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

Fossil-calibrated molecular clock data enable reconstruction of steps leading to differentiated multicellularity and anisogamy in the Volvocine algae

BACKGROUND

Throughout its nearly four-billion-year history, life has undergone evolutionary transitions in which simpler subunits have become integrated to form a more complex whole. Many of these transitions opened the door to innovations that resulted in increased biodiversity and/or organismal efficiency. The evolution of multicellularity from unicellular forms represents one such transition, one that paved the way for cellular differentiation, including differentiation of male and female gametes. A useful model for studying the evolution of multicellularity and cellular differentiation is the volvocine algae, a clade of freshwater green algae whose members range from unicellular to colonial, from undifferentiated to completely differentiated, and whose gamete types can be isogamous, anisogamous, or oogamous. To better understand how multicellularity, differentiation, and gametes evolved in this group, we used comparative genomics and fossil data to establish a geologically calibrated roadmap of when these innovations occurred.

RESULTS

Our ancestral-state reconstructions, show that multicellularity arose independently twice in the volvocine algae. Our chronograms indicate multicellularity evolved during the Carboniferous-Triassic periods in Goniaceae + Volvocaceae, and possibly as early as the Cretaceous in Tetrabaenaceae. Using divergence time estimates we inferred when, and in what order, specific developmental changes occurred that led to differentiated multicellularity and oogamy. We find that in the volvocine algae the temporal sequence of developmental changes leading to differentiated multicellularity is much as proposed by David Kirk, and that multicellularity is correlated with the acquisition of anisogamy and oogamy. Lastly, morphological, molecular, and divergence time data suggest the possibility of cryptic species in Tetrabaenaceae.

CONCLUSIONS

Large molecular datasets and robust phylogenetic methods are bringing the evolutionary history of the volvocine algae more sharply into focus. Mounting evidence suggests that extant species in this group are the result of two independent origins of multicellularity and multiple independent origins of cell differentiation. Also, the origin of the Tetrabaenaceae-Goniaceae-Volvocaceae clade may be much older than previously thought. Finally, the possibility of cryptic species in the Tetrabaenaceae provides an exciting opportunity to study the recent divergence of lineages adapted to live in very different thermal environments.

Sex and Gender in Population Neuroscience

To understand psychiatric and neurological disorders and the structural and functional properties of the human brain, it is essential to consider the roles of sex and gender. In this chapter, I first define sex and gender and describe studies of sex differences in non-human animals. In humans, I describe the sex differences in behavioral and clinical phenotypes and neuroimaging-derived phenotypes, including whole-brain measures, regional subcortical and cortical measures, and structural and functional connectivity. Although structural whole-brain sex differences are large, regional effects (adjusting for whole-brain volumes) are typically much smaller and often fail to replicate. Nevertheless, while an individual neuroimaging feature may have a small effect size, aggregating them in a "maleness/femaleness" score or machine learning multivariate paradigm may prove to be predictive and informative of sex- and gender-related traits. Finally, I conclude by summarizing emerging investigations of gender norms and gender identity and provide methodological recommendations to incorporate sex and gender in population neuroscience research.

Algal evolution: A touch of brown in a Paleozoic sea of greens and reds

Previous molecular clock studies indicated a Mesozoic origin for the brown algae (Phaeophyceae). New research based on phylogenetic evidence challenges this notion and provides novel insights into the origin and diversification of brown algae, which includes multiple transitions within the group from isogamy to oogamy (and back again!).

Hermaphroditic origins of anisogamy

Anisogamy-the size dimorphism of gametes-is the defining difference between the male and female sexual strategies. Game-theoretic thinking led to the first convincing explanation for the evolutionary origins of anisogamy in the 1970s. Since then, formal game-theoretic models have continued to refine our understanding of when and why anisogamy should evolve. Such models typically presume that the earliest anisogamous organisms had separate sexes. However, in most taxa, there is no empirical evidence to support this assumption. Here, we present a model of the coevolution of gamete size and sex allocation, which allows for anisogamy to emerge alongside either hermaphroditism or separate sexes. We show that hermaphroditic anisogamy can evolve directly from isogamous ancestors when the average size of spawning groups is small and fertilization is relatively efficient. Sex allocation under hermaphroditism becomes increasingly female-biased as group size decreases and the degree of anisogamy increases. When spawning groups are very small, our model also predicts the existence of complex isogamous organisms in which individuals allocate resources equally to two large gamete types. We discuss common, but potentially unwarranted, assumptions in the literature that could be relaxed in future models. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

Evolution of parental roles in phase portraits of bimatrix asymmetric games

In this paper, we address the evolutionary dynamic of parental roles using game theory. The main purpose of the article was to expand a classical list of evolutionary dynamic parental conflicts by adding some important cases which hitherto have not been intensively studied. Our models are apt to deliver some novel insights into the evolution of parental care. We also introduced several hypothetical events that served as illustrations of an arising alteration in cost-benefits for both parents and simulated a subsequent evolutionary endpoint. Our models revealed that evolutionary outcomes for reproductive decisions of both parents could be completely predicted by certain payoff matrices, which serve as proxies for a Darwinian fitness gain. In this sense, the result of a frequency-dependent selection on reproductive traits would inevitably depend on fitness costs and benefits arising for both parents in various circumstances. We demonstrated that population division could be a plausible evolutionary consequence for any human mating game where ‘reproductive defection’ represents the best response to any action by the reproductive opponent. We conclude that future evolutionary studies of human reproductive behavior should be more oriented on estimating a sex-biased asymmetry in potential fitness gains obtained by cooperative and deceptive parents in diverse environments and cultures.

Biological sex is binary, even though there is a rainbow of sex roles: Denying biological sex is anthropocentric and promotes species chauvinism: Denying biological sex is anthropocentric and promotes species chauvinism

Biomedical and social scientists are increasingly calling the biological sex into question, arguing that sex is a graded spectrum rather than a binary trait. Leading science journals have been adopting this relativist view, thereby opposing fundamental biological facts. While we fully endorse efforts to create a more inclusive environment for gender-diverse people, this does not require denying biological sex. On the contrary, the rejection of biological sex seems to be based on a lack of knowledge about evolution and it champions species chauvinism, inasmuch as it imposes human identity notions on millions of other species. We argue that the biological definition of the sexes remains central to recognising the diversity of life. Humans with their unique combination of biological sex and gender are different from non-human animals and plants in this respect. Denying the concept of biological sex, for whatever cause, ultimately erodes scientific progress and may open the flood gates to "alternative truths."

Sex in protists: A new perspective on the reproduction mechanisms of trypanosomatids

The Protist kingdom individuals are the most ancestral representatives of eukaryotes. They have inhabited Earth since ancient times and are currently found in the most diverse environments presenting a great heterogeneity of life forms. The unicellular and multicellular algae, photosynthetic and heterotrophic organisms, as well as free-living and pathogenic protozoa represents the protist group. The evolution of sex is directly associated with the origin of eukaryotes being protists the earliest protagonists of sexual reproduction on earth. In eukaryotes, the recombination through genetic exchange is a ubiquitous mechanism that can be stimulated by DNA damage. Scientific evidences support the hypothesis that reactive oxygen species (ROS) induced DNA damage can promote sexual recombination in eukaryotes which might have been a decisive factor for the origin of sex. The fact that some recombination enzymes also participate in meiotic sex in modern eukaryotes reinforces the idea that sexual reproduction emerged as consequence of specific mechanisms to cope with mutations and alterations in genetic material. In this review we will discuss about origin of sex and different strategies of evolve sexual reproduction in some protists such that cause human diseases like malaria, toxoplasmosis, sleeping sickness, Chagas disease, and leishmaniasis.

The origination events of gametic sexual reproduction and anisogamy

The evolution of gametic sex (meiosis and fertilization) and subsequent transition from isogamy (fusion between two equal-sized gametes) to anisogamy (dimorphism into eggs and sperm, namely, females and males) is one of the largest enigmas of evolutionary biology. Meiosis entails genome-dilution cost and anisogamy entails male-production cost. Despite much progress has been made for the maintenance mechanisms of sex, its origination events under such “twofold cost of sex” are still unsolved. Here, we posit two hypothetical scenarios as follows: the “Seesaw Effect” hypothesizes that automictic selfing between isogametes effectively purged deleterious mutations from an organism’s lineage and simultaneously fixed the sex-controlling allele and all other loci (no genome-dilution cost raised). The high relatedness among homoeologous cell colonies led to multicellularization. The “inflated isogamy” hypothesizes that multicellularity increased the reproductive investment of both mates, resulting in excessively large isogametes. This redundancy induced cheating of one sex (evolving to male) to reduce gamete size. However, the other sex (evolving to female) allowed this cheat because her cost did not change. Therefore, anisogamy originated as a kind of commensalism but turned into beneficial for females because it solved the gamete limitation problem inherent to isogamy. Thus, smooth transition to anisogamy had been attained.

Rampant nuclear-mitochondrial-plastid phylogenomic discordance in globally distributed calcifying microalgae

Incongruent phylogenies have been widely observed between nuclear and plastid or mitochondrial genomes in terrestrial plants and animals. However, few studies have examined these patterns in microalgae or the discordance between the two organelles. Here we investigated the nuclear-mitochondrial-plastid phylogenomic incongruence in Emiliania-Gephyrocapsa, a group of cosmopolitan calcifying phytoplankton with enormous populations and recent speciations. We assembled mitochondrial and plastid genomes of 27 strains from across global oceans and temperature regimes, and analyzed the phylogenomic histories of the three compartments using concatenation and coalescence methods. Six major clades with varying morphology and distribution are well recognized in the nuclear phylogeny, but such relationships are absent in the mitochondrial and plastid phylogenies, which also differ substantially from each other. The rampant phylogenomic discordance is due to a combination of organellar capture (introgression), organellar genome recombination, and incomplete lineage sorting of ancient polymorphic organellar genomes. Hybridization can lead to replacements of whole organellar genomes without introgression of nuclear genes and the two organelles are not inherited as a single cytoplasmic unit. This study illustrates the convoluted evolution and inheritance of organellar genomes in isogamous haplodiplontic microalgae and provides a window into the phylogenomic complexity of marine unicellular eukaryotes.

Bateman gradients from first principles

In 1948, Angus Bateman presented experiments and concepts that remain influential and debated in sexual selection. The Bateman gradient relates reproductive success to mate number, and Bateman presented this as the cause of intra-masculine selection. A deeper causal level was subsequently asserted: that the ultimate cause of sex differences in Bateman gradients is the sex difference in gamete numbers, an argument that remains controversial and without mathematical backup. Here I develop models showing how asymmetry in gamete numbers alone can generate steeper Bateman gradients in males. This conclusion remains when the further asymmetry of internal fertilisation is added to the model and fertilisation is efficient. Strong gamete limitation can push Bateman gradients towards equality under external fertilisation and reverse them under internal fertilisation. Thus, this study provides a mathematical formalisation of Bateman’s brief verbal claim, while demonstrating that the link between gamete number and Bateman gradients is not inevitable nor trivial.

In 1948, Bateman asserted that sexual selection is driven by the sex difference in gamete numbers. Lehtonen presents mathematical models broadly validating this controversial claim, while pointing out selection can be reversed under some conditions.

Sperm bauplan and function and underlying processes of sperm formation and selection

The spermatozoon is a highly differentiated and polarized cell, with two main structures: the head, containing a haploid nucleus and the acrosomal exocytotic granule, and the flagellum, which generates energy and propels the cell; both structures are connected by the neck. The sperm's main aim is to participate in fertilization, thus activating development. Despite this common bauplan and function, there is an enormous diversity in structure and performance of sperm cells. For example, mammalian spermatozoa may exhibit several head patterns and overall sperm lengths ranging from ∼30 to 350 µm. Mechanisms of transport in the female tract, preparation for fertilization, and recognition of and interaction with the oocyte also show considerable variation. There has been much interest in understanding the origin of this diversity, both in evolutionary terms and in relation to mechanisms underlying sperm differentiation in the testis. Here, relationships between sperm bauplan and function are examined at two levels: first, by analyzing the selective forces that drive changes in sperm structure and physiology to understand the adaptive values of this variation and impact on male reproductive success and second, by examining cellular and molecular mechanisms of sperm formation in the testis that may explain how differentiation can give rise to such a wide array of sperm forms and functions.

Asymmetric mating behavior of isogamous budding yeast

Anisogamy, the size difference between small male and large female gametes, is known to enable selection for sexual dimorphism and behavioral differences between sexes. Nevertheless, even isogamous species exhibit molecular asymmetries between mating types, which are known to ensure their self-incompatibility. Here, we show that different properties of the pheromones secreted by the MATa and MATα mating types of budding yeast lead to asymmetry in their behavioral responses during mating in mixed haploid populations, which resemble behavioral asymmetries between gametes in anisogamous organisms. MATa behaves as a random searcher that is stimulated in proportion to the fraction of MATα partner cells within the population, whereas MATα behaves as a short-range directional distance sensor. Mathematical modeling suggests that the observed asymmetric responses can enhance efficiency of mating and might thus provide a selective advantage. Our results demonstrate that the emergence of asymmetric mating behavior did not require anisogamy-based sexual selection.

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.

Switching environments, synchronous sex, and the evolution of mating types

While facultative sex is common in sexually reproducing species, for reasons of tractability most mathematical models assume that such sex is asynchronous in the population. In this paper, we develop a model of switching environments to instead capture the effect of an entire population transitioning synchronously between sexual and asexual modes of reproduction. We use this model to investigate the evolution of the number of self-incompatible mating types in finite populations, which empirically can range from two to thousands. When environmental switching is fast, we recover the results of earlier studies that implicitly assumed populations were engaged in asynchronous sexual reproduction. However when the environment switches slowly, we see deviations from previous asynchronous theory, including a lower number of mating types at equilibrium and bimodality in the stationary distribution of mating types. We provide analytic approximations for both the fast and slow switching regimes, as well as a numerical scheme based on the Kolmogorov equations for the system to quickly evaluate the model dynamics at intermediate parameters. Our approach exploits properties of integer partitions in number theory. We also demonstrate how additional biological processes such as selective sweeps can be accounted for in this switching environment framework, showing that beneficial mutations can further erode mating type diversity in synchronous facultatively sexual populations.

Drivers of Mating Type Composition in Tetrahymena thermophila

Sex offers advantages even in primarily asexual species. Some ciliates appear to utilize such reproductive strategy with many mating types. However, the factors determining the composition of mating types in the unicellular ciliate Tetrahymena thermophila are poorly understood, and this is further complicated by non-Mendelian determination of mating type in the offspring. We therefore developed a novel population genetics model to predict how various factors influence the dynamics of mating type composition, including natural selection. The model predicted either the coexistence of all seven mating types or fixation of a single mating type in a population, depending on parameter combinations, irrespective of natural selection. To understand what factor(s) may be more influential and to test the validity of theoretical prediction, five replicate populations were maintained in laboratory such that several factors could be controlled or measured. Whole-genome sequencing was used to identify newly arising mutations and determine mating type composition. Strikingly, all populations were found to be driven by strong selection on newly arising beneficial mutations to fixation of their carrying mating types, and the trajectories of speed to fixation agreed well with our theoretical predictions. This study illustrates the evolutionary strategies that T. thermophila can utilize to optimize population fitness.

Fitness differences suppress the number of mating types in evolving isogamous species

Sexual reproduction is not always synonymous with the existence of two morphologically different sexes; isogamous species produce sex cells of equal size, typically falling into multiple distinct self-incompatible classes, termed mating types. A long-standing open question in evolutionary biology is: what governs the number of these mating types across species? Simple theoretical arguments imply an advantage to rare types, suggesting the number of types should grow consistently; however, empirical observations are very different. While some isogamous species exhibit thousands of mating types, such species are exceedingly rare, and most have fewer than 10. In this paper, we present a mathematical analysis to quantify the role of fitness variation-characterized by different mortality rates-in determining the number of mating types emerging in simple evolutionary models. We predict that the number of mating types decreases as the variance of mortality increases.

Active pollination drives selection for reduced pollen-ovule ratios

PREMISE

Variation in pollen-ovule ratios is thought to reflect the degree of pollen transfer efficiency-the more efficient the process, the fewer pollen grains needed. Few studies have directly examined the relationship between pollen-ovule ratio and pollen transfer efficiency. For active pollination in the pollination brood mutualisms of yuccas and yucca moths, figs and fig wasps, senita and senita moths, and leafflowers and leafflower moths, pollinators purposefully collect pollen and place it directly on the stigmatic surface of conspecific flowers. The tight coupling of insect reproductive interests with pollination of the flowers in which larvae develop ensures that pollination is highly efficient.

METHODS

We used the multiple evolutionary transitions between passive pollination and more efficient active pollination to test if increased pollen transfer efficiency leads to reduced pollen-ovule ratios. We collected pollen and ovule data from a suite of plant species from each of the pollination brood mutualisms and used phylogenetically controlled tests and sister-group comparisons to examine whether the shift to active pollination resulted in reduced pollen-ovule ratios.

RESULTS

Across all transitions between passive and active pollination in plants, actively pollinated plants had significantly lower pollen-ovule ratios than closely related passively pollinated taxa. Phylogenetically corrected comparisons demonstrated that actively pollinated plant species had an average 76% reduction in the pollen-ovule ratio.

CONCLUSIONS

The results for active pollination systems support the general utility of pollen-ovule ratios as indicators of pollination efficiency and the central importance of pollen transfer efficiency in the evolution of pollen-ovule ratio.

Invasion and Extinction Dynamics of Mating Types Under Facultative Sexual Reproduction

In sexually reproducing isogamous species, syngamy between gametes is generally not indiscriminate, but rather restricted to occurring between complementary self-incompatible mating types. A longstanding question regards the evolutionary pressures that control the number of mating types observed in natural populations, which ranges from two to many thousands. Here, we describe a population genetic null model of this reproductive system, and derive expressions for the stationary probability distribution of the number of mating types, the establishment probability of a newly arising mating type, and the mean time to extinction of a resident type. Our results yield that the average rate of sexual reproduction in a population correlates positively with the expected number of mating types observed. We further show that the low number of mating types predicted in the rare-sex regime is primarily driven by low invasion probabilities of new mating type alleles, with established resident alleles being very stable over long evolutionary periods. Moreover, our model naturally exhibits varying selection strength dependent on the number of resident mating types. This results in higher extinction and lower invasion rates for an increasing number of residents.

Algal Sex Determination and the Evolution of Anisogamy

Algae are photosynthetic eukaryotes whose taxonomic breadth covers a range of life histories, degrees of cellular and developmental complexity, and diverse patterns of sexual reproduction. These patterns include haploid- and diploid-phase sex determination, isogamous mating systems, and dimorphic sexes. Despite the ubiquity of sexual reproduction in algae, their mating-type-determination and sex-determination mechanisms have been investigated in only a limited number of representatives. These include volvocine green algae, where sexual cycles and sex-determining mechanisms have shed light on the transition from mating types to sexes, and brown algae, which are a model for UV sex chromosome evolution in the context of a complex haplodiplontic life cycle. Recent advances in genomics have aided progress in understanding sexual cycles in less-studied taxa including ulvophyte, charophyte, and prasinophyte green algae, as well as in diatoms.

Evolution of asymmetric gamete signaling and suppressed recombination at the mating type locus

The two partners required for sexual reproduction are rarely the same. This pattern extends to species which lack sexual dimorphism yet possess self-incompatible gametes determined at mating-type regions of suppressed recombination, likely precursors of sex chromosomes. Here we investigate the role of cellular signaling in the evolution of mating-types. We develop a model of ligand-receptor dynamics, and identify factors that determine the capacity of cells to send and receive signals. The model specifies conditions favoring the evolution of gametes producing ligand and receptor asymmetrically and shows how these are affected by recombination. When the recombination rate evolves, the conditions favoring asymmetric signaling also favor tight linkage of ligand and receptor loci in distinct linkage groups. These results suggest that selection for asymmetric gamete signaling could be the first step in the evolution of non-recombinant mating-type loci, paving the road for the evolution of anisogamy and sexes.

Sexual reproduction, from birds to bees, relies on distinct classes of sex cells, known as gametes, fusing together. Most single cell organisms, rather than producing eggs and sperm, have similar sized gametes that fall into distinct ‘mating types’. However, only sex cells belonging to different mating types can fuse together and sexually reproduce.

At first glance, it seems illogical that cells from the same mating type cannot reproduce with each other, as this restricts eligible partners within a population and makes finding a mate more difficult. Yet the typical pattern amongst single cell organisms is still two distinct classes of sex cells, just as in birds and bees. How did the obsession with mating between two different types become favored during evolution?

One possibility is that cells with different mating types can recognize and communicate with each other more easily. Cells communicate by releasing proteins known as ligands, which bind to specific receptors found on the cell’s surface. Using mathematical modelling, Hadjivasiliou and Pomiankowski showed that natural selection typically favors ‘asymmetric’ signaling, whereby cells evolve to either produce receptor A with ligand B, or have the reverse pattern and produce receptor B with ligand A. These asymmetric mutants are favored because they avoid producing ligands that clog or activate the receptors on their own surface. As a result, different types of cells are better at recognizing each other and mating more quickly.

When cells sexually reproduce they exchange genetic material with each other to produce offspring with a combination of genes that differ to their own. However, if the genes coding for ligand and receptor pairs were constantly being ‘swapped’, this could lead to new combinations, and a loss of asymmetric signaling. Hadjivasiliou and Pomiankowski showed that for asymmetric signaling to evolve, natural selection favors the genes encoding these non-compatible ligand and receptor pairs to be closely linked within the genome. This ensures that the mis-matching ligand and receptor are inherited together, preventing cells from producing pairs which can bind to themselves.

This study provides an original way to address an evolutionary question which has long puzzled biologists. These findings raise further questions about how gametes evolved to become the sperm and egg, and how factors such as signaling between cells can determine the sex of more complex organisms, such as ourselves.

UV Chromosomes and Haploid Sexual Systems

The evolution of sex determination continues to pose major questions in biology. Sex-determination mechanisms control reproductive cell differentiation and development of sexual characteristics in all organisms, from algae to animals and plants. While the underlying processes defining sex (meiosis and recombination) are conserved, sex-determination mechanisms are highly labile. In particular, a flow of new discoveries has highlighted several fascinating features of the previously understudied haploid UV sex determination and related mating systems found in diverse photosynthetic taxa including green algae, bryophytes, and brown algae. Analyses integrating information from these systems and contrasting them with classical XY and ZW systems are providing exciting insights into both the universality and the diversity of sex-determining chromosomes across eukaryotes.

Multicellularity Drives the Evolution of Sexual Traits

From the male peacock's tail plumage to the floral displays of flowering plants, traits related to sexual reproduction are often complex and exaggerated. Why has sexual reproduction become so complicated? Why have such exaggerated sexual traits evolved? Early work posited a connection between multicellularity and sexual traits such as anisogamy (i.e., the evolution of small sperm and large eggs). Anisogamy then drives the evolution of other forms of sexual dimorphism. Yet the relationship between multicellularity and the evolution of sexual traits has not been empirically tested. Given their extensive variation in both multicellular complexity and sexual systems, the volvocine green algae offer a tractable system for understanding the interrelationship of multicellular complexity and sex. Here we show that species with greater multicellular complexity have a significantly larger number of derived sexual traits, including anisogamy, internal fertilization, and secondary sexual dimorphism. Our results demonstrate that anisogamy repeatedly evolved from isogamous multicellular ancestors and that anisogamous species are larger and produce larger zygotes than isogamous species. In the volvocine algae, the evolution of multicellularity likely drives the evolution of anisogamy, and anisogamy subsequently drives secondary sexual dimorphism. Multicellularity may set the stage for the overall diversity of sexual complexity throughout the Tree of Life.

Within-clutch variability in gamete size arises from the size variation in gametangia in the marine green alga Monostroma angicava

Within-clutch gamete size variability in Monostroma angicava. In many organisms, it is unclear how the size variation in gametes is generated in each clutch (i.e., total gametes produced by a gametophyte for a single spawning) or how gamete size is adjusted. Within-clutch variation in gamete size has been explained as a result of either physiological/developmental constraints or bet hedging during gametogenesis. These two explanations have been assumed to be mutually exclusive, and related observations are conflicting. The slightly anisogamous dioecious green alga Monostroma angicava employs a simple mechanism to produce gametes of each sex: each vegetative cell becomes a single gametangium cell, which synchronously divides to form equally sized gametes. The number of such cell divisions has several variations, which might vary gamete size. We measured the volume of gametangia in each clutch, counted the number of cell divisions in each gametangium and estimated the size of the gametes. We found that larger gametangia divided more times than smaller gametangia in both sexes, although male gametangia were smaller than female gametangia when they underwent the same number of cell divisions. Therefore, the variation in the number of cell divisions during gametogenesis serves to adjust gamete size in each sex rather than to vary it. Within-clutch gamete size variability originates in within-clutch variation in gametangium size: any factors that increase the variation in the size of gametangia can increase the within-clutch variation in gamete size.

Evolutionary divergence of the sex-determining gene MID uncoupled from the transition to anisogamy in volvocine algae

Volvocine algae constitute a unique comparative model for investigating the evolution of oogamy from isogamous mating types. The sex- or mating type-determining gene MID encodes a conserved RWP-RK transcription factor found in either the MT- or male mating locus of dioecious volvocine species. We previously found that MID from the isogamous species Chlamydomonas reinhardtii (CrMID) could not induce ectopic spermatogenesis when expressed heterologously in Volvox carteri females, suggesting coevolution of Mid function with gamete dimorphism. Here we found that ectopic expression of MID from the anisogamous species Pleodorina starrii (PsMID) could efficiently induce spermatogenesis when expressed in V. carteri females and, unexpectedly, that GpMID from the isogamous species Gonium pectorale was also able to induce V. carteri spermatogenesis. Neither VcMID nor GpMID could complement a C. reinhardtii mid mutant, at least partly owing to instability of heterologous Mid proteins. Our data show that Mid divergence was not a major contributor to the transition between isogamy and anisogamy/oogamy in volvocine algae, and instead implicate changes in cis-regulatory interactions and/or trans-acting factors of the Mid network in the evolution of sexual dimorphism.

The fitness effects of delayed switching to sex in a facultatively asexual insect

Facultative reproductive strategies that incorporate both sexual and parthenogenetic reproduction should be optimal, yet are rarely observed in animals. Resolving this paradox requires an understanding of the economics of facultative asexuality. Recent work suggests that switching from parthenogenesis to sex can be costly and that females can resist mating to avoid switching. However, it remains unclear whether these costs and resistance behaviors are dependent on female age. We addressed these questions in the Cyclone Larry stick insect, Sipyloidea larryi, by pairing females with males (or with females as a control) in early life prior to the start of parthenogenetic reproduction, or in mid- or late life after a period of parthenogenetic oviposition. Young females were receptive to mating even though mating in early life caused reduced fecundity. Female resistance to mating increased with age, but reproductive switching in mid- or late life did not negatively affect female survival or offspring performance. Overall, mating enhanced female fitness because fertilized eggs had higher hatching success and resulted in more adult offspring than parthenogenetic eggs. However, female fecundity and offspring viability were also enhanced in females paired with other females, suggesting a socially mediated maternal effect. Our results provide little evidence that switching from parthenogenesis to sex at any age is costly for S. larryi females. However, age-dependent effects of switching on some fitness components and female resistance behaviors suggest the possibility of context-dependent effects that may only be apparent in natural populations.

The varied ways of being male and female

Our understanding of sexual reproduction is mainly informed by research on gonochorists (i.e., species with separate sexes), including insects, birds, and mammals. But the male and female sexes are not two types of individuals; they actually represent two different reproductive strategies, and in many organisms, these two strategies are distributed among individuals in a population in a variety of ways. For example, sequential hermaphrodites (or sex-changers) exhibit one strategy early in life and later switch to the other, while simultaneous hermaphrodites exhibit both strategies at the same time. There are also many intermediate sexual systems that mix gonochorists and hermaphrodites in the same species and within many organismal groups, shifts occur between these sexual systems. A fascinating collection of six articles in this special issue on Hermaphroditism & Sex Determination impressively documents some important challenges to our understanding of sex determination, and the specification of male and female reproductive function when these need to occur within the same individual rather than in two separate individuals. Moreover, hermaphroditism changes how we need to think about reproductive allocation to sexual functions, how such allocation can be specified, as well as how the sexual system affects sexual conflict and the resulting antagonistic coevolution. Our understanding of sexual reproduction will profit greatly from exploring the varied ways of being male and female. Mol. Reprod. Dev. 84: 94-104, 2017. © 2017 Wiley Periodicals, Inc.

Give one species the task to come up with a theory that spans them all: what good can come out of that?

Does the progress in understanding evolutionary theory depend on the species that is doing the investigation? This question is difficult to answer scientifically, as we are dealing with an n = 1 scenario: every individual who has ever written about evolution is a human being. I will discuss, first, whether we get the correct answer to questions if we begin with ourselves and expand outwards, and second, whether we might fail to ask all the interesting questions unless we combat our tendencies to favour taxa that are close to us. As a whole, the human tendency to understand general biological phenomena via 'putting oneself in another organism's shoes' has upsides and downsides. As an upside, our intuitive ability to rethink strategies if the situation changes can lead to ready generation of adaptive hypotheses. Downsides occur if we trust this intuition too much, and particular danger zones exist for traits where humans are an unusual species. I argue that the levels of selection debate might have proceeded differently if human cooperation patterns were not so unique, as this brings about unique challenges in biology teaching; and that theoretical insights regarding inbreeding avoidance versus tolerance could have spread faster if we were not extrapolating our emotional reactions to incest disproportionately depending on whether we study animals or plants. I also discuss patterns such as taxonomic chauvinism, i.e. less attention being paid to species that differ more from human-like life histories. Textbooks on evolution reinforce such biases insofar as they present, as a default case, systems that resemble ours in terms of life cycles and other features (e.g. gonochorism). Additionally, societal norms may have led to incorrect null hypotheses such as females not mating multiply.

New-age ideas about age-old sex: separating meiosis from mating could solve a century-old conundrum

Ever since Darwin first addressed it, sexual reproduction reigns as the 'queen' of evolutionary questions. Multiple theories tried to explain how this apparently costly and cumbersome method has become the universal mode of eukaryote reproduction. Most theories stress the adaptive advantages of sex by generating variation, they fail however to explain the ubiquitous persistence of sexual reproduction also where adaptation is not an issue. I argue that the obstacle for comprehending the role of sex stems from the conceptual entanglement of two distinct processes - gamete production by meiosis and gamete fusion by mating (mixis). Meiosis is an ancient, highly rigid and evolutionary conserved process identical and ubiquitous in all eukaryotes. Mating, by contrast, shows tremendous evolutionary variability even in closely related clades and exhibits wonderful ecological adaptability. To appreciate the respective roles of these two processes, which are normally linked and alternating, we require cases where one takes place without the other. Such cases are rather common. The heteromorphic sex chromosomes Y and W, that do not undergo meiotic recombination are an evolutionary test case for demonstrating the role of meiosis. Substantial recent genomic evidence highlights the accelerated rates of change and attrition these chromosomes undergo in comparison to those of recombining autosomes. I thus propose that the most basic role of meiosis is conserving integrity of the genome. A reciprocal case of meiosis without bi-parental mating, is presented by self-fertilization, which is fairly common in flowering plants, as well as most types of apomixis. I argue that deconstructing sex into these two distinct processes - meiosis and mating - will greatly facilitate their analysis and promote our understanding of sexual reproduction.

Sexual Conflict, Facultative Asexuality, and the True Paradox of Sex

Theory suggests that occasional or conditional sex involving facultative switching between sexual and asexual reproduction is the optimal reproductive strategy. Therefore, the true 'paradox of sex' is the prevalence of obligate sex. This points to the existence of powerful, general impediments to the invasion of obligately sexual populations by facultative mutants, and recent studies raise the intriguing possibility that a key impediment could be sexual conflict. Using Bateman gradients we show that facultative asexuality can amplify sexual conflict over mating, generating strong selection for both female resistance and male coercion. We hypothesize that invasions are most likely to succeed when mutants have negative Bateman gradients, can avoid mating, and achieve high fecundity through asexual reproduction - a combination unlikely to occur in natural populations.

Social amoebae mating types do not invest unequally in sexual offspring

Unequal investment by different sexes in their progeny is common and includes differential investment in the zygote and differential care of the young. The social amoeba Dictyostelium discoideum has a sexual stage in which isogamous cells of any two of the three mating types fuse to form a zygote which then attracts hundreds of other cells to the macrocyst. The latter cells are cannibalized and so make no genetic contribution to reproduction. Previous literature suggests that this sacrifice may be induced in cells of one mating type by cells of another, resulting in a higher than expected production of macrocysts when the inducing type is rare and giving a reproductive advantage to this social cheat. We tested this hypothesis in eight trios of field-collected clones of each of the three D. discoideum mating types by measuring macrocyst production at different pairwise frequencies. We found evidence that supported differential contribution in only two of the 24 clone pairs, so this pattern is rare and clone-specific. In general, we did not reject the hypothesis that the mating types contribute cells relative to their proportion in the population. We also found a significant quadratic relationship between partner frequency and macrocyst production, suggesting that when one clone is rare, macrocyst production is limited by partner availability. We were also unable to replicate previous findings that macrocyst production could be induced in the absence of a compatible mating partner. Overall, mating type-specific differential investment during sex is unlikely in microbial eukaryotes like D. discoideum.