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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.
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