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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).
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