Hox C6 expression during development and regeneration of forelimbs in larval Notophthalmus viridescens

A central theme concerning the epimorphic regenerative potential of urodele amphibian appendages is that limb regeneration in the adult parallels larv al limb development. Results of previous research have led to the suggestio n that homeobox containing genes are "re-expressed" during the epimorphic r egeneration of forelimbs of adult Notophthalmus viridescens in patterns whi ch retrace larval limb development. However, to date no literature exists c oncerning expression patterns of any homeobox containing genes during larva l development of this species. The lack of such information has been a hind rance in exploring the similarities as well as differences which exist betw een limb regeneration in adults and limb development in larvae. Here we rep ort the first such results of the localization of Hox C6 (formerly, NvHBox- 1) in developing and regenerating forelimbs of N. viridescens larvae as dem onstrated by whole-mount in situ hybridization. Inasmuch as the pattern of Hox C6 expression is similar in developing forelimb buds of larvae and epim orphically regenerating forelimb blastemata of both adults and larvae, our results support the paradigm that epimorphic regeneration in adult newts pa rallels larval forelimb development. However, in contrast with observations which document the presence of Hox C6 in both intact, as well as regenerat ing hindlimbs and tails of adult newts, our results reveal no such Hox C6 e xpression during larval development of hindlimbs or the tail. As such, our findings indicate that critical differences in larval hindlimb and tail dev elopment versus adult expression patterns of this gene in these two appenda ges may be due primarily to differences in gene regulation as opposed to ge ne function. Thus, the apparent ability of urodeles to regulate genes in su ch a highly co-ordinated fashion so as to replace lost, differentiated, app endicular structures in adult animals may assist, at least in part, in bett er elucidating the phenomenon of epimorphic regeneration.