Healthcare and Medicine Reference
Defective touch cell
mec-7, mec-12, mec-17 expression
FIGURE 4.3 Intrinsic determinants at different steps on a neural lineage in the nematode. A. Normal
lineage. B. If Q1.p cannot express unc-86 , it becomes Q', a copy of its mother, Q. The result is a repeat of the
previous division, which results only in Q progenitors and ciliated sensory neurons. C. Mec-3 needs to be
expressed in the touch cell. If it is not, as in mec-3 mutants, this cell turns into an interneuron like its sister.
D. In mec-7, mec-12, and mec-17 mutants, the touch neuron is correctly specified but differentiates missing
critical components of its morphology or function.
of mec-3 , genes that are likely important for the function
of the mechanosensory neurons.
Another set of C. elegans neurons that have been
studied in detail are the hermaphrodite-specific egg-
laying neurons (Desai et al., 1988; Desai and Horvitz,
1989). Mutants in genes involved in the determination
or function of these neurons are unable to lay eggs. The
result is that the self-fertilized eggs hatch inside the
mother and begin to feed within their mother's uterus.
The larvae proceed to devour their mother from the
inside. Eventually, with only her epidermis intact, she
becomes a bag of wriggling larval worms that in their
hunger eventually eat through her cuticle into the
world. Mutant lines in these genes thus seem to give
their offspring a protected start in life characteristic of
viviparous species but at what appears to be a mother's
ultimate altruistic sacrifice. The “bag of worms”
mutants have a phenotype that is easy to detect, and so
a large collection of such mutants has been identified.
Twenty or so genes have been found to define hierarchi-
cal transcriptional cascades affecting egg-laying neuron
development. Surprisingly, only one of these is also nec-
essary for the proper development of the touch cells.
This is our friend unc-86 . The role of unc-86 in the deter-
mination of the egg-laying neurons, however, is quite
different. Instead of controlling lineage as it does in
mechanosensory cells, it regulates neurotransmitter
expression and axon outgrowth in the egg-laying
The fact that there is surprisingly little overlap in the
genes that are involved in these two systems suggests
that the molecular cascades of neuronal determination
must be complex and highly individualized. However,
there are similarities that are worth emphasizing. In the
case of both the egg-laying and the mechanosensory
neurons, there is a hierarchical pathway, rich in tran-
scription factors that operate through the specific
lineages. These factors regulate other intrinsic tran-
scription factors in a molecular cascade whereby the
lineage, the specification, the differentiation, and
finally the physiological properties of the neurons are
established through a series of successive stages. This
molecular strategy, we will see, is also used in the
determination of neurons in most other species.