Origin and Genetic Diversity of Diploid Parthenogenetic Artemia in Eurasia

There is wide interest in understanding how genetic diversity is generated and maintained in parthenogenetic lineages, as it will help clarify the debate of the evolution and maintenance of sexual reproduction. There are three mechanisms that can be responsible for the generation of genetic diversity of parthenogenetic lineages: contagious parthenogenesis, repeated hybridization and microorganism infections (e.g. Wolbachia). Brine shrimps of the genus Artemia (Crustacea, Branchiopoda, Anostraca) are a good model system to investigate evolutionary transitions between reproductive systems as they include sexual species and lineages of obligate parthenogenetic populations of different ploidy level, which often co-occur. Diploid parthenogenetic lineages produce occasional fully functional rare males, interspecific hybridization is known to occur, but the mechanisms of origin of asexual lineages are not completely understood. Here we sequenced and analysed fragments of one mitochondrial and two nuclear genes from an extensive set of populations of diploid parthenogenetic Artemia and sexual species from Central and East Asia to investigate the evolutionary origin of diploid parthenogenetic Artemia, and geographic origin of the parental taxa. Our results indicate that there are at least two, possibly three independent and recent maternal origins of parthenogenetic lineages, related to A. urmiana and Artemia sp. from Kazakhstan, but that the nuclear genes are very closely related in all the sexual species and parthenogegetic lineages except for A. sinica, who presumable took no part on the origin of diploid parthenogenetic strains. Our data cannot rule out either hybridization between any of the very closely related Asiatic sexual species or rare events of contagious parthenogenesis via rare males as the contributing mechanisms to the generation of genetic diversity in diploid parthenogenetic Artemia lineages.     

Introgression of apomixis into sexual species is inhibited by mentor effects and ploidy barriers in the Ranunculus auricomus complex

Background and Aims

Apomictic plants maintain functional pollen, and via pollination the genetic factors controlling apomixis can be potentially transferred to congeneric sexual populations. In contrast, the sexual individuals do not fertilize apomictic plants which produce seeds without fertilization of the egg cells. This unidirectional introgressive hybridization is expected finally to replace sexuality by apomixis and is thought to be a causal factor for the wide geographical distribution of apomictic complexes. Nevertheless, this process may be inhibited by induced selfing (mentor effects) of otherwise self-incompatible sexual individuals. Here whether mentor effects or actual cross-fertilization takes place between diploid sexual and polyploid apomictic cytotypes in the Ranunculus auricomus complex was tested via experimental crosses.

Methods

Diploid sexual mother plants were pollinated with tetra- and hexaploid apomictic pollen donators by hand, and the amount of well-developed seed compared with aborted seed was evaluated. The reproductive pathways were assessed in the well-developed seed via flow cytometric seed screen (FCSS).

Key Results

The majority of seed was aborted; the well-developed seeds have resulted from both mentor effects and cross-fertilization at very low frequencies (1·3 and 1·6 % of achenes, respectively). Pollination by 4x apomictic pollen plants results more frequently in cross-fertilization, whereas pollen from 6x plants more frequently induced mentor effects.

Conclusions

It is concluded that introgression of apomixis into sexual populations is limited by ploidy barriers in the R. auricomus complex, and to a minor extent by mentor effects. In mixed populations, sexuality cannot be replaced by apomixis because the higher fertility of sexual populations still compensates the low frequencies of potential introgression of apomixis.Key words: Apomixis, Ranunculus auricomus, evolution, geographical parthenogenesis, crossing experiments, flow cytometry     

Flow cytometry confirms reticulate evolution and reveals triploidy in Central European Diphasiastrum taxa (Lycopodiaceae, Lycophyta)

Background and Aims

Interspecific Diphasiastrum hybrids have been assumed to be homoploid and to produce well-formed spores serving sexual reproduction. If this were the case, forms intermediate between hybrids and parents or hybrid swarms should be expected. The purpose of this study was: (1) to check whether homoploidy consistently applies to the three hybrids throughout their Central European range; (2) to examine whether their genome sizes confirm their parentage as assumed by morphology; and (3) to perform a screening for detection of ploidy levels other than diploid and variation in DNA content due to backcrossing.

Methods

Flow cytometry was used first to measure the relative DNA values [with 4′,6-diamidino-2-phenylindole (DAPI) staining] and ploidy level as a general screening, and secondly to determine the absolute DNA 2C values [with propidium iodide (PI) staining] in a number of selected samples with the main focus on the hybrids.

Key Results

A considerable variation of DNA 2C values (5·26–7·52 pg) was detected between the three European Diphasiastrum species. The values of the diploid hybrids are highly constant without significant variation between regions. They are also intermediate between their assumed parents and agree closely with those calculated from their putative parents. This confirms their hybrid origin, assumed parentage and homoploid status. Considerably higher DNA amounts (9·48–10·30 pg) were obtained for three populations, suggesting that these represent triploid hybrids, an interpretation that is strongly supported by their morphology.

Conclusions

Diploid hybrids have retained their genetic and morphological identites throughout their Central European range, and thus no indications for diploid backcrossing were found. The triploid hybrids have probably originated from backcrossing between a diploid gametophyte of a hybrid (derived from a diplospore) and a haploid gametophyte of a diploid parental species. By repeated crossing events, reticulate evolution patterns arise that are similar to those known for a number of ferns.     

Loss of Sexual Reproduction and Dwarfing in a Small Metazoan

Background

Asexuality has major theoretical advantages over sexual reproduction, yet newly formed asexual lineages rarely endure. The success, or failure, of such lineages is affected by their mechanism of origin, because it determines their initial genetic makeup and variability. Most previously described mechanisms imply that asexual lineages are randomly frozen subsamples of a sexual population.

Methodology/Principal Findings

We found that transitions to obligate parthenogenesis (OP) in the rotifer Brachionus calyciflorus, a small freshwater invertebrate which normally reproduces by cyclical parthenogenesis, were controlled by a simple Mendelian inheritance. Pedigree analysis suggested that obligate parthenogens were homozygous for a recessive allele, which caused inability to respond to the chemical signals that normally induce sexual reproduction in this species. Alternative mechanisms, such as ploidy changes, could be ruled out on the basis of flow cytometric measurements and genetic marker analysis. Interestingly, obligate parthenogens were also dwarfs (approximately 50% smaller than cyclical parthenogens), indicating pleiotropy or linkage with genes that strongly affect body size. We found no adverse effects of OP on survival or fecundity.

Conclusions/Significance

This mechanism of inheritance implies that genes causing OP may evolve within sexual populations and remain undetected in the heterozygous state long before they get frequent enough to actually cause a transition to asexual reproduction. In this process, genetic variation at other loci might become linked to OP genes, leading to non-random associations between asexuality and other phenotypic traits.     

Autotetraploids of Vicia cracca show a higher allelic richness in natural populations and a higher seed set after artificial selfing than diploids

Background and Aims

Despite the great importance of autopolyploidy in the evolution of angiosperms, relatively little attention has been devoted to autopolyploids in natural polyploid systems. Several hypotheses have been proposed to explain why autopolyploids are so common and successful, for example increased genetic diversity and heterozygosity and the transition towards selfing. However, case studies on patterns of genetic diversity and on mating systems in autopolyploids are scarce. In this study allozymes were employed to investigate the origin, population genetic diversity and mating system in the contact zone between diploid and assumed autotetraploid cytotypes of Vicia cracca in Central Europe.

Methods

Four enzyme systems resolved in six putative loci were investigated in ten diploid, ten tetraploid and five mixed-ploidy populations. Genetic diversity and heterozygosity, partitioning of genetic diversity among populations and cytotypes, spatial genetic structure and fixed heterozygosity were analysed. These studies were supplemented by a pollination experiment and meiotic chromosome observation.

Key Results and Conclusions

Weak evidence of fixed heterozygosity, a low proportion of unique alleles and genetic variation between cytotypes similar to the variation among populations within cytotypes supported the autopolyploid origin of tetraploids, although no multivalent formation was observed. Tetraploids possessed more alleles than diploids and showed higher observed zygotic heterozygosity than diploids, but the observed gametic heterozygosity was similar to the value observed in diploids and smaller than expected under panmixis. Values of the inbreeding coefficient and differentiation among populations (ρ_ST) suggested that the breeding system in both cytotypes of V. cracca is mixed mating with prevailing outcrossing. The reduction in seed production of tetraploids after selfing was less than that in diploids. An absence of correlation between genetic and geographic distances and high differentiation among neighbouring tetraploid populations supports the secondary contact hypothesis with tetraploids of several independent origins in Central Europe. Nevertheless, the possibility of a recent in situ origin of tetraploids through a triploid bridge in some regions is also discussed.     

Genotyping of Burkholderia mallei from an Outbreak of Glanders in Bahrain Suggests Multiple Introduction Events

Background

Glanders, caused by the gram-negative bacterium Burkholderia mallei, is a highly infectious zoonotic disease of solipeds causing severe disease in animals and men. Although eradicated from many Western countries, it recently emerged in Asia, the Middle-East, Africa, and South America. Due to its rareness, little is known about outbreak dynamics of the disease and its epidemiology.

Methodology/Principal Findings

We investigated a recent outbreak of glanders in Bahrain by applying high resolution genotyping (multiple locus variable number of tandem repeats, MLVA) and comparative whole genome sequencing to B. mallei isolated from infected horses and a camel. These results were compared to samples obtained from an outbreak in the United Arab Emirates in 2004, and further placed into a broader phylogeographic context based on previously published B. mallei data. The samples from the outbreak in Bahrain separated into two distinct clusters, suggesting a complex epidemiological background and evidence for the involvement of multiple B. mallei strains. Additionally, the samples from Bahrain were more closely related to B. mallei isolated from horses in the United Arab Emirates in 2004 than other B. mallei which is suggestive of repeated importation to the region from similar geographic sources.

Conclusion/Significance

High-resolution genotyping and comparative whole genome analysis revealed the same phylogenetic patterns among our samples. The close relationship of the Dubai/UAE B. mallei populations to each other may be indicative of a similar geographic origin that has yet to be identified for the infecting strains. The recent emergence of glanders in combination with worldwide horse trading might pose a new risk for human infections.     

Phylogeography of a land snail suggests trans-Mediterranean neolithic transport

Background

Fragmented distribution ranges of species with little active dispersal capacity raise the question about their place of origin and the processes and timing of either range fragmentation or dispersal. The peculiar distribution of the land snail Tudorella sulcata s. str. in Southern France, Sardinia and Algeria is such a challenging case.

Methodology

Statistical phylogeographic analyses with mitochondrial COI and nuclear hsp70 haplotypes were used to answer the questions of the species'' origin, sequence and timing of dispersal. The origin of the species was on Sardinia. Starting from there, a first expansion to Algeria and then to France took place. Abiotic and zoochorous dispersal could be excluded by considering the species'' life style, leaving only anthropogenic translocation as parsimonious explanation. The geographic expansion could be dated to approximately 8,000 years before present with a 95% confidence interval of 10,000 to 3,000 years before present.

Conclusions

This period coincides with the Neolithic expansion in the Western Mediterranean, suggesting a role of these settlers as vectors. Our findings thus propose that non-domesticated animals and plants may give hints on the direction and timing of early human expansion routes.     

A Genome-Wide Study of Modern-Day Tuscans: Revisiting Herodotus's Theory on the Origin of the Etruscans

Background

The origin of the Etruscan civilization (Etruria, Central Italy) is a long-standing subject of debate among scholars from different disciplines. The bulk of the information has been reconstructed from ancient texts and archaeological findings and, in the last few years, through the analysis of uniparental genetic markers.

Methods

By meta-analyzing genome-wide data from The 1000 Genomes Project and the literature, we were able to compare the genomic patterns (>540,000 SNPs) of present day Tuscans (N = 98) with other population groups from the main hypothetical source populations, namely, Europe and the Middle East.

Results

Admixture analysis indicates the presence of 25–34% of Middle Eastern component in modern Tuscans. Different analyses have been carried out using identity-by-state (IBS) values and genetic distances point to Eastern Anatolia/Southern Caucasus as the most likely geographic origin of the main Middle Eastern genetic component observed in the genome of modern Tuscans.

Conclusions

The data indicate that the admixture event between local Tuscans and Middle Easterners could have occurred in Central Italy about 2,600–3,100 years ago (y.a.). On the whole, the results validate the theory of the ancient historian Herodotus on the origin of Etruscans.     

Present,past and future of the European rock fern Asplenium fontanum: combining distribution modelling and population genetics to study the effect of climate change on geographic range and genetic diversity

Background and Aims

Climate change is expected to alter the geographic range of many plant species dramatically. Predicting this response will be critical to managing the conservation of plant resources and the effects of invasive species. The aim of this study was to predict the response of temperate homosporous ferns to climate change.

Methods

Genetic diversity and changes in distribution range were inferred for the diploid rock fern Asplenium fontanum along a South–North transect, extending from its putative last glacial maximum (LGM) refugia in southern France towards southern Germany and eastern-central France. This study reconciles observations from distribution models and phylogeographic analyses derived from plastid and nuclear diversity.

Key Results

Genetic diversity distribution and niche modelling propose that genetic diversity accumulates in the LGM climate refugium in southern France with the formation of a diversity gradient reflecting a slow, post-LGM range expansion towards the current distribution range. Evidence supports the fern''s preference for outcrossing, contradicting the expectation that homosporous ferns would populate new sites by single-spore colonization. Prediction of climate and distribution range change suggests that a dramatic loss of range and genetic diversity in this fern is possible. The observed migration is best described by the phalanx expansion model.

Conclusions

The results suggest that homosporous ferns reproducing preferentially by outcrossing accumulate genetic diversity primarily in LGM climate refugia and may be threatened if these areas disappear due to global climate change.     

Mitochondrial DNA regionalism and historical demography in the extant populations of Chirocephalus kerkyrensis (Branchiopoda: Anostraca)

Background

Mediterranean temporary water bodies are important reservoirs of biodiversity and host a unique assemblage of diapausing aquatic invertebrates. These environments are currently vanishing because of increasing human pressure. Chirocephalus kerkyrensis is a fairy shrimp typical of temporary water bodies in Mediterranean plain forests and has undergone a substantial decline in number of populations in recent years due to habitat loss. We assessed patterns of genetic connectivity and phylogeographic history in the seven extant populations of the species from Albania, Corfu Is. (Greece), Southern and Central Italy.

Methodology/Principal Findings

We analyzed sequence variation at two mitochondrial DNA genes (Cytochrome Oxidase I and 16s rRNA) in all the known populations of C. kerkyrensis. We used multiple phylogenetic, phylogeographic and coalescence-based approaches to assess connectivity and historical demography across the whole distribution range of the species. C. kerkyrensis is genetically subdivided into three main mitochondrial lineages; two of them are geographically localized (Corfu Is. and Central Italy) and one encompasses a wide geographic area (Albania and Southern Italy). Most of the detected genetic variation (≈81%) is apportioned among the aforementioned lineages.

Conclusions/Significance

Multiple analyses of mismatch distributions consistently supported both past demographic and spatial expansions with the former predating the latter; demographic expansions were consistently placed during interglacial warm phases of the Pleistocene while spatial expansions were restricted to cold periods. Coalescence methods revealed a scenario of past isolation with low levels of gene flow in line with what is already known for other co-distributed fairy shrimps and suggest drift as the prevailing force in promoting local divergence. We recommend that these evolutionary trajectories should be taken in proper consideration in any effort aimed at protecting Mediterranean temporary water bodies.     

On the origin and spread of the Scab disease of apple: out of central Asia

Background

Venturia inaequalis is an ascomycete fungus responsible for apple scab, a disease that has invaded almost all apple growing regions worldwide, with the corresponding adverse effects on apple production. Monitoring and predicting the effectiveness of intervention strategies require knowledge of the origin, introduction pathways, and population biology of pathogen populations. Analysis of the variation of genetic markers using the inferential framework of population genetics offers the potential to retrieve this information.

Methodology/Principal Findings

Here, we present a population genetic analysis of microsatellite variation in 1,273 strains of V. inaequalis representing 28 orchard samples from seven regions in five continents. Analysis of molecular variance revealed that most of the variation (88%) was distributed within localities, which is consistent with extensive historical migrations of the fungus among and within regions. Despite this shallow population structure, clustering analyses partitioned the data set into separate groups corresponding roughly to geography, indicating that each region hosts a distinct population of the fungus. Comparison of the levels of variability among populations, along with coalescent analyses of migration models and estimates of genetic distances, was consistent with a scenario in which the fungus emerged in Central Asia, where apple was domesticated, before its introduction into Europe and, more recently, into other continents with the expansion of apple growing. Across the novel range, levels of variability pointed to multiple introductions and all populations displayed signatures of significant post-introduction increases in population size. Most populations exhibited high genotypic diversity and random association of alleles across loci, indicating recombination both in native and introduced areas.

Conclusions/Significance

Venturia inaequalis is a model of invasive phytopathogenic fungus that has now reached the ultimate stage of the invasion process with a broad geographic distribution and well-established populations displaying high genetic variability, regular sexual reproduction, and demographic expansion.     

Genetic structure of the tiger mosquito, Aedes albopictus, in Cameroon (Central Africa)

Background

Aedes albopictus (Skuse, 1884) (Diptera: Culicidae), a mosquito native to Asia, has recently invaded all five continents. In Central Africa it was first reported in the early 2000s, and has since been implicated in the emergence of arboviruses such as dengue and chikungunya in this region. Recent genetic studies of invasive species have shown that multiple introductions are a key factor for successful expansion in new areas. As a result, phenotypic characters such as vector competence and insecticide susceptibility may vary within invasive pest species, potentially affecting vector efficiency and pest management. Here we assessed the genetic variability and population genetics of Ae. albopictus isolates in Cameroon (Central Africa), thereby deducing their likely geographic origin.

Methods and Results

Mosquitoes were sampled in 2007 in 12 localities in southern Cameroon and analyzed for polymorphism at six microsatellite loci and in two mitochondrial DNA regions (ND5 and COI). All the microsatellite markers were successfully amplified and were polymorphic, showing moderate genetic structureamong geographic populations (F_ST = 0.068, P<0.0001). Analysis of mtDNA sequences revealed four haplotypes each for the COI and ND5 genes, with a dominant haplotype shared by all Cameroonian samples. The weak genetic variation estimated from the mtDNA genes is consistent with the recent arrival of Ae. albopictus in Cameroon. Phylogeographic analysis based on COI polymorphism indicated that Ae. albopictus populations from Cameroon are related to tropical rather than temperate or subtropical outgroups.

Conclusion

The moderate genetic diversity observed among Cameroonian Ae. albopictus isolates is in keeping with recent introduction and spread in this country. The genetic structure of natural populations points to multiple introductions from tropical regions.     

A single origin and moderate bottleneck during domestication of soybean (Glycine max): implications from microsatellites and nucleotide sequences

Background and Aims

It is essential to illuminate the evolutionary history of crop domestication in order to understand further the origin and development of modern cultivation and agronomy; however, despite being one of the most important crops, the domestication origin and bottleneck of soybean (Glycine max) are poorly understood. In the present study, microsatellites and nucleotide sequences were employed to elucidate the domestication genetics of soybean.

Methods

The genomes of 79 landrace soybeans (endemic cultivated soybeans) and 231 wild soybeans (G. soja) that represented the species-wide distribution of wild soybean in East Asia were scanned with 56 microsatellites to identify the genetic structure and domestication origin of soybean. To understand better the domestication bottleneck, four nucleotide sequences were selected to simulate the domestication bottleneck.

Key Results

Model-based analysis revealed that most of the landrace genotypes were assigned to the inferred wild soybean cluster of south China, South Korea and Japan. Phylogeny for wild and landrace soybeans showed that all landrace soybeans formed a single cluster supporting a monophyletic origin of all the cultivars. The populations of the nearest branches which were basal to the cultivar lineage were wild soybeans from south China. The coalescent simulation detected a bottleneck severity of K′ = 2 during soybean domestication, which could be explained by a foundation population of 6000 individuals if domestication duration lasted 3000 years.

Conclusions

As a result of integrating geographic distribution with microsatellite genotype assignment and phylogeny between landrace and wild soybeans, a single origin of soybean in south China is proposed. The coalescent simulation revealed a moderate genetic bottleneck with an effective wild soybean population used for domestication estimated to be ≈2 % of the total number of ancestral wild soybeans. Wild soybeans in Asia, especially in south China contain tremendous genetic resources for cultivar improvement.     

Spatiotemporal Phylogenetic Analysis and Molecular Characterisation of Infectious Bursal Disease Viruses Based on the VP2 Hyper-Variable Region

Background

Infectious bursal disease is a highly contagious and acute viral disease caused by the infectious bursal disease virus (IBDV); it affects all major poultry producing areas of the world. The current study was designed to rigorously measure the global phylogeographic dynamics of IBDV strains to gain insight into viral population expansion as well as the emergence, spread and pattern of the geographical structure of very virulent IBDV (vvIBDV) strains.

Methodology/Principal Findings

Sequences of the hyper-variable region of the VP2 (HVR-VP2) gene from IBDV strains isolated from diverse geographic locations were obtained from the GenBank database; Cuban sequences were obtained in the current work. All sequences were analysed by Bayesian phylogeographic analysis, implemented in the Bayesian Evolutionary Analysis Sampling Trees (BEAST), Bayesian Tip-association Significance testing (BaTS) and Spatial Phylogenetic Reconstruction of Evolutionary Dynamics (SPREAD) software packages. Selection pressure on the HVR-VP2 was also assessed. The phylogeographic association-trait analysis showed that viruses sampled from individual countries tend to cluster together, suggesting a geographic pattern for IBDV strains. Spatial analysis from this study revealed that strains carrying sequences that were linked to increased virulence of IBDV appeared in Iran in 1981 and spread to Western Europe (Belgium) in 1987, Africa (Egypt) around 1990, East Asia (China and Japan) in 1993, the Caribbean Region (Cuba) by 1995 and South America (Brazil) around 2000. Selection pressure analysis showed that several codons in the HVR-VP2 region were under purifying selection.

Conclusions/Significance

To our knowledge, this work is the first study applying the Bayesian phylogeographic reconstruction approach to analyse the emergence and spread of vvIBDV strains worldwide.     

The evolutionary history of sea barley (Hordeum marinum) revealed by comparative physical mapping of repetitive DNA

Background and Aims Hordeum marinum

is a species complex that includes the diploid subspecies marinum and both diploid and tetraploid forms of gussoneanum. Their relationships, the rank of the taxa and the origin of the polyploid forms remain points of debate. The present work reports a comparative karyotype analysis of six H. marinum accessions representing all taxa and cytotypes.

Methods

Karyotypes were determined by analysing the chromosomal distribution of several tandemly repeated sequences, including the Triticeae cloned probes pTa71, pTa794, pAs1 and pSc119·2 and the simple sequence repeats (SSRs) (AG)₁₀, (AAC)₅, (AAG)₅, (ACT)₅ and (ATC)₅.

Key Results

The identification of each chromosome pair in all subspecies and cytotypes is reported for the first time. Homologous relationships are also established. Wide karyotypic differences were detected within marinum accessions. Specific chromosomal markers characterized and differentiated the genomes of marinum and diploid gussoneanum. Two subgenomes were detected in the tetraploids. One of these had the same chromosome complement as diploid gussoneanum; the second subgenome, although similar to the chromosome complement of diploid H. marinum sensu lato, appeared to have no counterpart in the marinum accessions analysed here.

Conclusions

The tetraploid forms of gussoneanum appear to have come about through a cross between a diploid gussoneanum progenitor and a second, related—but unidentified—diploid ancestor. The results reveal the genome structure of the different H. marinum taxa and demonstrate the allopolyploid origin of the tetraploid forms of gussoneanum.     

Cytotype stability, facultative apomixis and geographical parthenogenesis in Ranunculus kuepferi (Ranunculaceae)

Background and Aims

Asexual organisms are more widespread in previously glaciated areas than their sexual relatives (‘geographical parthenogenesis’). In plants, this pattern is probably dependent on reproductive isolation and stability of cytotypes within their respective distribution areas. Both partial apomixis and introgressive hybridization potentially destabilize the spatial separation of sexual and apomictic populations. The wide distribution of apomicts may be further enhanced by uniparental reproduction which is advantageous for colonization. These factors are studied in the alpine species Ranunculus kuepferi.

Methods

Geographical distribution, diversity and mode of reproduction of cytotypes were assessed using flow cytometry and flow cytometric seed screening on samples from 59 natural populations of Ranunculus kuepferi. Seed set of cytotypes was compared in the wild.

Key Results

Diploid sexuals are confined to the south-western parts of the Alps, while tetraploid apomicts dominate in previously glaciated and in geographically isolated areas despite a significantly lower fertility. Other cytotypes (3x, 5x and 6x) occur mainly in the sympatric zone, but without establishing populations. The tetraploids are predominantly apomictic, but also show a partial apomixis via an uncoupling of apomeiosis and parthenogenesis in the seed material. Both pseudogamy and autonomous endosperm formation are observed which may enhance uniparental reproduction.

Conclusions

Diploids occupy a glacial relic area and resist introgression of apomixis, probably because of a significantly higher seed set. Among the polyploids, only apomictic tetraploids form stable populations; the other cytotypes arising from partial apomixis fail to establish, probably because of minority cytotype disadvantages. Tetraploid apomicts colonize previously devastated and also distant areas via long-distance dispersal, confirming Baker''s law of an advantage of uniparental reproduction. It is concluded that stability of cytotypes and of modes of reproduction are important factors for establishing a pattern of geographical parthenogenesis.     

The importance of Anatolian mountains as the cradle of global diversity in Arabis alpina, a key arctic-alpine species

Background and Aims

Anatolia is a biologically diverse, but phylogeographically under-explored region. It is described as either a centre of origin and long-term Pleistocene refugium, or as a centre for genetic amalgamation, fed from distinct neighbouring refugia. These contrasting hypotheses are tested through a global phylogeographic analysis of the arctic–alpine herb, Arabis alpina.

Methods

Herbarium and field collections were used to sample comprehensively the entire global range, with special focus on Anatolia and Levant. Sequence variation in the chloroplast DNA trnL-trnF region was examined in 483 accessions. A haplotype genealogy was constructed and phylogeographic methods, demographic analysis and divergence time estimations were used to identify the centres of diversity and to infer colonization history.

Key Results

Fifty-seven haplotypes were recovered, belonging to three haplogroups with non-overlapping distributions in (1) North America/Europe/northern Africa, (2) the Caucuses/Iranian Plateau/Arabian Peninsula and (3) Ethiopia–eastern Africa. All haplogroups occur within Anatolia, and all intermediate haplotypes linking the three haplogroups are endemic to central Anatolia and Levant, where haplotypic and nucleotide diversities exceeded all other regions. The local pattern of haplotype distribution strongly resembles the global pattern, and the haplotypes began to diverge approx. 2·7 Mya, coinciding with the climate cooling of the early Middle Pleistocene.

Conclusions

The phylogeographic structure of Arabis alpina is consistent with Anatolia being the cradle of origin for global genetic diversification. The highly structured landscape in combination with the Pleistocene climate fluctuations has created a network of mountain refugia and the accumulation of spatially arranged genotypes. This local Pleistocene population history has subsequently left a genetic imprint at the global scale, through four range expansions from the Anatolian diversity centre into Europe, the Near East, Arabia and Africa. Hence this study also illustrates the importance of sampling and scaling effects when translating global from local diversity patterns during phylogeographic analyses.     

Diversity in the reproductive modes of European Daphnia pulicaria deviates from the geographical parthenogenesis

Background

Multiple transitions to obligate parthenogenesis have occurred in the Daphnia pulex complex in North America. These newly formed asexual lineages are differentially distributed being found predominantly at high latitudes. This conforms to the rule of geographical parthenogenesis postulating prevalence of asexuals at high latitudes and altitudes. While the reproductive mode of high-latitude populations is relatively well studied, little is known about the reproduction mode in high altitudes. This study aimed to assess the reproductive mode of Daphnia pulicaria, a species of the D. pulex complex, from high altitude lakes in Europe.

Methodology/Principal Findings

Variation at eight microsatellite loci revealed that D. pulicaria from the High Tatra Mountains (HTM) had low genotype richness and showed excess of heterozygotes and significant deviations from Hardy-Weinberg expectations, and was thus congruent with reproduction by obligate parthenogenesis. By contrast, populations from the Pyrenees (Pyr) were generally in Hardy-Weinberg equilibrium and had higher genotypic richness, suggesting that they are cyclic parthenogens. Four lakes from lowland areas (LLaP) had populations with an uncertain or mixed breeding mode. All D. pulicaria had mtDNA ND5 haplotypes of the European D. pulicaria lineage. Pyr were distinct from LLaP and HTM at the ND5 gene. By contrast, HTM shared two haplotypes with LLaP and one with Pyr. Principal Coordinate Analysis of the microsatellite data revealed clear genetic differentiation into three groups. HTM isolates were intermediate to Pyr and LLaP, congruent with a hybrid origin.

Conclusion/Significance

Inferred transitions to obligate parthenogenesis have occurred only in HTM, most likely as a result of hybridizations. In contrast to North American populations, these transitions do not appear to involve meiosis suppressor genes and have not been accompanied by polyploidy. The absence of obligate parthenogenesis in Pyr, an environment highly similar to the HTM, may be due to the lack of opportunities for hybridization.     

Microsatellite and flow cytometry analysis to help understand the origin of Dioscorea alata polyploids

Background and Aims Dioscorea alata

is a polyploid species with a ploidy level ranging from diploid (2n = 2x = 40) to tetraploid (2n = 4x = 80). Ploidy increase is correlated with better agronomic performance. The lack of knowledge about the origin of D. alata spontaneous polyploids (triploids and tetraploids) limits the efficiency of polyploid breeding. The objective of the present study was to use flow cytometry and microsatellite markers to understand the origin of D. alata polyploids.

Methods

Different progeny generated by intracytotype crosses (2x × 2x) and intercytotype crosses (2x × 4x and 3x × 2x) were analysed in order to understand endosperm incompatibility phenomena and gamete origins via the heterozygosity rate transmitted to progeny.

Results

This work shows that in a 2x × 2x cross, triploids with viable seeds are obtained only via a phenomenon of diploid female non-gametic reduction. The study of the transmission of heterozygosity made it possible to exclude polyspermy and polyembryony as the mechanisms at the origin of triploids. The fact that no seedlings were obtained by a 3x × 2x cross made it possible to confirm the sterility of triploid females. Flow cytometry analyses carried out on the endosperm of seeds resulting from 2x × 4x crosses revealed endosperm incompatibility phenomena.

Conclusions

The major conclusion is that the polyploids of D. alata would have appeared through the formation of unreduced gametes. The triploid pool would have been built and diversified through the formation of 2n gametes in diploid females as the result of the non-viability of seeds resulting from the formation of 2n sperm and of the non-viability of intercytotype crosses. The tetraploids would have appeared through bilateral sexual polyploidization via the union of two unreduced gametes due to the sterility of triploids.