Revisiting the Karyotype Evolution of Neotropical Boid Snakes: A Puzzle Mediated by Chromosomal Fissions

Abstract

The Boidae family is an ancient group of snakes widely distributed across the Neotropical region, where several biogeographic events contributed towards shaping their evolution and diversification. Most species of this family have a diploid number composed of 2n = 36; however, among Booidea families, the Boidae stands out by presenting the greatest chromosomal diversity, with 2n ranging between 36 and 44 chromosomes and an undifferentiated XY sex chromosome system. Here, we applied a comparative chromosome analysis using cross-species chromosome paintings in five species representing four Boidae genera, to decipher the evolutionary dynamics of some chromosomes in these Neotropical snakes. Our study included all diploid numbers (2n = 36, 40, and 44) known for this family and our comparative chromosomal mappings point to a strong evolutionary relationship among the genera Boa, Corallus, Eunectes, and Epicrates. The results also allowed us to propose the cytogenomic diversification that had occurred in this family: a process mediated by centric fissions, including fission events of the putative and undifferentiated XY sex chromosome system in the 2n = 44 karyotype, which is critical in solving the puzzle of the karyotype evolution of boid snakes.

Karyotype Diversification and Chromosome Rearrangements in Squamate Reptiles

“…In Iguania, this hypothesis is supported by the high number of different taxa (species, genera and whole families) showing a fixed chromosome formula of 2n = 36 with 6 macro- and 12 microchromosome pairs ( Figure 1 ). Similarly, a karyotype composed of 2n = 36 with 8 macro- and 10 microchromosome pairs is highly conserved among many taxa in Serpentes and it is usually considered the ancestral state of the whole group [ 41 , 42 , 43 , 44 ].…”

“…Similarly, in Boidae, the putative snake ancestral karyotype of 2n = 36 is conserved in the genera Boa , Eunectes , Chilabothrus and Epicrates , but a higher chromosome number (2n = 40–44) has been described in Corallus [ 44 , 67 ]. These divergent karyotype formulas probably resulted from several chromosome-centric fissions which decreased the number of meta- and submetacentric elements and increased the count of acrocentric chromosomes.…”

“…These divergent karyotype formulas probably resulted from several chromosome-centric fissions which decreased the number of meta- and submetacentric elements and increased the count of acrocentric chromosomes. Notably, the primitive undifferentiated sex chromosomes (XY) originally found in Boa appear to be conserved in all the species with 2n = 36 chromosomes, while in Corallus caninus (with 2n = 44) they were split by a chromosome fission into the small acrocentric chromosome pairs, 11 and 12 [ 44 ]. There is currently no evidence on whether a single or a multiple-sex-chromosome system occurs in the genus Corallus .…”