Cytogenetic analysis of two Coprophanaeus species (Scarabaeidae) revealing wide constitutive heterochromatin variability and the largest number of 45S rDNA sites among Coleoptera

The Coleopterans of Scarabaeinae clade presents Coprophanaeus (Megaphanaeus) ensifer and C. (Coprophanaeus) cyanescens (Scarabaeidae) when they are studied cytogenetically by different techniques. The species present symmetric karyotypes, diploid number of 2n = 20, and meta-submetacentric chromosomes. C. (M.) ensifer present an XY sex-determining mechanism and C. (C.) cyanescens an XYp parachute mechanism. Analysis of constitutive heterochromatin (CH) in the two species revealed the presence of diphasic autosomes, with log arm heterochromatics. Moreover, an additional heterochromatic block in four autosomal bivalents were observed in C. (M.) ensifer. CMA3/DA/DAPI fluorochrome staining detected CMA3 positive heterochromatic blocks restricted to the sex chromosomes in C. (C.) cyanescens, whereas in C. (M.) ensifer CMA3 positive pericentromeric blocks were present in all autosomes, in the Y chromosome and in the four additional heterochromatic blocks. DAPI staining was neutral in both species. Silver nitrate (AgNO3) staining was inefficient for the detection of the nucleolar organizer region (NORs), but showed affinity for the heterochromatic regions. Fluorescence in situ hybridization (FISH) revealed the presence of 45S rDNA sites in the terminal region of the three autosomal bivalents of C. (C.) cyanescens and in seven bivalents and the Y chromosome of C. (M.) ensifer. These results contribute to a better understanding of chromosome evolution in the genus Coprophanaeus, and demonstrate a wide CH variability and the largest number of ribosomal sites among Coleoptera.