| Marsupial mammals are born at an embryonic state compared to their eutherian counterparts, yet certain features are accelerated to aid in survival. One particularly notable heterochrony is the steep gradient in development along the anterior-posterior axis in which anterior structures are generally well developed relative to posterior ones. For example, the forelimbs are well differentiated and relatively large and are considered to be essential for the journey to the teat immediately following birth. The developmental mechanisms by which the steep anterior-posterior gradient arises as well as how the forelimbs become so well developed at birth are unknown. In this thesis I show that the steep anterior-posterior gradient in the axis arises from changes in somitogenesis rate, and somitogenesis begins early in the opossum, Monodelphis domestica . Multiple, early changes to the limb development program, including both heterochrony and heterotopy, contribute to forelimb acceleration. Using Tbx5 and Tbx4 as fore- and hind limb field markers respectively, I have found that both limb fields arise extremely early during development of the opossum. Patterning of the limb buds is also accelerated, as Shh expression in the zone of polarizing activity (ZPA) appears early relative to the outgrowth of the bud itself. In addition, the forelimb fields have increased in relative size, increased myocyte allocation from the somites, and modified migration of the spinal nerves into the forelimb bud. This shift in the extent of the forelimb field is accompanied by shifts in Hox gene expression along the anterior-posterior axis. Furthermore, I found that both fore- and hind limb fields arise during gastrulation gradually at the posterior end of the embryo directly adjacent to the primitive streak, in contrast to the appearance of the limb fields in their entirety in all other known instances. These results show a surprising evolutionary flexibility in the early limb development program of amniotes and rule out the induction of the limb fields by mature structures such as the somites or mesonephrous. Additionally, I develop a tool and confidence measure for use in heterochrony studies. |
