Chromosomal evolution in giant water bugs

What is it about?

Telomeres are specialised nucleoprotein complexes localized at the ends of linear eukaryotic chromosomes. They maintain the stability and integrity of the chromosomes by protecting their ends from the action of exonucleases, end-to-end fusions, and gradual erosion during successive rounds of semi-conservative DNA replication. Besides keeping chromosome integrity, studies in several vertebrate species also suggest a potential role of telomeric repeats in karyotype evolution through additional intra-chromosomal sites, the so-called interstitial telomeric sequences (ITS). The giant water bugs Belostomatidae play an important role as biological agents in freshwater ecosystems because they are intermediate-stage predators in the food chain of their communities and are useful in the control of the most efficient vector species for malaria and dengue transmission, Aedes and Anopheles, given that they feed effectively on their larvae and pupae. To further explore the karyotype evolution in giant water bugs, we examined the presence and distribution of the TTAGG telomeric repeats and ITS in six Belostoma species by fluorescence in situ hybridization (FISH) with (TTAGG)n probes. The results obtained clearly support the hypothesis that the remarkable karyotype variability of Belostoma species was mainly caused by telomere-telomere fusions of the holokinetic chromosomes, which differentiated the karyotypes of extant species from a common ancestor with 2n = 26 + XY/XX.

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Why is it important?

We mapped the distribution of telomeric sequences and interstitial telomeric sequences (ITS) in Belostoma candidulum, B. dentatum, B. elegans, B. elongatum, B. micantulum, and B. oxyurum by FISH with the (TTAGG)n probes. Hybridization signals confirmed the presence of TTAGG repeats in the telomeres of all species examined. The three species with reduced chromosome numbers (B. candidulum, B. micantulum and B. oxyurum) showed additional hybridization signals in interstitial positions, indicating the occurrence of ITS. The ITS distribution between these closely related species supports the hypothesis that several telomere-telomere fusions of the chromosomes from an ancestral diploid chromosome number of 2n=26+XY/XX played a major role in the karyotype evolution of Belostoma. Consequently, our study provides valuable features that can be used to understand the karyotype evolution may contribute to a better understanding of taxonomic relationships and elucidate the high plasticity of nuclear genomes at the chromosomal level during the speciation processes.

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