Program of the 2004 East Coast Worm Meeting - Caenorhabditis ...

29. Genes Controlling Sensory Axon Patterning in the C. elegans Male Tail
Lingyun Jia, Scott W. Emmons
Albert Einstein College of Medicine, Bronx, 10461, USA
The C. elegans male exhibits sex-specific behaviors like mate searching and copulation. The
generation and modification of such behaviors depends on the precise connections between
male-specific neurons. However, how the male-specific circuits are established during
development is not well known. We initiated a genetic analysis by focusing on sensory neurons of
the rays to identify genes involved in this process. We first demonstrated by using fluorescent
protein that each ray axon takes a distinct but stereotyped pathway into the preanal ganglion and
is highly branched there, identical with previous study by Sulston(1980). In a genetic screen for
mutations with abnormal ray axon morphology, we isolated mutations in nine genes that cause a
diversity of ray axon morphology defects. A subset of these mutations define two non-sex-specific
genes: unc-27 and sax-2, and two potentially new genes: egl-35 and rax-2 with sex-specific
functions.
unc-27 encodes troponin I, a subunit of Troponin Complex. Our results show that UNC-27/TnI
controls axon pathway by ordering muscle position and the substrate for the axons to migrate
along, indicating that nerves and muscles are spatially coincidently regulated during
development. unc-27 loss of function results in axon wandering. Polarized microscope analysis
demonstrated that mutations in unc-27 distort the evenly spaced dense bodies, detaching muscle
from basement membrane and epidermis. Mutations in most genes encoding myofilament
proteins do not affect axon guidance except for mup-2/TnT, another subunit of the Troponin
Complex that is also required for muscle cell positioning during development. We showed by
mosaic analysis that UNC-27 expression in muscles but not in neurons or hypodermis rescued
the axon wandering; suggesting muscles provide a scaffold for axon pathfinding.
sax-2 encodes the C.elegans homologue of the drosophila furry, a conserved protein without
any known functional domains. sax-2 plays an important role in maintaining axon morphology.
The mutation sax-2(bx130) causes ray neurons to send supernumerary processes in the adult
stage and reduces the density of the presyanptic vesicles in the PAG. It also affects the stability
of amphid sensory neurons, as do other sax-2 alleles (Zallen et al, 1999). Other studies have
shown that furry is required for cellular morphogenesis and the polarity of cell division in the fly
and yeast. The disruption of neuronal morphology indicates furry is also required for maintaining
integrity of the mature nervous system. Consistently, the reduced density of presynaptic vesicles
and the low mating efficiency of males suggest that sax-2 may play a role in establishing neural
circuits important for copulation.
Finally, two genes, egl-35 and rax-1 (ray axon defect), specifically control ray axon guidance
and also appear to affect sex behaviors. Mutations in egl-35 and rax-1 cause ray axons to fail to
project into the PAG, but do not affect the non-sex- specific axons, consistent with normal
non-sex-specific behaviors like locomotion. However, egl-35(bx129) males loose attraction to
hermaphrodites and fail to initiate any steps of the mating program. Our assay for mate searching
(sexual motivation) suggests that bx129 males may have reduced sex drive. Males of
rax-1(bx132) also exhibit reduced mating frequency. Currently, we are cloning them and
analyzing the genetic interaction with known guidance factors and other rax genes.