In this study, we examined for the first time the distribution of the 28S ribosomal genes in beetles of the subfamily Cassidinae. More than 55% of the species in this subfamily have a similar karyotype, 2n = 16 + Xyp. For this work, we selected species belonging to the tribes Cassidini and Mesomphaliini, which have, respectively, the most conserved and diversified karyotype characteristics within the Cassidinae. An analysis of 11 species revealed that rDNA sites on one pair of autosomes is the most frequent pattern, occurring in 10 species. This condition occurs in the seven genera examined and in species of both of the tribes, Cassidini and Mesomphaliini. Nevertheless, the differences in the locations of 28S rDNA were more pronounced in the tribe Cassidini and among species with similar karyotype characteristics. On the other hand, in Mesomphaliini, the increase in the diploid number was not accompanied by an increase in the number of ribosomal sites. Moreover, the comparison of the number and localization of major rDNA sites with the distribution of constitutive heterochromatin indicates that there is no direct correlation between the dispersion of constitutive heterochromatin and 28S rDNA genes in Cassidinae.
In the present work, we have characterized the chromosomes of 13 Cassidinae beetles, belonging to four tribes, the broad aim being to increase the cytogenetic data and establish the mechanisms involved in chromosome evolution of this subfamily, which appear to be conserved karyotypically, i.e. 2n = 16 + Xyp. The analysis of mitotic and meiotic cells revealed a high diversity of diploid numbers (2n = 18, 2n = 22, 2n = 26, 2n = 32, 2n = 36, 2n = 40, 2n = 42), and the presence of sex chromosome system of the Xyp type in most species, with the exception of two representatives that exhibited Xyr and XY systems. C-banding showed constitutive heterochromatin predominantly localized in the pericentromeric region of the chromosomes, but differences regarding the number of chromosomes with positive C-bands, intensity of the blocks, and presence of additional bands in autosomes and/or sex chromosomes were observed among the species investigated. Our data revealed that the karyotype 2n = 16 + Xyp does not occur in all 13 tribes of the Cassidinae characterized cytogenetically, seeming to be only a shared feature among the species of the Cassidini. Variations in the C-band pattern, mainly in closely related species, suggest that the interspecific karyotype diversification occurred as a result of changes in the quantity and distribution of constitutive heterochromatin. The occurrence of the Xyp sex chromosome system in the tribe Mesomphaliini, which showed the highest diversity of simple and multiple systems among the coleopteran as a whole, reinforces the view that derived systems originated by chromosome rearrangements involving the Xyp ancestral system., Amália T. Lopes, Flávia R. Fernandes, Marielle C. Schneider., and Obsahuje bibliografii