PARALLEL EFFECTS OF A B-CHROMOSOME AND A MITE THAT DECREASE FEMALE FITNESS IN THE GRASSHOPPER EYPREPOCNEMIS-PLORANS

The effects of a genomic parasite (a B chromosome) and an ectoparasite (a mite) on the fitness of the host (the grasshopper Eyprepocnemis pl orans) have been analysed in 60 experimental females. These two parasi tes differ in their infectious transmission mode. B chromosomes are ve rtically transmitted from host-parents to offspring, but mites are hor izontally transmitted from one grasshopper to another within the same generation. The transmission mode can influence the virulence of these parasites, so that it should be expected that B chromosomes would be less virulent than mites. However, as mite transmission is linked to h ost mobility, some attenuation is also expected. Four egg pods were an alysed from each female, the first two egg pods were laid after a mati ng and the remaining two were not preceded by a mating. The results sh ow that B chromosomes severely decrease the proportion of eggs contain ing an embryo (egg fertility), mainly from the second egg pod onwards. Mites also decrease egg fertility but, in addition, they produced a d ecrease in the rate of embryo production over time (embryo productivit y), which might be derived from both the fertility decrease and a slig ht delay in egg production. The analysis of the relative effect of bot h parasites suggests that they have a synergistic effect on embryo clu tch size and egg fertility. Possible mechanisms for the observed effec ts are discussed.