No abstract

In Xenopus, localization of a rare class of mRNAs during oogenesis is believed to initiate pattern formation in the early embryo. We have determined the pattern of RNA localization for one of these RNAs, Xcat-2, which encodes a putative RNA-binding protein related to Drosophila nanos (Mosquera, L., Forristall, C., Zhou, Y. and King, M. L. (1993) Development 117, 377–386). Xcat-2 is exclusively localized to the mitochondrial cloud in stage I oocytes, moves with this body into the vegetal cortex during stage II and, later, partitions into islands consistent with it being a component of the germ plasm. As previously shown, Vg1 is not localized to the vegetal cortex until stage IV and distributes to all vegetal blastomeres during development. We found a direct correlation between the localized condition of these RNAs and their recovery in a detergent-insoluble fraction. We present evidence suggesting that differential RNA binding to a cytoskeletal component(s) in the vegetal cortex determines the pattern of inheritance for that RNA in the embryo.

Background: The inner ear develops from an ectodermal thickening known as the otic placode into a complex structure that is asymmetrical along both the anterior-posterior (A-P) and dorsal-ventral (D-V) axes. Embryological manipulations in Xenopus allow us to test regenerative potential along specific axes and timing of axis determination. We explore the role of Wnt signaling with gain and loss of function experiments. Results: In contrast to A or P half ablations, D or V half ablations almost never result in mirror duplications or normal ears. Instead there is a loss of structures, especially those associated with the ablated region. Rotation experiments inverting the D-V axis reveal that it is determined by stage 24-26 which is just before expression of the dorsal otic marker Wnt3a. Conditional blocking of canonical Wnt signaling results in reductions in the number of sensory organs and semicircular canals which could be placed in one of three categories, the most common phenotypes being similar to those seen after dorsal ablations. Conclusions: There is less regenerative potential along the D-V axis. Wnt3a protein alone is sufficient to rescue the severe loss of inner ear structures resulting from dorsal but not ventral half ablations. Developmental Dynamics 243:1262-1274,

mRNAs concentrated in specific regions of the oocyte have been found to encode determinants that specify cell fate. We show that an intermediate filament fraction isolated from Xenopus stage VI oocytes specifically contains, in addition to Vg1 RNA, a new localized mRNA, Xcat-2. Like Vg1, Xcat-2 is found in the vegetal cortical region, is inherited by the vegetal blasomeres during development, and is degraded very early in development. Sequence analysis suggests that Xcat-2 encodes a protein that belongs to the CCHC RNA-binding family of zinc finger proteins. Interestingly, the closest known relative to Xcat-2 in this family is nanos, an RNA localized to the posterior pole of the Drosophila oocyte whose protein product suppresses the translation of the transcription factor hunchback. The localized and maternally restricted expression of Xcat-2 RNA suggests a role for its protein in setting up regional differences in gene expression that occur early in development.