suboptimal or defective operation of this process is the basis for the
control of the developmental response of this life cycle to environmental
conditions. This seems unlikely, given the observed variation in develop-
mental potentials of isofemale lines (see above), which is the product of
natural selection, because it is unlikely that natural selection has operated
on a ‘mechanism’ controlling development where that ‘mechanism’ is the
suboptimal or defective operation of a cellular process. Rather, it is more
likely that the change in development of theS. ratti life cycle is an
accurate and controlled response to environmental conditions, in this
case the anti-S. rattihost immune response. In this respect, it is worth
remembering that the life-cycle changes of aphids that occur at times of
environmental stress are not induced or measured by environmental
‘stress’per se. Rather, since ‘stress’ occurs typically and repeatedly in
autumn and this season correlates with changes in day length, aphids use
day length as the proximate measure of autumn, and hence environmental
stress, in their environment (Hughes, 1989).
Thus, by analogy, what cue mayStrongyloidesuse to assess the host
immune response? One option is that parasitic females use the con-
centration of particular components of the host immune response as a
measure of an immune response acting against them. However, for this to
work, the cue would have to be a sufficiently reliable cue of an immune
response that has an anti-S. rattieffect, rather than any immune response
with a non- or lesser anti-S. rattieffect. Experimental observations suggest
that this is unlikely to happen, since the effect of the host immune
response on the development ofS. rattilarvae appears to depend on the
specificity of that response. Thus, challenge infections ofS. rattiin rats
previously immunized with nematodes other thanStrongyloidesor with
non-nematode antigens show less change in their developmental route,
compared with infections in rats previously immunized withS. ratti
(Westet al., 2001). Indeed, the greater the effect of the immunization on
the reduction in worm output (a measure of parasite fitness and thus of
the effectiveness of the anti-S. rattiimmune response) of the challenge
infection, the greater the proportion of larvae that developed into sexual
adults (Westet al., 2001). Therefore, while it is possible thatS. ratti
measures the host immune response directly, it is difficult to envisage
what components of the immune response are only produced during an
anti-S. rattiimmune response and not during an immune response against
any other infectious organisms or antigen sources.
There are alternatives. Parasitic females may measure other aspects of
the host physiology that are an accurate indicator of an anti-S. ratti
response that is occurring. However, it is again difficult to see what
components would be specific to an anti-S. rattiimmune response. A
third alternative is that the host immune response may bring about a
change in the internal physiology of the parasitic worms, and this change
may be sensed internally and used as a cue. This would seem to have
the advantage that this would be an accurate and specific measure of the
effectiveness of an anti-S. rattiimmune response.Environmental Control of Nematode Life Cycles 121