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Genetic effects on sperm design in the zebra finch


Sperm design and function are important determinants of male reproductive success and are expected to be under strong selection1,2. The way that spermatozoa phenotypes evolve is poorly understood, because there have been few studies of the quantitative genetics of sperm3,4,5. Here we show, in the zebra finch Taeniopygia guttata, an extraordinary degree of inter-male variation in sperm design that is independent of sperm swimming velocity. A quantitative genetics study using data from over 900 zebra finches in a complex breeding experiment showed that sperm head, mid-piece and flagellum length are heritable, that negative genetic correlations exist between sperm traits, and that significant indirect (maternal) genetic effects exist. Selection on the zebra finch sperm phenotype may be low because sperm competition is infrequent in this species6, and this, in combination with negative genetic correlations and maternal genetic effects, may account for the variation in sperm phenotype between males. These results have important implications for the evolution of sperm in other taxa.

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We are grateful to the following for technical assistance: A. Bamford, H. Basford, S. Bawden, S. Bradish, L. Birkhead, A. Blake, M. Hudson, K. Hutchence, R. Linacre, B. Mappin, A. MacDonald, the late O. Scott-Roberts, D. Rose, J. Shutt, K. Swinglehurst, L. Turton and E. Varsey. We thank F. M. Hunter, I. M. Matthews, N. Roddis and P. Young for help with the project, and A. Beckerman, J. D. Biggins, D. Coltman, J. Cummins, C. Haley, L. Keller, A. Moore, B. Sheldon, J. Slate, J. St John and D. Woolley for advice and/or comments. This study was funded by a NERC research grant to T.R.B.Authors' contributions T.R.B. managed the entire project and wrote the manuscript; E.J.P. managed the birds; P.B. and R.Y. measured the sperm; and H.C.-J. conducted the genetic analyses.

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Correspondence to T. R. Birkhead.

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Figure 1: Examples of sperm of approximately the same total length from two zebra finch males showing the marked difference in size of the mid-piece (that is, the mitochondrial helix (bright green and indicated by a horizontal line)).
Figure 2: Relationships between zebra finch sperm velocity (VAP) and sperm traits.
Figure 3: Relationship between sperm flagellum and mid-piece length.


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