TOPOGRAPHIC CHANGES IN NASCENT AND EARLY MESODERM IN AMPHIOXUS EMBRYOS STUDIED BY DIL LABELING AND BY IN-SITU HYBRIDIZATION FOR A BRACHYURYGENE

In amphioxus embryos, the nascent and early mesoderm (including chorda -mesoderm) was visualized by expression of a Brachyury gene (AmBra-2). A band of mesoderm is first detected encircling the earliest (vegetal plate stage) gastrula sub-equatorially. Soon thereafter, the vegetal plate invaginates. resulting in a cap-shaped gastrula with the mesoder m localized at the blastoporal lip and completely encircling the blast opore. As the gastrula stage progresses, DiI (a vital dye) labeling de monstrates that the entire mesoderm is internalized by a slight involu tion of the epiblast into the hypoblast all around the perimeter of th e blastopore. Subsequently. during the early neurula stage, the intern alized mesoderm undergoes anterior extension mid-dorsally (as notochor d) and dorsolaterally (in paraxial regions when segments will later fo rm). By the late neurula stage, AmBra-2 is no longer transcribed throu ghout the mesoderm as a whole; instead. expression is detectable only in the posterior mesoderm and in the notochord, but not in par axial m esoderm where definitive somites have formed.