Spatial and temporal transcription patterns of the forkhead related XFD-2/XFD-2' genes in Xenopus laevis embryos

Two novel fork head related cDNA sequences, termed XFD-2 and XFD-2', have been isolated from a Xenopus laevis gastrula stage cDNA library. XFD-2 and XFD-2' proteins share 88% sequence identity; a comparison of their fork head domains yields 96% identity. Such close homology suggests that the two genes represent pseudo-allelic variants of a common ancestor and probably arose by the ancient tetraploidization event in this species. Both genes are activated at midblastula transition. Main transcriptional activity is found during blastula and gastrula stages of development; thereafter, there is a gradual decrease of transcripts until somite segregation stages. Whole mount in situ hybridisation of blastula stage embryos reveals that the genes are initially transcribed within the animal hemisphere. Subsequently, we observe their transcription in a circumferential mode along the marginal zone, i.e., within the forming mesoderm. During gastrulation, these cells enter the blastoporus at the ventral, lateral and dorsal sites. At the end of gastrula and during neural stages transcripts are localized within somitogenic mesoderm, notochord, lateral and ventral mesoderm, neural floor plate, spinal cords and in the developing brain.