Chromosome mosaicism: Extreme karyotype variation in the genus ctenomys from Uruguay (Rodentia: Ctenomydae)

Abstract

The genus Ctenomys presents the highest variability in 2n (10 to 70) and number of chromosomal arms (NF) (20 to 84) in mammals, due mainly to the occurrence of a high number of rearrangements throughout their evolutionary history. Among these changes the more frequent are Robertsonian translocation and heterochromatin variation, which explain most of the 2n and NF changes. The Uruguayan coastal populations of Ctenomys are not an exception to this variability, presenting different 2n values along the shore of the Rio de la Plata River. Between them there are three well-defined chromosomal forms: two stable forms, one with 2n = 62 and NF = 78, and another with 2n = 64 and NF = 78; and, in an intermediate area, mosaic individuals presenting 2n = 62/63 and 2n = 63/64 karyotypes, both with NF =78. These individuals were only found in the contact area between the stable populations. The 2n = 63 karyotype differs in a heteromorphic pair formed by a metacentric chromosome and two acrocentric chromosomes homologous to each arm of the metacentric one. We determined that the proportion of the two cellular populations in the mosaic individuals was 1:1. Mosaicism is rarely reported in mammals except man; the explanations for this change are not obvious, but evidence from meiosis analysis seems to suggest the occurrence of an early mutation, before somatic and germ cell differentiation occurred. © 2012 Dipartimento di Biologia Evoluzionistica, Università di Firenze.