of or shortly after commitment to becoming a sexual form (Bruceet al.,
1990; Smithet al., 2000).LMC and Sex Determination
Several authors have suggested that the life cycle of malaria parasites
lends itself to LMC (Ghiselin, 1974; Readet al., 1992).Plasmodiumspp.
undergo obligate sexual reproduction within their mosquito vectors
during transmission. Fertilization therefore occurs between the gametes
produced from those gametocytes taken up in the mosquito blood meal
(~3μl). Although cross-fertilization between different clones occurs in the
laboratory (Wallikeret al., 1987), as well as in nature (Babikeret al., 1994;
Paulet al., 1995),Plasmodiumcan also effectively have a clonal mode of
reproduction. The propensity to self-fertilize will be, to a large extent,
determined by the number of overlapping infections in the human host.
The mean number of different parasite clones per infected person varies
considerably, according to the transmission intensity of the region.
In regions of intermediate transmission intensity, such as Papua New
Guinea (PNG) (Paulet al., 1995), the number of clones per infected person
is lower (range 1–3, mean 1.8) than in regions of high transmission
intensity (range 1–6, mean 3.2), such as Tanzania (Babikeret al., 1994).
Therefore the number of infected people harbouring only a single parasite
clone, and hence the likelihood of self-fertilization, will be higher in
regions of low or intermediate transmission than in regions of high trans-
mission. When applying LMC to such blood parasites, Hamilton’s basic
equation can be rearranged and formulated in terms of the selfing
rate,s(Readet al., 1992), which has been shown to be the equivalent
of Wright’s inbreeding coefficient F (Dye and Godfray, 1993). Thus
the optimum gametocyte sex ratio,r, relates to the selfing rate,s,by
r= (1−s)/2, and wheresis related to the number of clones per host,n,
bys= 1/n.
Examination of gametocyte sex ratios in PNG showed a female bias
and predicted an inbreeding coefficient, Wright’sF, of from 0.64 to 1
(Readet al., 1992). Genetic analyses of oocysts (zygotes), which contain
the haploid products of meiosis, from regions of high (Tanzania) and
intermediate (PNG) transmission intensity found that there was a signifi-
cant reduction in degree of heterozygosity from that expected under
random mating and that it was more extreme in the region of lower (PNG,
F= 0.9: Paulet al., 1995) than in that of higher (Tanzania,F= 0.3: Hill
et al., 1995) transmission intensity. The high incidence of self-fertilization
in PNG therefore confirmed the hypothesis, based on gametocyte sex
ratios, of high inbreeding in PNG malaria-parasite populations (Read
et al., 1992). The positive relationship between sex ratio and transmission
intensity (Readet al., 1992; Robertet al., 1996) suggests that the sex
ratio is adaptive and responds to the population genetic structure of the
parasite and hence inbreeding rate (LMC). Evidence supporting LMC hasParasite Sex Determination 209