Parallel effects of a B chromosome and a mite that decrease female fitness in the grasshopperEyprepocnemisplorans

The e¡ects of a genomic parasite (a B chromosome) and an ectoparasite (a mite) on the ¢tness of the host (the grasshopper Eyprepocnemis plorans) have been analysed in 60 experimental females. These two parasites di¡er in their infectious transmission mode. B chromosomes are vertically transmitted from host-parents to o¡spring, but mites are horizontally transmitted from one grasshopper to another within the same generation. The transmission mode can in£uence 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 host mobility, some attenuation is also expected. Four egg pods were analysed from each female, the ¢rst two egg pods were laid after a mating and the remaining two were not preceded by a mating. The results show that B chromosomes severely decrease the proportion of eggs containing an embryo (egg fertility), mainly from the second egg pod onwards. Mites also decrease egg fertility but, in addition, they produced a decrease in the rate of embryo production over time (embryo productivity), which might be derived from both the fertility decrease and a slight delay in egg production.The analysis of the relative e¡ect of both parasites suggests that they have a synergistic e¡ect on embryo clutch size and egg fertility. Possible mechanisms for the observed e¡ects are discussed.