Limb patterning genes and heterochronic development of the emu wing bud.

Kelly N. Roeszler,Peter G. Farlie,Claire Hirst,Craig A. Smith,Nadia M. Davidson,Alicia Oshlack,David M. Lambert

doi:10.1186/s13227-016-0063-5

Abstract

BackgroundThe forelimb of the flightless emu is a vestigial structure, with greatly reduced wing elements and digit loss. To explore the molecular and cellular mechanisms associated with the evolution of vestigial wings and loss of flight in the emu, key limb patterning genes were examined in developing embryos.MethodsLimb development was compared in emu versus chicken embryos. Immunostaining for cell proliferation markers was used to analyze growth of the emu forelimb and hindlimb buds. Expression patterns of limb patterning genes were studied, using whole-mount in situ hybridization (for mRNA localization) and RNA-seq (for mRNA expression levels).ResultsThe forelimb of the emu embryo showed heterochronic development compared to that in the chicken, with the forelimb bud being retarded in its development. Early outgrowth of the emu forelimb bud is characterized by a lower level of cell proliferation compared the hindlimb bud, as assessed by PH3 immunostaining. In contrast, there were no obvious differences in apoptosis in forelimb versus hindlimb buds (cleaved caspase 3 staining). Most key patterning genes were expressed in emu forelimb buds similarly to that observed in the chicken, but with smaller expression domains. However, expression of Sonic Hedgehog (Shh) mRNA, which is central to anterior–posterior axis development, was delayed in the emu forelimb bud relative to other patterning genes. Regulators of Shh expression, Gli3 and HoxD13, also showed altered expression levels in the emu forelimb bud.ConclusionsThese data reveal heterochronic but otherwise normal expression of most patterning genes in the emu vestigial forelimb. Delayed Shh expression may be related to the small and vestigial structure of the emu forelimb bud. However, the genetic mechanism driving retarded emu wing development is likely to rest within the forelimb field of the lateral plate mesoderm, predating the expression of patterning genes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13227-016-0063-5) contains supplementary material, which is available to authorized users.

Highlights

The forelimb of the flightless emu is a vestigial structure, with greatly reduced wing elements and digit loss
We found that emu forelimb buds do not proliferate to the extent of hindlimb buds during early bud outgrowth
Development of emu forelimb and hindlimb buds In the chicken embryo, both fore- and hindlimb limb buds first become demarcated from the body wall as small mesodermal bulges at stages 17–18 (Hamburger and Hamilton [25])

Summary

Introduction

The forelimb of the flightless emu is a vestigial structure, with greatly reduced wing elements and digit loss. Recent molecular analyses show that extant ratites are polyphyletic, most having evolved the loss of flight independently through dispersal followed by convergence [6,7,8]. This is consistent with the fact that the forelimb structures of living ratites vary significantly. Phylogenetic studies, together with the comparative anatomy, suggest that at least three different genetic mechanisms may mediate forelimb development in the different ratite groups. Understanding these mechanisms will shed light on questions of evolutionary developmental biology

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