We have analysed B chromosome frequency for three consecutive years, B transmission rate at population and
individual levels, clutch size, egg fertility and embryoYadult viability in a natural population of the grasshopper
Eyprepocnemis plorans containing two different B chromosome variants, i.e. B2 and B24, the second being
derived from the first and having replaced it in nearby populations. From 2002 to 2003 the relative frequency of
both variants changed, although the differences did not reach significance. A motherYoffspring analysis showed no significant effect of any of the two B variants on clutch size, egg fertility or embryoYadult viability, but B24 was more efficiently transmitted than B2 through males from the 2002 season, which explains the observed frequency change. Controlled crosses, at individual level, showed significant drive through some females for B24 but not for B2, suggesting that this difference in transmission rate might also be important for the substitution process. The analysis of relative fitness for B2 and B24 carriers for all fitness components, as a whole, showed a significantly better performance of B24-carrying individuals, suggesting that the cumulative effect of these slight differences might contribute to the replacement of B2 by B24.
