Variation inS. rattiDevelopment
The development of different isolates ofS. rattivaries.S. rattiisofemale
lines (a line of parasites derived from a single iL3) of isolates from differ-
ent geographical locations vary in the proportion of their larvae that
develop by the indirect and direct routes of development (Vineyet al.,
1992). Thus, some lines develop almost exclusively by the direct route of
development during an infection, with virtually no larvae developing into
free-living adults. Other lines show mixed direct and indirect develop-
ment throughout the infection. The development of isofemale lines can be
altered by selection. Thus, lines of parasites have been selected for direct
and indirect development (from a line that underwent mixed develop-
ment) by the repeated passage of iL3s that developed by these routes,
respectively. There is an effective response to this selection, which can be
very rapid (Viney, 1996). It is likely that the selection has acted on both
levels of control (sex determination and the developmental choice of
female larvae) in the life cycle, but the extent to which this occurred is not
known. However, from our understanding of the control of the life cycle,
selection for lines that develop by direct development must have at least
occurred by selection for a heavily female-biased sex ratio.
In view of the neuronal control of the developmental switches
ofStrongyloides, there are other possibilities for the basis of variation
between isofemale lines in their development. Thus, lines may vary
in their sensitivity to environmental conditionsper seand/or in the
transduction of the information in the environmental signal to the devel-
opmental response. The observed response of lines ofS. rattito artificial
selection for developmental propensity may therefore also be due to
selection for different sensitivities to environmental conditions and/or
variation in the transduction of that sensory information.
Different wild isolates ofC. elegansalso vary in their developmental
response to environmental conditions. In laboratory assays in which eggs
are placed in conditions likely to induce dauer larvae (limited food and
dauer pheromone (Golden and Riddle, 1984)), the proportion of larvae
that develop into dauer larvae, rather than into non-dauer L3s and
L4s, varies between different isolates (M.E. Viney, J.A. Jackson and
M.P. Gardner, unpublished observations). The developmental and/or
sensory basis of this is not known, but is ripe for investigation and is
more easily tractable inC. eleganscompared with many other species,
includingS. ratti.Why isStrongyloidesDevelopment Affected by Environmental
Conditions?
Overall, these findings show a remarkable sophistication in the control
of development in theStrongyloideslife cycle. The variation in the118 M.E. Viney