Embryonic development of the emu, Dromaius novaehollandiae

Abstract

The chick, Gallus gallus, is the traditional model in avian developmental studies. Data on other bird species are scarce. Here, we present a comparative study of the embryonic development of the chick and the emu Dromaius novaehollandiae, a member of Paleognathae, which also includes the ostrich, rhea, tinamou, kiwi, and cassowary. Emu embryos ranging from Hamburger and Hamilton (HH) equivalent stages 1 to 43 were collected and their gross morphology analyzed. Its early development was studied in detail with time-lapse imaging and molecular techniques. Emu embryos in general take 2-3 times longer incubation time to reach equivalent chicken stages, requiring 1 day for HH2, 2.5 days for HH4, 7 days for limb bud initiation, 23 days for feather germ appearance, and approximately 50-56 days for hatching. Chordin gene expression is similar in emu and chick embryos, and emu Brachyury is not expressed until HH3. Circulation is established at approximately the 27-to 30-somite stage. Forelimb buds are formed and patterned initially, but their growth is severely retarded. The size difference between an emu and a chick embryo only becomes apparent after limb bud formation. Overall, emu and chick embryogenesis proceeds through similar stages, but developmental heterochrony between these two species is widely observed.

Limb patterning genes and heterochronic development of the emu wing bud

“…In contrast, the forelimb and hindlimb buds of emu embryos show heterochronic growth. Nagai et al [26] reported that the hindlimb buds appear in the emu embryo at HH stages 17–18, as in chicken. However, forelimb bud development is delayed, first appearing at 20 [26].…”

“…Nagai et al [26] reported that the hindlimb buds appear in the emu embryo at HH stages 17–18, as in chicken. However, forelimb bud development is delayed, first appearing at 20 [26]. In the study described here, the forelimb bud first became apparent at stage 19, when the hindlimb bud was already more advanced (Fig.…”

“…Emu embryos were staged according to the morphological criteria of Hamburger and Hamilton [25], originally described for chicken and recently validated for emu embryos [26]. Nagai et al [26] noted that, proportional to its total incubation time, an emu embryo takes approximately 2–3 times longer to reach an equivalent chicken stage. As emu forelimb growth is retarded, embryos were staged using morphological development of the hindlimb and head/facial characters.…”

Saunders's framework for understanding limb development as a platform for investigating limb evolution

“…In particular, the molecular control of timing, particularly that determining when tetrapod limbs develop in various species, is poorly understood. One exception is the Emu, which has a marked delay in the development of the forelimb relative to chick embryos (Nagai et al, 2011). The transcription factor Tbx5 is required for forelimb development as its deletion in the forelimb lateral plate mesoderm results in mice lacking their forelimbs (Rallis et al, 2003).…”

Reconstruction and in vivo analysis of the extinct tbx5 gene from ancient wingless moa (Aves: Dinornithiformes)

“…Ostrich eggs were backlit to measure air-cell size reduction during embryonic growth and embryos were harvested after two weeks. (corresponding to approximate Hamburger Hamilton stage 38, (HH38; [43,44]) when most organ and limb building events are taking place). Emu, rhea, ostrich, and cassowary DNAs were a kind gift from Dr Joy Halverson, Zoogen Services, Sacramento, California, US.…”