Chromosomal organization of the 18S and 5S rRNAs and histone H3 genes in Scarabaeinae coleopterans: insights into the evolutionary dynamics of multigene families and heterochromatin

Abstract

BackgroundScarabaeinae beetles show a high level of macro-chromosomal variability, although the karyotypic organization of heterochromatin and multigene families (rDNAs and histone genes) is poorly understood in this group. To better understand the chromosomal organization and evolution in this group, we analyzed the karyotypes, heterochromatin distribution and chromosomal locations of the rRNAs and histone H3 genes in beetles belonging to eight tribes from the Scarabaeinae subfamily (Coleoptera, Scarabaeidae).ResultsThe number of 18S rRNA gene (a member of the 45S rDNA unit) sites varied from one to 16 and were located on the autosomes, sex chromosomes or both, although two clusters were most common. Comparison of the 45S rDNA cluster number and the diploid numbers revealed a low correlation value. However, a comparison between the number of 45S rDNA sites per genome and the quantity of heterochromatin revealed (i) species presenting heterochromatin restricted to the centromeric/pericentromeric region that contained few rDNA sites and (ii) species with a high quantity of heterochromatin and a higher number of rDNA sites. In contrast to the high variability for heterochromatin and 45S rDNA cluster, the presence of two clusters (one bivalent cluster) co-located on autosomal chromosomes with the 5S rRNA and histone H3 genes was highly conserved.ConclusionsOur results indicate that the variability of the 45S rDNA chromosomal clusters is not associated with macro-chromosomal rearrangements but are instead related to the spread of heterochromatin. The data obtained also indicate that both heterochromatin and the 45S rDNA loci could be constrained by similar evolutionary forces regulating spreading in the distinct Scarabaeinae subfamily lineages. For the 5S rRNA and the histone H3 genes, a similar chromosomal organization could be attributed to their association/co-localization in the Scarabaeinae karyotypes. These data provide evidence that different evolutionary forces act at the heterochromatin and the 45S rDNA loci compared to the 5S rRNA and histone H3 genes during the evolution of the Scarabainae karyotypes.

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

“…The presence of only one chromosome bivalent carrying the 18S rDNA clusters is the ancient condition in Scarabaeinae [Cabral-de-Mello et al, 2011a] and in the order Coleoptera, at least for Polyphaga representatives [Schneider et al, 2007;Cabral-de-Mello et al, 2011a]. The presence of more than one chromosome bivalent carrying clusters of 18S rDNA was previously observed in C .…”

“…These large heterochromatic blocks indicate that the chromosomes of the species suffered an expansion of repetitive DNA, involving amplification by different spreading processes as described for vertebrates and other insects [Charlesworth et al, 1994;Hancock, 1999;Landais et al, 2000;Li et al, 2002]. All the Phanaeini species analyzed showed a high amount of heterochromatin [Bione et al, 2005a;Oliveira et al, 2010;Cabral-de-Mello et al, 2011a;present work] which could indicate that the amplification of heterochromatin occurred early during group diversification.…”

“…(Coprophanaeus) cyanescens and C . (Megaphanaeus) ensifer , with 5 and 15 sites (the largest number of 18S rDNA sites of the order Coleoptera), respectively [Oliveira et al, 2010;Cabral-de-Mello et al, 2011a]. Among the Coprophanaeus species studied in this work, only C .…”

“…Among the Coprophanaeus species studied in this work, only C . [Oliveira et al, 2010;Cabral-de-Mello et al, 2011a;present work]. Intraspecific polymorphisms, the repositioning of the rDNA sites, increasing numbers of rDNA sites, and the movement of rDNA sites to different autosomes and sex chromosomes were observed [Oliveira et al, 2010;Cabral-de-Mello et al, 2011a].…”

Chromosomal localization and partial sequencing of the 18S and 28S ribosomal genes from Bradysia hygida (Diptera: Sciaridae)

“…Virkki and Denton (1987) described NORs associated with sex chromosomes in different coleopteran species: Alagoasa bicolor, Omophoita cyanipennis, and Omophoita albicollis. In Diabroctis minas, NORs were detected in autosomal and sex chromosomes, whereas in Digitonthophagus gazella and Isocopris inhiata they were in autosomal chromosomes only, and in Eurystermus caribaeus, variability in the 18S rDNA cluster size was observed between the X and Z chromosomes (Bione et al, 2005;Cabral-de-Mello et al, 2011b).…”