the difference in the proportion of female larvae that develop into free-
living females: (i) is greater in infections in rats previously immunized
withS. ratti, compared with infections in naïve, control rats; and (ii)
does not change as the infection progresses in infections in immuno-
compromised rats. There is no such effect on the male/female sex
determination switch, since this is not temperature-sensitive (see above).
The integration of these signals in controlling development is remark-
able, since it integrates environmental signals that probably operate at
different times in the life of developing larvae. Thus, the host immune
response can directly affect the parasitic female and her eggs and L1s in
the host intestine. Larvae are exposed to the environmental temperature
only from when they are passed from a host. This would suggest that, for
the host immune response and environmental temperature to interact,
larvae developing outside the host must have a ‘memory’ of the immune
status of the host from which they were passed.
There are several ways in which we may speculate how this happens.
Since the parasitic female is also subject to the host immune response,
when producing her eggs she may, in some way, preprogramme their
future development in respect of the state of the host immune response.
Alternatively, the host immune response may affect the physiological
conditions of the gut and hence faeces, such that larvae developing in
faeces are able to determine the immune status of the host from which
they came. This latter possibility seems unlikely to be correct, since
experiments in which larvae passed from newly infected (and thus not
mounting an anti-S. rattiimmune response) hosts which were transferred
to faeces collected from rats immune toS. rattidid not alter their develop-
ment (M. Crook and M.E. Viney, unpublished observations).
Transgenerational ‘memory’ of environmental conditions has been
investigated in the water-flea,Daphnia pulex. Its growth, development
and mode of reproduction of an individual are affected by the food
available in its environment. However, a mismatch (usually down) in the
quantity of food available in the environment of the flea compared with
that of its mother also results in a change in the development and,
importantly, reproduction of the progeny (LaMontagne and McCauley,
2001). The demonstration of this phenomenon elegantly shows how a
‘memory’ of conditions affecting one generation can affect the life of a sub-
sequent generation. It remains to be seen whether such mechanisms oper-
ate in the generational ‘memory’ ofStrongyloidesand other nematodes.Neuronal Control ofStrongyloidesDevelopment
In addition to these analyses of factors that affect development in the
Strongyloideslife cycle, there has been investigation of its neuronal basis.
All nematodes have bilateral, anterior sensory-rich structures, known as116 M.E. Viney