Spatial ecological and genetic structure of a mixed population of sexual diploid and apomictic triploid dandelions

Ecological differentiation is widely seen as an important factor enabling the stable coexistence of closely related plants of different ploidy levels. We studied ecological and genetic differentiation between co-occurring sexual diploid and apomictic triploid Taraxacum section Ruderalia by analysing spatial patterns both in the distribution of cytotypes and in the distribution of genetic variation within and between the cytotypes. A significant relationship between ploidy level and elevation was found. This mode of ecological differentiation however, was not sufficient to explain the significant spatial structure in the distribution of diploids and triploids within the population. Strong congruence was found between the spatial genetic patterns within the diploids and within the triploids. We argue that this congruence is an indication of gene flow between neighbouring plants of different ploidy levels.     

Morphological, developmental and electrophoretic variation within and between obligately apomictic Taraxacum species

Interclonal variation in six morphometric characters and in the onset and rate of capitulum production, together with inter-and intra-clonal variation in esterase zymograms was examined in each of the two obligately apomictic species, Taraxacum pseudohamatum (section Hamata ) and T. unguilobum (section Naevosa ). Interclonal and interspecific variation in esterase zymograms was also examined within and between seven additional obligately apomictic species within the section Hamata. Within T. pseudohamatum , interclonal variation in the morphometric and electrophoretic characters was extensive; no variation was recorded in the onset and rate of capitulum production. Within T. unguilobum , interclonal variation in the morphometric characters and in the onset and rate of capitulum production was recorded; five of the six clones electrophoretically assayed shared the same esterase zymogram but one of these clones contained an individual which exhibited an aberrant zymogram. Within the section Hamata , considerable variation in esterase zymograms was recorded; this variation was both inter-and intra-specific although one zymogram predominated, being represented in each species and in 65% of the clones.     

Asymmetric reproductive interference: The consequences of cross‐pollination on reproductive success in sexual–apomictic populations of Potentilla puberula (Rosaceae)

Apomixis evolves from a sexual background and usually is linked to polyploidization. Pseudogamous gametophytic apomicts, which require a fertilization to initiate seed development, of various ploidy levels frequently co‐occur with their lower‐ploid sexual ancestors, but the stability of such mixed populations is affected by reproductive interferences mediated by cross‐pollination. Thereby, reproductive success of crosses depends on the difference in ploidy levels of mating partners, that is, on tolerance of deviation from the balanced ratio of maternal versus paternal genomes. Quality of pollen can further affect reproductive success in intercytotype pollinations. Cross‐fertilization, however, can be avoided by selfing which may be induced upon pollination with mixtures of self‐ and cross‐pollen (i.e., mentor effects). We tested for reproductive compatibility of naturally co‐occurring tetraploid sexuals and penta‐ to octoploid apomicts in the rosaceous species Potentilla puberula by means of controlled crosses. We estimated the role of selfing as a crossing barrier and effects of self‐ and cross‐pollen quality as well as maternal: paternal genomic ratios in the endosperm on reproductive success. Cross‐fertilization of sexuals by apomicts was not blocked by selfing, and seed set was reduced in hetero‐ compared to homoploid crosses. Thereby, seed set was negatively related to deviations from balanced parental genomic ratios in the endosperm. In contrast, seed set in the apomictic cytotypes was not reduced in hetero‐ compared to homoploid crosses. Thus, apomictic cytotypes either avoided intercytotype cross‐fertilization through selfing, tolerated intercytotype cross‐fertilizations without negative effects on reproductive success, or even benefitted from higher pollen quality in intercytotype pollinations. Our experiment provides evidence for asymmetric reproductive interference, in favor of the apomicts, with significantly reduced seed set of sexuals in cytologically mixed populations, whereas seed set in apomicts was not affected. Incompleteness of crossing barriers further indicated at least partial losses of a parental genomic endosperm balance requirement.     

Dispersal and establishment traits provide a colonization advantage for a polyploid apomictic plant

Premise

Apomictic plants (reproducing asexually through seed) often have larger ranges and occur at higher latitudes than closely related sexuals, a pattern known as geographical parthenogenesis (GP). Explanations for GP include differences in colonizing ability due to reproductive assurance and direct/indirect effects of polyploidy (most apomicts are polyploid) on ecological tolerances. While life history traits associated with dispersal and establishment also contribute to the potential for range expansion, few studies compare these traits in related apomicts and sexuals.

Methods

We investigated differences in early life history traits between diploid-sexual and polyploid-apomictic Townsendia hookeri (Asteraceae), which displays a classic pattern of GP. Using lab and greenhouse experiments, we measured seed dispersal traits, germination success, and seedling size and survival in sexual and apomictic populations from across the range of the species.

Results

While theory predicts that trade-offs between dispersal and establishment traits should be common, this was largely not the case in T. hookeri. Apomictic seeds had both lower terminal velocity (staying aloft longer when dropped) and higher germination success than sexual seeds. While there were no differences in seedling size between reproductive types, apomicts did, however, have slightly lower seedling survival than sexuals.

Conclusions

These differences in early life history traits, combined with reproductive assurance conferred by apomixis, suggest that apomicts achieve a greater range through advantages in their ability to both spread and establish.

     

Development and characterization of nine new microsatellite markers in Taraxacum (Asteraceae)

This study aims at developing and characterizing new microsatellite primer pairs in Taraxacum officinale auct. to produce polymorphic markers for genetical and evolutionary studies on apomixis in this sexual‐apomictic complex. A total of 24 diploid plants were tested for allelic polymorphism and heterozygosity. Out of nine loci three deviated significantly from the Hardy–Weinberg equilibrium expectations, probably due to the presence of null‐alleles. Successful cross‐species amplification was obtained for all markers in 29 Taraxacum microspecies from five sections.     

Amplified fragment length polymorphism (AFLP) markers reveal that population structure of triploid dandelions (Taraxacum officinale) exhibits both clonality and recombination

Highly variable amplified fragment length polymorphism (AFLP) fingerprints of triploid apomictic dandelions obtained from three localities in an area where diploids are lacking were analysed to infer the predominant modes of reproduction. The distribution of markers was analysed using character compatibility to infer whether many genotypes agree with a tree-like structure in the data set. The presence of incompatible character state combinations (matrix incompatibility; MI) was used as a measure of genetic exchange. The detection of overrepresented genotypes, of which some were widespread, confirmed asexual reproduction. Not all genotypes were overrepresented; approximately half of the genotypes in the three localities were found only once. Because, in terms of genotype frequencies, only a part of the genetic variation is described, more important aspects of the molecular data such as relationships between markers or genotypes have been studied. The analysis of character compatibility indicated a disagreement of the data with a clonal structure. Nearly all genotypes contributed to MI and this contribution varied considerably among genotypes in each sampled locality. A gradual decrease of matrix incompatibility upon successive deletion of genotypes showing the highest contribution to MI indicated that marker distribution of virtually all genotypes disagreed with a tree-like structure in the data. This result suggested that many genotypes were separated by one or more sexual generations. Consistent with this conclusion was the fact that markers that show a low probability of contributing to MI are different in every sampled locality, which is most easily explained as the result of recombination. Apparently, asexual reproduction has resulted in overrepresented, widespread genotypes but sexual recombination has also substantially contributed to genetic variation in the sites studied.     

耐盐药蒲公英(Taraxacum officinale Weber)愈伤组织筛选及生理生化特性分析

为获得耐1.5% NaCl的药蒲公英(Taraxacum officinale Weber)愈伤组织, 以药蒲公英叶片外植体为材料诱导愈伤组织。以NaCl为选择因子, 从愈伤组织直接筛选。在选择培养基上, 大部分愈伤组织褐化死亡, 个别褐化死亡的愈伤组织周围有少量新的细胞团长出, 将其转接到新鲜的选择培养基上, 每3周继代一次, 经3个月继代筛选获得了耐1.5% NaCl的药蒲公英细胞团。以普通愈伤组织为对照, 发现随着NaCl浓度升高, 耐盐愈伤组织的相对生长率下降但显著高于对照; 且随着盐胁迫处理时间延长持续升高, 而普通愈伤组织对照几乎停止生长, 说明耐盐愈伤组织具有相对稳定的耐盐性。在蛋白水平上, 耐盐愈伤组织与对照愈伤组织差异明显, SDS-PAGE分析显示: 耐盐愈伤组织比对照多出一条34 kD大小的蛋白带, 且30 kD、18 kD左右的蛋白带明显上调。相同处理条件下耐盐愈伤组织脯氨酸的增加幅度高于对照。盐胁迫条件下, 耐盐愈伤组织的超氧化物歧化酶(Super oxidase dimutase, SOD)、过氧化物酶(Peroxidase, POD)和过氧化氢酶(Catalase, CAT)活性明显高于对照,且随着处理时间的延长和盐浓度的增加呈现升高的趋势, 而对照则呈现先升高后下降的趋势。结果说明耐盐愈伤组织一方面通过小分子有机溶质如脯氨酸的方式调节其渗透平衡, 另一方面还可通过提高抗氧化能力降低盐分造成的次级伤害。积累蛋白也可能是耐盐愈伤组织调节渗透平衡的一种方式。通过生理生化分析确定我们获得的耐盐愈伤组织为耐盐变异体。     

Towards a better understanding of polyploid Sorbus (Rosaceae) from Bosnia and Herzegovina (Balkan Peninsula), including description of a novel,tetraploid apomictic species

Sorbus subgenus Soraria encompasses taxa originating from spontaneous hybridization between members of subgenera Aria and Sorbus disjunctly distributed across Europe and Asia. Using molecular data (amplified fragment length polymorphisms, plastid DNA sequences and nuclear microsatellites), flow cytometry (allowing for the determination of ploidy and mode of reproduction) and morphology, we disentangled the relationships among polyploid cytotypes and explored their relationships with their diploid ancestors. Among others, we focused on a large, geographically isolated hybrid population in Bosnia and Herzegovina. Molecular and morphological analyses confirmed the distinct position of this population in relation to its parental (S. aria and S. aucuparia) and other hybridogenous taxa originating independently from the same parents in different parts of Europe. After establishing its genetic and morphological divergence, we describe the isolated Bosnian population as a new tetraploid apomictic species, S orbus bosniaca , discuss its taxonomic status and propose conservation measures to protect the locus classicus of this new Balkan endemic. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 178 , 670–685.     

Distribution, phenology and demography of sympatric sexual and asexual dandelions (Taraxacum officinale s.l.): geographic parthenogenesis on a small scale

In many plant and animal species, sexual and asexual forms have different geographical distributions ('geographic parthenogenesis'). The common dandelion Taraxacum officinale s.l. provides a particularly clear example of differing distributions: diploid sexuals are restricted to southern and central Europe, while triploid asexuals occur across Europe. To get a better understanding of the factors underlying this pattern, we studied the distribution and demography of sexuals and asexuals in a mixed population that was located at the northern distribution limit of the sexuals. In this population three adjacent, contrasting microhabitats were found: a foreland and south and north slopes of a river dike. Comparative analyses of the distribution, phenology and demography indicated that sexuals had a stronger preference for the south slope than did asexuals. We therefore propose that the large-scale geographic parthenogenesis in T. officinale is shaped by an environmental gradient which acts upon the sexuals.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 205–218.     

Chloroplast DNA variation and reticulate evolution in sexual and apomictic sections of dandelions

Sequencing of the trnL-trnF intergenic spacer in chloroplast DNA (cpDNA) from 237 sexual and apomictic species of dandelions (genus Taraxacum) from Europe, Asia and arctic North America revealed 46 haplotypes, which differed mainly by a variable number of polymorphic tRNA pseudogenes next to the trnF gene. The haplotypes could be divided into 20 cpDNA lineages, but independent duplications and deletions of the pseudogene copies made it difficult to further reconstruct the phylogeny. Intraspecific cpDNA variation was found in the primitive sexual T. serotinum. However, in contrast to a recent study, no cpDNA variation was detected within 12 apomictic species representing a variety of haplotypes. The cpDNA haplotype may therefore help to define these critical apomicts. On the other hand, the genetic variation may easily be overestimated, if the clones are not correctly identified, because some morphologically similar microspecies carried very different haplotypes. In all, 36 sections of the genus were sampled. Four primitive, mainly sexual, sections only displayed a group of ancient haplotypes, whereas morphologically more advanced sections often exhibited many different haplotypes from up to seven cpDNA lineages. In the latter cases, the lineages were rarely unique to a certain section. For example, the two most widespread haplotypes, belonging to different lineages, were found together in nine sections. This suggests that significant gene flow has occurred among the advanced sections, although sexual reproduction is not currently known in several of them. The result is consistent with the reticulate distribution of morphological characters among the sections.     

The role of hybridization, polyploidization and glaciation in the origin and evolution of the apomictic Ranunculus cassubicus complex

The Ranunculus cassubicus complex, comprising diploids and polyploids, is a good model for studying the role of hybridization and polyploidy in the origin of apomixis. Results from amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) analyses performed on 448 individuals were combined with evidence from morphology, isozymes, karyology and distribution. Our results indicated a unique hybrid origin for the apomictic hexaploid R. carpaticola from north-western Slovakia, involving two sexual parents: autotetraploid R. cassubicifolius from the northern pre-Alps, and diploid R. carpaticola from central Slovakia. The hybrids were intermediate to the parents, but unique alleles have resulted from genomic reorganisation in the allopolyploids, which might also have triggered apomixis. Their distribution patterns and estimated ages suggest that hybridization may be correlated with the last glacial period. Hybridization seems to be the major origination for apomicts in the R. cassubicus complex. Polyploidy creates novel sexual genotypes and acts as a springboard for the production of new hybrids, but it only results in a combination with hybridization in apomixis. In turn, asexuality has permitted the perpetuation and establishment of ecologically divergent hybrid genotypes.     

Difference in reproductive mode rather than ploidy explains niche differentiation in sympatric sexual and apomictic populations of Potentilla puberula

Apomicts tend to have larger geographical distributional ranges and to occur in ecologically more extreme environments than their sexual progenitors. However, the expression of apomixis is typically linked to polyploidy. Thus, it is a priori not clear whether intrinsic effects related to the change in the reproductive mode or rather in the ploidy drive ecological differentiation. We used sympatric sexual and apomictic populations of Potentilla puberula to test for ecological differentiation. To distinguish the effects of reproductive mode and ploidy on the ecology of cytotypes, we compared the niches (a) of sexuals (tetraploids) and autopolyploid apomicts (penta‐, hepta‐, and octoploids) and (b) of the three apomictic cytotypes. We based comparisons on a ploidy screen of 238 populations along a latitudinal transect through the Eastern European Alps and associated bioclimatic, and soil and topographic data. Sexual tetraploids preferred primary habitats at drier, steeper, more south‐oriented slopes, while apomicts mostly occurred in human‐made habitats with higher water availability. Contrariwise, we found no or only marginal ecological differentiation among the apomictic higher ploids. Based on the pronounced ecological differences found between sexuals and apomicts, in addition to the lack of niche differentiation among cytotypes of the same reproductive mode, we conclude that reproductive mode rather than ploidy is the main driver of the observed differences. Moreover, we compared our system with others from the literature, to stress the importance of identifying alternative confounding effects (such as hybrid origin). Finally, we underline the relevance of studying ecological parthenogenesis in sympatry, to minimize the effects of differential migration abilities.     

Germination responses of a seed population of Taraxacum officinale Weber to constant temperatures including the supra-optimal range

Abstract The germination responses of a nondormant fraction of a seed population of Taraxacum officinale Weber at constant temperatures in the range 7–34°C were analysed through a time-course study. Maximal percentage germination (approximately 90%) was attained at temperatures 10–18°C, where simple linear relationships were observed between the temperature and the germination rates, i.e. the reciprocals of the time taken to germinate by subpopulations with 20–80% germination. There was a variation in the required ‘thermal times’ (θ) which characterized the linear relationships, the distribution of which could be approximated for the seed population by the following distribution function: where m is the median of the distribution, and A is a shape parameter characterizing the pattern of the distribution. Final percentage germination decreased with increasing temperature from 20 to 32°C, where the final percentage germination vs. temperature plotted on a normal probability scale yielded a straight line, indicating the normality of the distribution of the upper limit temperature in the seed population. The estimated mean and standard deviation were 27.25 ± 3.75°C. The rate of germination for the subpopulation with 20–80% germination also decreased with increases in the temperature from 22 to 30°C. If the relationships between the temperature within this range and the rate for the subpopulations with 20–80% germination were approximated by the regression lines, the negative ‘thermal time’ characterizing the yielded linear relationship would have a distribution which could be approximated by the same function with the required thermal time for the relationship of suboptimal range. The parameters m and A for the negative ‘thermal time’ were determined to be 2870 K h and 1.7 × 10^-10 K^-3 h^-3.     

Neutral and selection-driven decay of sexual traits in asexual stick insects

Environmental shifts and lifestyle changes may result in formerly adaptive traits becoming non-functional or maladaptive. The subsequent decay of such traits highlights the importance of natural selection for adaptations, yet its causes have rarely been investigated. To study the fate of formerly adaptive traits after lifestyle changes, we evaluated sexual traits in five independently derived asexual lineages, including traits that are specific to males and therefore not exposed to selection. At least four of the asexual lineages retained the capacity to produce males that display normal courtship behaviours and are able to fertilize eggs of females from related sexual species. The maintenance of male traits may stem from pleiotropy, or from these traits only regressing via drift, which may require millions of years to generate phenotypic effects. By contrast, we found parallel decay of sexual traits in females. Asexual females produced altered airborne and contact signals, had modified sperm storage organs, and lost the ability to fertilize their eggs, impeding reversals to sexual reproduction. Female sexual traits were decayed even in recently derived asexuals, suggesting that trait changes following the evolution of asexuality, when they occur, proceed rapidly and are driven by selective processes rather than drift.     

Uncovering mechanisms of rubber biosynthesis in Taraxacum koksaghyz – role of cis‐prenyltransferase‐like 1 protein

The Russian dandelion Taraxacum koksaghyz synthesizes considerable amounts of high‐molecular‐weight rubber in its roots. The characterization of factors that participate in natural rubber biosynthesis is fundamental for the establishment of T. koksaghyz as a rubber crop. The cis‐1,4‐isoprene polymers are stored in rubber particles. Located at the particle surface, the rubber transferase complex, member of the cis‐prenyltransferase (cisPT) enzyme family, catalyzes the elongation of the rubber chains. An active rubber transferase heteromer requires a cisPT subunit (CPT) as well as a CPT‐like subunit (CPTL), of which T. koksaghyz has two homologous forms: TkCPTL1 and TkCPTL2, which potentially associate with the rubber transferase complex. Knockdown of TkCPTL1, which is predominantly expressed in latex, led to abolished poly(cis‐1,4‐isoprene) synthesis but unaffected dolichol content, whereas levels of triterpenes and inulin were elevated in roots. Analyses of latex from these TkCPTL1‐RNAi plants revealed particles that were similar to native rubber particles regarding their particle size, phospholipid composition, and presence of small rubber particle proteins (SRPPs). We found that the particles encapsulated triterpenes in a phospholipid shell stabilized by SRPPs. Conversely, downregulating the low‐expressed TkCPTL2 showed no altered phenotype, suggesting its protein function is redundant in T. koksaghyz. MS‐based comparison of latex proteomes from TkCPTL1‐RNAi plants and T. koksaghyz wild‐types discovered putative factors that convert metabolites in biosynthetic pathways connected to isoprenoids or that synthesize components of the rubber particle shell.     

Specific features of early embryogenesis in apomictic <Emphasis Type="Italic">Poa pratensis</Emphasis> L.

We studied the early stages of embryo formation in apomictic Poa pratensis L. It was shown that during transition to parthenogenesis, at least at the initial stages of embryogenesis, the algorithm of development of the sexual embryo is preserved. This could be due to the system of genetic control of embryogenesis, common for amphimixis and apomixis. We described asynchrony of developmental processes both within the efflorescence (asynchronous maturation of seed-buds) and within the seed-bud and even gametophyte (different timing of induction of apoarchesporic initials and oospores). This feature of pseudogamous apomicts allows them to produce simultaneously both sexual and apomictic progenies.     

Karyotype evolution in apomictic Boechera and the origin of the aberrant chromosomes

Chromosome rearrangements may result in both decrease and increase of chromosome numbers. Here we have used comparative chromosome painting (CCP) to reconstruct the pathways of descending and ascending dysploidy in the genus Boechera (tribe Boechereae, Brassicaceae). We describe the origin and structure of three Boechera genomes and establish the origin of the previously described aberrant Het and Del chromosomes found in Boechera apomicts with euploid (2n = 14) and aneuploid (2n = 15) chromosome number. CCP analysis allowed us to reconstruct the origin of seven chromosomes in sexual B. stricta and apomictic B. divaricarpa from the ancestral karyotype (n = 8) of Brassicaceae lineage I. Whereas three chromosomes (BS4, BS6, and BS7) retained their ancestral structure, five chromosomes were reshuffled by reciprocal translocations to form chromosomes BS1‐BS3 and BS5. The reduction of the chromosome number (from x = 8 to x = 7) was accomplished through the inactivation of a paleocentromere on chromosome BS5. In apomictic 2n = 14 plants, CCP identifies the largely heterochromatic chromosome (Het) being one of the BS1 homologues with the expansion of pericentromeric heterochromatin. In apomictic B. polyantha (2n = 15), the Het has undergone a centric fission resulting in two smaller chromosomes – the submetacentric Het′ and telocentric Del. Here we show that new chromosomes can be formed by a centric fission and can be fixed in populations due to the apomictic mode of reproduction.     

Molecular biogeography of clonal lineages in a high-Arctic apomictic Daphnia complex

An electrophoretic survey of 81 populations of arctic Daphnia pulex from around the Svalbard archipelago revealed the presence of 49 unique allozyme clones ( N = 3357). Two closely related clones accounted for 66% of the total sample, and were widespread across the archipelago. Restriction fragment length polymorphisms (RFLPs) of a 2.1-kb fragment of mtDNA (NADH-4 and NADH-5 subunits), amplified using the polymerase chain reaction (PCR), revealed the presence of eight mtDNA haplotypes. One haplotype was particularly widespread, and the two most abundant allozyme clones shared this haplotype. Nonrandom distribution patterns of clones were observed, and are most likely the result of historical events (i.e. founder effects) related to the past glacial history of the archipelago. The data are discussed with reference to past glaciation events, and attempts are made to discern the colonization history of this apomictic complex.     

Competition between two naturalized dandelions,Taraxacum officinale weber andTaraxacum laevigatum DC., in mixed cultures with different levels of soil moisture

Taraxacum officinala andTaraxacum laevigatum were grown in mixed stands at various plant densities and mixing ratios with various levels of soil moisture to formulate the effect of soil moisture on the competitive relationship between the species. In pure stands, the mean plant weight—plant density relation for each level of soil moisture could be described by the reciprocal equation of the crowding effect. On the other hand, the response of mean plant weight to soil moisture content followed the reciprocal equation for a repulsive growth factor at the respective levels of plant density. By introducing the density conversion factor, the results of mixed stands could be successfully formulated from similar reciprocal equations. The dependence of density conversion factor on soil moisture content was also formulated. From these relations, a comprehensive formula was developed to describe the effects of plant density and soil moisture content on the growth of two species in mixed stands. Changes in the biomass in mixed stands were, examined by means of calculations based on the experimental results.     

Patterns of morphometric variation and a new supraspecific classification of apomictic taxa of Hieracium (Asteraceae) from Denmark and southern Sweden

Patterns of variation among all micro-species of Hieracium sect. Hieracium and H. sect. Vulgata known from Denmark and southern Sweden are investigated. Forty-three, possibly independent morphological characters are used. Significant correlations were revealed among the majority of the characters. Multivariate analyses did not reveal any discontinuities in the variation or any discrete groups of species. However, there were obvious correlations between previous taxonomic classifications and the result of the multivariate analyses. Based on the statistical analyses, three sections and 38 informal species aggregates are recognized. H. sect. Bifidum T. Tyler is described anew. The characters that turned out to be most influential in the multivariate analyses, and thus most important for the classification, were the distribution of stellate tomentum on the phyllaries, the amount of glandular hairs and the amount and pigmentation of simple hairs on the phyllaries, the shape of the leaf-base and the regularity of the leaf dentation. The evolution of the group and the taxonomic treatment is thoroughly discussed in the light of the results.