Abstract
The voles of the Microtus thomasi/M. atticus species complex demonstrate a remarkable variability in diploid chromosomal number (2n = 38–44 chromosomes) and sex chromosome morphology. In the current study, we examined by in situ hybridization the topology of four satellite DNA motifs (Msat-160, Mth-Alu900, Mth-Alu2.2, TTAGGG telomeric sequences) and two transposons (LINE, SINE) on the karyotypes of nine chromosome races (i.e., populations with unique cytogenetic traits) of Microtus thomasi, and two chromosomal races of M. atticus. According to the topology of the repetitive DNA motifs, we were able to identify six types of biarmed chromosomes formed from either Robertsonian or/and tandem fusions. In addition, we identified 14 X chromosome variants and 12 Y chromosome variants, and we were able to reconstruct their evolutionary relations, caused mainly by distinct mechanisms of amplification of repetitive DNA elements, including the telomeric sequences. Our study used the model of the Microtus thomasi/M. atticus species complex to explore how repetitive centromeric content can alter from chromosomal rearrangements and can shape the morphology of sex chromosomes, resulting in extensive inter-species cytogenetic variability.
Highlights
Sex chromosomes evolved several times independently in different lineages
Our results show that pericentromeric heterochromatin of M. thomasi and M. atticus acrocentric chromosomes was highly complex and variable, with up to four different families of repeated DNA included
Our study examined the detailed contribution of four satellite DNA motifs (Msat-160, Mth-Alu900, Mth-Alu2.2, and TTAGGG telomeric sequences) and two transposons on the molecular differentiation of the karyotype variants of Microtus thomasi/atticus species complex
Summary
Sex chromosomes evolved several times independently in different lineages. The evolutionary dynamics of the X and Y chromosomes are defined by specific features in terms of gene composition as well as in evolutionary rates. X chromosome gene content and order are extremely conserved between distantly related species of mammals [1], with only minor exceptions described in some rodents [2]. The Y chromosome has been losing most of its original gene content over evolutionary time, a well-known process named “Y chromosome degeneration” [3]. This explains the enormous variation in size, gene content, and structural complexity observed even in closed related species [4]. The Y chromosome is small, nearly full heterochromatic, and almost depleted of genes in the majority of mammal species
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