0851996159

been found in the sex ratios of some related haemosporidians (e.g.

Leucocytozoonparasites of birds), but not in others (i.e.Haemoproteus

parasites of birds).Leucocytozoonspp. were found to produce sex ratios

optimal for the local inbreeding probabilities based on the prevalence

of infection, which reflects the transmission intensity and hence the

distribution of coinfections (Read etal., 1995). Haemoproteus spp.,

however, which are vectored byCulicoidesspp. (Diptera :Heleidae), were

found to produce fewer female-biased sex ratios than predicted (Shutler

etal., 1995), which was subsequently suggested to be the result of the

need to produce insurance males, because of the small size of the vector-

species blood meal compared with that taken by mosquitoes (Shutler and

Read, 1998), i.e. the insurance males compensate for the lower probability

of gametocytes being present in the diminutive blood meal. In situations

where there may be low numbers of gametocytes in the blood meal, LMC

predictions for the optimal sex ratio can be dramatically altered, reflecting

the influence of mating assurance (Westet al., 2002; Fig. 10.2).

Further evidence that LMC may operate on the sex determination of

Plasmodiumand related parasites has recently been found in a compara-

tive analysis of apicomplexan parasites that have syzygy. Syzygy is the

process by which a single male gametocyte pairs together with a single

female gametocyte, either in host cells or in the lumen of host organs, just

prior to gametogenesis (Barta, 1999) and therefore gametes from a single

210 R.E.L. Paul

0.5

0.4

0.3

0.2

0.1

0

Optimal sex ratio

0 0.25 0.5 0.75 1

Selfing rate

q = 2

q = 5

q = 10

q = 20

q = infinity

Fig. 10.2. The optimal sex ratio in relation to selfing rate for various gametocyte

numbers in the mating pool, whereqis the number of gametes able to interact.

Derived from Westet al. (2002) with permission from theJournal of Parasitology.