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Open Access Research article

The evolution of reproductive isolation in a simultaneous hermaphrodite, the freshwater snail Physa

Robert T Dillon1*, Amy R Wethington2 and Charles Lydeard34

Author affiliations

1 Department of Biology, College of Charleston, Charleston, SC 29424, USA

2 Department of Biology, Chowan University, 1 University Place, Murfreesboro, NC 27855, USA

3 Department of Biology, American University, 4400 Massachusetts Ave. NW, Washington, DC 20016, USA

4 Current Address: The National Science Foundation, Division of Environmental Biology, 4201 Wilson Boulevard Arlington, VA 22230, USA

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Citation and License

BMC Evolutionary Biology 2011, 11:144  doi:10.1186/1471-2148-11-144

Published: 27 May 2011

Abstract

Background

The cosmopolitan freshwater snail Physa acuta has recently found widespread use as a model organism for the study of mating systems and reproductive allocation. Mitochondrial DNA phylogenies suggest that Physa carolinae, recently described from the American southeast, is a sister species of P. acuta. The divergence of the acuta/carolinae ancestor from the more widespread P. pomilia appears to be somewhat older, and the split between a hypothetical acuta/carolinae/pomilia ancestor and P. gyrina appears older still.

Results

Here we report the results of no-choice mating experiments yielding no evidence of hybridization between gyrina and any of four other populations (pomilia, carolinae, Philadelphia acuta, or Charleston acuta), nor between pomilia and carolinae. Crosses between pomilia and both acuta populations yielded sterile F1 progeny with reduced viability, while crosses between carolinae and both acuta populations yielded sterile F1 hybrids of normal viability. A set of mate-choice tests also revealed significant sexual isolation between gyrina and all four of our other Physa populations, between pomilia and carolinae, and between pomilia and Charleston acuta, but not between pomilia and the acuta population from Philadelphia, nor between carolinae and either acuta population. These observations are consistent with the origin of hybrid sterility prior to hybrid inviability, and a hypothesis that speciation between pomilia and acuta may have been reinforced by selection for prezygotic reproductive isolation in sympatry.

Conclusions

We propose a two-factor model for the evolution of postzygotic reproductive incompatibility in this set of five Physa populations consistent with the Dobzhansky-Muller model of speciation, and a second two-factor model for the evolution of sexual incompatibility. Under these models, species trees may be said to correspond with gene trees in American populations of the freshwater snail, Physa.