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Both morphs are associated with natural variation at the npr-1 (neuropep-
tide receptor resemblance) locus, although several other loci may affect
this behavior, albeit in a lesser manner. This locus was identified using
a mutagenesis screen, and encodes a neuropeptide receptor, similar to
neuropeptide Y (NPY) receptors (G-proteins, regulating food consumption,
anxiety, and pain in mammals). These similarities suggest that npr-1 has a
neuropeptide ligand, however no clear NPY homolog has been found. Null
mutations in npr-1 transform solitary feeders into strong social feeders,
with a specific single amino acid change in npr-1 also found in 17 natural
isolates (
de Bono and Bargmann, 1998
).
The gene npr-1 regulates this aggregation through stimulating aversion to
noxious stimuli (
de Bono et al., 2002
), utilizing specific neuronal pathways
(the ASH and ADL neurons). If these neurons are ablated, the social aggre-
gation response is severely decreased. Aggregation is regulated by food in
C. elegans (
de Bono and Bargmann, 1998
), therefore food in this case is the
aversive stimulus, not surprising given that soil bacteria can kill C. elegans
(
Marroquin et al., 2000
). In mammals NPY has also been shown to have sed-
ative effects and decreases the sensitivity to nociceptive stimuli (
Naveilham,
2001
), showing a similar trend to the C. elegans model system with npr-1
activity suppressing the behavior induced by noxious stimuli.
Foraging in Drosophila Larvae and Apis mellifera
Another classic mutation affecting behavior, causing extreme effects and
largely attributable to a single gene locus is found in the for gene (
de Belle
and Sokolowksi, 1989; Sokolowksi, 1980
). Individuals with what is termed
the “rover” allele move greater distances whilst feeding than those homozy-
gous for the “sitter” allele, with neither showing any variation in behavior in
the absence of food (
Pereira and Sokolowksi, 1993
). Both of these behavioral
morphs exist at appreciable frequencies in the wild (70% rover; 30% sitter
(
Sokolowksi, 1980, 1982
). The for gene itself is in fact the gene dg2, one
of two cGMP-dependent protein kinase genes (PKG) in Drosophila, with
the cGMP signal transduction pathway implicated in foraging behavior in
D. melanogaster (
Osborne et al., 1997
). PKGs function in naturally occur-
ring variation in larval foraging behavior, with gene levels correlating with
larval foraging behavior (
Sokolowski, 1998
).
Sokolowski et al. (1997)
used
an empirical study comprising of three separate base populations, each
selected for either high- or low-population density. Those individuals in a
low-density selection environment had shorter foarging path lengths, whilst
those from high-density environments had longer path lengths.
The evolutionary benefits of these different phenotypes are therefore
apparent in varying-density environments. At high densities larvae are
required to navigate around other individuals to reach food patches, whilst
under low-density conditions this locomotion behavior is an unnecessary
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