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e.g. when solitary parasitoids parasitize gregarious hosts. The sex ratio

is found to be positively correlated with the number of coinfecting

foundresses, both within (Fig. 10.1; Werren, 1983) and across species

(Godfray, 1994), and with the number of hosts available (Scelionidae:

Waage, 1982); i.e. increasing number of foundresses decreases inbreeding

and hence LMC and increasing host density will increase the probability

of parasitism and thus of coinfecting with other foundresses. Similarly, in

hermaphrodites, LMC predicts an increase in the investment in the male

function (sperm) in comparison with the female function (ova) as the

number of mates increases. This has been demonstrated experimentally

in gut trematode infections of mice, where increasing mating-group size

correlated with an increase in testes at the expense of ovary size, and this

was independent of trematode body size (Trouvéet al., 1999), another

factor known to influence optimal sex allocation (cf. habitat quality).

Such adaptive adjustment in resource allocation requires cues enabling

assessment of the number of coinfecting trematodes. Echinostome

trematodes do produce a suite of pheromones involved in interindividual

attraction (for pairing and sexual stimulation) (Fried, 1986; Trouvé and

Coustau, 1999), which could convey coinfection density information. It

is also suggested that such facultative LMC occurs in natural cestode

infections of birds (Didyk and Burt, 1998): male–female pairs of the

cestodeShipleya inermispredominate in infections, with the occasional

occurrence of hermaphrodites. The absence of unisex pairs suggests that

the cestode is a protogynous hermaphrodite, where female development

occurs first and the next cestode arriving would be stimulated to become

male. As discussed later, there is broad support for the view that LMC also

applies to parasitic protozoa.

Parasite Sex Determination 205

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Foundress number

Sex ratio

Fig. 10.1. The offspring sex ratio (proportion of males) produced by differing

numbers of co-ovipositing foundresses (Nasonia vitripennis) (data from Werren,

1983). The solid line gives the optimal sex ratio (proportion of males) in respect of

foundress number under local mate competition (Hamilton, 1979).