Evolution of the arthropod prophenoloxidase/hexamerin protein family |
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Authors: | Austin L Hughes |
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Institution: | (1) Department of Biology and Institute of Molecular Evolutionary Genetics, The Pennsylvania State University, 208 Mueller Laboratory, University Park, PA 16803, USA, E-mail: austin@hugans3.bio.psu.edu, Tel.: +1-814-865-5013, Fax: +1-814-865-9131, US |
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Abstract: | Phylogenetic analysis of the prophenoloxidase/hexamerin family of arthropods revealed four well supported subfamilies: (1)
the arylphorin subfamily, including arylphorins, storage proteins, and other proteins of uncertain function from insects;
(2) the hemocyanins of branchiopod crustaceans, which are copper-binding proteins involved in oxygen transport; (3) the hemocyanins
of chelicerates; and (4) the prophenoloxidases (proPO) of both insects and branchiopods, which are copper-binding molecules
that play a role in sclerotization of cuticle and encapsulation of foreign particles. The phylogeny indicated that insect
and branchiopod proPO constitute a monophyletic group but that branchiopod and chelicerate hemocyanins do not constitute a
monophyletic group. Branchiopod hemocyanin and proPO diverged from each other prior to the divergence of insects from branchiopods
and probably prior to the divergence of chelicerates from the insect-branchiopod lineage. Likewise, the insect arylphorin
subfamily diverged from proPO prior to the divergence of insects from branchiopods and probably prior to the divergence of
chelicerates; thus, the results did not support the hypothesis that insect arylphorins represent hemocyanins freed to assume
a new function because the insect tracheal respiratory system removes the need for an oxygen-transport molecule. Nonetheless,
reconstruction of ancestral sequences by the maximum parsimony method suggested that the ancestors of the arylphorin family
were copper-binding. Regions corresponding to the copper-binding domains were found to have a faster rate of nonsynonymous
evolution in arylphorin subfamily genes than in other hexamerin family genes; this presumably reflects a relaxation of purifying
selection after the loss of copper-binding function.
Received: 25 March 1998 / Revised: 3 July 1998 |
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Keywords: | Arylphorin Hexamerin Invertebrate immunity Propheroloxidase |
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