Abstract
BackgroundTo contribute to the knowledge of coleopteran cytogenetics, especially with respect to the genomic content of B chromosomes, we analyzed the composition and organization of repetitive DNA sequences in the Coprophanaeus cyanescens karyotype. We used conventional staining and the application of fluorescence in situ hybridization (FISH) mapping using as probes C0t-1 DNA fraction, the 18S and 5S rRNA genes, and the LOA-like non-LTR transposable element (TE).ResultsThe conventional analysis detected 3 individuals (among 50 analyzed) carrying one small metacentric and mitotically unstable B chromosome. The FISH analysis revealed a pericentromeric block of C0t-1 DNA in the B chromosome but no 18S or 5S rDNA clusters in this extra element. Using the LOA-like TE probe, the FISH analysis revealed large pericentromeric blocks in eight autosomal bivalents and in the B chromosome, and a pericentromeric block extending to the short arm in one autosomal pair. No positive hybridization signal was observed for the LOA-like element in the sex chromosomes.ConclusionsThe results indicate that the origin of the B chromosome is associated with the autosomal elements, as demonstrated by the hybridization with C0t-1 DNA and the LOA-like TE. The present study is the first report on the cytogenetic mapping of a TE in coleopteran chromosomes. These TEs could have been involved in the origin and evolution of the B chromosome in C. cyanescens.
Highlights
To contribute to the knowledge of coleopteran cytogenetics, especially with respect to the genomic content of B chromosomes, we analyzed the composition and organization of repetitive DNA sequences in the Coprophanaeus cyanescens karyotype
The standard karyotype observed in C. cyanescens was 2n = 20, XYp (“p” refers to a “parachute” meiotic conformation between the X and Y), with meta-submetacentric chromosomes that showed a gradual reduction in size (Figure 1a)
The 18S ribosomal DNA (rDNA) clusters were observed at nine sites, and the 5S rDNA clusters were observed at two sites (Figure 1c)
Summary
To contribute to the knowledge of coleopteran cytogenetics, especially with respect to the genomic content of B chromosomes, we analyzed the composition and organization of repetitive DNA sequences in the Coprophanaeus cyanescens karyotype. We used conventional staining and the application of fluorescence in situ hybridization (FISH) mapping using as probes C0t-1 DNA fraction, the 18S and 5S rRNA genes, and the LOA-like non-LTR transposable element (TE). Eukaryote genomes are composed of classical genes and genetic elements, including transposable elements (TEs), B chromosomes and several cytoplasmic factors that do not follow Mendelian laws of inheritance [1]. B chromosomes have been observed in approximately 15% of living species [1,2,3,4]. Most B chromosomes are heterochromatic and composed of repetitive DNA sequences, supporting the idea that these chromosomes are non-coding.
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