Convergent Regulatory Evolution and Forelimb Heterochrony in Flightless Birds

Meeting Abstract

57-5  Saturday, Jan. 5 11:00 – 11:15  Convergent Regulatory Evolution and Forelimb Heterochrony in Flightless Birds GRAYSON, P*; YOUNG, JJ; EDWARDS, SV; TABIN, CJ; Harvard University, MCZ; Harvard Medical School, Genetics; Harvard University, MCZ; Harvard Medical School, Genetics

Palaeognathae, the avian clade comprising the flightless ratites, such as the emu (Dromaius novaehollandiae) and ostrich (Struthio camelus), alongside their volant (flight capable) relatives the tinamous, provides a unique setting to study both phenotypic and genomic aspects of repeated loss of a complex trait; a minimum of three convergent losses of flight are inferred within the group. The ratites exhibit the phenotypes most commonly associated with flightless birds, including reduced wings, whereas the tinamous have retained flight-associated traits. We find that in contrast to ratite embryos, the forelimb of the Chilean tinamou (Nothoprocta perdicaria) does not experience a delay in early development compared to the hindlimb. This suggests that ratite forelimb heterochrony results from selective relaxation for, or selection against, the early development of robust wings. The most extreme heterochony in ratite forelimb development is seen in the emu. We identified down-regulation of SALL1 in the emu forelimb at stage HH18 compared to the emu hindlimb, and both limbs of chicken (Gallus gallus) using RNA-seq. Comparative ATAC-seq on HH18 limbs and flank mesoderm identified an enhancer near SALL1 that is active in proliferating limbs, but inactive in non-proliferative mesoderm (both flanks and the emu forelimb). To examine whether convergent forelimb heterochrony in ratites is determined by similar gene expression and regulatory changes across the palaeognath tree, we are analyzing stage-matched RNA- and ATAC-seq for forelimb and hindlimb at HH18 and HH25 in the Chilean tinamou, ostrich, greater rhea (Rhea americana), emu and chicken. Our results thus far suggest that heterochronic shifts in flightless birds are driven in part by changes in enhancer activity and accessibility to transcription factors.

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