Preferential Occupancy of R2 Retroelements on the B Chromosomes of the Grasshopper Eyprepocnemis plorans

Abstract

R2 non-LTR retrotransposons exclusively insert into the 28S rRNA genes of their host, and are expressed by co-transcription with the rDNA unit. The grasshopper Eyprepocnemis plorans contains transcribed rDNA clusters on most of its A chromosomes, as well as non-transcribed rDNA clusters on the parasitic B chromosomes found in many populations. Here the structure of the E. plorans R2 element, its abundance relative to the number of rDNA units and its retrotransposition activity were determined. Animals screened from five populations contained on average over 12,000 rDNA units on their A chromosomes, but surprisingly only about 100 R2 elements. Monitoring the patterns of R2 insertions in individuals from these populations revealed only low levels of retrotransposition. The low rates of R2 insertion observed in E. plorans differ from the high levels of R2 insertion previously observed in insect species that have many fewer rDNA units. It is proposed that high levels of R2 are strongly selected against in E. plorans, because the rDNA transcription machinery in this species is unable to differentiate between R2-inserted and uninserted units. The B chromosomes of E. plorans contain an additional 7,000 to 15,000 rDNA units, but in contrast to the A chromosomes, from 150 to over 1,500 R2 elements. The higher concentration of R2 in the inactive B chromosomes rDNA clusters suggests these chromosomes can act as a sink for R2 insertions thus further reducing the level of insertions on the A chromosomes. These studies suggest an interesting evolutionary relationship between the parasitic B chromosomes and R2 elements.

Non-random expression of ribosomal DNA units in a grasshopper showing high intragenomic variation for the ITS2 region

“…Interestingly, in the E. plorans genome, R2 retroelements are preferentially located in B chromosomes, but Bs actually constitute a sink for R2 since the rDNA of the B chromosomes is rarely active (Montiel et al ., ) and, as shown here, the B‐specific ITS2 haplotype (Hap4) is highly repressed. The fact that the few Hap4 copies escaping silencing (thus observed in the cDNA) show flanking coding regions completely conserved suggests that the genome is extremely careful about which rDNA copies are active, especially in a genome with so much rDNA which only needs to express a small proportion of it.…”

“…This implied almost 4× coverage in the gDNA as the 0B males (i.e. those lacking B chromosomes) of this population carry ∼15 000 rDNA units and the B chromosome carries ∼3000 units (Montiel et al ., ).…”

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

“…Bearing also in mind that B chromosomes in this species can carry as many rDNA units as all A chromosomes together (Montiel et al 2014), we can conclude that the effect of rRNA contribution by the B chromosome is only residual and of minor significance. This conclusion is valid for males, which were the subject of the current experiments, and it is consistent with the fact that B chromosomes in E. plorans are near-neutral with no significant effects on male fitness (Camacho et al 1997).…”

Integration, Regulation, and Long-Term Stability of R2 Retrotransposons

“…Interestingly, variation in the ability of an organism to transcribe a region of the rDNA locus free of R2 insertions can explain why species have very different percentages of their rDNA units inserted with R2 (10,27,31,121,122). If an organism is less capable at differentiating between inserted and uninserted rDNA units, than all R2 inserts are potentially harmful and will be selected against.…”