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.
We report the results of a study on the neo-sex chromosome systems of six Neotropical Melanoplinae species for contributing to a better understanding of their origin and behaviour of these systems. Our analyses included detailed descriptions of the structure and behaviour of the sex chromosome configurations in male and female meiosis of species belonging to the genera Ronderosia, Dichromatos and Atrachelacris. Three species, R. forcipatus, R. malloi and A. unicolor, showed typical Robertsonian fusion-derived neo sex-chromosomes. However, the male metaphase I orientation of R. bergi sex pair indicated that more than one rearrangement was involved in its origin. The two species of Dichromatos presented a multiple neo-X(1)X(2)Y/X(1)X(1)X(2)X(2) sex system, with two Robertsonian fusions involved in their genesis. Observations of female meiosis, confirmed the nature of the sex-chromosomes analyzed. Our results also showed different degrees of homology divergence between the neo-sex chromosomes and emphasize the plasticity of the chromosome complement of the Neotropical Melanoplinae to establish Robertsonian fusions and generate novel sex-chromosome systems. We also discuss karyotypic diversity within this group in terms of the centromeric drive theory of chromosomal evolution.
We review the effects of abiotic factors on body size in two grasshopper species with large geographical distributions: Dichroplus pratensis and D. vittatus, inhabiting Argentina in diverse natural habitats. Geographical spans for both species provide an opportunity to study the effects of changes in abiotic factors on body size. The analyses of body size distribution in both species revealed a converse Bergmannian pattern: body size is positively correlated with latitude, altitude, and seasonality that influences time available for development and growth. Allen's rule is also inverted. Morphological variability increases towards the ends of the Bergmannian clines and, in D. pratensis, is related with a central-marginal distribution of chromosomal variants that influence recombination. The converse Bergmannian patterns influence sexual size dimorphism in both species but in different fashions. Body size variation at a microspatial scale in D. pratensis is extremely sensitive to microclimatic clines. We finally compare our results with those for other Orthopteran species.
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