Mitochondrial uniparental inheritance achieved after fertilization challenges the nuclear-cytoplasmic conflict hypothesis for anisogamy evolution

In eukaryotes, a fundamental phenomenon underlying sexual selection is the evolution of gamete size dimorphism between the sexes (anisogamy) from an ancestral gametic system with gametes of the same size in both mating types (isogamy). The nuclear-cytoplasmic conflict hypothesis has been one of the major theoretical hypotheses for the evolution of anisogamy. It proposes that anisogamy evolved as an adaptation for preventing nuclear-cytoplasmic conflict by minimizing male gamete size to inherit organelles uniparentally. In ulvophycean green algae, biparental inheritance of organelles is observed in isogamous species, as the hypothesis assumes. So we tested the hypothesis by examining whether cytoplasmic inheritance is biparental in Monostroma angicava, a slightly anisogamous ulvophycean that produces large male gametes. We tracked the fates of mitochondria in intraspecific crosses with PCR-RFLP markers. We confirmed that mitochondria are maternally inherited. However, paternal mitochondria enter the zygote, where their DNA can be detected for over 14 days. This indicates that uniparental inheritance is enforced by eliminating paternal mitochondrial DNA in the zygote, rather than by decreasing male gamete size to the minimum. Thus, uniparental cytoplasmic inheritance is achieved by an entirely different mechanism, and is unlikely to drive the evolution of anisogamy in ulvophyceans.

The early rise and spread of evolutionary game theory: perspectives based on recollections of early workers

Though the first attempts to introduce game theory into evolutionary biology failed, new formalism by Maynard Smith and Price in 1973 had almost instant success. We use information supplied by early workers to analyse how and why evolutionary game theory (EGT) spread so rapidly in its earliest years. EGT was a major tool for the rapidly expanding discipline of behavioural ecology in the 1970s; each catalysed the other. The first models were applied to animal contests, and early workers sought to improve their biological reality to compare predictions with observations. Furthermore, it was quickly realized that EGT provided a general evolutionary modelling method; not only was it swiftly applied to diverse phenotypic adaptations in evolutionary biology, it also attracted researchers from other disciplines such as mathematics and economics, for which game theory was first devised. Lastly, we pay attention to exchanges with population geneticists, considering tensions between the two modelling methods, as well as efforts to bring them closer. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

Adaptive division of growth and development between hosts in helminths with two-host life cycles

Parasitic worms (helminths) with complex life cycles divide growth and development between successive hosts. Using data from 597 species of acanthocephalans, cestodes, and nematodes with two-host life cycles, we found that helminths with larger intermediate hosts were more likely to infect larger, endothermic definitive hosts, although some evolutionary shifts in definitive host mass occurred without changes in intermediate host mass. Life-history theory predicts parasites to shift growth to hosts in which they can grow rapidly and/or safely. Accordingly, helminth species grew relatively less as larvae and more as adults if they infected smaller intermediate hosts and/or larger, endothermic definitive hosts. Growing larger than expected in one host, relative to host mass/endothermy, was not associated with growing less in the other host, implying a lack of cross-host trade-offs. Rather, some helminth orders had both large larvae and large adults. Within these taxa, however, size at maturity in the definitive host was unaffected by changes to larval growth, as predicted by optimality models. Parasite life-history strategies were mostly (though not entirely) consistent with theoretical expectations, suggesting that helminths adaptively divide growth and development between the multiple hosts in their complex life cycles.

Complex life-cycles in trophically transmitted helminths: Do the benefits of increased growth and transmission outweigh generalism and complexity costs?

Why do so many parasitic worms have complex life-cycles? A complex life-cycle has at least two hypothesized costs: (i) worms with longer life-cycles, i.e. more successive hosts, must be generalists at the species level, which might reduce lifetime survival or growth, and (ii) each required host transition adds to the risk that a worm will fail to complete its life-cycle. Comparing hundreds of trophically transmitted acanthocephalan, cestode, and nematode species with different life-cycles suggests these costs are weaker than expected. Helminths with longer cycles exhibit higher species-level generalism without impaired lifetime growth. Further, risk in complex life-cycles is mitigated by increasing establishment rates in each successive host. Two benefits of longer cycles are transmission and production. Longer cycles normally include smaller (and thus more abundant) first hosts that are likely to consume parasite propagules, as well as bigger (and longer-lived) definitive hosts, in which adult worms grow to larger and presumably more fecund reproductive sizes. Additional factors, like host immunity or dispersal, may also play a role, but are harder to address. Given the ubiquity of complex life-cycles, the benefits of incorporating or retaining hosts in a cycle must often exceed the costs.

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Comparing helminth species hints at the costs and benefits of complex life-cycles.

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Generalism and survival costs in longer life-cycles are weaker than expected.

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Longer life-cycles have growth and transmission benefits.

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The benefits of a complex life-cycle must often exceed the costs.

Evolution of Anisogamy in Organisms with Parthenogenetic Gametes

AbstractThe two sexes are defined by the sizes of the gametes they produce, anisogamy being the state with two differing gamete sizes (hence, females and males). The origin of this divergence has received much research interest, both theoretically and empirically. The gamete dynamics (GD) theory is a widely accepted theoretical explanation for anisogamy, and green algae have been an important empirical testing ground for the theory. However, some green and brown algae produce parthenogenetic gametes (gametes that can develop without fusing with another gamete), in contrast to an assumption in GD theory that unfused gametes do not develop. Here, we construct a GD model accounting for parthenogenetic gametes. We find that under conditions of panmixia and highly efficient fertilization (i.e., conditions of classical GD models from 1972 onward), the results remain largely unaltered by parthenogametes. However, under gamete-limited conditions anisogamy evolves less easily in the new model, and a novel result emerges: whereas previous models typically predict the evolution of either anisogamy or small isogamy, the current model shows that large isogamy can evolve when parthenogenetic gametes evolve under conditions of inefficient fertilization. Our analyses uncover unexplored complications relating to sex ratios under this relatively uncharted gametic system. We discuss limitations these complications impose on our models and suggest avenues for future research. We compare model results to algae with parthenogenetic gametes in nature.

A comparative test of the gamete dynamics theory for the evolution of anisogamy in Bryopsidales green algae

Gamete dynamics theory proposes that anisogamy arises by disruptive selection for gamete numbers versus gamete size and predicts that female/male gamete size (anisogamy ratio) increases with adult size and complexity. Evidence has been that in volvocine green algae, the anisogamy ratio correlates positively with haploid colony size. However, green algae show notable exceptions. We focus on Bryopsidales green algae. While some taxa have a diplontic life cycle in which a diploid adult (=fully grown) stage arises directly from the zygote, many taxa have a haplodiplontic life cycle in which haploid adults develop indirectly: the zygote first develops into a diploid adult (sporophyte) which later undergoes meiosis and releases zoospores, each growing into a haploid adult gametophyte. Our comparative analyses suggest that, as theory predicts: (i) male gametes are minimized, (ii) female gamete sizes vary, probably optimized by number versus survival as zygotes, and (iii) the anisogamy ratio correlates positively with diploid (but not haploid) stage complexity. However, there was no correlation between the anisogamy ratio and diploid adult stage size. Increased environmental severity (water depth) appears to drive increased diploid adult stage complexity and anisogamy ratio: gamete dynamics theory correctly predicts that anisogamy evolves with the (diploid) stage directly provisioned by the zygote.

How Soon Hath Time… A History of Two "Seminal" Publications

This review documents the history of the two papers written half a century ago that relate to this special issue of Cells. The first, “Sperm competition and its evolutionary consequences in the insects” (Biological Reviews, 1970), stressed that sexual selection continues after ejaculation, resulting in many adaptations (e.g., postcopulatory guarding phases, copulatory plugs, seminal fluid components that modify female reproduction, and optimal ejaculation strategies), an aspect not considered by Darwin in his classic treatise of 1871. Sperm competition has subsequently been studied in many taxa, and post-copulatory sexual selection is now considered an important sequel to Darwinian pre-copulatory sexual selection. The second, “The origin and evolution of gamete dimorphism and the male-female phenomenon” (Journal of Theoretical Biology, 1972) showed how selection, based on gamete competition between individuals, can give rise to anisogamy in an isogamous broadcast spawning ancestor. This theory, which has subsequently been developed in various ways, is argued to form the most powerful explanation of why there are two sexes in most multicellular organisms. Together, the two papers have influenced our general understanding of the evolutionary differentiation of the two forms of gametic cells, and the divergence of sexual strategies between males and females under sexual selection.

Trade-Offs with Growth Limit Host Range in Complex Life-Cycle Helminths

AbstractParasitic worms with complex life cycles have several developmental stages, with each stage creating opportunities to infect additional host species. Using a data set for 973 species of trophically transmitted acanthocephalans, cestodes, and nematodes, we confirmed that worms with longer life cycles (i.e., more successive hosts) infect a greater diversity of host species and taxa (after controlling for study effort). Generalism at the stage level was highest for middle life stages, the second and third intermediate hosts of long life cycles. By simulating life cycles in real food webs, we found that middle stages had more potential host species to infect, suggesting that opportunity constrains generalism. However, parasites usually infected fewer host species than expected from simulated cycles, suggesting that generalism has costs. There was no trade-off in generalism from one stage to the next, but worms spent less time growing and developing in stages where they infected more taxonomically diverse hosts. Our results demonstrate that life-cycle complexity favors high generalism and that host use across life stages is determined by both ecological opportunity and life-history trade-offs.

Conceptual developments in sperm competition: a very brief synopsis

The past half century has seen the development of the field of post-ejaculatory sexual selection, the sequel to sexual selection for mate-acquisition (pre-ejaculatory) described by Darwin. In richness and diversity of adaptations, post-ejaculatory selection rivals that of pre-ejaculatory sexual selection. Anisogamy-and hence two sexes-likely arose by primeval gamete competition, and sperm competition remains a major force maintaining high sperm numbers. The post-ejaculatory equivalent of male-male competition for matings, sperm competition was an intense ancestral form of sexual selection, typically weakening as mobility and internal fertilization developed in many taxa, when some expenditure became diverted into pre-ejaculatory competition. Sperm competition theory has been relatively successful in explaining variation in relative testes size and sperm numbers per ejaculate and is becoming more successful in explaining variation in sperm phenotype. Sperm competition has generated many other male adaptations such as seminal fluid proteins that variously modify female reproduction towards male interests, and copulatory plugs, prolonged copulations and post-ejaculatory guarding behaviour that reduce female remating probability, many of which result in sexual conflict. This short survey of conceptual developments is intended as a broad overview, mainly as a primer for new researchers. This article is part of the theme issue 'Fifty years of sperm competition'.

Evolutionary insight from a humble fly: sperm competition and the yellow dungfly

Studies of the yellow dungfly in the 1960s provided one of the first quantitative demonstrations of the costs and benefits associated with male and female reproductive behaviour. These studies advanced appreciation of sexual selection as a significant evolutionary mechanism and contributed to the 1970s paradigm shift toward individual selectionist thinking. Three behaviours in particular led to the realization that sexual selection can continue during and after mating: (i) female receptivity to remating, (ii) sperm displacement and (iii) post-copulatory mate guarding. These behaviours either generate, or are adaptations to sperm competition, cryptic female choice and sexual conflict. Here we review this body of work, and its contribution to the development of post-copulatory sexual selection theory. This article is part of the theme issue 'Fifty years of sperm competition'.

Endless forms of sexual selection

In recent years, the field of sexual selection has exploded, with advances in theoretical and empirical research complementing each other in exciting ways. This perspective piece is the product of a “stock-taking” workshop on sexual selection and sexual conflict. Our aim is to identify and deliberate on outstanding questions and to stimulate discussion rather than provide a comprehensive overview of the entire field. These questions are organized into four thematic sections we deem essential to the field. First we focus on the evolution of mate choice and mating systems. Variation in mate quality can generate both competition and choice in the opposite sex, with implications for the evolution of mating systems. Limitations on mate choice may dictate the importance of direct vs. indirect benefits in mating decisions and consequently, mating systems, especially with regard to polyandry. Second, we focus on how sender and receiver mechanisms shape signal design. Mediation of honest signal content likely depends on integration of temporally variable social and physiological costs that are challenging to measure. We view the neuroethology of sensory and cognitive receiver biases as the main key to signal form and the ‘aesthetic sense’ proposed by Darwin. Since a receiver bias is sufficient to both initiate and drive ornament or armament exaggeration, without a genetically correlated or even coevolving receiver, this may be the appropriate ‘null model’ of sexual selection. Thirdly, we focus on the genetic architecture of sexually selected traits. Despite advances in modern molecular techniques, the number and identity of genes underlying performance, display and secondary sexual traits remains largely unknown. In-depth investigations into the genetic basis of sexual dimorphism in the context of long-term field studies will reveal constraints and trajectories of sexually selected trait evolution. Finally, we focus on sexual selection and conflict as drivers of speciation. Population divergence and speciation are often influenced by an interplay between sexual and natural selection. The extent to which sexual selection promotes or counteracts population divergence may vary depending on the genetic architecture of traits as well as the covariance between mating competition and local adaptation. Additionally, post-copulatory processes, such as selection against heterospecific sperm, may influence the importance of sexual selection in speciation. We propose that efforts to resolve these four themes can catalyze conceptual progress in the field of sexual selection, and we offer potential avenues of research to advance this progress.

Evolution of the Two Sexes under Internal Fertilization and Alternative Evolutionary Pathways

Transition from isogamy to anisogamy, hence males and females, leads to sexual selection, sexual conflict, sexual dimorphism, and sex roles. Gamete dynamics theory links biophysics of gamete limitation, gamete competition, and resource requirements for zygote survival and assumes broadcast spawning. It makes testable predictions, but most comparative tests use volvocine algae, which feature internal fertilization. We broaden this theory by comparing broadcast-spawning predictions with two plausible internal-fertilization scenarios: gamete casting/brooding (one mating type retains gametes internally, the other broadcasts them) and packet casting/brooding (one type retains gametes internally, the other broadcasts packets containing gametes, which are released for fertilization). Models show that predictions are remarkably robust to these radical changes, yielding (1) isogamy under low gamete limitation, low gamete competition, and similar required resources for gametes and zygotes, (2) anisogamy when gamete competition and/or limitation are higher and when zygotes require more resources than gametes, as is likely as multicellularity develops, (3) a positive correlation between multicellular complexity and anisogamy ratio, and (4) under gamete competition, only brooders becoming female. Thus, gamete dynamics theory represents a potent rationale for isogamy/anisogamy and makes similar testable predictions for broadcast spawners and internal fertilizers, regardless of whether anisogamy or internal fertilization evolved first.

The evolution of gonad expenditure and gonadosomatic index (GSI) in male and female broadcast-spawning invertebrates

Sedentary broadcast-spawning marine invertebrates, which release both eggs and sperm into the water for fertilization, are of special interest for sexual selection studies. They provide unique insight into the early stages of the evolutionary succession leading to the often-intense operation of both pre- and post-mating sexual selection in mobile gonochorists. Since they are sessile or only weakly mobile, adults can interact only to a limited extent with other adults and with their own fertilized offspring. They are consequently subject mainly to selection on gamete production and gamete success, and so high gonad expenditure is expected in both sexes. We review literature on gonadosomatic index (GSI; the proportion of body tissue devoted to gamete production) of gonochoristic broadcast spawners, which we use as a proxy for gonad expenditure. We show that such taxa most often have a high GSI that is approximately equal in both sexes. When GSI is asymmetric, female GSI usually exceeds male GSI, at least in echinoderms (the majority of species recorded). Intriguingly, though, higher male GSI also occurs in some species and appears more common than female-biased GSI in certain orders of gastropod molluscs. Our limited data also suggest that higher male GSI may be the prevalent pattern in sperm casters (where only males release gametes). We explore how selection might have shaped these patterns using game theoretic models for gonad expenditure that consider possible trade-offs with (i) somatic maintenance or (ii) growth, while also considering sperm competition, sperm limitation, and polyspermy. Our models of the trade-off between somatic tissue (which increases survival) and gonad (which increases reproductive success) predict that GSI should be equal for the two sexes when sperm competition is intense, as is probably common in broadcast spawners due to synchronous spawning in aggregations. Higher female GSI occurs under low sperm competition. Sperm limitation appears unlikely to alter these conclusions qualitatively, but can also act as a force to keep male GSI high, and close to that of females. Polyspermy can act to reduce male GSI. Higher male than female GSI is predicted to be less common (as observed in the data), but can occur when ova/ovaries are sufficiently more resource-intensive to produce than sperm/testes, for which some evidence exists. We also show that sex-specific trade-offs between gonads and growth can generate different life-history strategies for males and females, with males beginning reproduction earlier. This could lead to apparently higher male GSI in empirical studies if immature females are included in calculations of mean GSI. The existence of higher male GSI nonetheless remains somewhat problematic and requires further investigation. When sperm limitation is low, we suggest that the natural logarithm of the male/female GSI ratio may be a suitable index for sperm competition level in broadcast spawners, and that this may also be considered as an index for internally fertilizing taxa.

INDIVIDUAL VARIATION IN SPERM COMPETITION SUCCESS OF YELLOW DUNG FLIES, SCATOPHAGA STERCORARIA

Intraspecific variation in the proportion of offspring sired by the second male to mate with a female (P₂ ) is an aspect of sperm competition that has received little attention. We examined variation in the sperm competition success of individual male dung flies, Scatophaga stercoraria. In unmanipulated matings, copula duration was dependent on male size with smaller males copulating for longer. A principal component analysis was used to generate uncorrelated scores based on a male's size and copula duration. Using these scores demonstrated that P₂ values were dependent both on the relative size and copula durations of competing males. When copula duration was held constant, the success of an individual male increased as his body size, relative to the first male, increased. We interrupted copulations of "large" and "small" second males and fitted the resultant P₂ values to a linear model of sperm competition with unequal ejaculates. The data fit well to a model of sperm displacement in which sperm mix quickly on introduction to the sperm stores. Furthermore, they show that "large" males have a greater rate of sperm displacement than "small" males. The levels of prey availability during testis maturation may influence a male's success in sperm competition although his immediate mating history does not. We show why an understanding of variation in sperm competition success is important for understanding the mechanisms and evolutionary significance of sperm competition.

What do isogamous organisms teach us about sex and the two sexes?

Isogamy is a reproductive system where all gametes are morphologically similar, especially in terms of size. Its importance goes beyond specific cases: to this day non-anisogamous systems are common outside of multicellular animals and plants, they can be found in all eukaryotic super-groups, and anisogamous organisms appear to have isogamous ancestors. Furthermore, because maleness is synonymous with the production of small gametes, an explanation for the initial origin of males and females is synonymous with understanding the transition from isogamy to anisogamy. As we show here, this transition may also be crucial for understanding why sex itself remains common even in taxa with high costs of male production (the twofold cost of sex). The transition to anisogamy implies the origin of male and female sexes, kickstarts the subsequent evolution of sex roles, and has a major impact on the costliness of sexual reproduction. Finally, we combine some of the consequences of isogamy and anisogamy in a thought experiment on the maintenance of sexual reproduction. We ask what happens if there is a less than twofold benefit to sex (not an unlikely scenario as large short-term benefits have proved difficult to find), and argue that this could lead to a situation where lineages that evolve anisogamy-and thus the highest costs of sex-end up being associated with constraints that make invasion by asexual reproduction unlikely (the 'anisogamy gateway' hypothesis).This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.

Why anisogamy drives ancestral sex roles

There is a clear tendency in nature for males to compete more strongly for fertilizations than females, yet the ultimate reasons for this are still unclear. Many researchers-dating back to Darwin and Bateman-have argued that the difference is ultimately driven by the fact that males (by definition) produce smaller and more numerous gametes than females. However, this view has recently been challenged, and a formal validation of the link between anisogamy and sex roles has been lacking. Here, we develop mathematical models that validate the intuition of Darwin and Bateman, showing that there is a very simple and general reason why unequal gamete numbers result in unequal investment in sexually competitive traits. This asymmetry does not require multiple mating by either sex, and covers traits such as mate searching, where the male bias has been difficult to explain. Furthermore, our models show males and females are predicted to diverge more strongly when the fertilization probability of each female gamete is high. Sex roles thus ultimately trace back to anisogamy and the resulting consequences for the fertilization process.

Gamete evolution and sperm numbers: sperm competition versus sperm limitation

Both gamete competition and gamete limitation can generate anisogamy from ancestral isogamy, and both sperm competition (SC) and sperm limitation (SL) can increase sperm numbers. Here, we compare the marginal benefits due to these two components at any given population level of sperm production using the risk and intensity models in sperm economics. We show quite generally for the intensity model (where N males compete for each set of eggs) that however severe the degree of SL, if there is at least one competitor for fertilization (N - 1 ≥ 1), the marginal gains through SC exceed those for SL, provided that the relationship between the probability of fertilization (F) and increasing sperm numbers (x) is a concave function. In the risk model, as fertility F increases from 0 to 1.0, the threshold SC risk (the probability q that two males compete for fertilization) for SC to be the dominant force drops from 1.0 to 0. The gamete competition and gamete limitation theories for the evolution of anisogamy rely on very similar considerations: our results imply that gamete limitation could dominate only if ancestral reproduction took place in highly isolated, small spawning groups.

Evolution of complex life cycles in trophically transmitted helminths. I. Host incorporation and trophic ascent

Links between parasites and food webs are evolutionarily ancient but dynamic: life history theory provides insights into helminth complex life cycle origins. Most adult helminths benefit by sexual reproduction in vertebrates, often high up food chains, but direct infection is commonly constrained by a trophic vacuum between free-living propagules and definitive hosts. Intermediate hosts fill this vacuum, facilitating transmission to definitive hosts. The central question concerns why sexual reproduction, and sometimes even larval growth, is suppressed in intermediate hosts, favouring growth arrest at larval maturity in intermediate hosts and reproductive suppression until transmission to definitive hosts? Increased longevity and higher growth in definitive hosts can generate selection for larger parasite body size and higher fecundity at sexual maturity. Life cycle length is increased by two evolutionary mechanisms, upward and downward incorporation, allowing simple (one-host) cycles to become complex (multihost). In downward incorporation, an intermediate host is added below the definitive host: models suggest that downward incorporation probably evolves only after ecological or evolutionary perturbations create a trophic vacuum. In upward incorporation, a new definitive host is added above the original definitive host, which subsequently becomes an intermediate host, again maintained by the trophic vacuum: theory suggests that this is plausible even under constant ecological/evolutionary conditions. The final cycle is similar irrespective of its origin (upward or downward). Insights about host incorporation are best gained by linking comparative phylogenetic analyses (describing evolutionary history) with evolutionary models (examining selective forces). Ascent of host trophic levels and evolution of optimal host taxa ranges are discussed.

The trophic vacuum and the evolution of complex life cycles in trophically transmitted helminths

Parasitic worms (helminths) frequently have complex life cycles in which they are transmitted trophically between two or more successive hosts. Sexual reproduction often takes place in high trophic-level (TL) vertebrates, where parasites can grow to large sizes with high fecundity. Direct infection of high TL hosts, while advantageous, may be unachievable for parasites constrained to transmit trophically, because helminth propagules are unlikely to be ingested by large predators. Lack of niche overlap between propagule and definitive host (the trophic transmission vacuum) may explain the origin and/or maintenance of intermediate hosts, which overcome this transmission barrier. We show that nematodes infecting high TL definitive hosts tend to have more successive hosts in their life cycles. This relationship was modest, though, driven mainly by the minimum TL of hosts, suggesting that the shortest trophic chains leading to a host define the boundaries of the transmission vacuum. We also show that alternative modes of transmission, like host penetration, allow nematodes to reach high TLs without intermediate hosts. We suggest that widespread omnivory as well as parasite adaptations to increase transmission probably reduce, but do not eliminate, the barriers to the transmission of helminths through the food web.

The sexual cascade and the rise of pre-ejaculatory (Darwinian) sexual selection, sex roles, and sexual conflict

After brief historic overviews of sexual selection and sexual conflict, I argue that pre-ejaculatory sexual selection (the form of sexual selection discussed by Darwin) arose at a late stage in an inevitable succession of transitions flowing from the early evolution of syngamy to the evolution of copulation and sex roles. If certain conditions were met, this "sexual cascade" progressed inevitably, if not, sexual strategy remained fixed at a given stage. Prolonged evolutionary history of intense sperm competition/selection under external fertilization preceded the rise of advanced mobility, which generated pre-ejaculatory sexual selection, followed on land by internal fertilization and reduced sperm competition in the form of postcopulatory sexual selection. I develop a prospective model of the early evolution of mobility, which, as Darwin realized, was the catalyst for pre-ejaculatory sexual selection. Stages in the cascade should be regarded as consequential rather than separate phenomena and, as such, invalidate much current opposition to Darwin-Bateman sex roles. Potential for sexual conflict occurs throughout, greatly increasing later in the cascade, reaching its peak under precopulatory sexual selection when sex roles become highly differentiated.

Polyandry: the history of a revolution

We give a historic overview and critical perspective of polyandry in the context of sexual selection. Early approaches tended to obfuscate the fact that the total matings (copulations) by the two sexes is equal, neglecting female interests and that females often mate with (or receive ejaculates from) more than one male (polyandry). In recent years, we have gained much more insight into adaptive reasons for polyandry, particularly from the female perspective. However, costs and benefits of multiple mating are unlikely to be equal for males and females. These must be assessed for each partner at each potential mating between male i and female j, and will often be highly asymmetric. Interests of i and j may be in conflict, with (typically, ultimately because of primordial sex differences) i benefitting and j losing from mating, although theoretically the reverse can also obtain. Polyandry reduces the sex difference in Bateman gradients, and the probability of sexual conflict over mating by: (i) reducing the potential expected value of each mating to males in inverse proportion to the number of mates per female per clutch, and also often by (ii) increasing ejaculate costs through increased sperm allocation. It can nevertheless create conflict over fertilization and increase conflict over parental investment. The observed mean mating frequency for the population (and hence the degree of polyandry) is likely, at least in part, to reflect a resolution of sexual conflict. Immense diversity exists across and within taxa in the extent of polyandry, and views on its significance have changed radically, as we illustrate using avian polyandry as a case study. Despite recent criticisms, the contribution of the early pioneers of sexual selection, Darwin and Bateman, remains generally valid, and should not, therefore, be negated; as with much in science, pioneering advances are more often amplified and refined, rather than replaced with entirely new paradigms.

Sperm competition games: a general model for precopulatory male-male competition

Reproductive males face a trade-off between expenditure on precopulatory male-male competition--increasing the number of females that they secure as mates--and sperm competition--increasing their fertilization success with those females. Previous sperm allocation models have focused on scramble competition in which males compete by searching for mates and the number of matings rises linearly with precopulatory expenditure. However, recent studies have emphasized contest competition involving precopulatory expenditure on armaments, where winning contests may be highly dependent on marginal increases in relative armament level. Here, we develop a general model of sperm allocation that allows us to examine the effect of all forms of precopulatory competition on sperm allocation patterns. The model predicts that sperm allocation decreases if either the "mate-competition loading,"a, or the number of males competing for each mating, M, increases. Other predictions remain unchanged from previous models: (i) expenditure per ejaculate should increase and then decrease, and (ii) total postcopulatory expenditure should increase, as the level of sperm competition increases. A negative correlation between a and M is biologically plausible, and may buffer deviations from the previous models. There is some support for our predictions from comparative analyses across dung beetle species and frog populations.

Inclusive fitness theory and eusociality

Arising from M. A. Nowak, C. E. Tarnita & E. O. Wilson 466, 1057-1062 (2010); Nowak et al. reply. Nowak et al. argue that inclusive fitness theory has been of little value in explaining the natural world, and that it has led to negligible progress in explaining the evolution of eusociality. However, we believe that their arguments are based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature. We will focus our comments on three general issues.

Sperm competition and ejaculate economics

Sperm competition was identified in 1970 as a pervasive selective force in post-copulatory sexual selection that occurs when the ejaculates of different males compete to fertilise a given set of ova. Since then, sperm competition has been much studied both empirically and theoretically. Because sperm competition often favours large ejaculates, an important challenge has been to understand the evolution of strategies through which males invest in sperm production and economise sperm allocation to maximise reproductive success under competitive conditions. Sperm competition mechanisms vary greatly, depending on many factors including the level of sperm competition, space constraints in the sperm competition arena, male mating roles, and female influences on sperm utilisation. Consequently, theoretical models of ejaculate economics are complex and varied, often with apparently conflicting predictions. The goal of this review is to synthesise the theoretical basis of ejaculate economics under sperm competition, aiming to provide empiricists with categorised model assumptions and predictions. We show that apparent contradictions between older and newer models can often be reconciled and there is considerable consensus in the predictions generated by different models. We also discuss qualitative empirical support for some of these predictions, and detail quantitative matches between predictions and observations that exist in the yellow dung fly. We argue that ejaculate economic theory represents a powerful heuristic to explain the diversity in ejaculate traits at multiple levels: across species, across males and within individual males. Future progress requires greater understanding of sperm competition mechanisms, quantification of trade-offs between ejaculate allocation and numbers of matings gained, further knowledge of mechanisms of female sperm selection and their associated costs, further investigation of non-sperm ejaculate effects, and theoretical integration of pre- and post-copulatory episodes of sexual selection.

Why do larval helminths avoid the gut of intermediate hosts?

In complex life cycles, larval helminths typically migrate from the gut to exploit the tissues of their intermediate hosts. Yet the definitive host's gut is overwhelmingly the most favoured site for adult helminths to release eggs. Vertebrate nematodes with one-host cycles commonly migrate to a site in the host away from the gut before returning to the gut for reproduction; those with complex cycles occupy sites exclusively in the intermediate host's tissues or body spaces, and may or may not show tissue migration before (typically) returning to the gut in the definitive host. We develop models to explain the patterns of exploitation of different host sites, and in particular why larval helminths avoid the intermediate host's gut, and adult helminths favour it. Our models include the survival costs of migration between sites, and maximise fitness (=expected lifetime number of eggs produced by a given helminth propagule) in seeking the optimal strategy (host gut versus host tissue exploitation) under different growth, mortality, transmission and reproductive rates in the gut and tissues (i.e. sites away from the gut). We consider the relative merits of the gut and tissues, and conclude that (i) growth rates are likely to be higher in the tissues, (ii) mortality rates possibly higher in the gut (despite the immunological inertness of the gut lumen), and (iii) that there are very high benefits to egg release in the gut. The models show that these growth and mortality relativities would account for the common life history pattern of avoidance of the intermediate host's gut because the tissues offer a higher growth rate/mortality rate ratio (discounted by the costs of migration), and make a number of testable predictions. Though nematode larvae in paratenic hosts usually migrate to the tissues, unlike larvae in intermediates, they sometimes remain in the gut, which is predicted since in paratenics mortality rate and migration costs alone determine the site to be exploited.

Sperm competition games: the risk model can generate higher sperm allocation to virgin females

We examine the risk model in sperm competition games for cases where female fertility increases significantly with sperm numbers (sperm limitation). Without sperm competition, sperm allocation increases with sperm limitation. We define 'average risk' as the probability q that females in the population mate twice, and 'perceived risk' as the information males gain about the sperm competition probability with individual females. If males obtain no information from individual females, sperm numbers increase with q unless sperm limitation is high and one of the two competing ejaculates is strongly disfavoured. If males can distinguish between virgin and mated females, greater sperm allocation to virgins is favoured by high sperm limitation, high q, and by the second male's ejaculate being disfavoured. With high sperm limitation, sperm allocation to virgins increases and to mated females decreases with q at high q levels. With perfect information about female mating pattern, sperm allocation (i) to virgins that will mate again exceeds that to mated females and to virgins that will mate only once, (ii) to virgins that mate only once exceeds that for mated females if q is high and there is high second male disadvantage and (iii) to each type of female can decrease with q if sperm limitation is high, although the average allocation increases at least across low q levels. In general, higher sperm allocation to virgins is favoured by: strong disadvantage to the second ejaculate, high sperm limitation, high average risk and increased information (perceived risk). These conditions may apply in a few species, especially spiders.

Sperm competition, mating rate and the evolution of testis and ejaculate sizes: a population model

There are various ways to estimate ejaculate expenditure. Ejaculate size or sperm number (s) is an absolute number of units of ejaculate. Relative ejaculate expenditure (E) is the expenditure on the ejaculate as the proportion of the total expenditure on all aspects of the mating, including finding and acquiring a female, and so on. Relative testis size or gonadosomatic index (sigma) is testes mass divided by body mass; it is assumed to reflect the product of mating rate (M) and ejaculate mass (s). In a new model, where mating rate, sperm competition and sperm allocation interact, and where the female's inter-clutch interval is assumed to be independent of s or M, we show that sigma is directly proportional to the mean E for a species; across species sigma and E increase monotonically with sperm competition. However, the relation between s and sperm competition across species depends on the range of sperm competition (low risk or high intensity): s increases with sperm competition at low risk levels, but decreases with sperm competition at high intensity levels. This situation arises because s alpha E/M; both E and M increase with sperm competition, but E increases differently with sperm competition in its two ranges.

Sexual conflict over mating and fertilization: an overview

Sexual conflict is a conflict between the evolutionary interests of individuals of the two sexes. The sexes can have different trait optima but this need not imply conflict if their optima can be attained simultaneously. Conflict requires an interaction between males and females (e.g. mating or parental care), such that the optimal outcomes for each sex cannot be achieved simultaneously. It is important to distinguish between battleground models, which define the parameter space for conflict and resolution models, which seek solutions for how conflicts are resolved. Overt behavioural conflict may or may not be manifest at resolution. Following Fisherian principles, an immediate (i.e. direct) benefit to a male that has a direct cost to his female partner can have an indirect benefit to the female via her male progeny. Female resistance to mating has been claimed to represent concurrence rather than conflict, due to female benefits via sons (males with low mating advantage are screened out by resistance). However, the weight of current evidence (both theoretical and empirical) supports sexual conflict for many cases. I review (i) conflicts over mate quality, encounters between males and females of genetically diverged subpopulations, mating rate and inbreeding, (ii) the special features of postcopulatory sexual conflict and (iii) some general features of importance for conflict resolution.

Male “Mixed” Reproductive Strategies in Biparental Species: Trivers Was Probably Right, but Why?

Trivers proposed that, if parental care by both sexes is advantageous, males should practice a "mixed" strategy of seeking extrapair copulations, while restricting their parental investment to offspring of social mates. We explore circumstances under which males should limit their parental care in the predicted manner. We find that Trivers's "mixed" strategy will generally be evolutionarily stable so long as either socially monogamous or polygynous males usually sire more offspring per brood from a social mate than they typically sire in broods of extrapair mates. Polygynous males should spread investment across their home nests unless the expected number of chicks sired in them differs widely. Whether polygynous males should restrict paternal care to social mates' offspring hinges additionally on resident male investment in broods containing extrapair young: if resident males contribute minimally, some investment by a polygynous extrapair male becomes more advantageous. Recently reviewed data on extrapair fertilization distributions within monogamous and polygynous passerines suggest that extrapair offspring often predominate numerically within their broods, consistent with sperm expenditure theory. Nevertheless, most species conform to the model's criterion regarding relative parentage levels in broods of social versus extrapair mates. Patterns of extrapair parentage thus appear sufficient to stabilize biparental care systems.

Evolution of complex life cycles in helminth parasites

The fundamental question of how complex life cycles--where there is typically more than one host-evolve in host--parasite systems remains largely unexplored. We suggest that complex cycles in helminths without penetrative infective stages evolve by two essentially different processes, depending on where in the cycle a new host is inserted. In 'upward incorporation', a new definitive host, typically higher up a food web and which preys on the original definitive host, is added. Advantages to the parasite are avoidance of mortality due to the predator, greater body size at maturity and higher fecundity. The original host typically becomes an intermediate host, in which reproduction is suppressed. In 'downward incorporation', a new intermediate host is added at a lower trophic level; this reduces mortality and facilitates transmission to the original definitive host. These two processes should also apply in helminths with penetrative infective stages, although the mathematical conditions differ.

Sperm competition games: sperm selection by females

We analyse a co-evolutionary sexual conflict game, in which males compete for fertilizations (sperm competition) and females operate sperm selection against unfavourable ejaculates (cryptic female choice). For simplicity, each female mates with two males per reproductive event, and the competing ejaculates are of two types, favourable (having high viability or success) or unfavourable (where progeny are less successful). Over evolutionary time, females can increase their level of sperm selection (measured as the proportion of unfavourable sperm eliminated) by paying a fecundity cost. Males can regulate sperm allocations depending on whether they will be favoured or disfavoured, but increasing sperm allocation reduces their mating rate. The resolution of this game depends on whether males are equal, or unequal. Males could be equal: each is favoured with probability, p, reflecting the proportion of females in the population that favour his ejaculate (the 'random-roles' model); different males are favoured by different sets of females. Alternatively, males could be unequal: given males are perceived consistently by all females as two distinct types, favoured and disfavoured, where p is now the frequency of the favoured male type in the population (the 'constant-types' model). In both cases, the evolutionarily stable strategy (ESS) is for females initially to increase sperm selection from zero as the viability of offspring from unfavourable ejaculates falls below that of favourable ejaculates. But in the random-roles model, sperm selection decreases again towards zero as the unfavourable ejaculates become disastrous (i.e. as their progeny viability decreases towards zero). This occurs because males avoid expenditure in unfavourable matings, to conserve sperm for matings in the favoured role where their offspring have high viability, thus allowing females to relax sperm selection. If sperm selection is costly to females, ESS sperm selection is high across a region of intermediate viabilities. If it is uncostly, there is no ESS in this region unless sperm limitation (i.e. some eggs fail to be fertilized because sperm numbers are too low) is included into the model. In the constant-types model, no relaxation of sperm selection occurs at very low viabilities of disfavoured male progeny. If sperm selection is sufficiently costly, ESS sperm selection increases as progeny viability decreases down towards zero; but if it is uncostly, there is no ESS at the lowest viabilities, and unlike the random-roles model, this cannot be stabilized by including sperm limitation. Sperm allocations in the ESS regions differ between the two models. With random roles, males always allocate more sperm in the favoured role. With constant types, the male type that is favoured allocates less sperm than the disfavoured type. These results suggests that empiricists studying cryptic female choice and sperm allocation patterns need to determine whether sperm selection is applied differently, or consistently, on given males by different females in the same population.

Optimal growth strategies of larval helminths in their intermediate hosts

We consider optimal growth of larval stages in complex parasite life cycles where there is no constraint because of host immune responses. Our model predicts an individual's asymptotic size in its intermediate host, with and without competition from conspecific larvae. We match observed variations in larval growth patterns in pseudophyllid cestodes with theoretical predictions of our model. If survival of the host is vital for transmission, larvae should reduce asymptotic size as intensity increases, to avoid killing the host. The life history strategy (LHS) model predicts a size reduction <1/intensity, thus increasing the parasite burden on the host. We discuss whether body size of competing parasites is an evolved LHS or simply reflects resource constraints (RC) on growth fixed by the host, leading to a constant total burden with intensity. Growth under competition appears comparable with "the tragedy of the commons", much analysed in social sciences. Our LHS prediction suggests that evolution generates a solution that seems cooperative but is actually selfish.

The evolution of anisogamy: a game-theoretic approach

A popular theory has proposed that anisogamy originated through disruptive selection acting on an ancestral isogamous population, though recent work has emphasized the importance of other factors in its evolution. We re-examine the disruptive selection theory, starting from an isogamous population with two mating types and taking into account the functional relationship, g(m), between the fitness of a gamete and its size, m, as well as the relationship, f(S), between the fitness of a zygote and its size, S. Evolutionary game theory is used to determine the existence and continuous stability of isogamous and anisogamous strategies for the two mating types under various models for the two functions g(m) and f(S). In the ancestral unicellular state, these two functions are likely to have been similar; this leads to isogamy whether they are sigmoidal or concave, though in the latter case allowance must be made for a minimal gamete size. The development of multicellularity may leave g(m) relatively unchanged while f(S) moves to the right, leading to the evolution of anisogamy. Thus, the disruptive selection theory provides a powerful explanation of the origin of anisogamy, though other selective forces may have been involved in the subsequent specialization of micro- and macrogametes.

Life history consequences of mammal sibling rivalry

Mammal life history traits relating to growth and reproduction are extremely diverse. Sibling rivalry may contribute to selection pressures influencing this diversity, because individuals that are relatively large at birth typically have an advantage in competition for milk. However, selection for increased growth rate is likely to be constrained by kin selection and physiological costs. Here, we present and test a model examining the ESS (evolutionarily stable strategy) balance between these constraints and advantages associated with increased prenatal growth in mammal sibling rivalry. Predictions of the model are supported by results of comparative analyses for the Carnivora and Insectivora, which demonstrate an increase in prenatal growth rate with increasing intensity of postnatal scramble competition, and a decrease in postnatal growth rate relative to size at birth. Because increased prenatal growth rates are predicted to select for reduced gestation length under certain conditions, our study also indicates that sibling rivalry may contribute to selection pressures influencing variation in altriciality and precociality among mammals.

Sexual conflict reduces offspring fitness in zebra finches

Parental care is often costly; hence, in sexually reproducing species where both male and female parents rear their offspring (biparental care), sexual conflict over parental investment can arise. Such conflict occurs because each care-giver would benefit from withholding parental investment for use with another partner, leading to a reduction in the amount of care given by one parent at the expense of the other. Here we report experiments to explore the prediction from theory that parents rearing offspring alone may provide greater parental investment than when rearing offspring together with a partner. We found that when the number of offspring per parent, and hence the potential workload, were kept constant, offspring received a greater per capita parental investment from single females than from both parents working together, and that males reared by single mothers were more sexually attractive as adults than their biparentally reared siblings. This difference between single- and two-parent families is due to a reduction in care provided by females when they care together with a male, rather than laziness by males or differences in the begging behaviour of chicks, supporting the claim that sexual conflict in biparental care can reduce the quality of offspring raised.