Morphometric and chromosomal variation in populations of Oryzomys albigularis (Muridae : Sigmodontinae) from Venezuela: multivariate aspects

The composite nature of the species Oryzomys albigularis Tomes 1860 has lon g been recognized from the karyological heterogeneity of diploid and fundam ental numbers, with at least two sets of populations in the eastern and nor thern Andes with 2n = 66 and 2n = 80. respectively The aim of this study wa s to test for differences among populations within the species using crania l morphometries. Populations for the multivariate analyses were defined acc ording to differences in autosomal arm numbers within a single modal diploi d number from six locations in two mountain systems of Venezuela, in northe rn South America. Principal component and canonical variate analyses clearl y discriminated among the karyomorphs, the variation at the interorbital, p alatal, and incisive foramen regions being the most informative. These trai ts explained most of the total variance, alter adjusting for size effects. We found no evidence for congruence between the patterns of morphometric va riation and geographical distance among karyomorphs, but similarity pattern s among samples resulted in congruence when morphometric and karyological d ata were considered. Since variation of these last two datasets can be cons idered independent of each other. the observed congruence is suggestive of a phylogenetic structure in the data. Our results are: consistent with a co mposite nature for the species, as most of the observed cranial variation a ppears to bt associated with major karyotypic differences. Given the theore tical relevance of the implied karyotypic changes as a means of reproductiv e isolation, morphometric evidence is used to support the splitting of the "albigularis" form in at least two distinct species.