The large size of the transparent early embryos and the powerful genetics of C. elegans, make it attractive for studies of cell division. Therefore, we have isolated and observed early embryonic cell divisions in approximately 600 temperature-sensitive, embryonic-lethal mutants in an ongoing screen in the lab. About 35 mutants appear defective in microtubule- and/or microfilament-dependent processes, including pronuclear migration, centrosome function, mitotic spindle assembly or orientation, and cytokinesis. We isolated ts-alleles of several genes known to affect these processes, including an aurora-like kinase (
air-2) and an MKLP1-like kinesin (
zen-4) [see abstract by A.F. Severson], as well as several previously unidentified genes [see abstracts by J.E. Gomes and R. Lyczak]. Here we describe two temperature-sensitive mutants,
spd-4 and
spd-5, that are required for mitotic spindle assembly and function (spd=spindle-defective).
spd-4 mutant embryos assemble bipolar mitotic spindles, but they are shorter than wild type and do not elongate.
spd-4 mutant embryos also have defects in DNA segregation and cytokinesis. Pronuclear migration is defective in
spd-5 mutant embryos, a mitotic spindle does not form, and the first cell division fails. Intriguingly,
spd-4 and
spd-5 show genetic interactions that suggest they function together in a complex to regulate mitotic spindle assembly in the early C. elegans embryo. From the map position and phenotype of
spd-4, as well as genetic complementation analysis (in collaboration with D. Schmidt, S. Strome, and W. Saxton, Indiana University) we believe that
spd-4 might encode a dynein heavy chain gene, although this still needs to be confirmed. Injection of dsRNA corresponding to the Genefinder locus F56A3.4 phenocopies the
spd-5 mutant phenotype. We are sequencing this locus to determine the molecular nature of the lesion. F56A3.4 encodes a coiled-coil protein with no significant similarity to other proteins apart from this motif. Therefore, if
spd-4 is dynein heavy chain,
spd-5 likely represents a novel dynein regulator.