Effect of a “B” chromosome on chiasma localisation and frequency in male Euthystira brachyptera

Summary

Euthystira brachyptera is one of several species of grasshopper noted for terminal localisation of chiasmata in the males. Chiasma frequency and distribution have been studied in samples collected in the French Alps in 1974 and 1978. Localisation was found to be unusual because two chiasmata occasionally occurred close together in the terminal region of the long bivalents. Chiasmata were not localised in the short bivalents. Some individuals were found to carry a supernumerary “B” chromosome (mean frequency 0·5 (1975) to 0·3 (1978) B's per individual) which apparently reduced localisation while raising the mean cell chiasma frequency and variation from 14·0 (s.e.m. 0·25) to 15·1 (s.e.m. 0·71). Large variations in chiasma frequency were found between samples collected from neighbouring sites (50 m to 300 m apart) showing that the population was very heterogeneous. While this makes it difficult to interpret the results simply, it suggests that chiasma frequency (and hence recombination) is not subject to strong standardising selection.