Male and female meiosis in a natural population of Dichroplus pratensis (Acrididae) polymorphic for Robertsonian translocations: A study of chiasma frequency and distribution

Chiasma frequency and distribution were comparatively analysed in males and females from an Argentinian population of the Acridid grasshopper Dichroplus pratensis. This population is polymorphic for four centric fusions among the six L-autosomes of the standard 2n = 19 male/20 female karyotype. Males and females carrying three fusions, L1/L2, L3/L4, and L5/L6 in homozygous or heterozygous condition, showed similar chiasma frequencies and distributions, which contrasted sharply From those of standard (all-telocentric) individuals from two different Patagonian populations. The fusions induce the same type of chiasma repatterning in both sexes, producing a significant decrease of chiasma frequency in all L-chromosome arms irrespective of relative size. Mean chiasma frequency per bivalent varies between 1.64 and 1.05 for the largest and smallest L-autosomes in standard individuals while, when fused, all have a frequency of about 1.0. Furthermore, males and females from Cerro Ceferino display the same distal chiasma pattern, while standard bivalents show a typical proximal-distal chiasma distribution. The X bivalent of females was analysed individually: it has a chiasma pattern not different from that of standard telocentric autosomes. Our results suggest that, in this species, Robertsonian translocations exert parallel effects on recombination in both sexes, and that, if there exist separate controls of recombination for males and females, these are overruled by the effects of the rearrangements, which, in turn, would control balanced orientation and segregation of trivalents and, thus, the maintenance of the polymorphisms in nature.