Complexity of developmental control: Analysis of embryonic cell lineage specification in Caenorhabditis elegans using pes-1 as an early marker

In the early Caenorhabditis elegans embryo five somatic founder cells are b orn during the first cleavages. The first of these founder cells, named AB, gives rise to 389 of the 558 nuclei present in the hatching larva. Very fe w genes directly involved in the specification of the AB lineage have been identified so far. Here we describe a screen of a large collection of mater nal-effect embryonic lethal mutations for their effect on the early express ion of a pes-1::lacZ fusion gene. This fusion gene is expressed in a charac teristic pattern in 14 of the 32 AB descendants present shortly after the i nitiation of gastrulation. Of the 37 mutations in 36 genes suspected to be required specifically during development, 12 alter the expression of the pe s-1::lacZ marker construct. The gene expression pattern alterations are of four types: reduction of expression, variable expression, ectopic expressio n in addition to the normal pattern, and reduction of the normal pattern to gether with ectopic expression. We estimate that similar to 100 maternal fu nctions are required to establish the pes-1 expression pattern in the early embryo.