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In order to provide evidence for a potential role of heterotrimeric GTP-binding proteins in the transduction of developmental signals, we prepared cDNAs from Xenopus laevis embryos and looked for fragments amplified between primers located in conserved sequences of the different subtypes of beta subunit. Using the amplified fragment as a probe, we cloned a member of the beta subunit family. The deduced protein sequence of the amphibian cDNA is highly homologous to the beta 1 subtype and, accordingly, we have named the Xenopus gene XG beta 1. In situ hybridization and RNase protection assay revealed that XG beta 1 mRNA is confined to the animal hemisphere of the mature oocyte. This localization of XG beta 1 mRNA is established at stage V during oogenesis. Following fertilization, the maternal mRNAs cosegregate with animal cells during cleavage stages. At gastrulation, transcripts are expressed in the dorsal ectoderm layer that will give rise to the central nervous system. Thus, XG beta 1 mRNA belongs to the small family of localized maternal mRNAs; as a transducing protein, its restriction to a subset of embryonic cells could mediate the distinct responsiveness which contributes to the patterning of the embryo.

We have developed and characterized a battery of specific polyclonal antibodies directed against specific portions of the alpha-chain of collagen type IV synthesized in Drosophila by the gene DCg1. Here, we describe the use of these antibodies together with in situ hybridization experiments in an attempt to study the expression and localization of collagen type IV during Drosophila oogenesis and early embryogenesis. The results clearly demonstrate that DCg1 is maternally expressed by follicle cells and that the collagen type IV chain produced is stockpiled in the growing oocyte. During the gastrulation stages, this component of Drosophila basement membranes concentrated on cells involved in the gradual invaginations leading to morphogenetic furrows. The presence of collagen type IV, which is an RGD-bearing molecule, during early stages of Drosophila development is discussed in comparison with the crucial, active role its vertebrate counterpart is supposed to play in morphogenetic processes.

During meiotic prophase I the nucleolus of the mouse oocyte assumes a reticulate structure of 'nucleolonema' type. This change coincides with the appearance of several secondary fibrillar centres. The number of these centres at diplotene (97-113), largely exceeds that of nucleolar organizers (4c DNA = 20 NORs). The quantitative analysis of autoradiographs after hybridization in situ with 3H-uridine labelled rRNA, enabled us to demonstrate that the multiplication of the fibrillar centres in mouse oocyte nucleolus during meiotic prophase I is not the result of an amplification of the rDNA. The number of silver grains in pachytene and diplotene nuclei was twice that counted for somatic cell and oogonium nuclei (2c DNA).