Chromosomal, genic, and morphological variation in the Oryzomys melanotis species group

Abstract

Oryzomys melanotis comprises at least three chromosomal races: a 2n=62, FN=68 cytotype in eastern and southern Mexico; a 2n=64, FN=70 cytotype on the Yucatan Peninsula; and a 2n=60, FN=116 cytotype in western Mexico. Analyses of standard and C-band karyotypes suggest that the 2n=62 and 2n=64 cytotypes are similar and form one karyotypic group, whereas the 2n=60 cytotype appears highly divergent and forms a second karyotypic group. Analyses of 27 presumptive genic loci using starch gel electrophoresis corroborate the association of the 2n=62 and 2n=64 cytotypes (average Roger's Similarity = 0.941). The 2n=60 cytotype is fixed for alternate electromorphs at three loci from the 2n=62 and 2n=64 cytotypes and the two karyotypic groups display pronounced frequency differences at other loci. The level of genic divergence between the karyotypic groups is similar to that often observed between distinct species of mammals (average Roger's Similarity = 0.828) and there is no allozymic evidence of intergradation. Three cytotypes of the O. alfaroi species group were used as outgroups in the genic analyses. Based on chromosomal and genic data, two species appear to be represented in the outgroup samples. However, the apparent species limits do not conform to presently recognized taxonomic boundaries.Morphological variation among 29 samples of O. melanotis was examined by univariate and multivariate analyses of mensural data. Three main groups of samples were delineated corresponding, in part, to the distributions of the three cytotypes. In contrast to results of genic and chromosomal analyses, samples from the Yucatan Peninsula were most similar in overall body and skull size to samples from western Mexico. However, trenchant meristic characters ally peninsular specimens with those from adjacent populations in eastern and southern Mexico. Congruence of the chromosomal, genic, and morphological data sets was demonstrated by significant matrix correlations. Because these character sets probably evolve independently, congruence suggests evolution in isolation and a vicariant origin of the three cytotypes. Zoogeographic implications of these data are discussed....