engleski

Evolution and organization of satellite DNAs in some insect species

Tandemly repetitive sequences or satellite DNAs are major components of heterochromatin in the eukaryotic genome. Although satellite sequences have been characterized in a number of species, no sequence specific role has been definitively established for them. They could be functional as a structural element in the nucleus influencing chromosome organization and centromere function by binding specific proteins. We have studied satellite DNA sequence evolution in insect species from family Tenebrionidae (Coleoptera). These species are characterized by a high amount of satellite DNAs, up to 50% of the genome. Major satellites building the species specific satellite profile in one species have highly homologous counterparts at low copy number in related species. The diversity of major satellites among a group of related species can be explained by differential amplification of unrelated sequences, which are in the form of master copies present in the genomes of related taxa. Studies of dynamics of satellite sequence change during evolution of species from the genus Pimelia (Tenebrionidae) revealed the nonconstant rate of sequence change. The periods of accelerated evolution alternate with the periods of sequence stability, probably as a consequence in changes in mutation rate and/or in a shift in concerted evolution mechanisms. The periods of satellite sequence stability can be very long like in the case of PRAT satellite DNA from the genus Palorus (Tenebrionidae) which remained unchanged for a period of around 60 Myr. The occurrence of identical satellite sequences among distant species is difficult to explain in terms of functional conservation over such a long time-span. It has been proposed that a small bias in gene conversion favouring the ancestral satellite variant during the process of concerted evolution would result in an extreme conservation of a repeat family. Recent horizontal transfer of PRAT satellite among different species could possibly be an alternative explanation. Whatever the mechanism, we suggest that highly homologous satellite DNAs shared by evolutionary distant species might be a widespread phenomenon, easily missed due to dramatic differences in copy number among species.

evolution; satellite DNA; heterochromatin

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