Karyotypic differentiation among two sibling species pairs of new world microtine rodents

Sibling species have been defined as "morphologically similar or identical natural populations that are reproductively isolated" (Mayr, 1963:34). Sibling species are found in all groups of animals (Mayr, 1963) and are differentiated as much genetically as other species (Avise, 1975). Thus, the lack of morphological distinctiveness between sibling species is not the result of a lack of concomitant genetic differences, but may rather be attributable to developmental homeostasis (Mayr, 1963). Several pairs of sibling species occur among North American voles of the genus Microtus, two of which are considered here. The first pair contains the wide-ranging mainland M. pennsylvanicus and the insular M. breweri (Hall, 1981). The second pair consists of M. montanus occurring in the Rocky Mountain system and in many of the smaller mountain ranges in the Great Basin of western North America and M. canicaudus which is restricted to the Willamette Valley of Oregon and adjacent parts of Washington (Hall, 1981). Based upon standard morphological criteria, M. breweri and M. canicaudus have been synonymized under M. pennsylvanicus and M. montanus, respectively (see Hall and Kelson, 1951; Tamarin and Kunz, 1974 for discussions). This paper presents conventionally and differentially stained chromosomal preparations for each of the four species. Interspecific karyotypic comparisons are interpreted with respect to anatomical evolution, speciation, and biogeography. Chromosomal preparations were taken from three different sources: (1) karyotypes from all M. pennsylvanicus, five M. breweri, and six M. montanus were derived from bone marrow cells following Lee and Elder (1980); (2) preparations from one M. pennsylvanicus, two M. breweri, and one M. montanus were obtained from fibroblast cell lines derived from ear explants grown in McCoy's 5a modified medium supplemented with 10% fetal calf serum; (3) splenic preparations were obtained from M. canicaudus following Modi (in press). The following specimens were examined: M. pennsylvanicus (37), Washington: 2 mi S Medical Lake, Spokane Co.; Wyoming: 7 mi N Jackson, Teton Co.; Wisconsin: 3 mi N Stoughton, Dane Co.; Illinois: 2 mi S, 2 mi E West Chicago, Du Page Co.; 1 mi S Champaign, Champaign Co.; Delaware: Hay Road, Wilmington, New Castle Co.; Virginia: Blacksburg, Montgomery Co.; laboratory colony (derived from parental stocks from Champaign Co., Illinois, and Delaware); M. breweri (7), Massachusetts: Muskeget Island, Nantucket Co.; M. montanus (7), Arizona: Apache Co.; Utah: 5 mi S Ogden, Davis Co.; Wyoming: 2 mi N Jackson, Teton Co.; M. canicaudus (4), Oregon: 3.5 mi W Eugene, Lane Co. The standard karyotypes of M. pennsylvanicus (2n = 46, NFa = 50) and M. breweri (2n = 46, NFa = 50) are presented in Fig. 1. The two species are identical with respect to diploid number, autosomal arm number, and sex chromosome morphology. These results agree with those of Fivush et al. (1975). C-banded karyotypes are illustrated in Fig. 2. No consistent interspecific differences were found in C-bands. Autosomal heterochromatin occurs centromerically on several pairs of elements in both species. The X chromosomes have C-bands located centromerically and also interstitially on the long arm. The Y chromosomes are completely heterochromatic in both species. G-banded karyotypes of M. pennsylvanicus and M. breweri are depicted in Fig. 3. Figure 3c portrays a composite prometaphase G-banded karyotype comparing the two species. The G-banded karyotypes of M. pennsylvanicus and M. breweri are identical, with one exception. The telomere of M. breweri chromosome 9 possesses an additional small distal euchromatic band (Fig. 3c). Based upon outgroup comparisons (Peromyscus chromosome 8) chromosomes 9 of M. pennsylvanicus and M. breweri represent the primitive and derived conditions, respectively (Modi, 1984). Figure 1 illustrates the standard karyotypes of M. montanus (2n = 24, NFa = 44) and M. canicaudus (2n = 24, NFa = 44). The karyotypes of the two species have identical diploid and autosomal arm numbers, but two differences are to be noted. First, the two smallest pairs of autosomes are dissimilar in size in M.