title: Sperm competition-induced plasticity in the speed of spermatogenesis.
abstract: BACKGROUND
  Sperm competition between rival ejaculates over the fertilization of ova typically selects for the production of large numbers of sperm. An obvious way to increase sperm production is to increase testis size, and most empirical work has focussed on this parameter. Adaptive plasticity in sperm production rate could also arise due to variation in the speed with which each spermatozoon is produced, but whether animals can respond to relevant environmental conditions by modulating the kinetics of spermatogenesis in this way has not been experimentally investigated.
  
  
  RESULTS
  Here we demonstrate that the simultaneously hermaphroditic flatworm Macrostomum lignano exhibits substantial plasticity in the speed of spermatogenesis, depending on the social context: worms raised under higher levels of sperm competition produce sperm faster.
  
  
  CONCLUSIONS
  Our findings overturn the prevailing view that the speed of spermatogenesis is a static property of a genotype, and demonstrate the profound impact that social environmental conditions can exert upon a key developmental process. We thus identify, to our knowledge, a novel mechanism through which sperm production rate is maximised under sperm competition.
authors.affiliation: Evolutionary Biology, Bielefeld University, Morgenbreede 45, 33615, Bielefeld, Germany. athina.giannakara@uni-bielefeld.de.
authors.forename: Athina
authors.initials: A
authors.lastname: Giannakara
authors.affiliation: Evolutionary Biology, Zoological Institute, University of Basel, Vesalgasse 1, 4051, Basel, Switzerland. lukas.scharer@unibas.ch.
authors.forename: Lukas
authors.initials: L
authors.lastname: Schärer
authors.affiliation: Evolutionary Biology, Bielefeld University, Morgenbreede 45, 33615, Bielefeld, Germany. steven.ramm@uni-bielefeld.de.
authors.forename: Steven A
authors.initials: SA
authors.lastname: Ramm
mesh_terms: D000818:Animals; D012734:Disorders of Sex Development; D005260:Female; D005306:Fertilization; D008297:Male; D010053:Ovary / Q000502:physiology; D010986:Platyhelminths / Q000502:physiology*; D013091:Spermatogenesis; D013094:Spermatozoa / Q000502:physiology*; D013737:Testis / Q000502:physiology