Generating high variability of B chromosomes in Eyprepocnemis plorans (grasshopper)

Abstract

Twenty-eight progeny analyses (PAs) performed on specimens of E. plorans collected from four natural Iberian populations have been informative about the transmission of rare B chromosome types or the de novo origin of some of them. At least 11 rare B-types have been found in addition to the predominant ones: B₁ in Daimuz, B₂ in Jete and Salobreña, and B₅ in Fuengirola. The presence in two controlled crosses of one embryo carrying a B-type which was absent in the parents suggests that these B variants (B_2iso and B_1f1) have originated de novo. Eleven other PAs suggest that new B derivatives are recurrently arising in these populations. The most frequent B chromosome mutation was centromere misdivision that originated four different B-types (B_2m1, B_1iso, B_2iso and B_mini). Other rearrangements were pericentric inversions (B_2i1, B_2i2 and B_2i3), inverse tandem fusion (B_2it1), centric fusion (B_1f1) and deletions (B_2d1 and B_2d2). The four B derivatives produced by centromeric misdivision are significantly eliminated during sexual transmission, most probably owing to deficiencies in the control of chromosome movement by their hemicentromeres. Those derived from translocations showed Mendelian transmission but deletion B variants showed a tendency to elimination. Our results suggest that B chromosome substitution of B₁ by B₂ in the Salobreña and Jete populations could be achieved by differences in relative transmission efficiency, as in one controlled cross, where the female carried 1 B₁ plus 1 B₂, B₂ was significantly overtransmitted and B₁, eliminated.