Published by the Royal Society. This article has been cited by other articles in PMC. Abstract Darwin was the first to recognize that sexual selection is a strong evolutionary force.
Exaggerated traits allow same-sex individuals to compete over access to mates and provide a mechanism by which mates are selected. It is relatively easy to appreciate how inter- and intrasexual selection work in organisms with the sensory capabilities to perceive physical or behavioural traits that signal mate quality or mate compatibility, and to assess the relative quality of competitors.
It is therefore not surprising that most studies of sexual selection have focused on animals with separate sexes and obvious adaptations that function in the context of reproductive competition. Yet, many sexual organisms are both male and female at the same time, often lack sexual dimorphism and never come into direct contact at mating. How does sexual selection act in such species, and what can we learn from them?
Here, we address these questions by exploring the potential for sexual selection in simultaneous hermaphrodites, sperm- and broadcast spawners, plants and fungi. Our review reveals a range of mechanisms of sexual selection, operating primarily after gametes have been released, which are common in many of these groups and also quite possibly in more familiar internally fertilizing and sexually dimorphic organisms.
Three implications are imbedded in these statements first used by Michiels [ 2 ] in his chapter on mating conflicts and sperm competition in hermaphrodites. First, simultaneous hermaphrodites are not subjected to sexual selection. And third, anything that is not an animal can safely be ignored with respect to the study of sexual selection.
Yet, as we outline here, studying exactly the kind of organisms Darwin implicitly and explicitly ignored provides fascinating insights into sexual selection.
After all, he only started to think about the ways organisms find sexual partners because his theory of evolution by natural selection could not account for the many traits that seemed to lower the survival of the organism that carried them. Hence, from the beginning the study of sexual selection has been biased towards the usual suspects: Moreover, it took the widespread use of DNA fingerprinting technologies to unequivocally determine paternity before the active role of females in mating was truly appreciated nicely summarized in Birkhead's book [ 3 ].
And lo and behold, females turned out to be active participants during mate choice, and furthermore were shown to pursue matings with multiple males within a single reproductive episode polyandry. Post-copulatory sexual selection, which encompasses sperm competition competition among sperm for fertilization [ 4 ] and cryptic female choice [ 5 ] where females influence the outcome of such contests [ 6 ] , provides a plausible explanation for many apparently bizarre reproductive traits such as giant sperm more than 5 cm in the fruit fly Drosophila bifurca [ 7 ] and the convoluted reproductive tracts of female waterfowl [ 8 ].
Thus, the realization that polyandry is widespread takes us back to where sexual selection begins, with competition among gametes over fertilization [ 9 ].
The definition thus acknowledges that traits subject to sexual selection may not be as visually spectacular as the elaborate displays of, say, a male peacock, but this does not make them any less important to an individual's reproductive success. Unlike the more conventional study organisms, all or most selection acts after the gametes have been released.
In this prospective review of sexual selection in these groups, we first discuss the sexual arena of simultaneous hermaphrodites before moving on to systems in which sexual partners have no direct interaction during reproduction spermcasters, broadcast spawners and wind-pollinated plants. From there, it is a relatively small step to move to plants that require a third force during reproduction i.
We end our taxonomic treatment of sexual selection by covering the fungi, a group displaying a bewildering array of breeding systems and reproductive characteristics. Our review highlights the enormous diversity of breeding systems displayed by these less-studied organisms, but also remarkable convergence in the ways that selection targets gametes when they compete or are selected for fertilization.
We conclude by arguing that the lessons learned from moving out of one's comfort zone and venturing into the lesser-known realms of sexual selection can provide critical insights into the mechanisms of gamete-level sexual selection in more familiar taxonomic groups.
When you are both male and female Here, we will mainly discuss simultaneous hermaphrodites hereafter simply referred to as hermaphrodites in fungi and mobile animals in which fertilization requires direct contact between mating partners. By definition, hermaphrodites produce both small, more numerous gametes sperm , and large, less numerous gametes ova.
If we then assume that ova, in general, are limiting while sperm are abundant, it follows that a hermaphrodite might prefer to donate sperm to a particular mating partner, while not necessarily wanting to receive sperm from that same partner [ 5 ]. Such preference for the male role can explain why the marine flatworm Macrostomum lignano appears to remove sperm by sucking their female genital pore after mating [ 11 ]. Such a high mating rate is clearly not necessary to fertilize the small number of eggs, so their main goal appears to be to donate sperm and to remove sperm from their mating partner.
Hence, the level of sperm sucking is probably directly related to the relative desirability of the partner as male-mate. But where exactly does this preference for the male role come from? Once an individual has received sperm after the first mating in the female role, in many instances reproductive output cannot be increased by remating shortly after as female, whereas remating in the male role can increase reproductive output. Basically, the Bateman gradient i.
When the latter fails, sperm from multiple partners are likely to compete for fertilization, setting the scene for sperm competition, cryptic female choice and conflicts over the fate of received ejaculates. The high mating rates of hermaphrodites and their frequent failure to avoid being inseminated selects for traits that maximize fertilization success during sperm competition.
Sperm morphology is most complex when competition for fertilization is fierce. Competition is fierce when sperm compete directly within the reproductive tract over fertilization. Traumatic insemination most likely evolved to circumvent cryptic female choice, but also allows an individual to inseminate while lowering the risk of being inseminated.
While sperm competition and cryptic female choice are not unique to hermaphrodites, the ability to influence the sex role of one's mating partner in other words change the allocation from male function to female function or vice versa is a unique feature in these systems.
Given that competition over the male role is fierce, anything an individual can do to reduce its mate's investment into male function can lower competition. Moreover, it pays to increase a partner's investment into female function, as that will increase the number of eggs that can be fertilized provided the organism stores sperm for later fertilization [ 17 ].
Likewise the seminal fluid of the freshwater snail Lymnaea stagnalis contains proteins that reduce both sperm transfer and fertilization success of a subsequent mating by the inseminated partner while at the same time manipulating female investment of that partner [ 18 ], similar to the anti-aphrodisiac found in seminal fluid of, for example, Drosophila [ 19 ].
Mating in one sex role can have significant effects on subsequent matings in the other sex role in mushroom-forming fungi. These investments reflect those of true anisogamous gametes female gametes contribute the cytoplasm and male gametes do not and are expected to have similar effects with respect to operational sex ratio and opportunities for multiple matings [ 20 ]. The male nuclei migrate and divide through the female mycelium, such that the latter becomes completely fertilized, but the nuclei within the mycelium do not fuse.
When this mycelium now meets an unfertilized haploid mycelium, the latter can become fertilized by either of the two nuclei from the former but the former can no longer accept a nucleus i.
The first mating, in the female role, thus reduces further male-mating success, as it introduces direct competition in later mating opportunities. Similar trade-offs between sex roles in which previous matings in one sex role affect the other sex role negatively are likely in other hermaphrodites too.
It does seem probable that such trade-offs are more probable when mating partners interact directly; when mating partners do not interact directly, there seems less scope for manipulation of any kind. An exception might be the gametophytes of several species of homosporous ferns. Without even getting into direct contact, gametophytes can influence the sex role of prospective mating partners. Spores in these species by default develop into either females or hermaphrodites and as they develop secrete hormones into their environment.
Spores that receive these hormones do not develop into females or hermaphrodites, but into males. The obvious difference between the ferns and mushroom-forming fungi is that both the male and female roles are likely to benefit from the production of and response to the hormones excreted by the fern spores [ 23 ].