For 10 points, name this amphibian genus of model organisms.
The zero-nu mutant of this organism fails to transcribe rRNA.  Gurdon developed a method for exogenous mRNA expression in this organism, and Keller explants are a system used to study cell movement in it.  Brachet's cleft and Spemann's organizer are developmental structures in this model organism, which is not genetically tractable but is often used to study gastrulation and embryonic development.  A protein complex containing p34Cdc2 and B-type cyclins, discovered in this vertebrate, was originally called maturation promoting factor. Microinjection of DNA into animal caps or oocytes is a common experimental technique in this organism, and species of this model organism include tropicalis and its tetraploid relative laevis.  For 10 points, name this amphibian genus of model organisms.
The zero-nu mutant of this organism fails to transcribe rRNA.  
Xenopus (also accept Xenopus laevis, Xenopus tropicalis, platanna, African clawed frog, or South African clawed toad, prompt on frog or toad)