Date of Award


Document Type


Degree Name

Master of Science (MS)

Legacy Department

Biological Sciences

Committee Member

Margaret B. Ptacek, Committee Chair

Committee Member

Michael J. Childress

Committee Member

J. Antonio Baeza


Variability in life history and morphology across conspecific populations is a widespread phenomenon and may be attributed to life history trade-offs in response to environmental variation. Such trade-offs between growth, maintenance, and reproduction often occur via developmental plasticity, which allows organisms to shift their developmental trajectories to maximize fitness in a given environment. Sailfin mollies (Poecilia latipinna) exhibit interpopulation variation in life history and morphological traits, and inhabit springs, tidal creeks, and estuaries that range from freshwater to seawater. Male size at maturity is smaller on average in freshwater springs than in brackish water marshes where these fish are most abundant. Such variation influences male fitness, because male size is fixed at maturity, highly variable, and correlated with the expression of alternative reproductive tactics. Large males erect an exaggerated dorsal sailfin in courtship displays, whereas small males perform sneaking mating behavior. In order to determine if developmental plasticity plays a role in interpopulation variation and the maintenance of size-associated mating tactics, I used a split brood design to rear siblings in low salinity (2 ppt) and brackish water (20 ppt) until sexual maturity. I found that growth and maturation rates of males and females did not differ in response to salinity. Mortality rate was somewhat higher in 20 ppt relative to 2 ppt. Females were smaller at maturity in brackish water, though their body condition did not differ in response to salinity treatment. Males also responded plastically to salinity environment, though the direction and magnitude depended largely on family genotype. Males exhibited better body condition in 20 than 2 ppt. The relative size (to body area) of the sexual ornament (dorsal sailfin) was smaller for males reared in low salinity than brackish water, suggesting that males in brackish water could allocate more energy resources towards producing larger sexual ornaments. Because males produce smallersexual ornaments and have reduced body condition in low salinity, small male body size and sneaking mating tactics may be favored in freshwater populations. Such a shift in the balance between natural and sexual selection between salinity environments may help to explain interpopulation variation in P. latipinna.



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