Orientation of cell division is critical for the partitioning of cytoplasmic factors and setting up cellular interactions necessary for proper development. In C. elegans , the first cleavage bisects the long axis of the embryo, resulting in an anterior AB cell and posterior P1 cell. Subsequent cleavages in the AB lineage are symmetrical and follow an orthogonal pattern of division. Divisions in the P1 lineage are asymmetric and occur repeatedly on the same axis, due to a 90 o rotation of the nuclear-centrosome complex. We are using time-lapsed video microscopy to study a number of maternal effect lethal mutations that alter spindle orientation in early embryos. The
spn-2 gene affects spindle orientation at the 2 and 4-cell stage.
spn-2 /sDf130 worms have defects in oocyte formation. Together these data suggest a continuing role for
spn-2 in spindle orientation during embryogenesis, as well as a possible role earlier in the germline. Some
spn-3 embryos misorient their spindle in the 1-cell, while others show defects at the 2 or 4-cell stage.
spn-3 /sDf127 embryos all have orientation defects during the first division. These data suggest that
spn-3 plays a role in spindle orientation during the first three divisions after fertilization. Variability in the phenotypes of
spn-2 and
spn-3 mutants, coupled with the stronger defects of each allele over a deficiency, suggest that we do not have null alleles for these genes. Future characterization of the null phenotypes, using either stronger alleles or RNAi, will provide more information as to the role these genes are playing in spindle orientation. Mutations in
spn-1 also cause defects in spindle orientation at the second and third cleavages. Double mutant
spn-1 ;
spn-3 embryos have stronger defects in spindle orientation than mutations in either gene alone, suggesting that these genes are either acting in the same pathway or in partially redundant pathways. Mapping of
spn-2 and
spn-3 is being performed as a first step towards cloning. The
spn-2 gene has been placed within sDf130, to the left of sDf121, on chromosome III.
spn-3 , also on chromosome III, has been placed within sDf127 between sDf125 and sDf135. Once cloned, a more detailed examination of these genes will be completed to elucidate their role in cell division orientation.