Project description:B-chromosomes are frequently argued to be genetically inert elements, but activity for some particular genes has been reported, especially for ribosomal RNA (rRNA) genes whose expression can easily be detected at the cytological level by the visualization of their phenotypic expression, i.e., the nucleolus. The B(24) chromosome in the grasshopper Eyprepocnemis plorans frequently shows a nucleolus attached to it during meiotic prophase I. Here we show the presence of rRNA transcripts that unequivocally came from the B(24) chromosome. To detect these transcripts, we designed primers specifically anchoring at the ITS-2 region, so that the reverse primer was complementary to the B chromosome DNA sequence including a differential adenine insertion being absent in the ITS2 of A chromosomes. PCR analysis carried out on genomic DNA showed amplification in B-carrying males but not in B-lacking ones. PCR analyses performed on complementary DNA showed amplification in about half of B-carrying males. Joint cytological and molecular analysis performed on 34 B-carrying males showed a close correspondence between the presence of B-specific transcripts and of nucleoli attached to the B chromosome. In addition, the molecular analysis revealed activity of the B chromosome rDNA in 10 out of the 13 B-carrying females analysed. Our results suggest that the nucleoli attached to B chromosomes are actively formed by expression of the rDNA carried by them, and not by recruitment of nucleolar materials formed in A chromosome nucleolar organizing regions. Therefore, B-chromosome rDNA in E. plorans is functional since it is actively transcribed to form the nucleolus attached to the B chromosome. This demonstrates that some heterochromatic B chromosomes can harbour functional genes.
Project description:For many years, parasitic B chromosomes have been considered genetically inert elements. Here we show the presence of ten protein-coding genes in the B chromosome of the grasshopper Eyprepocnemis plorans. Four of these genes (CIP2A, GTPB6, KIF20A, and MTG1) were complete in the B chromosome whereas the six remaining (CKAP2, CAP-G, HYI, MYCB2, SLIT and TOP2A) were truncated. Five of these genes (CIP2A, CKAP2, CAP-G, KIF20A, and MYCB2) were significantly up-regulated in B-carrying individuals, as expected if they were actively transcribed from the B chromosome. This conclusion is supported by three truncated genes (CKAP2, CAP-G and MYCB2) which showed up-regulation only in the regions being present in the B chromosome. Our results indicate that B chromosomes are not so silenced as was hitherto believed. Interestingly, the five active genes in the B chromosome code for functions related with cell division, which is the main arena where B chromosome destiny is played. This suggests that B chromosome evolutionary success can lie on its gene content.