Male reproductive success depends on the competitive ability of sperm to fertilize the ova, which should lead to strong selection on sperm characteristics. This raises the question of how heritable variation in sperm traits is maintained. Here we show that in zebra finches (Taeniopygia guttata) nearly half of the variance in sperm morphology is explained by an inversion on the Z chromosome with a 40% allele frequency in the wild. The sperm of males that are heterozygous for the inversion had the longest midpieces and the highest velocity. Furthermore, such males achieved the highest fertility and the highest siring success, both within-pair and extra-pair. Males homozygous for the derived allele show detrimental sperm characteristics and the lowest siring success. Our results suggest heterozygote advantage as the mechanism that maintains the inversion polymorphism and hence variance in sperm design and in fitness.

Department of Behavioural Ecology and Evolutionary Genetics Max Planck Institute for Ornithology Seewiesen 82319 Germany

Division of Evolutionary Biology Faculty of Biology Ludwig Maximilian University of Munich Planegg Martinsried 82152 Germany

Faculty of Science Charles University Prague 128 44 Czech Republic

Institute of Clinical Molecular Biology Christian Albrechts University Kiel 24105 Germany

Institute of Vertebrate Biology Czech Academy of Sciences Brno 603 65 Czech Republic

Nat Ecol Evol. 2017 Aug;1(8):1064-1065. doi: 10.1038/s41559-017-0261-0. PubMed

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