Phenotypic plasticity in sperm production rate: there’s more to it than testis size

Abstract

Evolutionary theory predicts that males should produce more sperm when sperm competition is high. Because sperm production rate is difficult to measure in most organisms, comparative and experimental studies have typically used testis size instead, while assuming a good correspondence between testis size and sperm production rate. Here we evaluate this common assumption using the marine flatworm Macrostomum lignano, in which we can estimate sperm production rate because the accumulation of produced sperm can be observed in vivo. In earlier studies we have shown that testis size is phenotypically plastic in M. lignano: worms can be induced to make larger testes by raising them in groups instead of pairs, and these larger testes have a higher cell proliferation activity (i.e. they are more energetically costly). Here we demonstrate that worms with such experimentally enlarged testes have a higher sperm production rate. Moreover, although testis size and sperm production rate were related linearly, worms with experimentally enlarged testes had a higher sperm production rate per unit testis size (i.e. a higher spermatogenic efficiency). We thus show that phenotypically plastic adjustment of sperm production rate includes a component that is independent of testis size. We discuss possible reasons for this novel finding, and suggest that the relationship between testis size and sperm production needs to be evaluated in other species as well.

Sperm competition affects sex allocation but not sperm morphology in a flatworm

“…Individuals of this species are capable of adjusting their sex allocation (i.e., the reproductive investment into the male versus the female sex function) in response to the social group size (i.e., the number of potential mates) that they experience. Such an adjustment is in agreement with a central prediction of sex allocation theory for simultaneous hermaphrodites (for a review, see Schärer 2009), and several studies have demonstrated for M. lignano that individuals that were raised in larger groups have bigger testes (e.g., Schärer and Ladurner 2003;Schärer et al 2005;Schärer and Vizoso 2007;Schärer and Janicke 2009).…”

“…Schärer and Ladurner 2003) or sperm production rate (e.g., Schärer and Vizoso 2007;Ramm and Stockley 2009).…”

Sperm competition and sperm length influence the rate of mammalian spermatogenesis

“…This presumably enables males to replenish sperm reserves more quickly, and thus maintain higher mating rates or larger ejaculate sizes. Hence, while testis size is a useful predictor of sperm production rate (Amann 1970;Møller 1989), there is also increasing evidence both from comparative analyses (Schärer et al 2008;Lü pold et al 2009; this study) and experimental studies of sperm production plasticity (Oppliger et al 1998;Bjork et al 2007;Schärer & Vizoso 2007;Ramm & Stockley 2009), that factors within the testis contributing to variation in sperm production rate should also be considered in the context of male adaptations to sperm competition.…”

Female monopolization mediates the relationship between pre- and postcopulatory sexual traits

“…It should be noted, however, that adaptations to sperm production in response to postcopulatory sexual selection can also involve increases in the density and functional efficiency of the sperm-producing tissue across testes of identical size [45][46][47][48] . Moreover, in addition to sperm competition, selection on testes size can occur through selection against sperm depletion 49 , with males that monopolize more females and thus mate more frequently losing paternity due to sperm depletion 50 .…”