The genetic inertness of supernumerary (B) chromosomes has recently been called into question after finding several cases of gene activity on them. The grasshopper Eyprepocnemis plorans harbors B chromosomes containing large amounts of ribosomal DNA (rDNA) units, some of which are eventually active, but the amount of rRNA transcripts contributed by B chromosomes, compared to those of the standard (A) chromosomes, is unknown. Here, we address this question by means of quantitative PCR (qPCR) for two different ITS2 amplicons, one coming from rDNA units located in both A and B chromosomes (ITS2A+B) and the other being specific to B chromosomes (ITS2B). We analyzed six body parts in nine males showing rDNA expression in their B chromosomes in the testis. Amplification of the ITS2B amplicon was successful in RNA extracted from all six body parts analyzed, but showed relative quantification (RQ) values four orders of magnitude lower than those obtained for the ITSA+B amplicon. RQ values differed significantly between body parts for the two amplicons, with testis, accessory gland and wing muscle showing threefold higher values than head, gastric cecum and hind leg. We conclude that the level of B-specific rDNA expression is extremely low even in individuals where B chromosome rDNA is not completely silenced. Bearing in mind that B chromosomes carry the largest rDNA cluster in the E. plorans genome, we also infer that the relative contribution of B chromosome rRNA genes to ribosome biogenesis is insignificant, at least in the body parts analyzed.
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Acknowledgments We thank an anonymous reviewer for helpful suggestions that substantially improved the manuscript, and Karl Meunier for language revision. This study was supported by a grant from the Spanish Ministerio de Ciencia e Innovación (CGL2009-11917) and Plan Andaluz de Investigación (CVI-6649), and was partially performed by FEDER funds. M Ruíz-Estévez was supported by a FPU fellowship from the Spanish Ministerio de Ciencia e Inno-vación. The authors also gratefully acknowledge the KU Leuven Research Foundation (GOA/11/02) and the Research Foundation of Flanders (Belgium) for financial support.
- B chromosome
- Gene expression
- rRNA genes