RESEARCH INTERESTS

My basic research program focuses on the origin and maintenance of sexual and parthenogenetic reproduction in a genus of freshwater snails (Campeloma) found throughout the southeastern United States. The main focus of our laboratory is testing competing hypotheses for the maintenance of sexual reproduction versus parthenogenetic reproduction. Recently, I have become intrigued with studying freshwater snails from Cuatro Cienegas in northern Mexico. In this protected region, there is a diverse array of aquatic habitats ranging from lakes, rivers, temporary streams, and sinks. There are unusually high levels of endemism in snails and fishes in this region. In some freshwater snails, some species show shell armature that rivals those of marine snails, and is probably a response to an endemic cichlid fish. Some phenotypes of this cichlid specialize on snails. I am studying various topics related to conservation issues in this basin: population structure and speciation of Mexipyrgus and coevolutionary interactions between cichlids and snails.

GRANT SUPPORT

• Undergraduate Mentoring in Environmental Biology: Mentoring Undergraduate Research in Conservation Biology and Ecology, PI, National Science Foundation, $438,902.  August 1, 2004- July 31, 2008
• Tests of competing hypotheses for the maintenance of sex and parthenogenesis in a southeastern US snail.  Population Biology Program, National Science Foundation ($153,000).   July 15, 2000 – August 31, 2006.

SELECTED PUBLICATIONS

• Vogel, L. S. and S. G. Johnson. 2008. Estimation of hybridization and introgression frequency in toads (Genus: Bufo) using DNA sequence variation at mitochondrial and nuclear loci. Journal of Herpetology 42:61-75.

• Johnson, S. G. and R. S. Howard. 2007. Contrasting Patterns of Synonymous and Nonsynonymous Sequence Evolution in Asexual and Sexual Freshwater Snail Lineages. Evolution 61: 2728-2735.

• Johnson, S.G., C. Darrin Hulsey, Francisco J. Garcia de Leon. 2007. Spatial mosaic evolution of snail defensive traits. BMC Evolutionary Biology 7:50

• Johnson, S.G. 2006. Geographic ranges, population structure and ages of parthenogenetic and sexual snail lineages. EEvolution 60:1417-1426.

• Johnson, S. G., 2005   Mode of origin differentially influences the fitness of parthenogenetic freshwater snails.  Proc. Roy. Soc. Lond. B. 272: 2149-2153.

• Johnson, S.G.  2005. Age, phylogeography and population structure of the microendemic banded spring snail, Mexipyrgus churinceanus.  Molecular Ecology 14:2299-2311.

• Johnson, S.G.  2000.  Population structure, parasitism and survivorship of sexual and parthenogenetic Campeloma limum (Gastropoda:Viviparidae).  Evolution 54:167-175.

• Johnson, S.G. and E. Bragg.  1999.  Age and polyphyletic origins of hybrid and spontaneous parthenogenetic Campeloma (Gastropoda: Viviparidae) from the southeastern United States.  Evolution 53:1769-1781.

• Johnson, S.G. and W. Leefe. 1999.  Clonal diversity and polyphyletic origins of hybrid and spontaneous parthenogenetic Campeloma (Gastropoda: Viviparidae) from the southeastern United States.  Journal of Evolutionary Biology 12:1056-1068.

• Johnson, S.G., R. Hopkins, and K. Goddard.  1999.  Constraints on elevated ploidy in hybrid and non-hybrid parthenogenetic snails.  Journal of Heredity 90: 659-662.