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Feather asymmetry development and evolution

From: Ben Creisler

A new online paper that may be of interest:

Teresa J. Feo and Richard O. Prum (2014)
Theoretical morphology and development of flight feather vane
asymmetry with experimental tests in parrots.
Journal of Experimental Zoology Part B: Molecular and Developmental
Evolution (advance online publication)
DOI: 10.1002/jez.b.22573

Asymmetry in flight feather vane width is a major functional
innovation associated with the evolution of flight in the ancestors of
birds. However, the developmental and morphological basis of feather
shape is not simple, and the developmental processes involved in vane
width asymmetry are poorly understood. We present a theoretical model
of feather morphology and development that describes the possible ways
to modify feather development and produce vane asymmetry. Our model
finds that the theoretical morphospace of feather shape is redundant,
and that many different combinations of parameters could be
responsible for vane asymmetry in a given feather. Next, we
empirically measured morphological and developmental model parameters
in asymmetric and symmetric feathers from two species of parrots to
identify which combinations of parameters create vane asymmetry in
real feathers. We found that both longer barbs, and larger barb angles
in the relatively wider trailing vane drove asymmetry in tail
feathers. Developmentally, longer barbs were the result of an offset
of the radial position of the new barb locus, whereas larger barb
angles were produced by differential expansion of barbs as the feather
unfurls from the tubular feather germ. In contrast, the helical angle
of barb ridge development did not contribute to vane asymmetry and
could be indicative of a constraint. This research provides the first
comprehensive description of both the morphological and developmental
modifications responsible for vane asymmetry within real feathers, and
identifies key steps that must have occurred during the evolution of
vane asymmetry.