Worm Breeder's Gazette 13(3): 27 (June 1, 1994)
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.
Mutations in the par genes disrupt a number of asymmetries in the early embryo (Kemphues et al., Cell 52, 311-320). The processes affected include cleavage spindle position and orientation, localization of P granules, and development of intestine and germline. A new par gene has been discovered during a screen for maternal effect lethal mutations in the Priess lab. The first allele of this gene, par-6 ( zu170 ),was isolated as a putative par-3 allele because its early cleavage pattern resembles that of par-3 mutants. However, zu170 complements par-3 ( it71 )and maps to the right arm of chromosome I. Subsequently, two more alleles of this gene were found in the same screen, zu174 and zu222 ,both of which complement par-3 and fail to complement par-6 ( zu170 ).
During wild type development, the asymmetric position of the first spindle in the one cell results in two different sized blastomeres. In par-6 ,as well as the other pars (with the exception of par-4 ),the spindle is not positioned asymmetrically, resulting in equal sized AB and P1 blastomeres. During the second cell cycle the spindles orient differently in the AB and P1 blastomeres. The spindle orients perpendicular to the anterior/posterior axis in the AB cell (transverse), and parallel to the A/P axis in the P1 cell (longitudinal). In unsquashed par-6 ( zu170 )mutant embryos, both the AB and P1 divide longitudinally 66% of the time. This is similar to the par-3 phenotype, where both blastomeres divide longitudinally 60-90% of the time, depending on the severity of the allele. Another early asymmetry which is disrupted in all of the par mutants is the localization of germline specific P granules. In four cell stage par-6 ( zu170 )embryos, P granules are localized improperly most of the time.
All of the pars also show defects in the germline and intestinal lineages. We have found only two embryos (out of 3500) from zu170 homozygotes that hatched and developed into adults, and both of these were Gon, consistent with a defect in the germline lineage. We have also scored for intestinal granules in late stage embryos of all three par-6 alleles and 25-37% of embryos are gut granule (-), indicating that they fail to differentiate intestine. This extent of intestinal differentiation failure is not nearly as severe as that caused by mutations in par-1 or par-4 ,but is similar to that of weak alleles of par-3 .
We were curious to determine the double mutant phenotypes of par-6 in combination with the other pars, since some other double mutant combinations show some interactions, such as enhancement (Kemphues lab, unpublished results). When we constructed the double mutant of par-6 and par-2 ,we found that one copy of the par-6 ( zu170 )mutation can suppress par-2 ( it5ts ).At 25°, close to 90% (after correcting for lethality caused by the hT2 translocation) of the embryos of worms of the genotype par-2 ( it5ts ) daf-7 ; par-6 ( zu170 70) unc-131 hT2 hatched and 60% of these worms were fertile as adults; only 8% of the embryos from the control par-2 ( it5ts ); hT2 worms hatched and all of these worms became adult Gons. The suppression is dominant, and gene specific, as one copy of par-6 ( zu170 )does not suppress mutations in par-l, par-3 , par-4 ,or par-5 However, the suppression is not specific to the it5ts allele. 77% of the embryos from worms of the genotype par-2 (1 w32 ) dpy-1 ; par-6 ( zu170 ) unc-13 /++hatched, although 95% of these worms grew up to be Gon. The lw32 allele is a putative null (Levitan et al., PNAS, in press). We are currently testing if the other alleles of par-6 can suppress par-2 ( it5ts )and par-2 (1 w32 ).