CONSERVATION OF BRACHYURY-(T) GENES IN AMPHIOXUS AND VERTEBRATES - DEVELOPMENTAL AND EVOLUTIONARY IMPLICATIONS

Homologues of the murine Brachyury (T) gene have been cloned from seve ral vertebrates, and are implicated in mesoderm formation and in diffe rentiation of the notochord, In contrast, the roles of the ascidian Br achyury gene may be restricted to presumptive notochord, To understand the evolution of Brachyury genes and their developmental roles, we ha ve searched for homologues in amphioxus, representing the third chorda te subphylum and the probable closest relative of the vertebrates. We report the isolation of two amphioxus cDNA clones with clear homology to Brachyury genes, and demonstrate that these derive from separate lo ci resultant from a recent gene duplication. This finding represents a n exception to the emerging consensus of an archetypal prevertebrate g enome in amphioxus, The spatial and temporal distribution of Brachyury transcripts during amphioxus development is remarkably similar to ver tebrate Brachyury, in presumptive mesoderm, posterior mesoderm and the notochord, Gene expression extends throughout the anteroposterior axi s of the notochord, despite the most rostral regions being a more rece nt specialization; it also persists into larval stages, despite differ entiation into contractile tissue, We propose that roles of Brachyury in notochord differentiation are more ancient than roles in mesoderm f ormation, and that the latter are shared by cephalochordates and all v ertebrates.