Evidence for equal size cell divisions during gametogenesis in a marine green alga Monostroma angicava

The correlation between anisogamy and sexual selection intensity-the broad theoretical predictions

Darwin and Bateman suggested that precopulatory sexual selection is more intense on males than females, and that this difference is due to anisogamy (i.e., dimorphism in gamete size and number). While a recent paper apparently presents empirical support for this hypothesis, another appears at first sight to present evidence against it. We argue that this is partly due to lack of transparent theoretical predictions, and discuss and analyze sexual selection theory in relation to anisogamy evolution. On one hand, we find that there exists relatively little theory that can directly address all the tested predictions; on the other, the picture painted by current theory indicates that both sets of empirical results broadly match predictions about the causal link between anisogamy and sexual selection, thus reconciling the two apparently opposing claims. We also discuss in a very broad, general sense how anisogamy is expected to affect sexual selection.

An ulvophycean marine green alga produces large parthenogenetic isogametes as predicted by the gamete dynamics model for the evolution of anisogamy

In eukaryotes, gamete size difference between the two sexes (anisogamy) evolved from gametes of equal size in both mating types (isogamy). The gamete dynamics (GD) model for anisogamy evolution combines gamete limitation and competition and predicts that if gametes of both mating types can develop parthenogenetically (i.e. without fusing with the opposite mating type), large isogamy can evolve under gamete-limited conditions. Ulvophycean marine green algae that have been claimed to exhibit various gametic systems from isogamy to anisogamy are important models for testing such theories. However, in most previous papers, whether a species is isogamous or anisogamous has not been examined statistically. Caution is necessary regarding claims of slight anisogamy because of gamete size variation. We reveal (i) that the gametic system of Struvea okamurae is large isogamy using a generalized linear mixed model, which accounted for the variation of gamete size among individual gametophytes, and (ii) that gametes of this alga can actually develop parthenogenetically, contrary to a previous report. Its habitat environments and protracted duration of gamete release suggest that this alga might experience gamete-limited conditions. Struvea okamurae seems to produce large parthenogenetic isogametes following GD model predictions, as an adaptation to deep waters.

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.

Identification of genomic differences between the sexes and sex-specific molecular markers in Monostroma angicava (Ulvophyceae)

There is little information available regarding genomic differences between sexes in ulvophycean green algae. The detection of these differences will enable the development of sex-discriminating molecular markers, which are useful for algae showing little apparent difference between sexes. In this study, we identified male- and female-specific DNA sequences in the ulvophycean marine green alga Monostroma angicava, which has a dioicous heteromorphic haplo-diplontic life cycle, via next-generation sequencing. Fluorescence in situ hybridization (FISH) showed that signals for the sex-specific sequences exist only in the nuclei of the corresponding sex, confirming the specificity of the sequences. Sex-specific molecular markers that targeted these sequences successfully distinguished the sex of gametophytes even in geographically distant populations, indicating that the sex-specific sequences are universal. These results consistently suggest that male and female gametophytes of M. angicava are genetically different, implying that sex may be determined genetically in this alga.

The Legacy of Parker, Baker and Smith 1972: Gamete Competition, the Evolution of Anisogamy, and Model Robustness

The evolution of anisogamy or gamete size dimorphism is a fundamental transition in evolutionary history, and it is the origin of the female and male sexes. Although mathematical models attempting to explain this transition have been published as early as 1932, the 1972 model of Parker, Baker, and Smith is considered to be the first explanation for the evolution of anisogamy that is consistent with modern evolutionary theory. The central idea of the model is ingenious in its simplicity: selection simultaneously favours large gametes for zygote provisioning, and small gametes for numerical competition, and under certain conditions the outcome is anisogamy. In this article, I derive novel analytical solutions to a 2002 game theoretical update of the 1972 anisogamy model, and use these solutions to examine its robustness to variation in its central assumptions. Combining new results with those from earlier papers, I find that the model is quite robust to variation in its central components. This kind of robustness is crucially important in a model for an early evolutionary transition where we may only have an approximate understanding of constraints that the different parts of the model must obey.

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.

Nuclear behavior and roles indicate that Codiolum phase is a sporophyte in Monostroma angicava (Ulotrichales, Ulvophyceae)

The life-cycle system of Ulotrichales, a major order of Ulvophyceae, remains controversial because it is unclear whether the Codiolum phase, a characteristic unicellular diploid generation in ulotrichalean algae, is a zygote or a sporophyte. This controversy inhibits the understanding of the diversified life cycles in Ulvophyceae. To distinguish between zygotes and sporophytes, we have to examine not only whether diploid generations function as sporophytes, but also whether mitosis occurs before meiosis in diploid generations. However, the nuclear behavior in the Codiolum phases is largely unknown, probably because no suitable methods are available. Using fluorescent microscopy with ethidium bromide and transmission electron microscopy of cell-wall-dissected specimens, we report the nuclear behavior in the Codiolum phases of an ulotrichalean alga with a representative life cycle, Monostroma angicava. Each vegetative Codiolum phase had a single polyploid nucleus due to endoreduplication, a type of mitosis without nuclear division. During zoosporogenesis, the nucleus had a structure that would be a meiosis-specific complex. We quantitatively showed that Codiolum phases grew extremely large and produced numerous zoospores. Our results suggest that an event comparable to mitosis occurs before meiosis in the Codiolum phase of M. angicava. This nuclear behavior and the functions (growth and zoospore production abilities) correspond to those of sporophytes. Therefore, the life-cycle system of M. angicava is a heteromorphic haplo-diplontic cycle. This system appears to be widely adopted among other ulotrichalean algae.

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.

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.

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'.