KARYOTYPES OF THE MEXICAN TROPICAL VOLES MICROTUS-QUASIATER AND M-UMBROSUS (ARVICOLINAE, MURIDAE)

The non-differentially stained karyotypes of the tropical voles Microt us quasiater (Coues, 1874) and M. umbrosus (Merriam, 1898) are describ ed by first time, and compared and contrasted with that of the hypothe tical primitive karyotype for Arvicolinae. The karyotype of M. quasiat er from Veracruz, Mexico, has 2n = 62 and FN = 66, while that for M. u mbrosus from Oaxaca, Mexico, has 2n = 56 and FN = 60. Their chromosoma l morphology comprises few two-armed chromosomes and a relatively larg e number of single-armed chromosomes. Both species share both a medium -sized metacentric X chromosome and a small telocentric Y chromosome. This karyotypical pattern deviates from the hypothetical primitive con dition. However, the karyotypes of other Microtus, including Mexican s pecies of temperate affinity, seem to be even more divergent from the ancestral karyotypical pattern.