Gonochorists or Hermaphrodites
For those parasites where sexual reproduction is obligatory, mating can
be assured by producing both male and female organs – hermaphrodites.
The adaptive significance of hermaphroditism (simultaneous rather than
sequential) has been considered to be its assurance of complementarity
between any two individuals mating at random in a low-density
population (Ghiselin, 1969). Indeed, in the extreme, facultative selfing
for reproductive assurance may help maintain hermaphroditism
(Charlesworth and Jarne, 1993). The importance of hermaphroditism
enabling facultative selfing is expected to be of particular relevance to
parasitic organisms, which, if gonochoristic (separate sexes), would have
to rely on coinfection for reproduction. The probability of coinfection is
determined by the epidemiology of the parasite, which can vary sub-
stantially for even a single host–parasite species interaction. Ensuring
coinfection is therefore certainly context-dependent, except when the
probability of coinfection is guaranteed: (i) when the foundress is free-
living and lays multiple offspring on the host (e.g. solitary parasitoids);
(ii) when there is cotransmission (e.g. filarial worms); or (iii) when
the duration of infection in the host is sufficiently long to ensure
superinfection (e.g. non-vector-borne nematodes). Such conditions are
permissive of the evolution or maintenance of gonochorism. Although
the predominance of hermaphroditism as a reproductive strategy within
the protozoa and platyhelminths could be explained by the uncertainty
of coinfection, there exist paradoxes (e.g. trematodes are hermaphrodites
but very rarely self-fertilize (Brusca and Brusca, 1990)). An alternative
(or rather complementary) hypothesis to mating assurance considers the
evolutionary stability of the hermaphroditic state to be the result of a
resource-allocation strategy.Resource Allocation
Sex-allocation theory unifies theories concerning sex ratio to generalize
across all organisms by considering how an individual should allocate its
resources in female/male progeny (gonochoristic species) or male/female
organs (simultaneous hermaphrodites) or when to shift from one sex
to another (i.e. state-sequential hermaphrodites) (Charnov, 1982). The
principle underlying sex-allocation theory is as follows. Organisms have
finite resources to partition among growth, survival and reproduction. Of
those made available to reproduction, the organism must further partition
its reproductive resources into male or female function (progeny, organs
or state). The optimal sex-allocation strategy will be that which produces
the greatest fitness returns. A foundress (mother or hermaphrodite) will
invest its resources in the production of a ratio of male to female function
(whether testes and ova in hermaphrodites or males and females in
gonochorists) that maximizes its fitness. The optimal strategy may be202 R.E.L. Paul