Sperm form and function: what do we know about the role of sexual selection?

in Reproduction

Sperm morphological variation has attracted considerable interest and generated a wealth of predominantly descriptive studies over the past three centuries. Yet, apart from biophysical studies linking sperm morphology to swimming velocity, surprisingly little is known about the adaptive significance of sperm form and the selective processes underlying its tremendous diversification throughout the animal kingdom. Here, we first discuss the challenges of examining sperm morphology in an evolutionary context and why our understanding of it is far from complete. Then, we review empirical evidence for how sexual selection theory applies to the evolution of sperm form and function, including putative secondary sexual traits borne by sperm.


    Society for Reproduction and Fertility

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    Sperm morphological diversity. Scanning electron micrographs of (A) Macrobiotus cf. hufelandi (Tardigrada: Macrobiotidae; L. Rebecchi, U. of Modena e Reggio Emilia), (B) Caenorhabditis elegans (Nematoda: Rhabditida; T Roberts, Florida State University, FL, USA), (C) Mytilocypris mytiloides (Crustacea: Ostracoda – posterior end of long, filiform sperm; R Matzke-Karasz, Ludwig Maximilian University, Munich, germany), (D) Drosophila bifurca (Insecta: Drosophilidae; R Dallai, University of Siena), (E) Patinopecten yessoensis (Mollusca: Ostreoida; from Li et al. 2000), (F) Iporangaia pustulosa (Arachnida: Opiliones; from Moya et al. 2007), (G) Trialeurodes vaporariorum (Insecta: Aleyrodidae; R Dallai), (H) Allacma fusca (Hexapoda: Sminthuridae; from Dallai et al. 2009), (I) Colostethus marchesianus (Anura: Aromobatidae; from Veiga-Menoncello et al. 2007), (J) Paralichthys olivaceus (Actinopterygii: Paralichthyidae; from Zhang et al. 2003), (K) Gopherus agassizii (Reptilia: Testudinata; L Liaw, Beckman Laser Institute at University of California Irvine, CA, USA), (L) Passer domesticus (Aves: Passeridae; R Dallai), (M) Phataginus tricuspi (Pholidota: Manidae; L Liaw), (N) Uromys caudimaculatus (Rodentia: Muridae; W Breed, University of Adelaide, Australia). All published photos reprinted with permission from Elsevier; all unpublished photos courtesy of authors in parentheses.

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    Variation in sperm length across the animal kingdom (in µm on a logarithmic scale). Each horizontal line spans the range of sperm lengths reported in the literature for each taxon. Line colors depict different fertilization modes (light gray: external fertilization, from broadcast spawning in marine invertebrates to female-directed sperm release in frogs; dark gray: spermcasting; black: internal fertilization). Dotted lines indicate considerable extension of the sperm length range by inclusion of an extreme outlier (e.g., Neoceratodus forsteri in the externally fertilizing bony fishes and Discoglossus pictus in the frogs). Data were retrieved from Pitnick et al. (2009a), with an extended range for seed shrimp based on Wingstrand (1988) and Smith et al. (2016).

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    Schematic illustration of a likely process of postcopulatory sexual selection on sperm length in Drosophila, involving aspects of both sperm competition and cryptic female choice. SS, sexual selection at both pre- and postmating stages.


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