Heterochrony in somitogenesis rate in a model marsupial, M onodelphis domestica

SUMMARY Marsupial newborns are highly altricial and also show a wide array of shifts in the rate or timing of developmental events so that certain neonatal structures are quite mature. One particularly notable feature is the steep gradient in development along the anterior–posterior axis such that anterior structures are generally well developed relative to posterior ones. Here, we study somitogenesis in the marsupial, M onodelphis domestica , and document two heterochronies that may be important in generating the unusual body plan of the newborn marsupial. First, we demonstrate a 4‐fold change in somitogenesis rate along the anterior–posterior axis, which appears to be due to somitogenesis slowing posteriorly. Second, we show that somitogenesis, particularly in the cervical region, initiates earlier in M onodelphis relative to other developmental events in the embryo. The early initiation of somitogenesis may contribute to the early development of the cervical region and forelimbs. Other elements of somitogenesis appear to be conserved. When compared to mouse, we see similar expression of genes involved in the clock and wavefront, and genes of the W nt, N otch, and fibroblast growth factor ( FGF ) pathways also cycle in M onodelphis . Further, we could not discern differences in somite maturation rate along the anterior–posterior axis in M onodelphis , and thus rate of maturation of the somites does not appear to contribute to the steep anterior–posterior gradient.