Convergent evolution of cysteine residues in sperm protamines of one genus of marsupials, the Planigales |
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Authors: | Retief JD; Rees JS; Westerman M; Dixon GH |
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Institution: | Department of Medical Biochemistry, University of Calgary. |
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Abstract: | A characteristic feature of the sperm P1 protamines of eutherian mammals is
the constant presence of six to nine cysteine residues per molecule. During
spermiogenesis these residues become oxidized to form a three-dimensional
network of disulfide bridges between, and within, protamine molecules in
the sperm chromatin. This covalent cross linking strongly stabilizes
eutherian sperm nuclei. In contrast, protamines sequenced from teleost
fish, birds, monotremes, and marsupials all lack cysteine residues and
their sperm nuclei, without the stabilizing cross links, are easily
decondensed in vitro. We have now found that one genus of tiny, shrewlike
dasyurid marsupials, the Planigales, possess P1 protamines containing five
to six cysteine residues. These residues appear to have evolved since the
divergence of Planigales from other members of the family Dasyuridae, such
as the marsupial mouse, Sminthopsis crassicaudata. We believe this
constitutes a case of convergent evolution in a subfamily of dasyurid
marsupials toward the cysteine-rich eutherian form of sperm protamine P1.
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