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P. J. Van Dijk – One of the best experts on this subject based on the ideXlab platform.

  • ecological and evolutionary opportunities of apomixis insights from taraxacum and chondrilla
    Philosophical Transactions of the Royal Society B, 2003
    Co-Authors: P. J. Van Dijk


    The ecological and evolutionary opportunities of apomixis in the short and the long term are considered, based on two closely related Apomictic genera: Taraxacum (dandelion) and Chondrilla (skeleton weed). In both genera Apomicts have a wider geographical distribution than sexuals, illustrating the short-term ecological success of apomixis. Allozymes and DNA markers indicate that Apomictic populations are highly polyclonal. In Taraxacum, clonal diversity can be generated by rare hybridization between sexuals and Apomicts, the latter acting as pollen donors. Less extensive clonal diversity is generated by mutations within clonal lineages. Clonal diversity may be maintained by frequency-dependent selection, caused by biological interactions (e.g. competitors and pathogens). Some clones are geographically widespread and probably represent phenotypically plastic ‘general-purpose genotypes’. The long-term evolutionary success of Apomictic clones may be limited by lack of adaptive potential and the accumulation of deleterious mutations. Although Apomictic clones may be considered as ‘evolutionary dead ends’, the genes controlling apomixis can escape from degeneration and extinction via pollen in crosses between sexuals and Apomicts. In this way, apomixis genes are transferred to a new genetic background, potentially adaptive and cleansed from linked deleterious mutations. Consequently, apomixis genes can be much older than the clones they are currently contained in. The close phylogenetic relationship between Taraxacum and Chondrilla and the similarity of their apomixis mechanisms suggest that apomixis in these two genera could be of common ancestry.

  • Comparative cyto-embryological investigations of sexual and Apomictic dandelions (Taraxacum) and their Apomictic hybrids
    Sexual Plant Reproduction, 2002
    Co-Authors: Peter Van Baarlen, Hans De Jong, P. J. Van Dijk


    In the autonomous Apomictic Taraxacum offinale (common dandelion), parthenogenetic egg cells develop into embryos and central cells into endosperm without prior fertilisation. Unreduced (2n) megaspores are formed via meiotic diplospory, a nonreductional type of meiosis. In this paper, we describe the normal developmental pathways of sexual and Apomictic reproduction and compare these with the development observed in the Apomictic hybrids. In sexual diploids, a standard type of megasporogenesis and embryo sac development is synchronised between florets in individual capitula. In contrast, we observed that megasporogenesis and gametogenesis proceeded asynchronously between florets within a single capitulum of natural triploid Apomicts. In addition, autonomous endosperm and embryo development initiated independently within individual florets. Parthenogenetic initiation of embryo development in outdoor Apomicts was found to be temperature-dependent. Egg cells produced in natural Apomicts were not fertilised after pollination with haploid pollen grains although pollen tubes were observed to grow into their embryo sacs. Both reductional and diplosporous megasporogenesis were observed in individual inflorescences of triploid Apomictic hybrids. Embryo and endosperm development initiated independently in natural and hybrid Apomicts. [KEYWORDS: apomixis, autonomous endosperm, diplospory embryo, development, parthenogenesis]

  • Meiotic recombination in sexual diploid and Apomictic triploid dandelions (Taraxacum officinale L.).
    Genome, 2000
    Co-Authors: P. Van Baarlen, P. J. Van Dijk, R. F. Hoekstra, J. H. De Jong


    Taraxacum officinale L. (dandelion) is a vigorous weed in Europe with diploid sexual populations in the southern regions and partially overlapping populations of diploid sexuals and triploid or tetraploid Apomicts in the central and northern regions. Previous studies have demonstrated unexpectedly high levels of genetic variation in the Apomictic populations, suggesting the occurrence of genetic segregation in the Apomicts and (or) hybridization between sexual and Apomictic individuals. In this study we analysed meiosis in both sexual diploid and Apomictic triploid plants to find mechanisms that could account for the high levels of genetic variation in the Apomicts. Microscopic study of microsporocytes in the triploid Apomicts revealed that the levels of chromosome pairing and chiasma formation at meiotic prophase I were lower than in that of the sexual diploids, but still sufficient to assume recombination between the homologues. Nomarski DIC (differential interference contrast) microscopy of optically cleared megasporocytes in the Apomicts demonstrated incidental formation of tetrads, which suggests that hybridization can occur in triploid Apomicts.

Timothy F. Sharbel – One of the best experts on this subject based on the ideXlab platform.

  • Chasing the Apomictic Factors in the Ranunculus auricomus Complex: Exploring Gene Expression Patterns in Microdissected Sexual and Apomictic Ovules.
    Genes, 2020
    Co-Authors: Marco Pellino, Elvira Horandl, Diego Hojsgaard, Timothy F. Sharbel


    Apomixis, the asexual reproduction via seeds, is associated to polyploidy and hybridization. To identify possible signatures of apomixis, and possible candidate genes underlying the shift from sex to apomixis, microarray-based gene expression patterns of live microdissected ovules at four different developmental stages were compared between Apomictic and sexual individuals of the Ranunculus auricomus complex. Following predictions from previous work on mechanisms underlying apomixis penetrance and expressivity in the genus, gene expression patterns were classified into three categories based on their relative expression in Apomicts compared to their sexual parental ancestors. We found evidence of misregulation and differential gene expression between Apomicts and sexuals, with the highest number of differences detected during meiosis progression and emergence of aposporous initial (AI) cells, a key developmental stage in the ovule of Apomicts where a decision between divergent reproductive pathways takes place. While most of the differentially expressed genes (DEGs) could not be annotated, gene expression was classified into transgressive, parent of origin and ploidy effects. Genes related to gametogenesis and meiosis demonstrated patterns reflective of transgressive and genome dosage effects, which support the hypothesis of a dominant factor controlling apomixis in Ranunculus and modulated by secondary modifiers. Three genes with probable functions in sporogenesis and gametogenesis development are identified and characterized for future studies.

  • Hybridization drives evolution of Apomicts in Rubus subgenus Rubus: evidence from microsatellite markers.
    Annals of botany, 2017
    Co-Authors: Petra Šarhanová, Timothy F. Sharbel, Radim J. Vašut, Martin Dančák, Michal Sochor, Bohumil Trávníček


    Background and Aims Rubus subgenus Rubus is a group of mostly Apomictic and polyploid species with a complicated taxonomy and history of ongoing hybridization. The only polyploid series with prevailing sexuality is the series Glandulosi , although the Apomictic series Discolores and Radula also retain a high degree of sexuality, which is influenced by environmental conditions and/or pollen donors. The aim of this study is to detect sources of genetic variability, determine the origin of Apomictic taxa and validate microsatellite markers by cloning and sequencing. Methods A total of 206 individuals from two central European regions were genotyped for 11 nuclear microsatellite loci and the chloroplast trn L- trn F region. Microsatellite alleles were further sequenced in order to determine the exact repeat number and to detect size homoplasy due to insertions/deletions in flanking regions. Key Results The results confirm that Apomictic microspecies of ser. Radula are derived from crosses between sexual series Glandulosi and Apomictic series Discolores , whereby the Apomict acts as pollen donor. Each Apomictic microspecies is derived from a single distinct genotype differing from the parental taxa, suggesting stabilized clonal reproduction. Intraspecific variation within Apomicts is considerably low compared with sexual series Glandulosi , and reflects somatic mutation accumulation. While facultative Apomicts produce clonal offspring, sexual species are the conduits of origin for new genetically different Apomictic lineages. Conclusions One of the main driving forces of evolution and speciation in the highly Apomictic subgenus Rubus in central Europe is sexuality in the series Glandulosi . Palaeovegetation data suggest that initial hybridizations took place over different time periods in the two studied regions, and that the successful origin and spread of Apomictic microspecies of the series Radula took place over several millennia. Additionally, the cloning and sequencing show that standard evaluations of microsatellite repeat numbers underestimate genetic variability considering homoplasy in allele size.

  • Mutation Accumulation in an Asexual Relative of Arabidopsis.
    PLoS genetics, 2017
    Co-Authors: John T. Lovell, Robert J. Williamson, Stephen I. Wright, John K. Mckay, Timothy F. Sharbel


    Asexual populations experience weaker responses to natural selection, which causes deleterious mutations to accumulate over time. Additionally, stochastic loss of individuals free of deleterious mutations can lead to an irreversible increase in mutational load in asexuals (the “click” in Muller’s Ratchet). Here we report on the genomic divergence and distribution of mutations across eight sympatric pairs of sexual and Apomictic (asexual) Boechera (Brassicaceae) genotypes. We show that Apomicts harbor a greater number of derived mutations than sympatric sexual genotypes. Furthermore, in phylogenetically constrained sites that are subject to contemporary purifying selection, the ancestral, conserved allele is more likely to be retained in sexuals than Apomicts. These results indicate that Apomictic lineages accumulate mutations at otherwise conserved sites more often than sexuals, and support the conclusion that deleterious mutation accumulation can be a powerful force in the evolution of asexual higher plants.

Gerdien De Jong – One of the best experts on this subject based on the ideXlab platform.

  • The effect of intra‐specific competition on seedlings of sexual and Apomictic Taraxacum officinale
    Oikos, 2001
    Co-Authors: Carolien G.f. De Kovel, Gerdien De Jong


    Because of their higher evolvability, sexuals may have an advantage relative to asexual organisms in a competitive environment with many biotic interactions. We tested this idea using sexual and Apomictic Taraxacum, dandelions. Taraxacum seedlings were grown without competition and in different competing combinations in a greenhouse. Apomicts had more and longer leaves than sexuals, but the same dry weight at harvest as sexuals. Competition reduced growth to the same extent in both Apomicts and sexuals. Therefore, we conclude that sexual dandelions are no superior competitors relative to Apomicts. In Taraxacum, new Apomictic lineages spin off from the sexual population with some unknown frequency. This may enable the Apomictic community to keep up with the sexual population.

  • Responses of Sexual and Apomictic Genotypes of Taraxacum officinale to Variation in Light
    Plant Biology, 1999
    Co-Authors: Carolien G.f. De Kovel, Gerdien De Jong


    : The mode of reproduction, sexual or asexual, will influence the way populations respond to selective pressures. This can cause genetic and ecological divergence between sexual and asexual forms of the same species. Here we examine differences in morphology and phenology between sexual and Apomictic types of dandelion, Taraxacum officinale. Sexual and Apomictic dandelions were collected from a mixed population on the banks of the river Rhine, The Netherlands. Clonal copies of both sexual and Apomictic genotypes were planted in an experimental garden under two light levels. Sexual plants flowered four days later on average than Apomicts, but the number of capitula was the same. Apomicts had longer leaves and were heavier than sexual plants, especially under shaded conditions. In Apomicts plasticity for leaf length and height was larger than in sexuals, but for most other measured traits no differences in plasticity were observed. Trait values of Apomicts were within the same range as those of sexual plants.