The pattern of early cleavage of the marsupial frog Gastrotheca riobambae.

Comparison of early development of the marsupial frog Gastrotheca riobambae with Xenopus laevis suggests that the cleavage pattern of Xenopus and the tight coupling of events at the midblastula transition are features of the accelerated development of small amphibian eggs with aquatic reproduction, rather than generalized features of amphibian development. The large eggs of the marsupial frog Gastrotheca riobambae not only display an atypical holoblastic pattern of cleavage and a very slow rate of development, but the events of the midblastula transition are uncoupled, suggesting that amphibians may have a diversity of developmental patterns. Early cleavage of the egg, which measures about 3 mm in diameter, occurs mostly by meridional and vertical furrows. The first cleavage cycle takes about 16 to 20 h and about four days may be required to reach the midblastula stage. Cleavage becomes asynchronous at about the third cleavage cycle evidenced by the formation of cleavage furrows. However, during cleavage (up to 342-cells), the majority of the nuclei divide synchronously and only 15 to 40% of the nuclei of a given embryo have a different cleavage schedule. At the 8-cell stage, nucleoli become visible (approximately 24 h after amplexus), signaling that transcription of rRNA has started at this early stage. Cell motility was detected in three- to four-day old embryos and seems to be associated with changes in cell shape and with expansion of the blastocoel at this stage.