THE MATERNAL PAR GENES AND THE SEGREGATION OF CELL FATE SPECIFICATIONACTIVITIES IN EARLY CAENORHABDITIS-ELEGANS EMBRYOS

After fertilization in C. elegans, activities encoded by the maternall y expressed par genes appear to establish cellular and embryonic polar ity Loss-of-function mutations in the par genes disrupt anterior-poste rior (a-p) asymmetries in early embryos and result in highly abnormal patterns of cell fate. Little is known about how the early asymmetry d efects are related to the cell fate patterning defects in par mutant e mbryos, or about how the par gene products affect the localization and activities of developmental regulators known to specify the cell fate patterns made by individual blastomeres, Examples of such regulators of blastomere identity include the maternal proteins MEX-3 and GLP-1, expressed at high levels anteriorly, and SKN-1 and PAL-1, expressed at high levels posteriorly in early embryos. To better define par gene f unctions, we examined the expression patterns of MEX-3, PAL-1 and SKN- 1, and we analyzed mex-3, pal-1, skn-1 and glp-1 activities in par mut ant embryos, We have found that mutational inactivation of each par ge ne results in a unique phenotype, but in no case do we observe a compl ete loss of a-p asymmetry, We conclude that no one par gene is require d for all a-p asymmetry and we suggest that, in some cases, the par ge nes act independently of each other to control cell fate patterning an d polarity. Finally, we discuss the implications of our findings for u nderstanding how the initial establishment of polarity in the zygote b y the par gene products leads to the proper localization of more speci fically acting regulators of blastomere identity.