Structural and Evolutionary Relationships in the Giant Sex Chromosomes of Three Microtus Species

Abstract

The genus Microtus has high karyotypic diversity. The existence of notable differences in the length of its sex chromosomes contributes to this variation. Variations in size are attributed to the enlargement of their heterochromatin content, which is of such magnitude in some species that they are referred to as "giant sex chromosomes". Here, we perform an intra- and interspecific analysis of the molecular composition of the heterochromatic blocks in three species with giant sex chromosomes (Microtus chrotorrhinus, M. cabrerae and M. agrestis). Our results show that the heterochromatic content is very similar in both the X and Y chromosomes of M. chrotorrhinus, and that their molecular composition is more closely related to the heterochromatic blocks of M. agrestis than to the sex heterochromatin of M. cabrerae; however, species-specific differences do clearly exist. Interestingly, the euchromatic regions of the X chromosome of all three of these species share a homologous region composed of heterochromatic-related sequences. Our results therefore reinforce the idea that certain similarities in the original organization of these X chromosomes could have facilitated their later enlargement.

Keywords: Microtus; chromosome painting; giant sex chromosomes; repeated DNA sequences; sex chromosome heterochromatin.

Figure 1

Chromosome painting with the probe of the X chromosome heterochromatin (a–c) and of the whole Y chromosome (d–f) of M. chrotorrhinus on metaphases of the same species (a,d), of M. cabrerae (b,e) and of M. agrestis (c,f). Red colour probes hybridizations and blue colour DAPI (4′,6-Diamidine-2′-phenylindole dihydrochloride) staining; X and Y, denoted X and Y chromosomes respectively. Scale bars 4 µm.

Figure 2

Fluorescent in-situ hybridization (FISH) with different repeat DNA sequences, non-tandem repetitive sequence pMAHAE2 (a–c), L1 retrotransposon pMAECO14 (d–f), telomeric sequences (g–i), and satellite DNA Msat-160 (j–l). Metaphases of M. chrotorrhinus (a,d,g,j), M. cabrerae (b,e,h,k) and M. agrestis (c,f,i,l). The insert in e is the Y chromosome of another metaphase of M. cabrerae hybridized with the same probe. Green colour probes hybridizations and blue colour DAPI staining; X and Y, denoted X and Y chromosomes respectively. Scale bars 4 µm.