Karyotype Evolution in Aeshna (Aeshnidae, Odonata)

On the Origin of Neo-Sex Chromosomes in the Neotropical Dragonflies Rhionaeschna bonariensis and R. planaltica (Aeshnidae, Odonata)

Odonata have holokinetic chromosomes. About 95% of species have an XX/X0 sex chromosome system, with heterogametic males. There are species with neo-XX/neo-XY sex chromosomes resulting from an X chromosome/autosome fusion. The genus Rhionaeschna includes 42 species found in the Americas. We analyzed the distribution of the nucleolar organizer region (NOR) using FISH with rDNA probes in Rhionaeschna bonariensis (n = 12 + neo-XY), R. planaltica (n = 7 + neo-XY), and Aeshna cyanea (n = 13 + X0). In R. bonariensis and A. cyanea, the NOR is located on a large pair of autosomes, which have a secondary constriction in the latter species. In R. planaltica, the NOR is located on the ancestral part of the neo-X chromosome. Meiotic analysis and FISH results in R. planaltica led to the conclusion that the neo-XY system arose by insertion of the ancestral X chromosome into an autosome. Genomic in situ hybridization, performed for the first time in Odonata, highlighted the entire neo-Y chromosome in meiosis of R. bonariensis, suggesting that it consists mainly of repetitive DNA. This feature and the terminal chiasma localization suggest an ancient origin of the neo-XY system. Our study provides new information on the origin and evolution of neo-sex chromosomes in Odonata, including new types of chromosomal rearrangements, NOR transposition, and heterochromatin accumulation.

A checklist of chromosome numbers and a review of karyotype variation in Odonata of the world

The ancient insect order Odonata is divided into three suborders: Anisoptera and Zygoptera with approximately 3000 species worldwide each, and Anisozygoptera with only four extant species in the relict family Epiophlebiidae. An updated list of Odonata species studied regarding chromosome number, sex chromosome mechanism and the occurrence of m-chromosomes (= microchromosomes) is given. Karyotypes of 607 species (198 genera, 23 families), covering approximately 10% of described species, are reported: 423 species (125 genera, 8 families) of the Anisoptera, 184 species (72 genera, 14 families) of the Zygoptera, and one species of the Anisozygoptera. Among the Odonata, sex determination mechanisms in males can be of X(0), XY and X1X2Y types, and diploid chromosome numbers can vary from 6 to 41, with a clear mode at 2n = 25(60%) and two more local modes at 2n = 27(21%) and 2n = 23(13%). The karyotype 2n = 25(24A + X) is found in each of the three suborders and is the most typical (modal) in many families, including the best-covered Libellulidae, Corduliidae (Anisoptera), Lestidae, Calopterygidae, and Platycnemididae (Zygoptera). This chromosome set is considered ancestral for the Odonata in general. Chromosome rearrangements, among which fusions and fissions most likely predominated, led to independent origins of similar karyotypes within different phylogenetic lineages of the order. The karyotype 2n = 27(26A + X) prevails in Aeshnidae and Coenagrionidae, whereas the karyotype 2n = 23(22A + X) is modal in Gomphidae and Chlorocyphidae, in both pairs of families one being from the Anisoptera while the other from the Zygoptera.

Cytogenetic characterization of Eurysternus caribaeus (Coleoptera: Scarabaeidae): evidence of sex-autosome fusion and diploid number reduction prior to species dispersion

Mitotic and meiotic chromosomes of several populations of Eurysternus caribaeus (Coleoptera: Scarabaeidae) were analysed through conventional staining, C-banding, base-specific fluorochromes, silver nitrate staining and fluorescent in situ hybridization (FISH). All specimens showed 2n = 8 in their karyotypes, with a neo-XY sex system (Y is a submetacentric and X a metacentric) and three pairs of submetacentric autosomes. The analysis of constitutive heterochromatin (CH) revealed small blocks located in the centromeric region of all chromosomes which do not present positive staining under the fluorochromes CMA3 and DAPI. Silver nitrate staining revealed that the nucleolar organizer region (NORs) is associated with the sex chromosomes. The FISH technique revealed that rDNA sites in the X and Y are different in size. Data from different populations indicate that the diploid number reduction (2n = 8) observed in E. caribaeus is established and presumably has preceded the dispersion of this species. Moreover, this reduction occasioned the translocation of rDNA sites to the sex chromosomes, X and Y, an uncommon pattern in Scarabaeidae that was observed for the first time by the FISH in this work.