development of isofemale lines shows that different developmental
strategies are appropriate in different geographical settings. It is relatively
easy to envisage that the developmental response of female larvae to
temperature is an adaptation in which one can hypothesize that it is
used as a seasonal cue, etc., which in turn may predict, for example, the
probability of survival and reproduction as a free-living female or the
probability of an iL3 encountering a new host.
A principal difference between indirect and direct development is
that indirect development includes sexual reproduction, alternating with
the asexual reproduction of the parasitic females, making the develop-
ment of this life cycle effectively cyclically parthenogenetic. In direct
development, only asexual, parthenogenetic reproduction occurs. In
other words, sexual reproduction is facultative in theStrongyloideslife
cycle, and its occurrence varies in response to conditions within and
outside the host. The selective advantages of sexual reproduction in
nature are a continuing area of debate and investigation (Barton and
Charlesworth, 1998). However, its widespread occurrence has been taken
to show its general selective advantage. This advantage, whatever it is,
presumably applies equally toStrongyloidesspp. It is notable that many
organisms that have facultative sexual reproduction generally indulge in
it at times of stress. For example, aphids reproduce parthenogenetically
during the summer, which results in rapid population increases.
However, when environmental conditions deteriorate and food supplies
become limiting, sexual reproduction occurs, the resulting progeny
of which are resistant, overwintering stages (Hughes, 1989). Similarly,
temperate populations ofDaphniaspp. alternate between sexual and
parthenogenetic reproduction, with sexual reproduction favoured at
times of environmental stress (Herbert, 1987). With this perspective, the
favouring of the development of free-living adults of S. ratti and
the consequent sexual reproduction is consistent with the general trend
of sexual reproduction occurring at times of environmental stress. It is
notable withS. rattithat the environmental stress of the host immune
response acts against the stages within the host, whereas the forms that
undergo sexual reproduction are in the external environment. This may
suggest that, in this respect, the host immune response is being used as a
predictor of the state of a host environment in which future iL3s may find
themselves.
InC. elegansall reproduction is sexual, the only variation being
inbreeding during hermaphrodite sex and potential outcrossing during
male/hermaphrodite sex. ThereforeC. eleganshas no sexual response to
environmental stress. Instead, its developmental response to stress is to
form dauer larvae. These are long-lived, environmentally resistant stages
which, biologically, are presumably the dispersive stage of the life cycle.
Thus, here, the strategic response to stress is to wait until the unfavour-
able conditions have passed.Environmental Control of Nematode Life Cycles 119