amphids, which are principally involved in chemo- and thermosensation.
The ultrastructure of the amphidial neurons ofS. stercoralishave recently
been reconstructed from serial electron-microscope sections (Ashton and
Schad, 1996). Their role in larval development has been investigated by
ablating neurons using a laser microbeam and assessing the effect of this
on worm development. This has shown that inS. stercoralisthe develop-
mental decision of female L1s to develop into free-living females requires
the presence of two (known as ASF and ASI) amphidial neurons (Ashton
et al., 1998). Thus, larvae in which these pairs of neurons were killed
develop directly into infective L3s, whereas untreated larvae or those in
which only one pair of neurons were ablated develop into free-living
females (Ashtonet al., 1998). This also suggests that the default develop-
ment ofS. stercoralisfemale L1s is direct development into iL3s, and that
this can be altered by environmental signals so that larvae develop into
free-living females. It is probable that young larvae ofS. rattisense their
environmental conditions, including temperature, via similar amphidial
mechanisms. Temperature sensation inC. elegansis known to occur
via amphidial neurons (Bargmann and Mori, 1997). Parasitic females of
Strongyloidesspp. also have amphids and it is tempting to speculate that
these are used for the sensation of the host immune response, which is
used to affect the developmental potential of the progeny of a parasitic
female.
This control ofS. stercoralisdevelopment is analogous to the control
of a developmental switch in the life cycle of many free-living nematodes,
includingC. elegans. C. elegans larvae have a developmental choice:
when there is a limited supply of food and a high conspecific population
density, larvae develop into long-lived, arrested third-stage dauer larvae;
when there is a plentiful supply of food and a low conspecific population
density, ‘normal’, non-dauer development continues (Riddle and Albert,
1997). The environmental signals (food availability and conspecific
population density) are sensed by the amphids and this signal is
transduced via a number of signalling pathways, which unite to bring
about the physical and physiological development into dauer larvae,
rather than into ‘normal’, non-dauer L3s. Experimental ablation of
amphidial neurons has identified those which participate in this signal-
ling inC. elegans(Riddle and Albert, 1997). The hypothesized analogy of
the control of the developmental switches of these two species (Riddle
and Albert, 1997; Viney, 1999) is supported by the demonstration that
apparently analogous amphidial neurons ofS. stercoralisandC. elegans
control the respective developmental switches. However, it is noteworthy
that inC. elegansboth hermaphrodites and males can develop into dauer
larvae. Thus, the environmental control of development is not restricted
to one sex, as it is inS. ratti. This suggests that, if during evolution there
has been conservation of the molecular basis of theC. elegansandS. ratti
developmental switches, then the point in the life cycle at which this can
operate has changed during the evolution of these species.Environmental Control of Nematode Life Cycles 117