Renn Lab Thesis Project
Red Fish, Blue Fish. One Fish, Two Fish. Testing Speciation by Sexual Selection in Astatotilapia burtoni
Sexual selection has played an important role in the extraordinary species radiations of haplochromine cichlid fish in the Great Lakes of Africa. In these fish species barriers are maintained by sexual selection of mate color. Many cichlid species appear to be differentiated by little more than color. These observations have led to many studies of whether sexual selection might cause accelerated speciation. Sexual selection causes directional fixation of alleles. Two geographically isolated populations could easily diverge in sexually selected characteristics. The differential female preferences responsible for this divergence would also create a gene flow barrier, which would further isolate the populations, accelerating divergence and leading to speciation. This process has been proven theoretically in modeling experiments, but never documented in a living organism. It is difficult to test for speciation directly, but evidence for population specific mate choice would support a hypothesis of speciation by sexual selection. This study tested whether by population-specific mate choice occurred among three geographically isolated populations of the haplochromine cichlid fish Astatotilapia burtoni.
Astatotilapia burtoni is thought to be the closest living species
mouth-brooding riverine cichlid that diverged into at least 110 species
in Lake Tanganyika (Lowe-McConnell, 1991), (Ribbink, 1991). A. burtoni is also the closest relative to the Lake Victoria and Malawi mouthbrooding flocks according to the trees based on mitochondrial sequencing data put together by Sturmbauer and Meyer (1993). The putative ancestors of the species flocks of both lakes are also in the genus Astatotilapia.
Astatotilapia burtoni is a mouthbrooding cichlid in the Haplochrome tribe. It is a generalist species native to Lake Tanganyika and its surrounding swamps and rivers. In the lake, these fish live in shallow shore pools, and have a lek-like social system, with dominant males guarding territory, and females and subordinate males schooling together (Parikh et al., 2006b).
A series of behavioral mate-choice tests were set up, in which a female was allowed to choose between a male of her own population and a male from a foreign population. If females exhibited strong preference for males of their own stocks, it would be evidence for divergence in mating preferences, and would support the idea of a home team advantage. Male attributes and behavior were recorded and used to determine what factors were important in determining a female’s choice.
There was no preferential mate choice among the populations, which show that speciation by sexual selection is not ongoing in this species. Genetic data agree with this finding and suggest that the populations were not genetically isolated despite large geographical separation.
Figure3: Mean Preference of Different Female Stocks. Based on a simple ANOVA, there was no significant difference among the stocks for whether females preferred males of their own stock or of a foreign stock (ANOVA: F=1.1850, DF=2, P=0.3285). Preferences ranged from-30(complete preference for foreign males) to 30 (complete preference for same stock males). Error bars represent one standard deviation of the mean.
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