Evolution of Sceloporus Grammicus Complex (Sauria: Iguanidae) in Central Mexico: Population Cytogenetics

Abstract

The iguanid lizard Sceloporus grammicus is one of the most chromosomally variable of all nonmammalian vertebrates, and is an often-cited example of stasipatric or cascading chromosomal speciation in progress. We report on a sampling effort focused on populations in central Mexico of this complex that was designed to describe the (1) extent of karyotypic variability, (2) interpopulation divergence as defined by chromosomal markers, and (3) levels of within-sample chromosomal polymorphism. A total of 597 lizards karyotyped from 42 localities revealed extensive inter- and intrapopulational variation, with a minimum of 42 distinct karyotypes being scored by conventional staining techniques. These ranged in diploid number from 2N = 31 to 2N = 46, and are most parsimoniously explained as being derived primarily by the fixation of a sequence of fission rearrangements in the macrochromosomes. Summary statistics of genetic similarity and distance coefficients were used to group populations by overall chromosomal similarity, and this resulted in an objective designation of seven distinct cytotypes. Distributional evidence indicates that the ranges of the most derived cytotypes are internal to ranges of the more ancestral races. Of 14 chromosomally polymorphic populations analyzed, all conformed to Hardy-Weinberg expectations. Implications of these findings for models of chromosomally-mediated speciation are discussed.