Developmental origins of precocial forelimbs in marsupial neonates

Abstract

SUMMARYMarsupial mammals are born in an embryonic state, as compared with their eutherian counterparts, yet certain features are accelerated. The most conspicuous of these features are the precocial forelimbs, which the newborns use to climb unaided from the opening of the birth canal to the teat. The developmental mechanisms that produce this acceleration are unknown. Here we show that heterochronic and heterotopic changes early in limb development contribute to forelimb acceleration. Using Tbx5 and Tbx4 as fore-and hindlimb field markers, respectively, we have found that, compared with mouse, both limb fields arise notably early during opossum development. Patterning of the forelimb buds is also accelerated, as Shh expression appears early relative to the outgrowth of the bud itself. In addition, the forelimb fields and forelimb myocyte allocation are increased in size and number, respectively, and migration of the spinal nerves into the forelimb bud has been modified. This shift in the extent of the forelimb field is accompanied by shifts in Hox gene expression along the anterior-posterior axis. Furthermore, we found that both fore-and hindlimb fields arise gradually during gastrulation and extension of the embryonic axis, in contrast to the appearance of the limb fields in their entirety in all other known cases. Our 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 mesonephros.

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity

“…Our finding that limb, not axially derived, Shh expression controls Myf5 provides a powerful mechanism to coordinate Myf5 transcription with the arrival of MPCs in the limb bud mesenchyme. Consistent with this, precocious development of forelimb muscles in marsupial embryos correlates with a much earlier onset of limb Shh expression (Keyte and Smith 2011).…”

Regulation of limb bud initiation and limb‐type morphology

“…Perhaps surprisingly, hindlimb formation also occurs earlier although its subsequent outgrowth is delayed (Sears, 2009). Consistent with these observations, expression of markers of forelimb and hindlimb initiation, Tbx5/Tbx4 and their targets Fgf10 and Fgf8, are detected at earlier developmental stages in the marsupial, the Opossum, Monodelphis domestica when compared to the mouse (Keyte and Smith, 2010). It remains unclear whether the subsequent difference in timing of forelimb and hindlimb development arises through acceleration of the forelimb developmental programme or delay of the hindlimb programme or though a combination of these two processes.…”

The developmental basis of bat wing muscle

“…Interestingly, Fgf10 continued to be expressed in the lateral plate-derived mesenchyme at the anterior-proximal part of the forelimb, flank and posterior-proximal part of hind limb of bat embryos at stage 13L and 14 (Figs 4d,f and 5a-c; Supplementary Fig. S2), although expression of Fgf10 ceased in corresponding tissues of stage-matched mouse, marsupial opossum and chick embryos [17][18][19] . We also examined expression of Fgfr2IIIb that encodes the receptor of Fgf10 in bat embryos, and detected its expression in epidermis over the flank region of Fgf10-expressing lateral plate mesoderm, as well as in epidermis of the limb buds and lateral plate mesoderm anterior to the forelimb bud at stage 13L (Fig.…”