Meeting Abstract

22.3  Sunday, Jan. 4  The highly divergent developmental pathways of marsupial fore- and hind limbs: Evidence from the AER DOROBA, C.K.*; SEARS, K.E.; University of Illinois; University of Illinois carolyn.doroba@gmail.com

Marsupials give birth after short gestations to neonates that must immediately crawl to the teat using only their precociously developed forelimbs (FL). As their hind limbs (HL) play no role in the crawl, they are relatively undeveloped at birth. We hypothesize that these differential selective pressures on the marsupial FL and HL resulted in the divergence of their developmental pathways. As a first step in testing this hypothesis, we examined apical ectodermal ridge (AER) development in marsupials. The AER manifests as an epithelial ridge along the dorsoventral boundary (DVB) of the embryonic limb and is a primary signaling center for limb outgrowth. All amniotes (e.g., reptiles, birds, mammals) possess a well-defined AER, with the exception of two groups with greatly reduced limbs, whales and snakes. Using scanning election microscopy, we compared the AER of the Mus musculus (mouse) and Monodelphis domestica (possum) FL and HL at comparable stages. The possum FL AER is greatly reduced and characterized by seemingly random clumps of epithelial cells along the DVB, although the DVB appears to be intact. The possum HL AER is well defined and comparable to that of mouse limbs. These results suggest that the development of the marsupial FL and HL are more divergent than in any known mammal in which both limbs are present. We also hypothesize that differences in expression of one or more gene families with a role in AER formation (e.g., Wnt‘s, Fgf‘s, and Bmp‘s) are responsible for the relative reduction of the marsupial FL AER. Gross Fgf8 expression in the possum FL is similar to that in the possum HL and mouse limbs. We are currently testing for differences in Fgf8 expression at the cellular level, and for differences in expression of Wnt3 and Bmp2/4.