exploit their hosts and evade host defence systems, sexually reproducing
parasites must further divert resources to sexual reproduction, thus
introducing a further subdivision of investment into males and
females. Whether sex-allocation strategies can evolve depends on how
sex is determined. In parasitoids, the foundress determines the sex of
its offspring. For hermaphrodites, the adult can invest differentially in
male/female function in response to environmental stimuli. Although
chromosomal sex determination reduces genetic variance in sex ratio and
will tend to restrain the evolution of an allocation strategy, environmental
factors can strongly influence the sex ratio, whether primary or secondary.
In short, parasites exhibit a tremendous range of sex-determination
mechanisms that enable the evolution of strategies. Such strategies
tend to reflect three basic principles: mating assurance in low-density
conditions; resource optimization reflecting coinfection probabilities;
and condition-dependent fitness differences in the sexes. Such principles
do appear to explain the wide variety of sex ratios observed in parasitic
systems.
Our comprehension of how malaria parasites modulate their resource
allocation to maximize reproduction remains limited. For malaria
parasites in particular, where transmission and sexual reproduction
are simultaneous, strategies that ensure fertilization are expected to be
under very strong selection. The tremendous variability in host quality is
expected to select for a very plastic resource-allocation strategy, and
further cues of habitat quality in both malaria and other parasites are
anticipated to exist. How long-term evolutionary selection, such as that
observed under LMC, can further influence parasite sex-determining
behaviour and resource allocation in the complex environment of
the vertebrate host remains a relatively unexplored realm with exciting
possibilities. As well as evaluating, for example, classical sex-allocation
theory, one interesting novelty considers the value of such plastic ESD
not only in ensuring fertilization but also in promoting cross-fertilization.
If clones vary in their response to haematological cues, for example,
coinfecting clones can have different sex ratios in the same host environ-
ment. Sexual-stage proteins are notably conserved and hence host
immunological responses will be shared across clones. If males are indeed
the limiting sex, then a clone poorly responding to cues will transmit
poorly and be outcompeted by a good responder. However, when the two
clones (poor and good responder) are coinfecting, their differing sex ratios
will lead to a relative increase in the rate of cross- vs. self-fertilization.
Cross-fertilization not only leads to recombination, but also guarantees
the presence of both clones in the same mosquito – that is, co-
transmission. The majority of infected mosquitoes in the field contain
one or very few oocysts, each the product of a single fertilization event,
and therefore cotransmission is strongly determined by cross-fertilization
frequencies. Although recombinant parasites may have an advantage over216 R.E.L. Paul