Chromosomal distribution of interstitial telomeric sequences as signs of evolution through chromosome fusion in six species of the giant water bugs (Hemiptera, Belostoma)

Abstract

Tandem arrays of TTAGG repeats show a highly conserved location at the telomeres across the phylogenetic tree of arthropods. In giant water bugs Belostoma, the chromosome number changed during speciation by fragmentation of the single ancestral X chromosome, resulting in a multiple sex chromosome system. Several autosome-autosome fusions and a fusion between the sex chromosome pair and an autosome pair resulted in the reduced number in several species. We mapped the distribution of telomeric sequences and interstitial telomeric sequences (ITSs) in Belostoma candidulum (2n = 12 + XY/XX; male/female), B. dentatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. elegans (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. elongatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. micantulum (2n = 14 + XY/XX), and B. oxyurum (2n = 6 + XY/XX) 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 showed additional hybridization signals in interstitial positions, indicating the occurrence of ITS. From the comparison of all species here analyzed, we observed inverse relationships between chromosome number and chromosome size, and between presence/absence of ITS and chromosome number. 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 2n = 26 + XY/XX played a major role in the karyotype evolution of Belostoma. Consequently, our study provide valuable features that can be used to understand the karyotype evolution, may contribute to a better understanding of taxonomic relationships, and also elucidate the high plasticity of nuclear genomes at the chromosomal level during the speciation processes.

Keywords: chromosomal fusion; interstitial telomeric repeats; karyotype evolution; telomere FISH.

Figure 1

Belostoma giant water bugs from Argentina cytogenetically analyzed. Bar = 1 cm

Figure 2

FISH of the (TTAGG)_n telomere probe (red signals) in mitotic and meiotic chromosomes of six species of Belostoma counterstained with DAPI (blue). (a) Oogonial metaphase of B. dentatum, 2n = 26 + X₁X₁X₂X₂. (b) Male diakinesis of B. elegans, 2n = 26 + X₁X₂Y, n = 13 + X₁X₂Y. (c) Spermatogonial metaphase of B. elongatum, 2n = 26 + X₁X₂Y. (d) Spermatogonial prometaphase of B. candidulum, 2n = 12 + XY. (e) Spermatogonial prometaphase of B. micantulum, 2n = 14 + XY. (f) Spermatogonial prometaphase of B. oxyurum, 2n = 6 + XY. (g) Male diakinesis of B. candidulum, n = 6 + XY. (h) Male diakinesis/metaphase I of B. micantulum, n = 7 + XY. (i) Male metaphase I of B. oxyurum, n = 3 + XY. N: nucleolus. X, Y: sex chromosomes. Arrows show the secondary constriction in the X chromosomes. Arrowheads indicate the interstitial hybridization signals. Asterisks in schematic drawings of selected bivalents in B, G, H, and I indicate positions of chiasmata and red dots positions of telomeric and ITS signals. Bar = 10 μm

Figure 3

Hypothetical pattern of the karyotype evolution in the genus Belostoma. (a) Ancestral diploid karyotype (2n = 26 + XY/XX). (b) Karyotype of B. dentatum, B. elegans, and B. elongatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂). (c) Karyotype of B. micantulum and B. candidulum from Brazil (2n = 14 + XY/XX). (d) Karyotype of B. candidulum from Argentina (2n = 12 + XY/XX). (e) Karyotype of B. oxyurum (2n = 6 + XY/XX). Interstitial telomeric sequences (ITS, green) detected by FISH represent remnants of true telomeres (red) as a result of end‐to‐end chromosome fusion