Abstract
The G- and C-banded karyotypes of Sigmodon ochrognathus (2n = 52) and S. fulviventer (2n = 28, 29, 30) are presented and compared to S. hispidus (2n = 52), which we regard as representative of the ancestral chromosomal type. Sigmodon ochrognathus differs from S. hispidus only by the addition of heterochromatic short arms on eight pairs of chromosomes. Sigmodon fulviventer possesses three unique chromosomes and lacks homologous counterparts for ten chromosomes found in S. hispidus. The available cytogenetic and immunological evidence suggests that S. fulviventer represents an evolutionary lineage well removed from all other North American species. We propose that S. hispidus, S. leucotis, S. alleni, S. ochrognathus, S. arizonae, and S. mascotensis comprise a species group and that S. fulviventer should be recognized as a separate monotypic species group.
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