In animal development, epidermal tissues undergo complex structural changes during morphogenesis. During morphogenesis in C. elegans, the embryo elongates from a ball of cells into the cylindrical worm like shape. This elongation process occurs by complex cytoskeletal rearrangements in the hypodermis that lead to cell shape changes in the embryo (Priess and Hirsh, 1986 Dev. Biol. 117: 156-173; Costa et al., 1997 Dev. Biol. 184: 373-384). We have isolated a mutant,
imo-1(
ra409) (incomplete morphogenesis), that fails to elongate entirely (1.5 - 2-fold) and arrests just after hatching. Although all tissues types examined have differentiated in these mutants, many do not appear wild type. In particular, the body wall muscle and the overlying basement membrane are 2-fold wider than in wild type animals. In addition, muscle filaments in
imo-1(
ra409) animals are abnormally oriented to the longitudinal axis of the embryo. This may be a consequence of either the lack of complete elongation or the expansion of muscle cells or basement membrane. Also, the pharynx is much shorter, has abnormal pharyngeal myofilaments and labors during contraction. The actin filament organization and the antigen recognized by MH4 (intermediate filament like antigen) in the hypodermis both appear normal, with the exception that the MH4 staining pattern is two fold wider corresponding to the wider muscle quadrants in this mutant.
sma-1 animals, although viable, resemble
imo-1(
ra409) mutants during late embryogenesis and early L1. J. Austin and colleagues have recently identified
sma-1 as a beta-spectrin (McKeown, C., Praitis, V. and Austin, J. 1997 Worm Meeting abstract). This is of interest because the region on the X chromosome where
imo-1(
ra409) was mapped contains an alpha-spectrin (C. elegans genome sequencing consortium). Since beta- spectrin forms heterodimers with alpha-spectrin, we felt this alpha- spectrin a likely candidate gene for
imo-1. To demonstrate that the
imo-1(
ra409) lesion is in the alpha- spectrin gene we used RNA interference, transformation rescue and Tc1 interruption of the alpha-spectrin gene.
imo-1 is ~58% identical to the vertebrate non-erythroid alpha-spectrin and ~63% identical to the Drosophila alpha-spectrin. From scanning the genomic sequence generated to date, this is the only alpha-spectrin found in the genome. To investigate whether there is any maternal contribution of the lone alpha spectrin gene,
imo-1(
ra409) germline mosaics were obtained using an unstable rescued line. The progeny from these mosaics did not demonstrate a more severe phenotype than
imo-1(
ra409) homozygotes from heterozygous parents. This suggests that there is no significant maternal contribution and that the spectrin cytoskeleton is not necessary before the onset of morphogenesis during embryogenesis. Spectrin molecules are major components of the membrane cytoskeleton and are important for maintaining cell shape and polarity.
imo-1 and other alpha spectrins contain several spectrin repeats, an SH3 domain and a calcium binding region which are all potential sites for protein- protein interaction. Spectrins role in non erythrocytes are far from being understood, therefore studying their role in C. elegans morphogenesis should shed some light on their function.