Constraints on the reproductive value of vertical transmission for a microsporidian parasite and its female-killing behaviour

1. Several species of microsporidian parasites use a mixture of vertical an d horizontal transmission among their respective mosquito host species. The se relationships vary in how hosts are exploited for either vertical or hor izontal transmission. An important factor is that only females can provide vertical transmission. Two types of relationship have particularly been con trasted; those where vertically infected larvae of both sexes are killed la te in their development and contribute towards the parasite's horizontal tr ansmission, and those where only male larvae are killed while females exper ience benign infections, become adults, and contribute towards the parasite 's vertical transmission success. The selective killing of males in these l atter relationships leads to female-biased adult sex ratios. These differen ces among relationships have been suggested to depend on the environmental conditions they experience and on the relative efficiency of vertical or ho rizontal transmission. 2. The transmission behaviour of the microsporidian parasite Edhazardia aed is (Becnel, Fukuda & Sprague) following vertical transmission was studied a s a function of larval food availability to its host, the mosquito Aedes ae gypti (L.). The number of vertically infected mosquitoes dying before reach ing adulthood increased as larval food availability became less. However, p roportionately more females died as food availability decreased and adult m osquito populations became increasingly male-biased. 3. The fate of vertically infected mosquitoes was closely related to their larval growth rate. As food availability decreased and larval growth rates slowed, the parasite's infection had more time to produce its spores and in duce the host's mortality before emergence. This disproportionately affecte d female mosquitoes as they pupate later than males, especially as larval g rowth rates slow, AS late ages at pupation are associated with smaller adul t size (approximate to lower fecundity), the potential vertical transmissio n offered by slowly growing female larvae is limited and more success may b e gained by exploiting them for horizontal transmission. 4. Our results indicate that the relative reproductive value of vertical or horizontal transmission altered with an ecological parameter (larval food availability), Constraints in the life-history traits of each organism help ed to explain why the observed sex ratio was biased towards males rather th an females.