An unusual karyotype with low chromosome number in Megacephalini, a basal group of tiger beetles (Coleoptera, Cicindelidae): cytogenetic characterisation by C-banding and location of rDNA genes

Abstract

Mitotic and meiotic chromosomes of Megacephala brasiliensis, a representative of the tribe Megacephalini-a basal group in the family Cicindelidae-were studied using conventional Giemsa-staining, C-banding and fluorescence in-situ hybridization (FISH) with 18S rDNA probe. The diploid chromosome number was 2n=12 and the karyotype contained five pairs of metacentric and submetacentric autosomes and one pair of sex chromosomes, XY and XX in male and female, respectively. The X chromosome was metacentric while the Y was subtelocentric. All autosomes and the X showed distinct, large C-positive blocks around the centromere indicating extensive distribution of constitutive heterochromatin. The Y chromosome showed a conspicuous large C-positive block in the subterminal region as well as a weak interstitial band. FISH with a rDNA probe gave consistently positive signals in the pericentromeric region of the fourth pair of chromosomes. Size polymorphism of this rDNA-positive site was always observed. The karyotype of this beetle exhibits the smallest 2n known for any tiger beetle of the genus Megacephala and of other basal groups of the family though it shares single sex chromosome system with them. The extreme reduction in the autosome number appears to be the result of massive and combined rearrangenments of various types. The discovered case might parallel the famous “Muntjac scandal” in mammal cytogenetics.