Description
In
C. elegans
, the dauer larva (DL) is a non-feeding and stress-resistant stage that can respond to environmental cues differently from adults. For example,
C. elegans
adults avoid CO
2
while DL are attracted to CO
2
(Hallem and Sternberg 2008; Hallem
et al.
2011). Similarly, adults of the nematode
Pristionchus pacificus
are mildly attracted to a beetle host pheromone while its dispersive DL are highly attracted to the pheromone (Carstensen
et al.
2021).
To determine if genes that affect dauer development also modulate olfactory behavior in
C. elegans
, we compared the odor response profiles of young adults versus DL in
C. elegans
of two well-studied Daf-c mutants,
daf-2
and
daf-7
(
Riddle
et al.
1981; Swanson and Riddle 1981; Kimura
et al.
1997
)
towards three odorants. In
C. elegans
adults, the AWA neurons detect 2,3-butanedione (diacetyl), while the AWC neurons sense isoamyl alcohol (IAA) and 2,3-pentanedione (Sengupta
et al.
1996; Chalasani
et al.
2007). We found that wild-type DL showed less odor attraction than corresponding adults for 2,3-butanedione, 2,3-pentanedione, and IAA, consistent with the results of a previous study that included 2,3-butanedione (Hallem
et al.
2011)(
Fig. 1A-C
). However, this adult-DL difference was not observed in
daf-2
towards IAA or
daf-7
towards 2,3-butanedione and 2,3-pentanedione. Furthermore, the response to IAA changed from attractive to repulsive in the DL of two
daf-7
alleles
(e1372, ok3125)
. To evaluate if the avoidance response to IAA is due to hypersensitivity to IAA in the
daf-7
DL, we also tested their response to a 10-fold lower IAA concentration (0.1%). We found that the strong avoidance response was no longer observed, and instead the dauer IAA attraction became significantly reduced in both
daf-7
alleles compared to adults, resembling the wildtype DL (
Fig. 1D
). In
C. elegans
, odor concentration-dependent preference shift has been observed for IAA for adult worms and is dependent on
odr-3
function (Yoshida
et al.
2012). Thus,
C. elegans
wild-type and Daf-c DL showed decreased attraction compared to adults to all three odorants tested, with the
daf-7
mutations producing an accentuated concentration-dependent response to IAA in the DL, suggesting that the TGF-ß pathway is involved in both dauer regulation and dauer-specific response to IAA.
While remodeling of the AWC dendritic ends that increase their surface area have been speculated to heighten odor sensitivity in DL (Albert and Riddle 1988), our results show that only
daf-7
alleles exhibited a potentially higher sensitivity to isoamyl alcohol, while wild-type and
daf-2
DL were actually less attracted than adults to several odors. Mutation of the
daf-2
gene also mostly eliminated acute CO
2
avoidance in adult
C. elegans
, while the
P. pacificus
Daf-c mutant
Ppa-hsd-2
exhibited greatly enhanced adult attraction to a host odor (Hallem and Sternberg 2008; Carstensen
et al.
2021). These results suggest that dauer development has multiple effects on olfactory behavior, such that both wild-type and Daf-c DL should be assessed when surveying species-specific responses.