Two pathways of maternal RNA localization at the posterior-vegetal cytoplasm in early ascidian embryos

A cDNA library prepared from fertilized eggs of the ascidian Halocynthia ro retzi was screened for prelocalized mRNAs in the early embryo by means of w hole-mount in situ hybridization using a digoxigenin-labeled antisense RNA of each clone. Random mass screening of 150 cDNAs in a fertilized egg yield ed six different clones which showed mRNA localization in the posterior-veg etal cytoplasm of the 8-cell embryo. An in situ hybridization study of the detailed spatial distribution of each mRNA in embryos of various stages rev ealed that there are, in contrast to the identical localization in embryos after the 16-cell stage, two distinct patterns of RNA distribution at earli er stages. One is colocalization with the myoplasm from the prefertilizatio n stage to the 8-cell stage (type I postplasmic RNAs). The other is delayed accumulation of RNA at the posterior-vegetal cytoplasm after fertilization (type II postplasmic RNAs). We found that both types of RNAs associate wit h the cytoskeleton, but that they show different sensitivities to inhibitor s of the cytoskeleton; translocation of the type I RNAs is dependent upon m icrofilaments during the first phase of ooplasmic segregation and dependent upon microtubules during the second phase of segregation, whereas transloc ation of the type II RNAs is dependent upon microfilaments throughout oopla smic segregation. These results show that there are two pathways for the lo calization of the RNAs at the posterior-vegetal cytoplasm in the 8-cell emb ryo of the ascidian H. roretzi. (C) 2000 Academic Press.