Genic Variation and Systematics of Rice Rats (Oryzomys palustris Species Group) in Southern Texas and Northeastern Tamaulipas, Mexico

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Abstract
Two taxa within the Oryzomys palustris species group, O. p. texensis and O. couesi aquaticus, occur sympatrically in southern Texas and northeastern Tamaulipas, Mexico. Tissues of 379 individuals from sympatric and allopatric populations within this area and allopatric populations outside the area were analyzed using starch-gel electrophoresis to describe genic variation in the contact zone within each taxon, determine the level of gene flow, if any, between the two taxa, and resolve the taxonomic status of the two rice rats. Populations of O. palustris in and near the contact zone have lower mean heterozygosity than populations from regions to the north and east, whereas O. couesi does not show lower heterozygosity within the contact zone relative to adjacent regions. Within the contact zone, O. palustris has a more linear (coastal) pattern of gene flow with greater differentiation among local populations than does O. couesi. Within and outside the zone of contact, the two taxa were fixed for alternate alleles at four loci, and unique alleles were expressed by one or the other of the two taxa at eight loci. Based on the lack of evidence of hybridization or introgression in sympatry, O. palustris and O. couesi are recognized as distinct species.

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