Stable inversion clines in a grasshopper species group despite complex geographical history.

Chromosomal inversions are known to play roles in adaptation and differentiation in many species. They involve clusters of correlated genes (i.e., loci in linkage disequilibrium, LD) possibly associated with environmental variables. The grasshopper "species complex" Trimerotropis pallidipennis comprises several genetic lineages distributed from North to South America in arid and semi-arid high-altitude environments. The southernmost lineage, Trimerotropis sp., segregates for four to seven putative inversions that display clinal variation, possibly through adaptation to temperate environments. We analysed chromosomal, mitochondrial and genome-wide single nucleotide polymorphism data in 19 Trimerotropis sp. populations mainly distributed along two altitudinal gradients (MS and Ju). Populations across Argentina comprise two main chromosomally and genetically differentiated lineages: one distributed across the southernmost border of the "Andes Centrales," adding evidence for a differentiation hotspot in this area; and the other widely distributed in Argentina. Within the latter, network analytical approaches to LD found three clusters of correlated loci (LD-clusters), with inversion karyotypes explaining >79% of the genetic variation. Outlier loci associated with environmental variables mapped to two of these LD-clusters. Furthermore, despite the complex geographical history indicated by population genetic analyses, the clines in inversion karyotypes have remained stable for more than 20 generations, implicating their role in adaptation and differentiation within this lineage. We hypothesize that these clines could be the consequence of a coupling between extrinsic postzygotic barriers and spatially varying selection along environmental gradients resulting in a hybrid zone. These results provide a framework for future investigations about candidate genes implicated in rapid adaptation to new environments.

Phylogeny and chromosomal diversification in the Dichroplus elongatus species group (Orthoptera, Melanoplinae).

In an attempt to track the chromosomal differentiation in the Dichroplus elongatus species group, we analyzed the karyotypes of four species with classical cytogenetic and mapping several multigene families through fluorescent in situ hybridization (FISH). We improved the taxon sampling of the D. elongatus species group adding new molecular data to infer the phylogeny of the genus and reconstruct the karyotype evolution. Our molecular analyses recovered a fully resolved tree with no evidence for the monophyly of Dichroplus. However, we recovered several stable clades within the genus, including the D. elongatus species group, under the different strategies of tree analyses (Maximum Parsimony and Maximum Likelihood). The chromosomal data revealed minor variation in the D. elongatus species group's karyotypes caused by chromosome rearrangements compared to the phylogenetically related D. maculipennis species group. The karyotypes of D. intermedius and D. exilis described herein showed the standard characteristics found in most Dichroplini, 2n = 23/24, X0♂ XX♀, Fundamental number (FN) = 23/24. However, we noticed two established pericentric inversions in D. intermedius karyotype, raising the FN to 27♂/28♀. A strong variation in the heterochromatic blocks distribution was evidenced at interespecific level. The multigene families' mapping revealed significant variation, mainly in rDNA clusters. These variations are probably caused by micro chromosomal changes, such as movement of transposable elements (TEs) and ectopic recombination. These observations suggest a high genomic dynamism for these repetitive DNA sequences in related species. The reconstruction of the chromosome character "variation in the FN" posits the FN = 23/24 as the ancestral state, and it is hypothesized that variations due to pericentric inversions has arisen independently three times in the evolutionary history of Dichroplus. One of these independent events occurred in the D. elongatus species group, where D. intermedius is the unique case with the highest FN described in the tribe Dichroplini.

Neo-sex Chromosomes in the Maculipennis Species Group (Dichroplus: Acrididae, Melanoplinae): The Cases of D. maculipennis and D. vittigerum.

South American melanopline grasshoppers display a disproportionate number of derived karyotypes, including many cases of neo-sex chromosome systems. This is especially true of the genus Dichroplus and its Maculipennis species group. We analyzed the karyotype and neo-sex chromosomes in mitosis and meiosis of Dichroplus maculipennis and D. vittigerum from Argentina using conventional and fluorescent cytogenetic protocols in order to elucidate the behavior and origin of these neo-XY systems in relation to the current phylogeny of this group. Our results showed that D. maculipennis (2n = 22♂/22♀; neoXY/neoXX) and D. vittigerum, whose karyotype is described here for the first time (2n = 18♂/18♀; neoXY/neoXX), show highly evolved neo-XY systems, although with significant differences between them. Furthermore, both species differ for two autosomal fixed Robertsonian fusions present in D. vittigerum. Analysis of karyotypic character state optimization strongly suggests the independent origin and evolution of neo-sex systems within this species group.

Neo-sex chromosome diversity in Neotropical melanopline grasshoppers (Melanoplinae, Acrididae).

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.