The contribution of haploids, diploids and clones to fine-scale population structure in the seaweed Cladophoropsis membranacea (Chlorophyta)

Local populations of Cladophoropsis membranacea exist as mats of coalesced thalli composed of free-living haploid and diploid plants including clonally reproduced plants of either phase. None of the phases are morphologically distinguishable. We used eight microsatellite loci to explore clonality and fine-scale patch structure in C. membranacea at six sites on the Canary Islands. Mats were always composites of many individuals; not single, large clones. Haploids outnumbered diploids at all sites (from 2:1 to 10:1). In both haploid and diploid plants, genetic diversity was high and there was no significant difference in allele frequencies. Significant heterozygote deficiencies were found in the diploid plants at five out of six sites and linkage disequilibrium was associated with the haploid phase at all sites. Short dispersal distances of gametes/spores and small effective population sizes associated with clonality probably contribute to inbreeding. Spatial autocorrelation analysis revealed that most clones were found within a radius of approximately 60 cm and rarely further than 5 m. Dominance of the haploid phase may reflect seasonal shifts in the relative frequencies of haploids and diploids, but may alternatively reflect superiority of locally adapted and competitively dominant, haploid clones; a strategy that is theoretically favoured in disturbed environments. Although sexual reproduction may be infrequent in C. membranacea, it is sufficient to maintain both life history phases and supports theoretical modelling studies that show that haploid-diploid life histories are an evolutionarily stable strategy.     

The evolution of life cycles with haploid and diploid phases

Sexual eukaryotic organisms are characterized by an alternation between haploid and diploid phases. In vascular plants and animals, somatic growth and development occur primarily in the diploid phase, with the haploid phase reduced to the gametic cells. In many other eukaryotes, however, growth and development occur in both phases, with substantial variability among organisms in the length of each phase of the life cycle. A number of theoretical models and experimental studies have shed light on factors that may influence life cycle evolution, yet we remain far from a complete understanding of the diversity of life cycles observed in nature. In this paper we review the current state of knowledge in this field, and touch upon the many questions that remain unanswered. BioEssays 20 :453–462, 1998. © 1998 John Wiley & Sons, Inc.     

What controls haploid—diploid ratio in the red alga,Gracilaria verrucosa?

The conditions for maintenance of a haploid—diploid life cycle in the species Gracilaria verrucosa were studied. This species is a red alga, where haploid plants have separate sexes. In the two natural populations studied, male and female haploid individuals were in equal proportions, and the frequency of diploid individuals reached 0.5. A two-fold advantage in viability for diploid relative to haploid juveniles was observed in the field. This advantage can account for a frequency of 0.5 of diploid individuals in natural populations. Different types of anomalies in the reproduction of diploid individuals were observed, all of which lead to a reduction of the haploid stage.     

Evolutionary limits to the frequency of aggression between related or unrelated conspecifics in diploid species with simple mendelian inheritance

Game theory has been used by some authors to analyse evolutionary limits to the expression of aggression in theoretical haploid parthenogenetic species. Others have examined frequency dependent selection, of which aggression may be a case, by applying population genetic models to diploid species. A model is presented which attempts to combine these two approaches. Game theory is used to determine evolutionarily stable strategies and corresponding stable polymorphisms for a two-strategy game played by members of a diploid sexual species, when choice of strategy is determined by two alleles at a single locus. Results are given for dominant, co-dominant and recessive determination of choice of the more aggressive of two strategies, for two levels of relationship: unrelated players and sibs. It is found that for a range of models of single locus inheritance the evolutionarily stable strategy (ESS) determined for haploid species remains the stable population strategy for diploid sexual species, when players are unrelated. In sibling contestants aggression is reduced. The mixed strategy haploid ESS underestimates, but the pure strategy haploid ESS provides a good indication of the degree to which relatedness lessens aggression in diploid species. For both haploid and diploid species there may be a considerable advantage to confining conflicts to kin.     

PLOIDY AND EVOLUTION BY SEXUAL SELECTION: A COMPARISON OF HAPLOID AND DIPLOID FEMALE CHOICE MODELS NEAR FIXATION EQUILIBRIA

We compare the stability properties of haploid and diploid models of Fisherian sexual selection (with male contribution limited to sperm) by examining both models at equilibria for which a male trait is fixed or absent. Haploid and diploid two locus diallelic models share the property that the stability of such fixation equilibria is determined by the relationship between the harmonic mean of relative preference values for the common male trait, weighted by the frequency of the preferences, and the relative viability associated with the common male trait. When diploid females with heterozygotic-based preferences express preference strengths intermediate between homozygote-based preferences, then boundary equilibria of haploid and diploid models share many stability properties. However, even with intermediate heterozygote preferences, haploid and diploid models do differ: (1) for a particular frequency of the preference allele, both fixation boundaries can be stable for the diploid model, and (2) with over- or underdominance at the preference locus (a possibility precluded in the haploid model), a fixation boundary in the diploid model may show two switches in its stability state for increasing frequencies of one of the preference alleles. These differences are due not just to the impossibility of dominance in haploid models, but also to the larger number of diploid genotypes.     

Population Studies in Microorganisms I. Evolution of Diploidy in SACCHAROMYCES CEREVISIAE

The relative adaptation of isogenic haploid and diploid strains of yeast was investigated in different sets of physiological conditions. When all nutrients were present in excess, no difference in the reproductive rates of isogenic haploid and diploid strains of yeast was detected in both optimal and non-optimal growth conditions. Competition between haploid and diploid strains of yeast was observed when growth was limited by the concentration of a single nutrilite. Under certain conditions when fitness (reproductive rate) is determined by transport of an essential nutrilite that exists in very low concentrations, diploid cells were selected against. These environmental conditions are similar to those found in offshore marine environments where nutrients are often present in extremely low concentrations. The fitness of diploid cells was estimated to be.93 +/-.02 (haploid fitness = 1). The reduced fitness of diploid cells in this environment can be explained by the reduced surface area/volume ratio possessed by diploid cells in comparison to haploid cells. The fitnesses of haploid and diploid cells in these environments are closely correlated with geometric variations in these strains. These results are consistent with the hypothesis that diploid cells are simply double haploids, and diploidy per se does not confer any direct adaptive advantage. The mechanism of the evolution of diploidy as a dominant phase in the life cycle of higher plants and animals remains obscure.     

Invasion of novel habitats uncouples haplo‐diplontic life cycles

Baker's Law predicts uniparental reproduction will facilitate colonization success in novel habitats. While evidence supports this prediction among colonizing plants and animals, few studies have investigated shifts in reproductive mode in haplo‐diplontic species in which both prolonged haploid and diploid stages separate meiosis and fertilization in time and space. Due to this separation, asexual reproduction can yield the dominance of one of the ploidy stages in colonizing populations. We tested for shifts in ploidy and reproductive mode across native and introduced populations of the red seaweed Gracilaria vermiculophylla. Native populations in the northwest Pacific Ocean were nearly always attached by holdfasts to hard substrata and, as is characteristic of the genus, haploid–diploid ratios were slightly diploid‐biased. In contrast, along North American and European coastlines, introduced populations nearly always floated atop soft‐sediment mudflats and were overwhelmingly dominated by diploid thalli without holdfasts. Introduced populations exhibited population genetic signals consistent with extensive vegetative fragmentation, while native populations did not. Thus, the ecological shift from attached to unattached thalli, ostensibly necessitated by the invasion of soft‐sediment habitats, correlated with shifts from sexual to asexual reproduction and slight to strong diploid bias. We extend Baker's Law by predicting other colonizing haplo‐diplontic species will show similar increases in asexuality that correlate with the dominance of one ploidy stage. Labile mating systems likely facilitate colonization success and subsequent range expansion, but for haplo‐diplontic species, the long‐term eco‐evolutionary impacts will depend on which ploidy stage is lost and the degree to which asexual reproduction is canalized.     

Ploidy controls the success of mutators and nature of mutations during budding yeast evolution

BACKGROUND: We used the budding yeast Saccharomyces cerevisiae to ask how elevated mutation rates affect the evolution of asexual eukaryotic populations. Mismatch repair defective and nonmutator strains were competed during adaptation to four laboratory environments (rich medium, low glucose, high salt, and a nonfermentable carbon source). RESULTS: In diploids, mutators have an advantage over nonmutators in all conditions, and mutators that win competitions are on average fitter than nonmutator winners. In contrast, haploid mutators have no advantage when competed against haploid nonmutators, and haploid mutator winners are less fit than nonmutator winners. The diploid mutator winners were all superior to their ancestors both in the condition they had adapted to, and in two of the other conditions. This phenotype was due to a mutation or class of mutations that confers a large growth advantage during the respiratory phase of yeast cultures that precedes stationary phase. This generalist mutation(s) was not selected in diploid nonmutator strains or in haploid strains, which adapt primarily by fixing specialist (condition-specific) mutations. In diploid mutators, such mutations also occur, and the majority accumulates after the fixation of the generalist mutation. CONCLUSIONS: We conclude that the advantage of mutators depends on ploidy and that diploid mutators can give rise to beneficial mutations that are inaccessible to nonmutators and haploid mutators.     

CHPA, a cysteine- and histidine-rich-domain-containing protein, contributes to maintenance of the diploid state in Aspergillus nidulans

The alternation of eukaryotic life cycles between haploid and diploid phases is crucial for maintaining genetic diversity. In some organisms, the growth and development of haploid and diploid phases are nearly identical, and one might suppose that all genes required for one phase are likely to be critical for the other phase. Here, we show that targeted disruption of the chpA (cysteine- and histidine-rich-domain- [CHORD]-containing protein A) gene in haploid Aspergillus nidulans strains gives rise to chpA knockout haploids and heterozygous diploids but no chpA knockout diploids. A. nidulans chpA heterozygous diploids showed impaired conidiophore development and reduced conidiation. Deletion of chpA from diploid A. nidulans resulted in genome instability and reversion to a haploid state. Thus, our data suggest a vital role for chpA in maintenance of the diploid phase in A. nidulans. Furthermore, the human chpA homolog, Chp-1, was able to complement haploinsufficiency in A. nidulans chpA heterozygotes, suggesting that the function of CHORD-containing proteins is highly conserved in eukaryotes.     

Evolution of the alternation of haploid and diploid phases in life cycles. II. Maintenance of the haplo-diplontic cycle

The relative duration of the haploid and the diploid phases during the reproductive cycle varies greatly between organisms. This paper addresses the question of the evolution of haploid, diploid, and haplo-diplontic life cycles. When the life span of haploid and diploid individuals is constant whatever their cycle, we show that the haplo-diplontic cycle has an advantage, which depends on the sex-ratio in anisogamous species and on the probability of fertilization in isogamous species. This is because meiosis and fertilization occur half as often in the haplo-diplontic cycle as in haploid or diploid cycles, for the same number of generations of individuals. This argument is demonstrated using a model which considers a genetic determination of the cycle, and fixed haploid and diploid fitnesses. The relevance of measures of fitness of haploid and diploid individuals in predicting the evolution of life cycles is discussed. Measures obtained in algae are compared with theoretical predictions.     

Differential ecological responses to environmental stress in the life history phases of the isomorphic red alga Gracilaria chilensis (Rhodophyta)

In order to better understand the alternation of generations that characterizes haploid–diploid life cycles, we assessed the existence of ecological differences between the two phases (haploid gametophyte and diploid tetrasporophyte) in Gracilaria chilensis, a rhodophyte with a typical Polysiphonia-type life cycle. We investigated the effect of light intensity and salinity on viability and growth of both phases at different ontogenetic stages: juveniles and adults. In our study, the survival of juvenile gametophytes (n) was higher than the survival of juvenile tetrasporophytes (2n) despite culture conditions; however, low salinity had greater effect on carpospores (2n) than on tetraspores (n). On the other hand, a complex interaction between salinity and light intensity within each life history phase generated observed differences between juvenile growth rates. Low light was shown to trigger early onset of alteration of the holdfast growing pattern. In addition, adult tetrasporophytes showed, despite the conditions, a faster vegetative growth than female and male gametophytes. These differences between phases could have led to the complete dominance of tetrasporophyte fragments of fronds observed in G. chilensis farms. We hypothesize that Chilean fishers could have unknowingly selected for tetrasporophyte thalli during domestication of the species, thus enhancing the natural trend of tetrasporophytes dominance already present in estuarine natural populations of free-floating plants.     

Somatic Mutation Favors the Evolution of Diploidy

Explanations of diploidy have focused on advantages gained from masking deleterious mutations that are inherited. Recent theory has shown that these explanations are flawed. Indeed, we still lack any satisfactory explanation of diploidy in species that are asexual or that recombine only rarely. Here I consider a possibility first suggested by EFROIMSON in 1932, by MULLER in 1964 and by CROW and KIMURA in 1965: diploidy may provide protection against somatic, not inherited, mutations. I both compare the mean fitness of haploid and diploid populations that are asexual and investigate the invasion of ``diploidy' alleles in sexual populations. When deleterious mutations are partially recessive and somatic mutation is sufficiently common, somatic mutation provides a clear advantage to diploidy in both asexual and sexual species.     

DELETERIOUS MUTATION AND THE EVOLUTION OF GENETIC LIFE CYCLES

It is often proposed that the ability of diploids to mask deleterious mutations leads to an evolutionary advantage over haploidy. In this paper, we studied the evolution of the relative duration of haploid and diploid phases using a model of recurrent deleterious mutations across the entire genome. We found that a completely diploid life cycle is favored under biologically reasonable conditions, even when prolonging the diploid phase reduces a population's mean fitness. A haploid cycle is favored when there is complete linkage throughout the genome or when mutations are either highly deleterious or partially dominant. These results hold when loci interact multiplicatively and for synergistic epistasis. The strength of selection generated on the life cycle can be substantial because of the cumulative effect of selection against mutations across many loci. We did not find conditions that support cycles that retain both phases, such as those found in some plants and algae. Thus, selection against deleterious mutations may be an important force in the evolution of life cycles but may not be sufficient to explain all the patterns of life cycles seen in nature.     

Evolution of alternation of haploid and diploid phases in life cycles

Eukaryotic sex leads to an alternation of haploid and diploid nuclear phases. Because all multicellular animals are diploid, diploidy is often considered a 'biological success' and many arguments have been advanced to explain the evolution of a prolonged diploid phase. Nevertheless, among eukaryotes three basic situations are encountered, where the vegetative individuals are diploid or haploid or both. These three basic life cycles are widely distributed among kingdoms and in some taxa the occurrence of different life cycles within the same species has been reported. This article briefly summarizes the different hypotheses on the evolution of reproductive life cycles and underlines how possibilities of variation for this trait may open new perspectives for research.     

Seasonal and spatial patterns of population density in the marine macroalga Mazzaella splendens (Gigartinales, Rhodophyta)

Insight into demographic processes that operate at larger spatial scales can be achieved through studying local populations when a particular species of interest is examined over time, by many investigators, in a variety of locations. On the west coast of North America, Mazzaella splendens (Setchell et Gardner) Fredericq is such a species of interest. A synthesis of local demographic studies of M. splendens from the late 1960s to the present reveals a pattern that is potentially common to the larger natural populations. This is the pattern: population density is high in summer and low in winter for both alternate free‐living life history phases of M. splendens. The magnitude of this seasonal change decreases in increasingly wave‐exposed habitats. In wave‐sheltered habitats there is a seasonal alternation from summer haploid to winter diploid dominance. This alternation gradually changes to constant diploid dominance as wave exposure in the habitat increases. Changes in population density are primarily a function of appearances and disappearances of perennating basal crusts (genets), as modules are produced or lost, rather than differential module production by genets of one phase over those of the other. To test the generality of this pattern, we examined seasonal changes in density, in local populations of M. splendens, in both a wave‐sheltered and a wave‐exposed habitat at Second Beach, Barkley Sound. Greater seasonal fluctuation in population density at wave‐sheltered, compared to wave‐exposed habitats is supported as a pattern potentially common to the natural populations of M. splendens. A change from summer haploid dominance in wave‐sheltered areas to summer diploid dominance in wave‐exposed areas is similarly supported. All changes in population density were the result of appearances and disappearances of genets rather than differential module production by haploid versus diploid basal crusts, also consistent with previous observations. A seasonal alternation in phase dominance, however, was absent from the wave‐sheltered site at Second Beach, Barkley Sound for 3 consecutive years. Seasonal alternation in phase dominance of M. splendens appears dependent on local conditions and is not common to all natural populations.     

ALTERNATION OF GENERATIONS IN ALGAE: ITS COMPLEXITY, MAINTENANCE AND EVOLUTION

Life histories of photosynthetic eukaryotes traditionally-termed algae exhibit a considerably greater degree of complexity than those of ‘higher cryptogams.’ Some algae have a so-called ‘obligate’alternation between spore-producing and gamete-producing phases, but the majority seem capable of following other pathways depending upon environmental conditions. In only four algal classes do life histories show a change in morphological and/or nuclear phases. The following basic life histories are recognized in the Chlorophyceae, Phaeophyceae and Rhodophyceae:(a) monophasic, a diploid or haploid phase, (b) two or more phases, most commonly an alternation of an isomorphic or heteromorphic haploid gametangial phase and a diploid sporangial phase, and (c) three phases (unique to florideophyte Rhodophyceae), with a diploid spore-producing phase (carposporophyte) developing on the gametangial phase, a diploid phase (tetrasporophyte if meiosis is sporic) and a haploid gametangial phase. Evidence from recent research indicates that in many algae there is an uncoupling of the morphological and nuclear phases. The dominance of one phase and suppression of another has been suggested to be due to the common occurrence in algae of apogamy, apomeiosis and parthenogenesis. Free-living morphs in heteromorphic life histories may be morphologically so dissimilar that formerly they were attributed to different genera. Evolution of the carposporangial phase in red algae is speculated to be a means of achieving zygotic amplification to compensate for the infrequency of syngamy. Such amplification allows the production of a large number of dispersible products from a single fertilization. The direct development of a free-living tetrasporangial phase is considered another mechanism for achieving amplification. In freshwater red algae the growth of an upright phase from a perennial microscopic one is considered an adaptation for maintaining their upstream position. Life history pathways in algae are controlled by subtle environmental influences (e.g. photoperiodism, temperature, light quality, nutrients). Experimental evidence is lacking to support the contention that spatial and/or temporal partitioning of the environment is a mechanism favouring the maintenance of heteromorphy. Herbivory is known to be an important selective force suppressing some morphs and accentuating the seasonal dominance of others. Differential resistance of morphs to herbivory in environments where grazing intensity is predictable may lead to the selective maintenance of heteromorphy. Algal life history patterns are unexplored in terms of evolutionary processes. Various models for the evolution of biphasic or polyphasic life histories stress the importance of the capacity for both asexual dispersal of successful genotypes and for the generation of new genotypes via meiosis and syngamy. All evidence points to the fact that many life history processes operative in algae differ significantly from those described for other cryptogams.     

Selection and the Evolution of Genetic Life Cycles

The evolution of haploid and diploid phases of the life cycle is investigated theoretically, using a model where the relative length of haploid and diploid phases is under genetic control. The model assumes that selection occurs in both phases and that fitness in each phase is a function of the time spent in that phase. The equilibrium and stability conditions that allow for all-haploid, all-diploid, or polyphasic life cycles are considered for general survivorship functions. Types of stable life cycles possible depend on the form of the viability selection. If mortality rates are constant, either haploidy or diploidy is the only stable life cycle possible. Departures from constant mortality can give qualitatively different results. For example, when survivorship in each phase is a linear, decreasing function of the time spent in the phase, stable haploid, diploid or polyphasic life cycles are possible. The addition of genetic variation at a coevolving viability locus does not qualitatively affect the outcome with respect to the maintenance of polyphasic cycles but can lead to situations where more than one life cycle is concurrently stable. These results show that trade-offs between the advantages of being diploid and of being haploid may help explain the patterns of life cycles found in nature and that the type of selection may be critical to determining the results.     

Ploidally antagonistic selection maintains stable genetic polymorphism

Understanding the maintenance of genetic variation in the face of selection remains a key issue in evolutionary biology. One potential mechanism for the maintenance of genetic variation is opposing selection during the diploid and haploid stages of biphasic life cycles universal among eukaryotic sexual organisms. If haploid and diploid gene expression both occur, selection can act in each phase, potentially in opposing directions. In addition, sex-specific selection during haploid phases is likely simply because male and female gametophytes/gametes tend to have contrasting life histories. We explored the potential for the maintenance of a stable polymorphism under ploidally antagonistic as well as sex-specific selection. Furthermore, we examined the role of the chromosomal location of alleles (autosomal or sex-linked). Our analyses show that the most permissible conditions for the maintenance of polymorphism occur under negative ploidy-by-sex interactions, where stronger selection for an allele in female than male diploids is coupled with weaker selection against the allele in female than male haploids. Such ploidy-by-sex interactions also promote allele frequency differences between the sexes. With constant fitness, ploidally antagonistic selection can maintain stable polymorphisms for autosomal and X-linked genes but not for Y-linked genes. We discuss the implications of our results and outline a number of biological settings where the scenarios modeled may apply.     

Similar Performance of Diploid and Haploid Males in an Ant Species without Inbreeding Avoidance

Under haplodiploidy, a characteristic trait of all Hymenoptera, females develop from fertilised eggs, and males from unfertilised ones. Males are therefore typically haploid. Yet, inbreeding can lead to the production of diploid males that often fail in development, are sterile or are of lower fertility. In most Hymenoptera, inbreeding is avoided by dispersal flights of one or both sexes, leading to low diploid male loads. We investigated causes for the production of diploid males and their performance in a highly inbred social Hymenopteran species. In the ant Hypoponera opacior, inbreeding occurs between wingless sexuals, which mate within the mother nest, whereas winged sexuals outbreed during mating flights earlier in the season. Wingless males mate with queen pupae and guard their mating partners. We found that they mated randomly with respect to relatedness, indicating that males do not avoid mating with close kin. These frequent sib‐matings lead to the production of diploid males, which are able to sire sterile triploid offspring. We compared mating activity and lifespan of haploid and diploid wingless males. As sexual selection acts on the time of emergence and body size in this species, we also investigated these traits. Diploid males resembled haploid ones in all investigated traits. Hence, albeit diploid males cannot produce fertile offspring, they keep up with haploid males in their lifetime mating success. Moreover, by fathering viable triploid workers, they contribute to the colonies' work force. In conclusion, the lack of inbreeding avoidance led to frequent sib‐matings of wingless sexuals, which in turn resulted in the regular production of diploid males. However, in contrast to many other Hymenopteran species, diploid males exhibit normal sexual behaviour and sire viable, albeit sterile daughters.     

Sexual conflict and the alternation of haploid and diploid generations

Land plants possess a multicellular diploid stage (sporophyte) that begins development while attached to a multicellular haploid progenitor (gametophyte). Although the closest algal relatives of land plants lack a multicellular sporophyte, they do produce a zygote that grows while attached to the maternal gametophyte. The diploid offspring shares one haploid set of genes with the haploid mother that supplies it with resources and a paternal haploid complement that is not shared with the mother. Sexual conflict can arise within the diploid offspring because the offspring's maternal genome will be transmitted in its entirety to all other sexual and asexual offspring that the mother may produce, but the offspring's paternally derived genes may be absent from these other offspring. Thus, the selective forces favouring the evolution of genomic imprinting may have been present from the origin of modern land plants. In bryophytes, where gametophytes are long-lived and capable of multiple bouts of asexual and sexual reproduction, we predict strong sexual conflict over allocation to sporophytes. Female gametophytes of pteridophytes produce a single sporophyte and often lack means of asexual reproduction. Therefore, sexual conflict is predicted to be attenuated. Finally, we explore similarities among models of mate choice, offspring choice and segregation distortion.