Worm Breeder's Gazette 14(2): 83 (February 1, 1996)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

sma-1 is required for morphogenesis of the C. elegans embryo

Caroline McKeown, Alpa Patel , Vida Praitis, Judith Austin

Department of Molecular Genetics and Cell Biology University of Chicago 920 East 58th St. Chicago, IL 60637

Morphogenesis is the process by which an embryo is transformed from a
ball of cells into its mature form.  At the start of C. elegans
morphogenesis, the actin cytoskeleton in hypodermal cells reorganizes
into a highly ordered structure which, if disrupted, prevents embryonic
elongation (Priess & Hirsch, Dev. Bio. 117:156-173).  We are interested
in how this cytoskeletal reorganization occurs, what cytoskeletal
components are required for morphogenesis, and how the process of
elongation is regulated.

We have isolated embryos defective in morphogenesis that carry mutations
in the sma-1 gene.  In sma-1 mutant embryos elongation is initiated but
ceases prematurely, resulting in short, fat larvae with round heads. 
The length of newly hatched sma-1 larvae ranges from 50% of wild type
for severe alleles to 75% of wild type for weaker alleles.  The
reduction in length occurs along the entire animal, with head, body and
tail lengths decreased proportionately.  Interestingly, the pharynx of
sma-1 mutants is also short and fat.  Additional sma-1 phenotypes
include a defect in extension of the excretory canal (see Buechner,
Hall, and Hedgecock, WBG 13(5):75), and a twisted cuticle which produces
a weak roller phenotype in adults.  Preliminary results from phalloidin
staining experiments indicate sma-1 embryos have abnormalities in the
actin cytoskeleton during morphogenesis.  We have not yet determined
whether any of the sma-1 alleles we have isolated is a null, and are
using complementation screens to isolate additional alleles including
sma-1 deletions.  

The sma-1 mutant phenotype is rescued by injection of either the entire
cosmid C10C2 or an 11 kb subclone.  Partial sequence from the rescuing
subclone contains homology to a number of interesting cytoskeletal
protein domains, including an SH3 domain, multiple spectrin repeats, and
a pleckstrin homology domain.  The order and sequences of these domains
show strong homology to the Drosophila  ßH isoform of spectrin. 
Drosophila   ßH-spectrin expression is developmentally regulated and
restricted to musculature and ectodermally-derived epithelia.  In
general, ßH-spectrin protein localizes to the apicolateral portions of
cells, but in cells undergoing morphogenesis, an apical cap of
ßH-spectrin is observed (Thomas & Kiehart, Dev. 120: 2039-50).  We are
continuing to sequence the 11 kb rescuing fragment and flanking regions
to determine if this molecule, like ß-spectrins, contains an actin
binding domain, and is the C. elegans homolog of ßH-spectrin.  

We have performed in situ hybridizations in C. elegans embryos using a 2
kb subclone of the 11 kb rescuing fragment.  RNA staining first appears
at the lima bean stage of early morphogenesis in the dorsal hypodermis. 
By comma stage, we observe decreased RNA staining intensity in dorsal
hypodermis and new RNA staining in ventral hypodermis and gut.  RNA
staining in the hypodermis is nearly gone by the 1.5-fold stage, while
the gut still contains RNA staining.  We observe no RNA staining in
two-fold and later stage embryos.  We are continuing to examine the RNA
expression pattern of the gene encoded by the rescuing fragment, both by
Northern blot and in situ analysis, in larval and adult stages.  We are
also planning to generate antibodies to examine the expression pattern
and intracellular localization of the sma-1 protein in C. elegans.