as some parasitoids, the foundress can facultatively determine its off-
spring’s sex, using cues (host size, superparasitism). In many circum-
stances, however, sex is determined later by some environmental
influence. Environmental sex determination (ESD) is well known to occur
in reptiles, where temperature plays an important role in determining the
sex of the developing egg (for a recent review, see Shine, 1999). Analogous
to the condition-dependent sex allocation proposed by Charnov (1979)
and Bull (1981), ESD is favoured when an individual’s fitness is strongly
influenced by environmental conditions, where an offspring will enter an
environment away from the parent and where the individual has little
control over the environment it will experience (Charnov and Bull, 1977).
ESD, as either the major or the secondary determining influence enabling
facultative control of sex ratio, is probably widespread in both gono-
choristic and hermaphroditic parasites (Charnov and Bull, 1977). ESD is
known to occur in parasitic copepods and mermithid nematodes, where
adults are free-living and the larval forms grow and attain sexual maturity
within their host. In these cases, sex is determined according to the
number of coinfecting larvae (Blackmore and Charnov, 1989; Charnov,
1993). Similarly, both trematode and cestode platyhelminths adjust their
male : female investment according to the number of coinfecting individ-
uals (Didyk and Burt, 1998; Trouvéet al., 1999). In both cases, LMC and
probably host quality play a part. In addition, ESD is important when
reductions in host quality affect reproductive successper se– that is,
when mating assurance is jeopardized. In the examples given above, this
is probably not the case. However, examples from parasitic protozoa,
discussed later, suggest that mating assurance can play a major role and
that immune responses targeting sexual reproduction provide a selective
force for the evolution of ESD.Sex Determination, Phenotype Plasticity and Host Heterogeneity
Variability either in host quality or in the intensity of coinfection is
expected to select for great plasticity in sex allocation. Although the
theories have been largely based on parasitoid systems, it is clear that sex
determination in other parasite systems follows the adaptive predictions
from resource-allocation theory. The extension of this framework to
parasite–vertebrate host systems offers great potential, because it intro-
duces further elements, most notably those based on the relative longevity
of the host–parasite interactions and the increased importance of host
immunity and parasite epidemiology. This is of particular consequence
for vector-borne protozoan parasites, where the transmission season (and
hence sexual reproduction) can vary considerably for a single host–
parasite species interaction. The complex nature of such host–parasite
systems, where the host is of highly variable quality (e.g. degree of immu-
nity) and the epidemiology often unstable, provides the parasite with a
challenging diversity of circumstance, where no single sex-allocationParasite Sex Determination 207