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Cytogenetic Studies of Three Chromosomal Races of Pocket Gophers (Geomys bursarius Complex) at Hybrid Zones

Robert C. Dowler
Journal of Mammalogy
Vol. 70, No. 2 (May, 1989), pp. 253-266
DOI: 10.2307/1381506
Stable URL: http://www.jstor.org/stable/1381506
Page Count: 14
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Cytogenetic Studies of Three Chromosomal Races of Pocket Gophers (Geomys bursarius Complex) at Hybrid Zones
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Abstract

Cytogenetic relationships among three races of pocket gophers (Geomys bursarius complex) at two hybrid zones in eastern Texas were examined by use of chromosome-banding and meiotic analyses. Race E (2n = 74, FN = 72) interacts with races F (2n = 70, FN = 74) and G (2n = 70, FN = 72) at a narrow (0.8 km) hybrid zone in Burleson Co., Texas. Of 78 pocket gophers collected there, three (3.8%) were F1 hybrids and none was a backcross or F2 hybrid. Races F and G contacted one another at a second hybrid zone 35 km in width, located in Milam and Burleson counties, Texas. Backcross and F2 hybrids between races F and G could not be distinguished chromosomally from F1 hybrids, but distribution patterns of parental and hybrid pocket gophers suggest that hybrids included backcross or F2 animals. G-band chromosome analysis revealed that races F and G differ from race E by at least two centric fission-fusion events. Further chromosomal rearrangements distinguishing race E from the other two races were indicated by G-bands, but could not be identified specifically. C-banded karyotypes revealed heterochromatic differences between race E and races F and G. Races F and G differed in one whole-arm heterochromatic change, as evidenced by C-banding, and by a paracentric inversion, demonstrated by G-banding. Chromosomal distinctions among the races were supported by meiotic analyses of hybrids. Degree of hybridization among the three races and the width of the hybrid zones reflect the extent of cytogenetic divergence of the races. The narrow zone between the 2n = 74 form and the 2n = 70 forms is most likely the result of meiotic imbalances and associated decreases in viability from a minimum of two and probably more rearrangements. The broad zone of hybridization between the two 2n = 70 forms is characterized by much higher hybrid production. The identified rearrangements in the karyotypes of races F and G and their hybrids have not resulted in the degree of reduced fitness evident in hybrids involving race E. Races F and G, and race E interact as species, recognized as Geomys attwateri and G. breviceps, respectively.

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