9508642 Slocum The process by which a single fertilized egg gives rise to specialized cell types and structures in an adult organism remains one of the major unsolved mysteries in developmental biology. In plants, differentiated tissues and organ systems are defined early in plant development and the occurrence of these differentiated structures reflects the initial establishment of embryonic polarity and subsequent differentiation of specific cell types. The body of knowledge concerning the mechanisms of cell differentiation in early plant embryogenesis is scant, in part due to difficulties in accessing embryonic tissue in higher plants. The molecular basis of polarity establishment and cell differentiation is being studied in the brown alga, Pelvetia. In Pelvetia, eggs are abundant and easily obtained. Unfertilized eggs are phenotypically apolar, but express increasing asymmetry during the first cell cycle. The first zygotic division gives rise to two distinct cell types with different developmental fates. Experiments are being conducted to test the hypothesis that mRNAs are segregated to different regions of the developing Pelvetia zygote and are then unequally partitioned at the first cell division. Subsequently, proteins translated from the localized mRNAs determine the fates of the embryonic cells in which they reside. Spatially localized mRNAs are being characterized to determine how proteins from these mRNAs specify cell fate and to determine how they get targeted to and localized in distinct regions of the cytoplasm. ***
