B chromosomes showing active ribosomal RNA genes contribute insignificant amounts of rRNA in the grasshopper Eyprepocnemis plorans

Abstract

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 (ITS2(A+B)) and the other being specific to B chromosomes (ITS2(B)). We analyzed six body parts in nine males showing rDNA expression in their B chromosomes in the testis. Amplification of the ITS2(B) 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 ITS(A+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.

Uncovering the Ancestry of B Chromosomes in Moenkhausia sanctaefilomenae (Teleostei, Characidae)

“…Remarkably, MS3 and MS7 nucleotide diversity is lower in B 1 , perhaps because of its higher possibility of sequence homogenization due to its greater age and population frequency. In the grasshopper Eyprepocnemis plorans , males with two or more B chromosomes form chiasmated B-bivalents during meiosis [ 59 ], which helps to explain the observed variation in the amount of distally located 45S ribosomal DNA [ 60 ]. Likewise, in M .…”

Genomic Characterization of a B Chromosome in Lake Malawi Cichlid Fishes

“…The C-KIT gene in two Canidae species [24], rRNA genes and thousands of genes and gene fragments in the fish Astatotilapia latifasciata [12,25], and protein coding genes in the grasshopper Eyprepocnemis plorans [26] are examples of genic sequences detected on B chromosomes. Furthermore, transcription has been characterized for an rRNA gene in the smooth hawksbeard Crepis capillaris [27], rRNA genes and a pseudogene in the grasshopper E. plorans [28,29,30], pseudogenes in rye Secale cereale [31], and protein coding genes in maize Zea mays [32,33].…”

Rye B chromosomes encode a functional Argonaute‐like protein with in vitro slicer activities similar to its A chromosome paralog

“…Regardless of the conceivable intra‐ or interspecies origin of Bs, A‐ or sex chromosome‐derived sequences (including genic sequences) are potential components of any B, for example, the inactive ribosomal RNA (rRNA) genes of the daisy Brachycome dichromosomatica (Donald et al ., ), the Chinese raccoon dog Nyctereutes procyonoides (Szczerbal & Switonski, ) and the African cichlid fish Haplochromis obliquidens (Poletto et al ., ). Active B‐derived rRNA has been demonstrated in Crepis capillaris (Leach et al ., ) and the grasshopper Eyprepocnemis plorans (Ruiz‐Estévez et al ., , ).…”