Karyotypic Variation in the Pocket Mice Perognathus amplus and P. longimembris

We studied the karyotypic variation of Perognathus amplus and P. longimembris in Arizona and adjacent Nevada and Utah. We extend the distribution of previously known fundamental-number (FN) variants. C-banded karyotypes showed that the FN variants do not result from addition or deletion of heterochromatic arms. We hypothesize that karyotypic evolution in these pocket mice proceeded in three directions from a presumed ancestral FN = 88 karyotype shared by both species. In both P. longimembris and P. amplus , parallel decreases in FN led to low-FN populations. In P. amplus , a separate lineage is characterized by increasing FN. The short-term cell-culture technique we used was superior to in vivo-colchicine, hypotonic methods for the production of chromosome spreads from pocket mice.