Sexual selection after gamete release in broadcast spawning invertebrates

Abstract

Broadcast spawning invertebrates offer highly tractable models for evaluating sperm competition, gamete-level mate choice and sexual conflict. By displaying the ancestral mating strategy of external fertilization, where sexual selection is constrained to act after gamete release, broadcast spawners also offer potential evolutionary insights into the cascade of events that led to sexual reproduction in more ‘derived’ groups (including humans). Moreover, the dynamic reproductive conditions faced by these animals mean that the strength and direction of sexual selection on both males and females can vary considerably. These attributes make broadcast spawning invertebrate systems uniquely suited to testing, extending, and sometimes challenging classic and contemporary ideas in sperm competition, many of which were first captured in Parker's seminal papers on the topic. Here, we provide a synthesis outlining progress in these fields, and highlight the burgeoning potential for broadcast spawners to provide both evolutionary and mechanistic understanding into gamete-level sexual selection more broadly across the animal kingdom. This article is part of the theme issue ‘Fifty years of sperm competition’.

How sperm competition shapes the evolution of testes and sperm: a meta-analysis

“…Further, our analysis revealed stronger relationships of RTS with sperm competition in internal compared to external fertilizers. This difference could result from a higher risk of sperm limitation in external fertilizers, particularly broadcast spawners [66], compared to internal fertilizers with more targeted sperm release [7,67]. If so, even if total selection on testes size is stronger in external than internal fertilizers, sperm competition would explain a relatively smaller portion royalsocietypublishing.org/journal/rstb Phil.…”

The role of female reproductive fluid in sperm competition

“…More recently, the burgeoning field of cryptic female choice [2,3], defined as female-mediated mechanisms that bias fertilization toward the sperm of specific males, emphasizes the critical role that females play in moderating the outcome of sperm competition to suit their reproductive interests. As highlighted in this special issue, the definitions of sperm competition and cryptic female choice have now been broadened considerably to include externally fertilizing species and those with less well-studied mating systems ( [4], see also [5]).…”

Selection drives the evolution of convergent gene expression changes during transitions to co-sexuality in haploid sexual systems

“…Therefore, the link between gene expression evolution and sexual selection is uncertain for these organisms, and may reflect a lower degree of sexual selection in the brown algae compared with animals. Brown algae have relatively low levels of sexual dimorphism 9,19 and are broadcast spawners so the opportunities for mate choice and/or mating competition are mainly constrained to interactions involving male and female gametes and not gametophytes 30 . Consistent with the idea that gamete sexual selection may occur (and perhaps not so much gametophyte sexual selection), it has been shown recently that in the absence of males, female gametes of brown alga populations lose their sexual morphological characteristics, e.g.…”