Heterochromatin, Sex Chromosomes and rRNA Gene Clusters in <b><i>Coprophanaeus</i></b> Beetles (Coleoptera, Scarabaeidae)

Abstract

Repetitive DNA sequences constitute a high fraction of eukaryotic genomes and are considered a key component for the chromosome and karyotype evolution. For a better understanding of their evolutionary role in beetles, we examined the chromosomes of 5 species of the genus Coprophanaeus by C-banding, fluorochrome staining CMA₃/DA/DAPI, and fluorescence in situ hybridization (FISH) with probes for 18S and 5S rRNA genes. The Coprophanaeus species have identical chromosome numbers and a conserved chromosome morphology. However, they show different sex chromosome forms, XY, Xy, XY_p, and heterochromatin seems to be involved in the origin and diversification of these forms. C-banding showed primarily the presence of diphasic chromosomes in all species examined. After CMA₃/DA/DAPI staining, 1–9 autosomal pairs showed CMA₃-positive blocks depending on the species, while DAPI-positive blocks were detected only in Coprophanaeusdardanus. FISH mapping revealed 5S rDNA signals in one autosomal pair in each species, whereas the number of pairs with 18S rDNA signals varied from 1–8 between the Coprophanaeus species. Our results suggest that distinct genetic mechanisms had been involved in the karyotype evolution of Coprophanaeus species, i.e. mechanisms maintaining the conserved number of 5S rDNA clusters and those generating variability in the amount of heterochromatin, sex chromosome forms, and distribution of 18S rDNA clusters.

Chromosome mapping of 28S ribosomal genes in 11 species of Cassidinae (Coleoptera: Chrysomelidae)

“…The species with interstitial and/or centromeric rDNA cistrons are mainly members of the families Scarabaeidae and Chrysomelidae (subfamily Alticinae). As recorded in the present study, the changes in the location of the sites of rDNA are described in species with similar karyotype characteristics (Virkki, 1983;Yadav et al, 1992;Bione et al, 2005;Almeida et al, 2006Almeida et al, , 2010Arcanjo et al, 2009Arcanjo et al, , 2013Silva et al, 2009;Cabral de Mello et al, 2010, 2011bOliveira et al, 2010Oliveira et al, , 2012b. The differences in the location of rDNA in closely related species with similar chromosomal characteristics may be a result of small chromosomal rearrangements, which change the position of the ribosomal site without modifying the metacentric chromosomal morphology.…”

“…Among the Polyphaga, most of the data on the distribution of rDNA is for the family Scarabaeidae, with more than 120 species characterized to date. The occurrence of two autosomal NORs is also widespread within this family, but closely related species do differ in the presence of a ribosomal cluster on more than one autosomal pair and/or on sex chromosomes (Colomba et al, 2006;Silva et al, 2009;Cabral-Mello et al, 2010, 2011aOliveira et al, 2010Oliveira et al, , 2012b.…”

Chromosomal diversification of diploid number, heterochromatin and rDNAs in two species of Phanaeus beetles (Scarabaeidae, Scarabaeinae)

“…This hypothesis is reinforced by the occurrence of two species in the tribe with 18S rDNA sites in one bivalent, Phanaeus ( N. ) chalcomelas , analyzed here, and Coprophanaeus ( Metallophanaeus ) pertyi (Oliveira et al , 2012). As observed for instance in the Coprophanaeus species (Oliveira et al , 2010, 2012) and P. ( N. ) splendidulus , the differences in number and location of major rDNA sites appear to be related to processes such as amplification/dispersion rather than to macro-chromosomal rearrangements like translocations.…”

“…Among the 20 species studied, three different diploid numbers (2n = 12, 19 and 20) and six sex chromosome mechanisms (Xy p , XY p , Xy, XY, X0 and neo-XY) were observed, contrasting with the majority of Scarabaeinae species that display the karyotype 2n = 20, Xy p , considered modal and primitive for the group (Cabral-de-Mello et al , 2008; Oliveira et al , 2010, 2012). So far, six representatives of genus Phanaeus (corresponding to 14.29% of all described species) were cytogenetically studied, four belonging to the subgenus Phanaeus s.str., and two to the subgenus Notiophanaeus .…”

“…Differential cytogenetic techniques, such as C-banding, base-specific fluorochrome staining, silver nitrate (AgNO 3 ) staining and fluorescent in situ hybridization (FISH) using major ribosomal DNA (45S rDNA) as probe, were applied only in nine Phanaeini representatives (Bione et al , 2005a; Oliveira et al , 2010, 2012; Cabral-de-Mello et al , 2011a). The use of these techniques has enabled a more detailed chromosomal analysis of different Scarabaeinae species, helping to identify major events involved in the karyotype evolution of the group (Bione et al , 2005a,b; Silva et al , 2009; Cabral-de-Mello et al , 2010a,b, 2011a).…”