VIRULENCE, PARASITE MODE OF TRANSMISSION, AND HOST FLUCTUATING ASYMMETRY

Horizontally transmitted parasites are broadly predicted to be more vi rulent, or costly to host fitness, than those with vertical transmissi on. This is mainly because vertical transmission, from host parent to offspring, explicitly links the reproductive interests of both parties . Underlying this prediction is a general assumption that parasite tra nsmission success is positively correlated with its virulence. We repo rt results where infection of larval yellow fever mosquitoes Aedes aeg ypti with the microsporidian Edhazardia aedis was experimentally manip ulated. The parasite's complex life cycle allowed comparisons between estimates of horizontal and vertical transmission on host fitness. Our measure of virulence was the fluctuating asymmetry (FA) of adult fema le wings. Hosts harbouring spores showed higher FAs than controls. Hor izontally transmitting spores were associated with higher FAs than ver tically transmitting spores. Furthermore, within hosts FA correlated p ositively with the number of horizontally transmitting spores: while n o relation was seen with the number of vertically transmitting spores. A developmental mechanism uncoupling the relationship between vertica l transmission and virulence is proposed.