Chromosome puzzle in the southernmost populations of the medically important scorpion Tityus bahiensis (Perty 1833) (Buthidae), a polymorphic species with striking structural rearrangements

Abstract

The scorpion Tityus bahiensis is the most common scorpion species of medical importance with sexual reproduction in South America. Many of the populations cytogenetically studied so far are polymorphic both for chromosome number and meiotic configuration in males. In this work, we perform cytogenetic analysis of three of the most meridional populations of T. bahiensis occurring in north-eastern Argentina (Misiones province), which showed numerical and structural variations with respect to previously studied populations. We describe four new diploid numbers in males for this species, from 2n = 12 to 2n = 15, seven different cytotypes, and we analyze the chromosome rearrangements involved in the different multivalent associations observed during meiosis (II, III, V, VI, VII and IX). Ribosomal DNA is invariably present at one terminal region on each of two chromosomes. Blocks of constitutive heterochromatin of different sizes are found at the terminal regions of every chromosome in the different cytotypes, along with subterminal and interstitial blocks in some chromosomes. Based on the rDNA and constitutive heterochromatin localization, we analyzed chromosome evolution within these populations. We propose that all the cytotypes could have originated from a hypothetical ancestral karyotype with 2n = 18 and 9 bivalents at meiosis I but followed two pathways with different sequences of mainly fusions and some translocations from the ancestral karyotype. Furthermore, we review all the cytogenetically studied populations of this species and their distribution to discuss the chromosomal evolution of this polymorphic species.