Molecular and Physical Characterization of the Complex Pericentromeric Heterochromatin of the Vole Species Microtus thomasi

Abstract

A new repeated DNA from Microtus thomasi, Mth-Alu2.2, was cloned and characterized and is presented here for the first time. Digestion of genomic DNA from M. thomasi with AluI restriction enzyme revealed a 2.2-kb repetitive DNA sequence with a high AT content (69%). This sequence consists of a tandemly repeated nonanucleotide of the consensus sequence CACAATGTA, which constitutes approximately 93-95% of the total unit length. The location of the Mth-Alu2.2 sequence in the karyotype was determined by FISH, demonstrating strong hybridization signals in the pericentromeric regions of all chromosomes and in the heterochromatin blocks of several X chromosome variants. In addition, the distribution of the 4 pericentromeric repeat sequences Msat-160, Mth-Alu900, Mth-Alu2.2, and interstitial telomeric repeats was analyzed by in situ hybridization in M. thomasi, in order to shed light on the complex composition of the chromosomal pericentromeric regions in this species. The order and organization of these sequences in the pericentromeric regions are conserved, with slight variations in both the degree of overlapping and the amount of each repeated DNA in the chromosomes. Specifically, Mth-Alu2.2 is localized in the terminal regions of the chromosomes, with Msat-160 occupying the immediately inner region, partially intermixed with Mth-Alu2.2. The sequence Mth-Alu900 is found in internal positions below Msat-160, and the interstitial telomeric repeats are located close to the long-arm euchromatin of the chromosomes.

Extensive Sex Chromosome Polymorphism of Microtus thomasi/Microtus atticus Species Complex Associated with Cryptic Chromosomal Rearrangements and Independent Accumulation of Heterochromatin

“…In particular, 4 repetitive DNA sequences located on the pericentromeric chromosomal regions have been characterized in M. thomasi : Msat-160 [Acosta et al, 2010], Mth-Alu900 [Acosta et al, 2009], Mth-Alu2.2 [Rovatsos et al, 2014], and interstitial telomeric sequences [Rovatsos et al, 2011b]. In sex chromosomes, the highly repetitive satellite sequence Mth-Alu900 was located in the pericentromeric heterochromatin but not in the proximal or interstitial C-bands of the acrocentric X chromosome in this species.…”

“…Nine chromosomal races in M. thomasi and 2 in M. atticus were described [Giagia-Athanasopoulou and Stamatopoulos, 1997;Mitsainas et al, 2009;Rovatsos et al, 2011aRovatsos et al, , 2014Rovatsos and Giagia-Athanasopoulou, 2012]. The karyotype of the chromosomal race assigned as M. thomasi 'thomasi' with all chromosomes being acrocentric (2n = 44, FN = 44) was considered to represent a likely ancestral karyotype for this species complex [Giagia-Athanasopoulou and Stamatopoulos, 1997].…”

“…The karyotype of the chromosomal race assigned as M. thomasi 'thomasi' with all chromosomes being acrocentric (2n = 44, FN = 44) was considered to represent a likely ancestral karyotype for this species complex [Giagia-Athanasopoulou and Stamatopoulos, 1997]. The variation in chromosome number among chromosomal races can be probably attributed to Robertsonian and tandem fusions [Giagia-Athanasopoulou et al, 1995;Giagia and Ondrias, 1973;Giagia, 1985;Rovatsos et al, 2011aRovatsos et al, , 2014Rovatsos and Giagia-Athanasopoulou, 2012].…”

“…C-banding technique demonstrated a significant variation in the amount of pericentromeric heterochromatin and in the size and position of heterochromatic bands or blocks. Nine different X chromosomes have been described with either acrocentric (X 0 -X 5 ) or subtelocentric (X st0 -X st2 ) morphology and variable amounts of heterochromatin [Acosta et al, 2009;Mitsainas et al, 2009;Rovatsos et al, 2011aRovatsos et al, , 2014. Similarly, 8 Y chromosome variants exist, either acrocentric (Y 0 -Y 4 ) or metacentric (Y m and 2 Y m1 ); all of them are almost completely heterochromatic and also highly variable in size [Acosta et al, 2009;Mitsainas et al, 2009;Rovatsos et al, 2011a;Rovatsos and Giagia-Athanasopoulou, 2012].…”

“…These investigations have characterized different families of complex repeated DNAs, satellite DNAs, retroelements, and pseudogenes as components of the sex chromosome heterochromatin [Marchal et al, 2003[Marchal et al, , 2004b[Marchal et al, , 2006Acosta et al, 2008Acosta et al, , 2009Rovatsos et al, 2014]. Many of these sequences are significantly accumulated in the heterochromatic blocks of the giant sex chromosomes and display patterns of distribution, organization, and amplification that are highly variable within species.…”