interactions, which results in differential fitness for both hosts and
parasites (for a review, see Ladle, 1992). Sex will increase the evolution-
ary rate of hosts, whose reproductive (and hence evolutionary) rate is
generally slower than that of their parasites (Jaenike, 1978; Hamilton,
1980). Therefore, sex accelerates the evolution of the host population,
thus generating an ever-changing environment (e.g. the host immune
system) for the parasite population (Seger and Hamilton, 1988; Hamilton
et al., 1990). In response, parasites are expected to adapt to this changing
environment, resulting in a coevolutionary arms race between the parasite
and the host (Hamilton, 1993). Despite the higher intrinsic evolutionary
rate of the parasite, intrinsic to its rapid life cycle, sex in parasites
will provide an additional increase, enabling the parasite population
to track the host population. As will become clear, the parasite’s
intrinsic evolutionary rate advantage does appear to enable more
flexibility in reproductive strategies, which are strongly influenced
by short-term transmission requirements, as well as the long-term
advantages of sex.
Several parasitic protozoa of medical importance, notablyLeishmania
andTrypanosoma, although able to undergo sexual reproduction (in
laboratory crosses) (Gibson and Garside, 1991), rarely, if ever, do so in the
field (Tibayrencet al., 1990). Although the persistence of clonal lineages
of such parasites may seem to contradict the theories of sexual repro-
duction, only occasional sex is necessary to remove the disadvantages of
asexual reproduction (Falconer, 1981). Disposing of sexual reproduction
is, however, rather rare. Parasitic Protozoa and Platyhelminthes undergo
obligate sex and tend to be hermaphrodites, which in principle allows
transmission, following self-fertilization, even when the parasite finds
itself alone in a host (but see next section). The majority of parasitoids
and nematodes, however, are gonochorists (separate sexes), although
such taxa do exhibit certain particular reproductive characteristics.
Hymenopteran parasitoids have a haplodiploid genetic system, where
unfertilized (haploid) eggs become males and fertilized (diploid) eggs are
females; hence the offspring sex ratio can be determined by the ovi-
positing female. Parthenogenesis occurs not infrequently in parasitoids,
where unfertilized females produce diploid daughters (thelytoky).
Although it is tempting to consider that thelytoky has evolved as a
mating-assurance adaptation in low-density populations (Price, 1980),
there is good evidence that parthenogenesis is actually controlled by
infecting microorganisms (Stouthameret al., 1990). Similarly, although
the majority of parasitic nematodes are gonochorists, sex determination
can be under environmental control; one well-studied species,Strong-
yloides ratti, alternates between parthenogenetic and fully sexual accord-
ing to the immune state of its host (Gemmillet al., 1997; Viney, 1999).
Thus parasites exhibit tremendous variability in reproductive strategy,
but to what extent can these strategies be interpreted as those favoured by
natural selection?Parasite Sex Determination 201