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

Abstract

A comparison of the G- and C-banded chromosomes from O. torridus torridus to those of other known karyotypic forms of peromyscine rodents results in a revision of the proposed primitive karyotype for Onychomys. The primitive karyotype for Onychomys probably contained 5 pairs of biarmed autosomes with euchromatic short arms and 17 pairs of acrocentric autosomes. Morphology of 1 pair is uncertain; it was either biarmed with a heterochromatic short arm or acrocentric. If the primitive karyotype is as hypothesized and currently accepted systematic arrangements are correct, then 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 O. arenicola differ by 2, G- and C-band data reveal that as many as 15 heterochromatic short arm differences may distinguish the 2 karyotypes.