Spreading of heterochromatin and karyotype differentiation in two Tropidacris Scudder, 1869 species (Orthoptera, Romaleidae)

Abstract

Tropidacris Scudder, 1869 is a genus widely distributed throughout the Neotropical region where speciation was probably promoted by forest reduction during the glacial and interglacial periods. There are no cytogenetic studies of Tropidacris, and information allowing inference or confirmation of the evolutionary events involved in speciation within the group is insufficient. In this paper, we used cytogenetic markers in two species, T. collaris (Stoll, 1813) and T. cristata grandis (Thunberg, 1824), collected in different Brazilian biomes. Both species exhibited 2n=24,XX for females and 2n=23,X0 for males. All chromosomes were acrocentric. There were some differences in the karyotype macrostructure, e.g. in the chromosome size. A wide interspecific variation in the chromosome banding (C-banding and CMA3/DAPI staining) indicated strong differences in the distribution of repetitive DNA sequences. Specifically, T. cristata grandis had a higher number of bands in relation to T. collaris. FISH with 18S rDNA revealed two markings coinciding with the NORs in both species. However, two analyzed samples of T. collaris revealed a heterozygous condition for the rDNA site of S10 pair. In T. collaris, the histone H3 genes were distributed on three chromosome pairs, whereas in T. cristata grandis, these genes were observed on 14 autosomes and on the X chromosome, always in terminal regions. Our results demonstrate that, although the chromosome number and morphology are conserved in the genus, T. cristata grandis substantially differs from T. collaris in terms of the distribution of repetitive sequences. The devastation and fragmentation of the Brazilian rainforest may have led to isolation between these species, and the spreading of these repetitive sequences could contribute to speciation within the genus.