Adaptive Plasticity of the Penis in a Simultaneous Hermaphrodite

Abstract

Acorn barnacles are important model organisms for the study of sex allocation. They are sessile, nonselfing hermaphrodites that copulate with penises that have been suggested to be phenotypically plastic. On wave-exposed shores, Semibalanus balanoides develop penises with relatively greater diameter whereas in wave-protected sites they are thinner. A reciprocal transplant experiment between wave-exposed and protected sites tested whether these exposure-specific morphologies have adaptive value. Mating success was compared over a range of distances to compare the ability of barnacles to reach mates. Barnacles that grew in the wave-protected site and mated in the wave-protected site fertilized more broods at increasing distances than those transplanted to the wave-exposed site. For barnacles that developed in the wave-exposed site, there was no difference in the ability to fertilize neighbors between sites of differing exposure. This study demonstrates the adaptive value of plasticity in penis morphology. The results suggest a trade-off between development of a penis adapted to wave exposure and the ability to fertilize distant mates. Barnacles in different physical environments are limited by different factors, which may limit numbers of potential mates, constrain optimal sex allocation strategies and alter reproductive behavior.

Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection, Mate Choice, and Sexual Conflict

“…Although previous work in insects showed that development of genitalia are canalized and show little evidence of plasticity (House and Simmons 2007), experiments in barnacles showed that genitalia can respond in a phenotypically plastic manner to wave action (Neufeld and Palmer 2008), and social environment, in which individuals grow shorter penises when colonies are more dense (Hoch 2009;Neufeld 2011). Similarly, we have found that some species of ducks have phenotypically plastic male genitalia in at least some species in response to their social environment (PLR Brennan, K Zyscowski, and RO Prum, unpubl.…”

Sex allocation and investment into pre- and post-copulatory traits in simultaneous hermaphrodites: the role of polyandry and local sperm competition

“…Recent models have investigated optimal allocation towards different male post-copulatory components in both gonochorists [86,87] and hermaphrodites [80,88], and there are recent empirical studies showing variation in allocation towards traits other than sperm production [56,89]. In contrast, models that integrate pre-and post-copulatory components have until recently been rather simplistic.…”

Phenotypic plasticity in sex allocation for a simultaneously hermaphroditic coral reef fish

“…Multiple lines of evidence suggest that phenotypic plasticity in simultaneous hermaphrodites is adaptive, variable in form, and highly responsive to competition for male-role mating opportunities (Crean and Marshall 2008;Hoch 2009;Janicke and Schärer 2010). Yet, there is a general lack of field data on plasticity in reproductive traits across variable densities (Schärer 2009;but see Raimondi and Martin 1991;Crean and Marshall 2008), particularly for fishes.…”