The Expression of Brachyury (T) during Gastrulation in the Marsupial FrogGastrotheca riobambae

AbstractGastrulation in the marsupial frogGastrotheca riobambaehas been analyzed by the distribution of the Brachyury (T) protein. Comparison with other amphibians provides mechanistic insights, sinceG. riobambaedevelops slowly and has the most divergent mode of amphibian gastrulation, producing an embryonic disk. The T pattern indicates that the prospective mesoderm is superficial, as in many amphibians. The dorsal blastopore lip could not be identified by the expression ofT, or by morphological criteria, thus it is unknown whetherGastrothecaembryos have a dorsal organizer before or after blastopore closure. The circumblastoporal and notochordal expression ofT, which are temporally contiguous inXenopus,are separated inGastrotheca,implying that distinct regulatory mechanisms may control the expression ofTin its two domains. The separation of the T pattern also indicates that involution at the blastopore is separate from notochord formation. In addition, extension of the archenteron and notochord occurs after blastopore closure, suggesting that dorsal convergence and extension have been delayed until after blastopore closure. Therefore, dorsal convergence and extension need not be the cause of blastopore closure inGastrotheca.The separation of gastrulation events in embryos that have not been experimentally manipulated, such as those ofGastrotheca,helps in understanding the distinct nature of gastrulation processes