Karyotype Diversification and Chromosome Rearrangements in Squamate Reptiles

Abstract

Karyotype diversification represents an important, yet poorly understood, driver of evolution. Squamate reptiles are characterized by a high taxonomic diversity which is reflected at the karyotype level in terms of general structure, chromosome number and morphology, and insurgence of differentiated simple or multiple-sex-chromosome systems with either male or female heterogamety. The potential of squamate reptiles as unique model organisms in evolutionary cytogenetics has been recognised in recent years in several studies, which have provided novel insights into the chromosome evolutionary dynamics of different taxonomic groups. Here, we review and summarize the resulting complex, but promising, general picture from a systematic perspective, mapping some of the main squamate karyological characteristics onto their phylogenetic relationships. We highlight how all the major categories of balanced chromosome rearrangements contributed to the karyotype evolution in different taxonomic groups. We show that distinct karyotype evolutionary trends may occur, and coexist, with different frequencies in different clades. Finally, in light of the known squamate chromosome diversity and recent research advances, we discuss traditional and novel hypotheses on karyotype evolution and propose a scenario of circular karyotype evolution.

Keywords: cytogenetics; evolution; macromutations; phylogeny; reptiles.

Figure 1

Phylogenetic relationships of lizard families + Sphenodon superimposed with karyotype data. nsk = number of species with a described karyotype; nst = number of described species; c = continuous karyotype; d = discontinuous karyotype; m = mix of continuous and discontinuous karyotypes; micro = number of microchromosome pairs; macro = number of macrochromosome pairs (chromosome data were gathered from [6] and therein references). Phylogenetic relationships redrawn from [29]. Dashed lines represent phylogenetic relationships by * [30], ** [31] and *** [32].

Figure 2

Phylogenetic relationships of snake families superimposed with karyotype data. nsk = number of species with a described karyotype; nst = number of described species; c = continuous karyotype; d = discontinuous karyotype; m = mix of continuous and discontinuous karyotypes; micro = number of microchromosome pairs; macro = number of macrochromosome pairs (chromosome data were gathered from [6] and therein references). Phylogenetic relationships redrawn from [33,34].

Figure 3

Different evolutionary trends and hypothesized circular karyotype evolutionary pathways in chameleons. Solid boxes = observed karyotype states. Dashed boxes = putative karyotype states. Solid lines = documented chromosome rearrangements. Dashed lines = hypothesized chromosome rearrangements. Blue lines = chromosome fusions. Red lines = chromosome fissions.