The offspring size/fecundity trade-off and female fitness in the Atlantic molly (Poecilia mexicana, Poeciliidae)
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Across a variety of taxa, large offspring have been demonstrated to have a fitness advantage over smaller offspring of the same species. However, producing large offspring often comes at the cost of having to produce fewer young, and the payoff (and thus, evolutionary outcome) of this trade-off is expected to vary between environments. Atlantic mollies (Poecilia mexicana: Poeciliidae, Teleostei), inhabiting a sulfidic cave and various non-sulfidic surface habitats in Tabasco (Mexico), are reproductively isolated and evolved divergent female life-history traits: females of the cave ecotype produce considerably fewer, but larger offspring. Stressful (sulfidic) environments may favor the production of larger offspring, as they are better able to cope with chemical stressors. It remains to be determined though if increased offspring survival outweighs the fitness cost of producing fewer but larger offspring even under benign laboratory conditions. We tested 30-day newborn survival of offspring from wild-caught P. mexicana females from diverging populations in a low-density, no predation, no cannibalism, and ad-libitum-food, benign laboratory environment. Survival rates were highly skewed towards larger cave molly offspring; however, surface molly females still had a higher fitness than cave molly females in terms of higher total numbers of surviving offspring. Our study provides evidence for an innate fitness advantage of larger cave molly offspring. Furthermore, the observed differences in life-history strategies could promote further divergence and reproductive isolation among these ecotypes of P. mexicana, because cave molly females immigrating into the adjacent surface habitats would most likely be selected against.
KeywordsCave fish Ecological speciation Extremophile teleost Immigrant fitness Local adaptation Offspring size/number trade-off
We thank J. Curtis for help with fish care, T. J. Colston for help in the field, and the Mexican Government (Permiso de Pesca de Fomento: DGOPA.06192.240608.-1562) for kindly providing permits. We appreciate the support rendered to this project by F.J. García de León, and R. Brian Langerhans helped improve a previous version of this manuscript. The experiments comply with the current laws on animal experimentation of the United States of America (AUS-IACUC approved protocol: R06-026). Funding came from the National Science Foundation of America (DEB-0743406).
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