Abstract
BackgroundThe accumulation of repetitive DNA during sex chromosome differentiation is a common feature of many eukaryotes and becomes more evident after recombination has been restricted or abolished. The accumulated repetitive sequences include multigene families, microsatellites, satellite DNAs and mobile elements, all of which are important for the structural remodeling of heterochromatin. In grasshoppers, derived sex chromosome systems, such as neo-XY♂/XX♀ and neo-X1X2Y♂/X1X1X2X2♀, are frequently observed in the Melanoplinae subfamily. However, no studies concerning the evolution of sex chromosomes in Melanoplinae have addressed the role of the repetitive DNA sequences. To further investigate the evolution of sex chromosomes in grasshoppers, we used classical cytogenetic and FISH analyses to examine the repetitive DNA sequences in six phylogenetically related Melanoplinae species with X0♂/XX♀, neo-XY♂/XX♀ and neo-X1X2Y♂/X1X1X2X2♀ sex chromosome systems.ResultsOur data indicate a non-spreading of heterochromatic blocks and pool of repetitive DNAs (C0t-1 DNA) in the sex chromosomes; however, the spreading of multigene families among the neo-sex chromosomes of Eurotettix and Dichromatos was remarkable, particularly for 5S rDNA. In autosomes, FISH mapping of multigene families revealed distinct patterns of chromosomal organization at the intra- and intergenomic levels.ConclusionsThese results suggest a common origin and subsequent differential accumulation of repetitive DNAs in the sex chromosomes of Dichromatos and an independent origin of the sex chromosomes of the neo-XY and neo-X1X2Y systems. Our data indicate a possible role for repetitive DNAs in the diversification of sex chromosome systems in grasshoppers.
Highlights
The accumulation of repetitive DNA during sex chromosome differentiation is a common feature of many eukaryotes and becomes more evident after recombination has been restricted or abolished
We focused mainly on the dynamics of repetitive DNA incorporation into new sex chromosomes as an evolutionary force that contributes to the chromosomal diversification of this group, and we examined the evidence for independent or common origins of the neosex chromosome systems in the analyzed species
Meiosis and karyotypes Different diploid numbers were observed in the six species studied: 2n=23♂/24♀ in Chlorus vittatus and Eurotettix brevicerci, 2n=19♂/20♀ in Ch. chiquitensis, 2n=22♂/22♀ in E. minor and 2n=21♂/22♀ in Dichromatos lilloanus and D. schrottkyi (Figure 1; Table 1)
Summary
The accumulation of repetitive DNA during sex chromosome differentiation is a common feature of many eukaryotes and becomes more evident after recombination has been restricted or abolished. Based on evidence obtained from molecular studies in different taxa, DNA sequence restructuring occurs within new sex chromosome regions (Y or W) during the early evolution of the sex chromosomes, and this process involves modifications to the chromatin structure and the insertion of repetitive DNA sequences. These morphological and genetic changes are consistent with the abolition of recombination, which precedes the genetic degeneration of neo-Y or neo-W chromosomes with unknown fates [1,15,16,17,18,19,20,21]. The accumulation of repetitive sequences, even in young sex chromosomes, has been observed in other organisms, such as Drosophila miranda [19], Silene latifolia [29,30,31] and Rumex acetosa [23]
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