The Effect of the Host Immune Response on Development in the
S. rattiLife Cycle
Analysis of the proportion of larvae that develop by these two develop-
mental routes (Boxes 1 and 2 in Fig. 6.1) shows that as anS. rattiinfection
progresses two changes occur. First, the proportion of larvae that develop
into free-living males increases, i.e. the sex ratio of the progeny of a
parasitic female becomes increasingly male-biased (Harveyet al., 2000).
Secondly, the proportion of female larvae produced by an infection that
develop by the indirect route of development into free-living females
increases (Harveyet al., 2000). As an infection progresses two things are
happening: parasitic female worms are becoming older and the host is
developing an anti-S. rattiimmune response. This immune response will
eventually completely eliminate the infection, after which rats remain
strongly immune to reinfection (Kimuraet al., 1999). To determine which
(if any) of these factors alters the sex ratio and/or the development of
female larvae, development via these routes was measured in infections of
hosts of different immune status. The sex ratio of larvae passed from rats
previously immunized withS. rattiwere more male-biased compared
with infections in naïve, control rats (Harveyet al., 2000). Furthermore,
the sex ratio of larvae passed from immune-compromised rats (which
cannot mount an anti-S. rattiimmune response) did not change as the
infection progressed. Combined, these observations show that the host
immune response, and not the age of the parasitic female, affects the sex
ratio of the progeny of the parasitic females, with a change in the ratio
towards males occurring in hosts mounting an anti-S. ratti immune
response (Harveyet al., 2000). By the same analysis, it was found that the
host immune response also affects the development of female larvae, with
development by the indirect route into free-living females favoured by
larvae passed from rats with an anti-S. rattiimmune response (Harvey
et al., 2000). The host immune response therefore has a coordinated effect
on the development of the free-living phase of theS. ratti life cycle,
namely, that development into both free-living adult males and females is
favoured by larvae passed from hosts mounting an immune response.
It is not clear how the host immune response brings about this effect
onS. rattisex determination. It is almost certain that it occurs before eggs
are laid, since newly laid eggs contain fully developed L1s and it is
difficult to envisage that sex can be changed at this late stage of develop-
ment. This suggests that the negative effects which the host immune
response has on the fitness of parasitic females (see below; Kimuraet al.,
1999) also affects the behaviour of the X chromosomes during the repro-
duction of the parasitic female. InC. elegans, the sex ratio is usually very
heavily biased towards hermaphrodites (XX), with males (XO) comprising
less than 0.5% of the population (Hodgkinet al., 1979). These XO males
are produced as a result of rare, spontaneous non-disjunction of the X
chromosomes during meiosis (Hodgkinet al., 1979). The rate of non-
disjunction and hence the proportion of the progeny which are males114 M.E. Viney