additive model best explained inheritance of species differences in these morphological traits, but this model did not explain all of the variance observed. However, when comparing the relationship between species-specific morphological traits and mating behaviors, I found a positive relationship between dorsal fin size and courtship display rate, but no relationship between gonopodium length and gonopodial thrust rate. These results suggest that divergence between sailfin and shortfin mollies occurred as a result of genetic changes to both morphological and behavioral features of mating signals, which allowed for a switch from a mating system based primarily on male-male competition to one of male courtship and female cooperation.

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