Is maternal mRNA a determinant of tissue-specific proteins in ascidian embryos?

INTRODUCTION The generation of different cell types during embryonic development is thought to be mediated by the combined activity of cytoplasmic factors (determinants), which are localized in the egg, and inductive interactions, which occur between different embryonic cells and tissues. Ascidians, animals that exhibit rapid and exceptionally autonomous development (reviewed by Jeffery, 1985), appear to employ cytoplasmic determinants to generate embryonic cell diversity. Although determinants have not been identified in ascidians or other animals, it is hypothesized that they function in at least two different ways. First, as initially pointed out by Morgan (1934), determinants may be regulatory factors which promote differential gene expression in specific cell lineages. Consistent with this possibility, inhibitors of transcription, added prior to gastrulation, block the appearance of some ascidian tissue-specific enzymes and morphological markers whose expression is regulated by the activity of cytoplasmic determinants (Whittaker, 1973; Crowther & Whittaker, 1984). Second, determinants may be localized factors which promote cell diversification independent of zygotic gene expression. This class of determinants is implicated in specifying crucial embryonic events that occur before the onset of zygotic transcription. Evidence for the second class of determinants in ascidians was first obtained from interspecific hybridization studies. These studies showed that haploid andromerogones, which developed from anucleate eggs of one ascidian species fertilized with sperm of another species, exhibited larval adhesive papillae that were morphologically identical to those of the maternal species (Minganti, 1959). This paper reviews the class of cytoplasmic determinants that function independently of embryonic gene expression in ascidian embryos. The review focuses on the possibility that maternal mRNA molecules serve as determinants for the expression of alkaline phosphatase in the endodermal cell lineage and the expression of muscle actin in the muscle cell lineage.