Growth Cone Guidance and Neuronal Recognition in Drosophila 
the following section) . Five of the six glycopro- 
teins are dynamically expressed on overlapping 
subsets of growth cones, axon fascicles, and glia 
during embryonic development; the sixth, neuro- 
glian, is more broadly expressed on the surface of 
most axons and glia. Fasciclin II, neuroglian, and 
fasciclin III are members of the immunoglobulin 
superfamily; connectin is a member of the leu- 
,cine-rich repeat family; fasciclin I and fasciclin 
rv are neither related to each other nor thus far to 
anything else in the data bank. Five of the six 
proteins (except for fasciclin IV) can function as 
homophilic cell adhesion molecules. 
Genetic analysis has shown that fasciclin II is 
indeed a neuronal recognition molecule that 
plays an important role in specific growth cone 
guidance. The fasciclin II protein is normally ex- 
pressed on a subset of growth cones and axons 
that pioneer and selectively fasciculate in the 
MPl axon fascicle; later it is expressed on several 
other axon pathways. In embryos that are mutant 
for the fas II gene, although these specific 
growth cones extend, they do not properly recog- 
nize one another and fail to fasciculate. These 
studies are the first molecular confirmation of the 
existence of functional labels on specific axon 
pathways in the developing nervous system. 
A major current goal is to determine the molec- 
ular machinery whereby the activation of fasci- 
clin II, by either another fasciclin II molecule or 
a different ligand on the surface of neighboring 
neurons, instructs a fasciclin Il-expressing 
growth cone to extend toward and along these 
specific cells. We have established a highly sensi- 
tive assay that utilizes a mutation in the fas II 
gene that reduces the amount of fasciclin II pro- 
tein to the minimal amount required for fas II 
function. Using this assay we are looking for other 
genes in which a 50 percent reduction in the 
level of their protein product results in a failure 
of fas II function. In this way we are in the pro- 
cess of identifying genes that appear to encode 
products that interact with the fas II gene and 
thus function in the events of neuronal 
recognition. 
Our previous studies have shown that the ex- 
pression of surface recognition molecules is dy- 
namic and regional on the surface of individual 
neurons; i.e., parts of the cell are differentially 
labeled in accordance with the processes around 
it for which it has a selective affinity. We wish to 
uncover the signals that instruct a neuron, as it 
navigates from one pathway to another, to change 
the expression of surface recognition molecules 
on its growth cone. For example, the growth 
cones of many commissural interneurons extend 
right past longitudinal axon pathways on their 
own side; once they cross the midline, they dis- 
play a high affinity for one of these axon pathways 
on the other side of the CNS. This change in the 
behavior of the growth cone is accompanied by 
an equally dramatic change in its expression of 
surface recognition molecules; neurons express 
different molecules on the surface of their longi- 
tudinal and commissural processes. We are inves- 
tigating whether extension across the midline is 
required for growth cones to make the appro- 
priate changes in surface expression and subse- 
quent pathway choices, by examining the behav- 
ior of specific growth cones in commissureless 
mutant embryos in which guidance toward the 
midline does not occur. Thus far it appears that 
some aspects of longitudinal guidance are per- 
turbed in these mutant embryos. 
Target Recognition 
Having navigated along a series of pathways, 
growth cones are ultimately capable of recogniz- 
ing their correct target cells. In the Drosophila 
embryo, the specificity of neuronal growth cones 
for target cells is most clearly studied in the abil- 
ity of motoneuron growth cones to recognize spe- 
cific muscle fibers. A large-scale molecular ge- 
netic screen has led to the identification of 
several genes that are expressed by different sub- 
sets of undiff^erentiated muscle fibers prior to in- 
nervation. A new gene, connectin, and a previ- 
ously identified gene. Toll, encode membrane 
proteins that are members of the leucine-rich re- 
peat family of cell adhesion and signaling mole- 
cules. The connectin gene encodes a protein that 
is expressed on the surface of eight (of the 31) 
muscle fibers in each abdominal segment and on 
the grovvT:h cones and peripheral axons of the 
very motoneurons that innervate these eight mus- 
cle fibers. The growth cones of this subset of mo- 
toneurons, and the undifferentiated muscle 
fibers they innervate, express the connectin pro- 
tein prior to contact and synapse formation. In 
vitro studies show that the connectin protein is 
capable of functioning as a homophilic cell adhe- 
sion molecule. Thus this protein is a good candi- 
date to be a bona fide target recognition molecule 
involved in the formation of specific synaptic 
connections. A genetic analysis of connectin 
function is presently in progress. In addition, a 
large-scale genetic screen is under way to identify 
mutations that perturb the guidance of motoneu- 
ron growth cones. 
170 
