Status of RNAs, localized in Xenopus laevis oocytes, in the frogs Rana pipiens and Eleutherodactylus coqui

Abstract

Early development in the frog model, Xenopus laevis, is governed by RNAs, localized to the vegetal cortex of the oocyte. These RNAs include Xdazl RNA, which is involved in primordial germ cell formation, and VegT RNA, which specifies the mesoderm and endoderm. In order to determine whether orthologues of these RNAs are localized and have similar functions in other frogs, we cloned RpDazl and RpVegT from Rana pipiens, a frog that is phylogenetically distant from X. laevis. RNAs from both genes are localized to the vegetal cortex of the R. pipiens oocyte, indicating that the vegetal localization is likely the basal state. The animal location of EcVegT RNA in Eleutherodactylus coqui that we found previously (Beckham et al., 2003) is then a derived state, probably due to the great increase in egg size required for direct development of this species. To answer the question of function, we injected RpVegT or EcVegT RNAs into X. laevis embryos, and assayed animal caps for gene expression. Both of these RNAs induced the expression of endodermal, mesodermal, and organizer genes, showing that the function of RpVegT and EcVegT as meso-endodermal determinants is conserved in frogs. The RNA localizations and the function of VegT orthologues in germ layer specification may be synapomorphies for anuran amphibians.

Transcription factors Mix1 and VegT, relocalization of vegt mRNA, and conserved endoderm and dorsal specification in frogs

“…These findings demonstrated that the function of vegt as meso-endodermal determinant is conserved in frogs (16). The RNA localizations and functions of vegt orthologs in germ-layer specification may be synapomorphies for anuran amphibians (16).…”

“…A high degree of vegt conservation was detected in these anurans (8). Moreover, when Ec_vegt or Rp_vegt mRNAs were injected into two-cell embryos of X. laevis, endodermal, mesodermal, and organizer genes were induced (16). These findings demonstrated that the function of vegt as meso-endodermal determinant is conserved in frogs (16).…”

“…1 and 2). Moreover, the orthologous genes Ec_vegt and Rp_vegt can induce expression of mix1 in X. laevis embryo assays (16). Thus, the function of vegt and mix1 in frog endoderm specification is conserved.…”

“…Moreover, when Ec_vegt or Rp_vegt mRNAs were injected into two-cell embryos of X. laevis, endodermal, mesodermal, and organizer genes were induced (16). These findings demonstrated that the function of vegt as meso-endodermal determinant is conserved in frogs (16). The RNA localizations and functions of vegt orthologs in germ-layer specification may be synapomorphies for anuran amphibians (16).…”

“…The vegt ORF has been sequenced in the amphibians E. coqui (Ec_vegt), Rana pipiens (Rp_vegt), and Ambystoma mexicanum (Am_vegt) (16)(17)(18) and partially sequenced in Epipedobates machalilla (Cm_vegt), Pipa pipa (Pp_vegt), and Gastrotheca riobambae (Gr_vegt) (8). A high degree of vegt conservation was detected in these anurans (8).…”