Vegetal localization of maternal mRNAs is disrupted by VegT depletion

VegT is an essential maternal regulator of germ layer specification in Xenopus. The localization of VegT mRNA to the vegetal cortex of the oocyte during oogenesis ensures its inheritance by vegetal and not animal cells, and directs the differentiation of vegetal cells into endoderm. Similarly localized mRNAs, Vg1 and Bicaudal-C, are also inherited by vegetal cells, while germ plasm-associated mRNAs, such as Xcat2, become incorporated into vegetally derived primordial germ cells. Although mRNA localization is clearly important for tissue specification, the mechanism of mRNA anchoring to the oocyte vegetal cortex is not understood. Here, we examine the role of VegT in cortical localization. We report that depletion of VegT mRNA caused the release of Vg1 mRNA from the vegetal cortex and a reduction of Vg1 protein, without affecting the total amount of Vg1 transcript. Furthermore, we found that Bicaudal-C and Wnt11 mRNAs were also dispersed, but not degraded, by VegT depletion, while the localization of Xcat2 and Xotx1 mRNAs was unaffected. This effect was specific to the loss of VegT mRNA and not VegT protein, since a morpholino oligo against VegT, that blocked translation without degrading mRNA, did not disperse the vegetally localized mRNAs. Therefore, a subset of localized mRNAs is dependent on VegT mRNA for anchoring to the vegetal cortex, indicating a novel function for maternal VegT mRNA.