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

SIMPLE SUMMARY: Dragonflies and damselflies (Odonata) are very interesting insects from a cytogenetic point of view. Their chromosomes do not have a typical centromere and their meiosis process differs in some respects from the canonical meiosis process. Sex in Odonata is usually determined by two X chromosomes in females and only one X chromosome in males (a Y chromosome is not present). In this work, we studied sex chromosome evolution in two dragonfly species of the genus Rhionaeschna that have a derived sex chromosome system: neo-XX in females and neo-XY in males. This variation is the result of chromosome rearrangements. In R. planaltica, meiotic analysis and fluorescence in situ hybridization with a ribosomal DNA probe revealed that the original X chromosome was inserted into the smallest autosome, giving rise to the neo-X chromosome, while the homologous autosome became a neo-Y chromosome. In contrast, the neo-X chromosome in R. bonariensis evolved by a terminal fusion of the original X chromosome with the largest autosome, whose homolog became the neo-Y chromosome. Our results suggest an independent origin of neo-sex chromosomes in these dragonfly species and contribute to our understanding of the distinct mechanisms of sex chromosome evolution. ABSTRACT: 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.