Variation in the chromosomes of five species of pocket gophers in the genus Geomys as detected by differential staining, and the phylogenetic implications

Abstract

Two isolated populations of pocket gophers were discovered outside the previously known distribution of geomyids in Texas. These represent a new taxon, which is related to Geomys texensis in biochemical properties, chromosome and cranial morphologies. Because of the morphological distinctiveness and the extreme spatial separation (120 km) from populations of G. texensis confined to the central basin of the Edwards Plateau, this new taxon is an isolated, relictual population of G. texensis, and is a new subspecies. The chromosomes of six taxa, representing five species of Geomys (G. attwateri, G. breviceps, G. personatus, G. texensis, G. bursarius) were analyzed for variation in G-band, R-band, sequential R-/DAPI-AMD, and sequential R-/C-banding patterns. Eleven chromosomes have structural rearrangements resulting from duplication/deletion/addition events. Fission/fusion rearrangements occur in two chromosomes. A pericentric inversions is seen in chromosome 1, and tandem fusions were not detected. Eighteen of 34 autosomes have constitutive heterochromatin, with variation seen in the presence or absence, position, and quantity. The heterochromatic differences are seen both among species and between two subspecies of G. personatus. Chromomycin fluorescent staining identifies G-C rich regions in 29 of the autosomes, with variation seen in presence or absence, position, and quantity. The X-chromosome is highly variable, and has differences attributable to structural rearrangements and chromomycin bright staining. G- and R-banded analyses indicate the chromosomes are more conservative than suggested for highly fragmented populations of species with low vagility and low effective population sizes. The types of chromosomal rearrangements seen in Geomys are not those that are proposed to produce meiotic problems or act as post-mating reproductive isolating barriers. The phylogeny based on structural rearrangements, and patterns of constitutive heterochromatin and chromomycin bright staining in the chromosomes of the five species of Geomys are congruent with those previously reported from molecular and biochemical data. Three clades are recognized: (1) G. breviceps; (2) G. attwateri - G. personatus; (3) G. bursarius - G. texensis. Very few rearrangements separate these three clades. Variation in constitutive heterochromatin and chromomycin-bright staining patterns identifies extensive cryptic differences among the taxa.