A meiotic analysis of the genus Reithrodontomys in the United States

Abstract

The frequency and distribution of chiasma in meiotic of four species of Reithrodontomys (fulvescens, humulis, megalotis and montanus) were examined using standard light microscopy. R. fulvescens and R. humulis have the highest Rl values and R. megalotis and R. montanus have the lowest. Among these species, the reduction in the number of linkage groups appears to indicate an increase in specialization. In all species examined in this study, a non-random distribution of chiasma was found. Four individuals of R humulis had a n = 27 cytotype and two of these had the two smallest chromosome sets unpaired in all cells examined. In both individuals, an examination of prophase II cells indicated a normal chromosomal complement. The sex chromosomes of each species have an end-to-end association, however, the presence or absence of chiasma was not determined. The X chromosomes were heteropycnotic in each species with the short arms being stained more than the short arms. In R. megalotis, chiasma frequency did not differ significantly between cells whh and whhout supernumeraries. Cells that had more than one supernumerary contained less proximal chiasmata than those with one or none.