Heterochrony in the Germ Ring Closure and Tail Bud Formation in Embryonic Development of Rainbow Trout (Oncorhynchus mykiss)

Due to their large yolk size, salmonid embryos take a longer time for epiboly movements and germ ring closure compared with most other teleost species. Here we analyzed the germ ring closure, tail bud formation and development of the notochord and somites in rainbow trout using live embryo imaging and in situ hybridization with the rt-ntl probe. Rt-ntl is expressed in the germ ring (blastula, gastrula and somitogenesis stage), notochord, tail bud and somites (somitogenesis stage). When epiboly covers half the yolk, a tail bud-like structure is formed and somitogenesis starts. By the time epiboly is completed, the yolk covered and the germ ring closed, the embryo has already reached the 20 somite stage. Therefore, the timing of germ ring closure and tail bud formation is reversed in trout embryos compared with zebrafish and other small model fish embryos (heterochrony). Based on this result, we re-examined the definition of tail bud formation.