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.

Chromosome evolution in tiger beetles: Karyotypes and localization of 18S rDNA loci in Neotropical Megacephalini (Coleoptera, Cicindelidae)

“…We found that Megacephala species have higher and more variable number of autosomal pairs as compared to the Cicindelini with most Megacephala species having from 12 to 15 autosomal pairs, although Proença et al (2002b) found an unusually low number of 5 pairs for M. brasiliensis which, however, was attributed to multiple Robertsonian fusions between the autosomes that might have occurred secondarily in the evolutionary process. This variability in autosome numbers is also reflected by our observation of aneuploidy in M. rutilans.…”

“…It thus appears that there is a trend to even smaller numbers of rDNA loci in the Cicindelidae, which accompanied the reduction in the number of autosomes. One exception to this model is the description of only two autosomal clusters in Megacephala brasiliensis, attributable to the unusually small number of autosomal pairs (Proença et al, 2002b). Recently, the localization of rDNA clusters in Cicindela littoralis and Cicindela flexuosa revealed the presence of interpopulation polymorphisms regarding the number of chromosomes with ribosomal genes and their localization within the genome (Proença and Galián, 2003).…”

“…The most primitive Cicindelidae tribes are Manticorini, Omini and Megacephalini, all of which seem to be characterized by the presence of simple sex chromosome mechanisms of the XY or X0 types (Pearson and Vogler, 2001;Galián et al, 2002;Proença et al, 2002b). On the other hand, Cicindelidae species belonging to the Collyrini, Ctenostomini and Cicindelini tribes have been described as having a diverse multiple sex chromosome system, XnY, where n varies from two to four (Serrano and Galián, 1998;Galián and Hudson, 1999;Proença et al, 1999aProença et al, , b, 2002aProença et al, , 2004Galián et al, 2002;Zacaro et al, 2004).…”

Cytogenetic variability in three species of the genus Cicindela (s.l.) (Coleoptera, Cicindelidae): karyotypes and localization of 18S rDNA genes

“…The most primitive tribes Manticorini, Omini and Megacephalini seem to be characterized by the presence of simple sex chromosome mechanisms of the X0 or XY types (Pearson and Vogler, 2001;Galián et al, 2002;Proença et al, 2002b). Multiple sex chromosome systems, XnY (where n varies from two to four) have been described in the Cicindelini and Collyrini tribes and were considered an apomorphy of a large clade that includes these two tribes (Galian et al, 2002).…”

New contributions to the cytotaxonomy of tiger beetles (Coleop-tera, Cicindelidae) from the Afrotropical Region: cytogenetic characterization of Prothyma concinna, Elliptica lugubris and Ropaloteres cinctus

“…Although the mechanisms involved in the chromosome evolution of cicindelids and in the development of the multiple sex systems remain an open question, Robertsonian fusions have been proposed to explain major autosome reduction in some species (Proença et al 2002a).…”