increase with the age of the parent hermaphrodite (Goldstein and Curtis,
1987). The sex ratio can also be changed towards the production of males
by environmental stresses, such as heat shock (Sulston and Hodgkin,
1988). However, precisely how and why the temperature experienced by a
hermaphrodite or its age affects the cytology of the X chromosomes during
meiosis are not known. Importantly, though, the observations inC. elegans
andS. rattiare analogous in that the sex ratio of progeny of worms can
facultatively change in response to apparent stresses on parent worms.Other Environmental Effects on Development in theS. rattiLife
Cycle
Many observations have been made on the effect of environmental
conditions external to the host on the development of the free-living
phase of theS. rattilife cycle. Generally, this has involved placing freshly
passed faeces containing young larvae into different environmental
conditions and assessing the effect of these treatments on the proportion
of larvae that develop by the different developmental routes. Examples of
such treatments include the incubation of faecal cultures in different
gaseous environments (Taylor and Weinstein, 1990) and experiments in
which various concentrations of a range of fatty acids were added to faecal
cultures (Minematsuet al., 1989). However, one of the most thoroughly
investigated environmental manipulations is the environmental temper-
ature external to the host (Viney, 1996). This has shown that temperature
only affects the female developmental choice, with direct development
to iL3s favoured at lower temperatures and indirect development into
free-living adult females favoured at higher temperatures (Viney, 1996;
Harveyet al., 2000). Temperature pulse-chase experiments have shown
that these larvae retain a developmental response to environmental
temperature until the end of the L2 stage (Viney, 1996). Temperature
does not affect the sex ratio of the progeny of a parasitic female.The Interaction of the Host Immune Response and Temperature
The host immune response and the environmental temperature interact
in their effect on the development of female larvae ofS. ratti. The temper-
ature sensitivity of the developmental choice of female larvae can be
expressed as the difference in the proportion of female larvae that develop
into free-living females between two temperatures (19 and 25oC). The
greater this difference in proportion, the greater the developmental
sensitivity of larvae to environmental temperature. This difference in
proportion increases as an infection progresses, i.e. during an infection
the developmental choice of female larvae becomes increasingly sensitive
to environmental temperature. This change in developmental sensitivity
to temperature is affected by the host anti-S. rattiimmune response. Thus,Environmental Control of Nematode Life Cycles 115