Chromosomal variation in the Sceloporus grammicus complex (Sauria, Phrynosomatidae)

Abstract

The Sceloporus grammicus complex offers a valuable resource for studies of interrelated questions about population structure, the origin and fixation of new chromosomal rearrangements, the fitness consequences of structural chromosomal heterozygosity in hybrid zones, and the possible role of such rearrangements in reducing gene flow between chromosomally pure types in regions of parapatric contact. This complex, which ranges from extreme southern Texas south across most of mainland Mexico (see map in Sites et al., 1987), is composed of a number of distinct chromosome races (= cytotypes) with different 2n’s resulting largely from fixed simple Robertsonian rearrangements. Some of these cytotypes have diverged to the species level, as assessed by independent molecular markers, while others are less distinct. Similar chromosomal polytypy has been described in other vertebrates, notably eutherian mammals (Searle, 1992), but in contrast to most of these cases, many cytotypes in the S. grammicus complex exhibit extensive within-population polymorphism as well as between-population fixation for the same kinds of rearrangements. These observations, coupled with estimates of very low levels of isozyme divergence (see below), suggest that processes relevant to the origin, spread, and fixation (or maintenance as polymorphisms) of chromosomal rearrangements may still be actively occurring in this group. The apparent narrowness of a number of hybrid zones also suggests that even pairs of cytotypes capable of hybridizing and backcrossing are behaving as distinct gene pools, and that the between-race chromosomal differences may contribute significantly to speciation potential in this group (Hall, 1983). This paper reviews some aspects of recent work on a number of these issues by myself, students, and colleagues.