engleski

What a difference "junk" DNAs make...

The sibling species Tribolium audax and T. madens (Insecta, Coleoptera) greatly resemble each other to the point that for a long time they were classified as the single species T. madens. There are, however, some slight differences, and the two species produce a few sterile hybrids. Satellite DNAs are highly abundant sequences, tandemly repeated in the (peri)centromeric/telomeric regions of constitutive heterochromatin. Since no unambiguous function was assigned to them, for decades they were proclaimed to be a useless "junk" DNA. Nowadays their importance in chromosomal organization and centromere function is becoming more and more evident. Analyzing satellite DNAs in the genome of the beetle T. audax, we have discovered the two abundant repetitive DNA families, named TAUD1 and TAUD2. TAUD1 is a typical centromeric satellite DNA, composed of 112 bp repetitive units, which comprises 40% of the genome. 19% of the whole genome belongs to the TAUD2 sequences, which are interspersed sporadically among the arrays of tandemly repeated TAUD1 units. The complex TAUD2 sequence, 1412 bp long, consists of the two unusually long, reversely oriented segments, which could provoke formation of a thermodynamically stable dyad structure. Putative dyad structures could considerably contribute to the compact state and stability of the centromeric heterochromatin. Interestingly, the TAUD1 repetitive unit can be recognized as the building element of the TAUD2 family. Even more surprisingly, the TAUD1 and TAUD2 repetitive families show remarkable similarity to the two satellite DNAs detected previously in the centromeric heterochromatin on all chromosomes of T. madens. Despite certain differences in nucleotide sequence and substructure of repetitive units, the satellite DNAs of the two sibling species derived undoubtedly from a common ancestral satellite family. Concerning the portion of the genome that these "junk" DNAs comprise, an acquisition of changes in the satellite DNAs could result in genetically reproductive barriers, leading thus to the speciation process. In other words, it is highly probable that satellite DNAs played a role in the evolutionary separation of T. audax and T. madens. Furthermore, in the case of these morphologically hardly distinguishable species, satellite DNAs can be used as a reliable discriminative marker.

repetitive sequences; centromeric heterochromatin; concerted evolution; inverted repeats

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