EVALUATION OF ELEVATED PLOIDY AND ASEXUAL REPRODUCTION AS ALTERNATIVE EXPLANATIONS FOR GEOGRAPHIC PARTHENOGENESIS IN EUCYPRIS VIRENS OSTRACODS

Transitions from sexual to asexual reproduction are often coupled with elevations in ploidy. As a consequence, the importance of ploidy per se for the maintenance and spread of asexual populations is unclear. To examine the effects of ploidy and asexual reproduction as independent determinants of the success of asexual lineages, we sampled diploid sexual, diploid asexual, and triploid asexual Eucypris virens ostracods across a European wide range. Applying nuclear and mitochondrial markers, we found that E. virens consists of genetically highly differentiated diploid sexual populations, to the extent that these sexual clades could be considered as cryptic species. All sexual populations were found in southern Europe and North Africa and we found that both diploid asexual and triploid asexual lineages have originated multiple times from several sexual lineages. Therefore, the asexual lineages show a wide variety of genetic backgrounds and very strong population genetic structure across the wide geographic range. Finally, we found that triploid, but not diploid, asexual clones dominate habitats in northern Europe. The limited distribution of diploid asexual lineages, despite their shared ancestry with triploid asexual lineages, strongly suggests that the wider geographic distribution of triploids is due to elevated ploidy rather than to asexuality.     

Gene Flow among Populations of Two Rare Co-Occurring Fern Species Differing in Ploidy Level

Differences in ploidy levels among different fern species have a vast influence on their mating system, their colonization ability and on the gene flow among populations. Differences in the colonization abilities of species with different ploidy levels are well known: tetraploids, in contrast to diploids, are able to undergo intra-gametophytic selfing. Because fertilization is a post-dispersal process in ferns, selfing results in better colonization abilities in tetraploids because of single spore colonization. Considerably less is known about the gene flow among populations of different ploidy levels. The present study examines two rare fern species that differ in ploidy. While it has already been confirmed that tetraploid species are better at colonizing, the present study focuses on the gene flow among existing populations. We analyzed the genetic structure of a set of populations in a 10×10 km study region using isoenzymes. Genetic variation in tetraploid species is distributed mainly among populations; the genetic distance between populations is correlated with the geographical distance, and larger populations host more genetic diversity than smaller populations. In the diploid species, most variability is partitioned within populations; the genetic distance is not related to geographic distance, and the genetic diversity of populations is not related to the population size. This suggests that in tetraploid species, which undergo selfing, gene flow is limited. In contrast, in the diploid species, which experience outcrossing, gene flow is extensive and the whole system behaves as one large population. Our results suggest that in ferns, the ability to colonize new habitats and the gene flow among existing populations are affected by the mating system.     

Gametophyte-sporophyte coalescence in populations of the intertidal carrageenophyte <Emphasis Type="Italic">Mazzaella laminarioides</Emphasis> (Rhodophyta)

Mazzaella laminarioides has consistently been reported as a typical coalescent/clump species with a triphasic life history of the Polysiphonia-type in which the haploid gametophyte is the predominant phase with respect to the diploid sporophyte. Preliminary observations of intertidal populations revealed that, in some instances, cystocarpic and tetrasporic fronds emerged from the same clump (G-T clumps), implying a coalescent process of haploid and diploid thalli by fusion of their corresponding adjacent basal holdfasts. Population surveys at three sites in Coliumo Bay, central Chile, were carried out to characterize frond demography as well as to asses the frequency of gametophyte-tetrasporophyte (G-T) coalescence. Visual and resorcinol methods were employed to determine the phases of the fronds collected over central transects of 15 randomly sampled clumps. Coalescence of G-T clumps was infrequent, with gametophytes dominating over tetrasporophyte thalli.     

Clarifying Baker's Law

Background

Baker''s Law states that colonization by self-compatible organisms is more likely to be successful than colonization by self-incompatible organisms because of the ability for self-compatible organisms to produce offspring without pollination agents. This simple model has proved very successful in plant ecology and has been applied to various contexts, including colonizing or ruderal species, islands colonizers, invasive species or mating system variation across distribution ranges. Moreover, it is one of the only models in population biology linking two traits of major importance in ecology, namely dispersal and mating system. Although Baker''s Law has stimulated a large number of empirical studies reporting the association of self-fertilization and colonizing ability in various contexts, the data have not established a general pattern for the association of traits.

Scope

In this paper, a critical position is adopted to discuss and clarify Baker''s Law. From the literature referring to Baker''s Law, an analysis made regarding how mating success is considered in such studies and discrepancies with population genetics theory of mating systems are highlighted. The data reporting the association of self-fertilization and colonizing ability are also briefly reviewed and the potential bias in interpretation is discussed. Lastly, a recent theoretical model analysing the link between colonizing ability and self-fertilization is considered.

Conclusions

Evolutionary predictions are actually more complex than Baker''s intuitive arguments. It appears that Baker''s Law encompasses a variety of ecological scenarios, which cannot be considered a priori as equivalent. Questioning what has been considered as self-evident for more than 50 years seems a reasonable objective to analyse in-depth dispersal and mating system traits.     

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.     

The gynogenetic reproduction of diploid and triploid hybrid spined loaches (<Emphasis Type="Italic">Cobitis</Emphasis>: Teleostei), and their ability to establish successful clonal lineages—on the evolution of polyploidy in asexual vertebrates

Polyploidisation is assumed to have played a significant role in the evolution of hybrid asexual lineages. The virtual absence of natural asexual systems in which more than a single ploidy level successfully establishes successful independent clonal lineages is generally explained by the strong effects of polyploidisation on fitness. Experimental crosses were made between diploid and triploid asexual Cobitis elongatoides × C. taenia hybrids (female) and both parental spined loach species (male). Genotyping of the progeny using allozymes and multilocus DNA fingerprinting, along with flow cytometric measurement of ploidy level, demonstrated the occurrence of gynogenetic reproduction in both female biotypes. The incorporation of the sperm genome occurred in some progeny, giving rise to a higher ploidy level, but the rate of polyploidisation differed significantly between the diploid and triploid females. These outcomes are consistent with the existence of developmental constraints on tetraploidy, which determine the rarity of tetraploids in natural populations. No cases of ploidy level reduction were observed. Since diploid and triploid hybrid populations occur where the lack of potential progenitor excludes the possibility of de novo origin, it is probable that both diploid and triploid females can establish successful clonal lineages. Spined loaches represent a unique example, among asexual vertebrates, where more than one ploidy level can establish persistent clonal lineages, which are reproductively independent of one another.     

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.     

Evolution and maintenance of haploid–diploid life cycles in natural populations: The case of the marine brown alga Ectocarpus

The evolutionary stability of haploid–diploid life cycles is still controversial. Mathematical models indicate that niche differences between ploidy phases may be a necessary condition for the evolution and maintenance of these life cycles. Nevertheless, experimental support for this prediction remains elusive. In the present work, we explored this hypothesis in natural populations of the brown alga Ectocarpus. Consistent with the life cycle described in culture, Ectocarpus crouaniorum in NW France and E. siliculosus in SW Italy exhibited an alternation between haploid gametophytes and diploid sporophytes. Our field data invalidated, however, the long‐standing view of an isomorphic alternation of generations. Gametophytes and sporophytes displayed marked differences in size and, conforming to theoretical predictions, occupied different spatiotemporal niches. Gametophytes were found almost exclusively on the alga Scytosiphon lomentaria during spring whereas sporophytes were present year‐round on abiotic substrata. Paradoxically, E. siliculosus in NW France exhibited similar habitat usage despite the absence of alternation of ploidy phases. Diploid sporophytes grew both epilithically and epiphytically, and this mainly asexual population gained the same ecological advantage postulated for haploid–diploid populations. Consequently, an ecological interpretation of the niche differences between haploid and diploid individuals does not seem to satisfactorily explain the evolution of the Ectocarpus life cycle.     

Transcriptome analysis of functional differentiation between haploid and diploid cells of Emiliania huxleyi, a globally significant photosynthetic calcifying cell

Background  

Eukaryotes are classified as either haplontic, diplontic, or haplo-diplontic, depending on which ploidy levels undergo mitotic cell division in the life cycle. Emiliania huxleyi is one of the most abundant phytoplankton species in the ocean, playing an important role in global carbon fluxes, and represents haptophytes, an enigmatic group of unicellular organisms that diverged early in eukaryotic evolution. This species is haplo-diplontic. Little is known about the haploid cells, but they have been hypothesized to allow persistence of the species between the yearly blooms of diploid cells. We sequenced over 38,000 expressed sequence tags from haploid and diploid E. huxleyi normalized cDNA libraries to identify genes involved in important processes specific to each life phase (2N calcification or 1N motility), and to better understand the haploid phase of this prominent haplo-diplontic organism.     

Inbreeding depression by recessive deleterious genes affecting female fecundity of a haplo‐diploid mite

The effect of inbreeding on haplo‐diploid organisms has been regarded as very low, because deleterious recessive genes on hemizygous (haploid) males were immediately purged generation by generation. However, we determined such recessive genes to decrease female fecundity in a population of Schizotetranychus miscanthi Saito which is known in the Acari as a subsocial species with haplo‐diploidy. In mother–son inbreeding experiments, there was no depression in egg hatchability nor in the larval survival of progeny over four generations. There was, on the other hand, significant inbreeding depression in the fecundity with increasing f‐value. Crosses between two lineages, one having deleterious effects on the fecundity and the other having no such effects, established during the inbreeding, revealed heterosis, and backcrosses showed that the depression was caused by deleterious recessive(s). These results strongly suggest the existence of some deleterious genes governing only the traits of adult females in wild populations of haplo‐diploid organisms.     

Cryptic fitness advantage: diploids invade haploid populations despite lacking any apparent advantage as measured by standard fitness assays

Ploidy varies tremendously within and between species, yet the factors that influence when or why ploidy variants are adaptive remains poorly understood. Our previous work found that diploid individuals repeatedly arose within ten replicate haploid populations of Saccharomyces cerevisiae, and in each case we witnessed diploid takeover within ~1800 asexual generations of batch culture evolution in the lab. The character that allowed diploids to rise in frequency within haploid populations remains unknown. Here we present a number of experiments conducted with the goal to determine what this trait (or traits) might have been. Experiments were conducted both by sampling a small number of colonies from the stocks frozen every two weeks (~ 93 generations) during the original experiment, as well through sampling a larger number of colonies at the two time points where polymorphism for ploidy was most prevalent. Surprisingly, none of our fitness component measures (lag phase, growth rate, biomass production) indicated an advantage to diploidy. Similarly, competition assays against a common competitor and direct competition between haploid and diploid colonies isolated from the same time point failed to indicate a diploid advantage. Furthermore, we uncovered a tremendous amount of trait variation among colonies of the same ploidy level. Only late-appearing diploids showed a competitive advantage over haploids, indicating that the fitness advantage that allowed eventual takeover was not diploidy per se but an attribute of a subset of diploid lineages. Nevertheless, the initial rise in diploids to intermediate frequency cannot be explained by any of the fitness measures used; we suggest that the resolution to this mystery is negative frequency-dependent selection, which is ignored in the standard fitness measures used.     

The eco-evolutionary importance of reproductive system variation in the macroalgae: Freshwater reds as a case study

The relative frequency of sexual versus asexual reproduction governs the distribution of genetic diversity within and among populations. Most studies on the consequences of reproductive variation focus on the mating system (i.e., selfing vs. outcrossing) of diploid-dominant taxa (e.g., angiosperms), often ignoring asexual reproduction. Although reproductive systems are hypothesized to be correlated with life-cycle types, variation in the relative rates of sexual and asexual reproduction remains poorly characterized across eukaryotes. This is particularly true among the three major lineages of macroalgae (green, brown, and red). The Rhodophyta are particularly interesting, as many taxa have complex haploid–diploid life cycles that influence genetic structure. Though most marine reds have separate sexes, we show that freshwater red macroalgae exhibit patterns of switching between monoicy and dioicy in sister taxa that rival those recently shown in brown macroalgae and in angiosperms. We advocate for the investigation of reproductive system evolution using freshwater reds, as this will expand the life-cycle types for which these data exist, enabling comparative analyses broadly across eukaryotes. Unlike their marine cousins, species in the Batrachospermales have macroscopic gametophytes attached to filamentous, often microscopic sporophytes. While asexual reproduction through monospores may occur in all freshwater reds, the Compsopogonales are thought to be exclusively asexual. Understanding the evolutionary consequences of selfing and asexual reproduction will aid in our understanding of the evolutionary ecology of all algae and of eukaryotic evolution generally.     

Genetic structure of natural populations of Gelidium species: A re-evaluation of results

Twenty-two loci were re-evaluated to assess genetic variation and differentiation in three natural populations (two from Gran Canaria and one from Tenerife) of Gelidium (G. canariensis and G. arbuscula). The new data using exclusively the diploid subpopulation gene frequencies confirm that dispersal was restricted over short distances for the two species, but contrary to previous conclusions, the data reveal that these two closely related species differed markedly by their mating systems and patterns of genetic differentiation. Genetic differentiation among populations was twice as high in G. arbuscula as in G. canariensis. It was confirmed that the mean way of reproduction is asexual in G. arbuscula and a discussion included as to how clonal propagation may explain the difference in haploid and diploid allele frequencies in this species. There was no evidence for asexual reproduction in G. canariensis. Heterozygote deficiency could be explained simply by spatial sub-structuring within populations. The importance of the sampling design in determining the level and pattern of genetic differentiation within a species is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hybridization between two cryptic filamentous brown seaweeds along the shore: analysing pre‐ and postzygotic barriers in populations of individuals with varying ploidy levels

We aimed to study the importance of hybridization between two cryptic species of the genus Ectocarpus, a group of filamentous algae with haploid–diploid life cycles that include the principal genetic model organism for the brown algae. In haploid–diploid species, the genetic structure of the two phases of the life cycle can be analysed separately in natural populations. Such life cycles provide a unique opportunity to estimate the frequency of hybrid genotypes in diploid sporophytes and meiotic recombinant genotypes in haploid gametophytes allowing the effects of reproductive barriers preventing fertilization or preventing meiosis to be untangle. The level of hybridization between E. siliculosus and E. crouaniorum was quantified along the European coast. Clonal cultures (568 diploid, 336 haploid) isolated from field samples were genotyped using cytoplasmic and nuclear markers to estimate the frequency of hybrid genotypes in diploids and recombinant haploids. We identified admixed individuals using microsatellite loci, classical assignment methods and a newly developed Bayesian method (XPloidAssignment), which allows the analysis of populations that exhibit variations in ploidy level. Over all populations, the level of hybridization was estimated at 8.7%. Hybrids were exclusively observed in sympatric populations. More than 98% of hybrids were diploids (40% of which showed signs of aneuploidy) with a high frequency of rare alleles. The near absence of haploid recombinant hybrids demonstrates that the reproductive barriers are mostly postzygotic and suggests that abnormal chromosome segregation during meiosis following hybridization of species with different genome sizes could be a major cause of interspecific incompatibility in this system.     

Nucleolus size varies with sex,ploidy and gene dosage in insects

The nucleolus constitutes a cytologically visible phenotype for ribosomal DNA (rDNA). Nucleolar size, as determined by silver staining, is a good indicator of cell proliferation rate and biosynthetic activity. Nevertheless, the relationship between rDNA content and sexual dimorphism for nucleolar size is not well documented. In the present study, the impact of sex and ploidy level on nucleolar size is investigated in three haplo/diploid and three diplo/diploid species of insect. Nucleolar sizes are found to be proportional to ploidy level in the haplo/diploid hymenopterans Trypoxylon albitarse and Nasonia vitripennis. Conversely, in the ant Messor barbarus, nucleolar sizes are larger in haploid males (winged) than diploid females (apterous). Among the diplo/diploid species, evidence for gene dosage compensation on nucleolar activity is suggested by the absence of sex differences in Drosophila simulans, a species in which rDNA is limited to the X chromosome. By contrast, in the grasshopper Stenobothrus festivus, another species with rRNA genes restricted to the X chromosome, the size of the nucleolus is significantly larger in females than in males. Additionally, in the grasshopper Chorthippus parallelus, where rDNA is distributed evenly on several autosomes of males and females, the females also show larger nucleoli than males. In both grasshopper species, the magnitude of the female/male ratio for nucleolus area is very similar to the body size ratio, suggesting that body size, as well as sex, ploidy, gene dosage and physiological activity, may be an important determinant of nucleolus area.     

Complex spatial clonal structure in the macroalgae Fucus radicans with both sexual and asexual recruitment

In dioecious species with both sexual and asexual reproduction, the spatial distribution of individual clones affects the potential for sexual reproduction and local adaptation. The seaweed Fucus radicans, endemic to the Baltic Sea, has separate sexes, but new attached thalli may also form asexually. We mapped the spatial distribution of clones (multilocus genotypes, MLGs) over macrogeographic (>500 km) and microgeographic (<100 m) scales in the Baltic Sea to assess the relationship between clonal spatial structure, sexual recruitment, and the potential for natural selection. Sexual recruitment was predominant in some areas, while in others asexual recruitment dominated. Where clones of both sexes were locally intermingled, sexual recruitment was nevertheless low. In some highly clonal populations, the sex ratio was strongly skewed due to dominance of one or a few clones of the same sex. The two largest clones (one female and one male) were distributed over 100–550 km of coast and accompanied by small and local MLGs formed by somatic mutations and differing by 1–2 mutations from the large clones. Rare sexual events, occasional long‐distance migration, and somatic mutations contribute new genotypic variation potentially available to natural selection. However, dominance of a few very large (and presumably old) clones over extensive spatial and temporal scales suggested that either these have superior traits or natural selection has only been marginally involved in the structuring of genotypes.     

Heterozygote Advantage and the Evolution of a Dominant Diploid Phase

The life cycle of eukaryotic, sexual species is divided into haploid and diploid phases. In multicellular animals and seed plants, the diploid phase is dominant, and the haploid phase is reduced to one, or a very few cells, which are dependent on the diploid form. In other eukaryotic species, however, the haploid phase may dominate or the phases may be equally developed. Even though an alternation between haploid and diploid forms is fundamental to sexual reproduction in eukaryotes, relatively little is known about the evolutionary forces that influence the dominance of haploidy or diploidy. An obvious genetic factor that might result in selection for a dominant diploid phase is heterozygote advantage, since only the diploid phase can be heterozygous. In this paper, I analyze a model designed to determine whether heterozygote advantage could lead to the evolution of a dominant diploid phase. The main result is that heterozygote advantage can lead to an increase in the dominance of the diploid phase, but only if the diploid phase is already sufficiently dominant. Because the diploid phase is unlikely to be increased in organisms that are primarily haploid, I conclude that heterozygote advantage is not a sufficient explanation of the dominance of the diploid phase in higher plants and animals.     

Polyploidy‐associated genomic instability in Arabidopsis thaliana

Formation of polyploid organisms by fertilization of unreduced gametes in meiotic mutants is believed to be a common phenomenon in species evolution. However, not well understood is how species in nature generally exist as haploid and diploid organisms in a long evolutionary time while polyploidization must have repeatedly occurred via meiotic mutations. Here, we show that the ploidy increased for two consecutive generations due to unreduced but viable gametes in the Arabidopsis cyclin a1;2‐2 (also named tardy asynchronous meiosis‐2) mutant, but the resultant octaploid plants produced progeny of either the same or reduced ploidy via genomic reductions during meiosis and pollen mitosis. Ploidy reductions through sexual reproduction were also observed in independently generated artificial octaploid and hexaploid Arabidopsis plants. These results demonstrate that octaploid is likely the maximal ploidy produced through sexual reproduction in Arabidopsis. The polyploidy‐associated genomic instability may be a general phenomenon that constrains ploidy levels in species evolution. genesis 48:254–263, 2010. © 2010 Wiley‐Liss, Inc.     

Extensive variation in chromosome number and genome size in sexual and parthenogenetic species of the jumping‐bristletail genus Machilis (Archaeognatha)

Parthenogenesis in animals is often associated with polyploidy and restriction to extreme habitats or recently deglaciated areas. It has been hypothesized that benefits conferred by asexual reproduction and polyploidy are essential for colonizing these habitats. However, while evolutionary routes to parthenogenesis are manifold, study systems including polyploids are scarce in arthropods. The jumping‐bristletail genus Machilis (Insecta: Archaeognatha) includes both sexual and parthenogenetic species, and recently, the occurrence of polyploidy has been postulated. Here, we applied flow cytometry, karyotyping, and mitochondrial DNA sequencing to three sexual and five putatively parthenogenetic Eastern‐Alpine Machilis species to investigate whether (1) parthenogenesis originated once or multiply and (2) whether parthenogenesis is strictly associated with polyploidy. The mitochondrial phylogeny revealed that parthenogenesis evolved at least five times independently among Eastern‐Alpine representatives of this genus. One parthenogenetic species was exclusively triploid, while a second consisted of both diploid and triploid populations. The three other parthenogenetic species and all sexual species were diploid. Our results thus indicate that polyploidy can co‐occur with parthenogenesis, but that it was not mandatory for the emergence of parthenogenesis in Machilis. Overall, we found a weak negative correlation of monoploid genome size (Cx) and chromosome base number (x), and this connection is stronger among parthenogenetic species alone. Likewise, monoploid genome size decreased with elevation, and we therefore hypothesize that genome downsizing could have been crucial for the persistence of alpine Machilis species. Finally, we discuss the evolutionary consequences of intraspecific chromosomal rearrangements and the presence of B chromosomes. In doing so, we highlight the potential of Alpine Machilis species for research on chromosomal and genome‐size alterations during speciation.