Karyotypic Orthoselection for Additions of Heterochromatic Short Arms in Grasshopper Mice (Onychomys: Cricetidae)

Abstract

A comparison of the G- and C-banded chromosomes from Onychomys torridus tor- ridus to those of other known karyotypic forms of peromyscine rodents results in a revision of the proposed primitive karyotype for the genus Onychomys. The primitive karyotype for Onychomys probably contained five pairs of biarmed autosomes with euchromatic short arms and 17 pairs of acrocentric autosomes. Morphology of one pair is uncertain; however, it was either biarmed with a heterochromatic short arm or acrocentric. If the primitive karyotype is as hypothesized and cur- rently accepted systematic arrangements are correct, many convergent heterochromatic short arm additions or deletions must have occurred during the evolution of the cytotypes of Onychomys in order to explain their distribution in living taxa. Although the respective fundamental numbers of O. t. torridus and 0. arenicola differ by two, G- and C-band data reveal that as many as 15 hete- rochromatic short arm differences may distinguish the two karyotypes. All karyotypes reported for grasshopper mice of the genus Onychomys have a constant diploid number of 48, but variation in fundamental number ranges from 72 to 92 (Hsu and Benirschke, 1967, 1968; Hinesley, 1979). Baker et al. (1979) reported that all variation among the FN = 92, FN = 80, FN = 78 and FN = 72 karyotypic forms can be explained by the presence or absence of heterochromatic short arms with no alterations of the euchro- matic segments. Those authors concluded that the primitive karyotype for Onychomys was biarmed in chromosomes 1, 2, 3, 9, 14, 19, 22, and 23 and that chromosomes 2, 3, and 14 had heterochromatic short arms. Baker et al. (1979) concluded that convergent additions of heterochromatic short arms must have occurred if the present distribution of heterochromatic short arms among these taxa is to be explained. A third cytotype of 0. torridus (FN = 74) has been described from standard karyotypes (Hinesley, 1979). The number of arms in the autosomal complement (fundamental number) can be related most parsimoniously to the standard karyotype of 0. arenicola (FN = 72) by a single addition or deletion of a heterochromatic short arm. How- ever, G-band data presented in this paper indicate that explanation of the differences in these cytotypes requires several additional events which, if the currently accepted systematic arrangements are accurate, means an even greater amount of convergent addition of heterochromatic short arms has occurred in the genus. Data from the FN = 74 cytotype also result in a revi- sion of the proposed primitive karyotype for the genus. Additionally, G- band data from this cytotype provide evidence in support of the proposed primitive peromyscine karyotypes (Greenbaum and Baker, 1979; Yates et al., 1979; Baker et al., 1979).