MEXICAN RODENTS (MURIDAE)

Karyotypes of six species of murid rodents from Mexico are reported for the first time, including: Nyctomys sumichrasti, Peromyscus hylocetes, Peromyscus perfulvus, Peromyscus banderanus, Baiomys musculus, and Neotoma goldmani. The chromosomes of some species clearly suggest affinities within the appropriate taxa. Identity of the sex chromosomes and an apparent automosomal polymorphism are shown for Neotoma alleni. The karyotype of Microtus mexicanus from western Mexico is polymorphic for two pairs of chromosomes and, more importantly, differs signifi- cantly from that of presumed M. mexicanus occurring north of Mexico. During the past several years, we have routinely karyotyped small mammals col- lected in western Mexico. Among these are examples of six species whose karyotypes, to our knowledge, have not been previously published. We also find the karyotypes of two species either to be at variance with those initially described or to add signifi- cantly to the cytotaxonomic information already known. Standard karyograms together with their descriptions and comments regarding their taxonomic-evolutionary signifi- cance form the basis of this report. MATERIALS AND METHODS Somatic metaphase chromosomal material was obtained from wild-captured individuals using a modification of the procedure outlined by Lee (1969). The two major deviations from this technique are a reduction in time from 1 to 4 hours to 30 to 45 minutes(min) between administer- ing the mitotic inhibitor (0.008 percent Velban, Eli Lilly & Co.) and sacrificing the animal, and the use of a 0.56 percent KCl solution as the hypotonic medium (12 to 15 min). Chromosome mor- phology in this report is classified as either biarmed or acrocentric. In those instances where more refined distinctions are required, we have utilized the standard terminology (see Zimmerman and Lee, 1968). AN denotes total number of autosomal arms. In certain of the accounts the term simple is used for karyotypes that comprise all or mostly acrocentric chromosomes. Chromosome preparations for most of the species herein reported were either too old or had been prepared and stored such that they were unsuitable for differential staining procedures. Museum study skins with skulls, skulls only, or fluid preserved specimens were prepared for all individuals examined. With the exception of a single specimen of Microtus mexicanus in the mammal collection at North Texas State University, all specimens are deposited in the Museum of Natural History, University of Illinois. Numbers of individuals and localities of capture for all specimens examined follow each species account.