XK endo B is preferentially expressed in several induced embryonic tissues during the development of Xenopus laevis

XK endo B is a type I keratin that was originally identified by its preferential expression in the embryonic notochord of the amphibian Xenopus laevis. A peptide identical to a short region of its predicted amino acid sequence was used to generate antibodies against the XK endo B protein. This paper reports an immunocytochemical study of the spatial expression pattern of XK endo B during development. The protein was observed in the notochord and endoderm as predicted from previous RNA analysis. In addition, XK endo B was detected in the cement gland, in the pituitary, olfactory and pharyngeal pouch rudiments, and in a nonuniform distribution in the neural tube as well as the inner sensorial layer of the ectoderm. XK endo B expression is not limited to any germ layer or any particular cell type, but is nevertheless highly restricted in its distribution in the embryo. Its expression in several different embryonic tissues requiring inductive interactions for differentiation makes XK endo B a valuable tool with which to study the regulation of induced gene expression during embryogenesis.     

A novel function for the Sm proteins in germ granule localization during C. elegans embryogenesis

General mRNA processing factors are traditionally thought to function only in the control of global gene expression. Here we show that the Sm proteins, core components of the splicesome, also regulate germ granules during early C. elegans development. Germ granules are large cytoplasmic particles that localize to germ cells and their precursors during embryogenesis of diverse organisms. In C. elegans, germ granules, called P granules, are segregated to the germline precursor cells during embryogenesis by asymmetric cell division, and they remain in germ cells at all stages of development. We found that at least some Sm proteins are components of P granules. Moreover, disruption of Sm activity caused defects in P granule localization to the germ cell precursors during early embryogenesis. In contrast, loss of other splicing factor activities had no effect on germ granule control in the embryo. These observations suggest that the Sm proteins control germ granule integrity and localization in the early C. elegans embryo and that this role is independent of pre-mRNA splicing. Thus, a highly conserved splicing factor may have been adapted to control both snRNP biogenesis and the localization of components important for germ cell function.     

Non-canonical functions of hunchback in segment patterning of the intermediate germ cricket Gryllus bimaculatus

In short and intermediate germ insects, only the anterior segments are specified during the blastoderm stage, leaving the posterior segments to be specified later, during embryogenesis, which differs from the segmentation process in Drosophila, a long germ insect. To elucidate the segmentation mechanisms of short and intermediate germ insects, we have investigated the orthologs of the Drosophila segmentation genes in a phylogenetically basal, intermediate germ insect, Gryllus bimaculatus (Gb). Here, we have focused on its hunchback ortholog (Gb'hb), because Drosophila hb functions as a gap gene during anterior segmentation, referred as a canonical function. Gb'hb is expressed in a gap pattern during the early stages of embryogenesis, and later in the posterior growth zone. By means of embryonic and parental RNA interference for Gb'hb, we found the following: (1) Gb'hb regulates Hox gene expression to specify regional identity in the anterior region, as observed in Drosophila and Oncopeltus; (2) Gb'hb controls germband morphogenesis and segmentation of the anterior region, probably through the pair-rule gene, even-skipped at least; (3) Gb'hb may act as a gap gene in a limited region between the posterior of the prothoracic segment and the anterior of the mesothoracic segment; and (4) Gb'hb is involved in the formation of at least seven abdominal segments, probably through its expression in the posterior growth zone, which is not conserved in Drosophila. These findings suggest that Gb'hb functions in a non-canonical manner in segment patterning. A comparison of our results with the results for other derived species revealed that the canonical hb function may have evolved from the non-canonical hb functions during evolution.     

Temporal complexity within a translational control element in the nanos mRNA

Translational control of gene expression plays a fundamental role in the early development of many organisms. In Drosophila, selective translation of nanos mRNA localized to the germ plasm at the posterior of the embryo, together with translational repression of nanos in the bulk cytoplasm, is essential for development of the anteroposterior body pattern. We show that both components to spatial control of nanos translation initiate during oogenesis and that translational repression is initially independent of Smaug, an embryonic repressor of nanos. Repression during oogenesis and embryogenesis are mediated by distinct stem loops within the nanos 3' untranslated region; the Smaug-binding stem-loop acts strictly in the embryo, whereas a second stem-loop functions in the oocyte. Thus, independent regulatory modules with temporally distinct activities contribute to spatial regulation of nanos translation. We propose that nanos evolved to exploit two different stage-specific translational regulatory mechanisms.