CAM evolution is associated with gene family expansion in an explosive bromeliad radiation.

The subgenus Tillandsia (Bromeliaceae) belongs to one of the fastest radiating clades in the plant kingdom and is characterized by the repeated evolution of Crassulacean acid metabolism (CAM). Despite its complex genetic basis, this water-conserving trait has evolved independently across many plant families and is regarded as a key innovation trait and driver of ecological diversification in Bromeliaceae. By producing high-quality genome assemblies of a Tillandsia species pair displaying divergent photosynthetic phenotypes, and combining genome-wide investigations of synteny, transposable element (TE) dynamics, sequence evolution, gene family evolution, and temporal differential expression, we were able to pinpoint the genomic drivers of CAM evolution in Tillandsia. Several large-scale rearrangements associated with karyotype changes between the 2 genomes and a highly dynamic TE landscape shaped the genomes of Tillandsia. However, our analyses show that rewiring of photosynthetic metabolism is mainly obtained through regulatory evolution rather than coding sequence evolution, as CAM-related genes are differentially expressed across a 24-h cycle between the 2 species but are not candidates of positive selection. Gene orthology analyses reveal that CAM-related gene families manifesting differential expression underwent accelerated gene family expansion in the constitutive CAM species, further supporting the view of gene family evolution as a driver of CAM evolution.

Repeat Dynamics across Timescales: A Perspective from Sibling Allotetraploid Marsh Orchids (Dactylorhiza majalis s.l.).

To provide insights into the fate of transposable elements (TEs) across timescales in a post-polyploidization context, we comparatively investigate five sibling Dactylorhiza allotetraploids (Orchidaceae) formed independently and sequentially between 500 and 100K generations ago by unidirectional hybridization between diploids D. fuchsii and D. incarnata. Our results first reveal that the paternal D. incarnata genome shows a marked increased content of LTR retrotransposons compared to the maternal species, reflected in its larger genome size and consistent with a previously hypothesized bottleneck. With regard to the allopolyploids, in the youngest D. purpurella both genome size and TE composition appear to be largely additive with respect to parents, whereas for polyploids of intermediate ages we uncover rampant genome expansion on a magnitude of multiple entire genomes of some plants such as Arabidopsis. The oldest allopolyploids in the series are not larger than the intermediate ones. A putative tandem repeat, potentially derived from a non-autonomous miniature inverted-repeat TE (MITE) drives much of the genome dynamics in the allopolyploids. The highly dynamic MITE-like element is found in higher proportions in the maternal diploid, D. fuchsii, but is observed to increase in copy number in both subgenomes of the allopolyploids. Altogether, the fate of repeats appears strongly regulated and therefore predictable across multiple independent allopolyploidization events in this system. Apart from the MITE-like element, we consistently document a mild genomic shock following the allopolyploidizations investigated here, which may be linked to their relatively large genome sizes, possibly associated with strong selection against further genome expansions.

Reference standards for flow cytometric estimation of absolute nuclear DNA content in plants.

The estimation of nuclear DNA content has been by far the most popular application of flow cytometry in plants. Because flow cytometry measures relative fluorescence intensities of nuclei stained by a DNA fluorochrome, ploidy determination, and estimation of the nuclear DNA content in absolute units both require comparison to a reference standard of known DNA content. This implies that the quality of the results obtained depends on the standard selection and use. Internal standardization, when the nuclei of an unknown sample and the reference standard are isolated, stained, and measured simultaneously, is mandatory for precise measurements. As DNA peaks representing G1 /G0 nuclei of the sample and standard appear on the same histogram of fluorescence intensity, the quotient of their position on the fluorescence intensity axis provides the quotient of DNA amounts. For the estimation of DNA amounts in absolute units, a number of well-established standards are now available to cover the range of known plant genome sizes. Since there are different standards in use, the standard and the genome size assigned to it has always to be reported. When none of the established standards fits, the introduction of a new standard species is needed. For this purpose, the regression line approach or simultaneous analysis of the candidate standard with several established standards should be prioritized. Moreover, the newly selected standard organism has to fulfill a number of requirements: it should be easy to identify and maintain, taxonomically unambiguous, globally available, with known genome size stability, lacking problematic metabolites, suitable for isolation of sufficient amounts of nuclei, and enabling measurements with low coefficients of variation of DNA peaks, hence suitable for the preparation of high quality samples.

Population structure in Neotropical plants: Integrating pollination biology, topography and climatic niches.

Animal pollinators mediate gene flow among plant populations, but in contrast to well-studied topographic and (Pleistocene) environmental isolating barriers, their impact on population genetic differentiation remains largely unexplored. Comparing how these multifarious factors drive microevolutionary histories is, however, crucial for better resolving macroevolutionary patterns of plant diversification. Here we combined genomic analyses with landscape genetics and niche modelling across six related Neotropical plant species (424 individuals across 33 localities) differing in pollination strategy to test the hypothesis that highly mobile (vertebrate) pollinators more effectively link isolated localities than less mobile (bee) pollinators. We found consistently higher genetic differentiation (FST ) among localities of bee- than vertebrate-pollinated species with increasing geographical distance, topographic barriers and historical climatic instability. High admixture among montane populations further suggested relative climatic stability of Neotropical montane forests during the Pleistocene. Overall, our results indicate that pollinators may differentially impact the potential for allopatric speciation, thereby critically influencing diversification histories at macroevolutionary scales.

Isolation of plant nuclei for estimation of nuclear DNA content: Overview and best practices.

A critical aspect for obtaining accurate, reliable, and high-resolution estimates of nuclear DNA content is the release of nuclei from the cytoplasm in sufficient amounts, while maintaining their integrity throughout the analysis, protecting their DNA from degradation by endonucleases, and enabling stoichiometric DNA staining. In embryophytes, the most common method consists of chopping the plant material with a sharp razor blade to release nuclei into an isolation buffer, filtering the homogenate, and staining the nuclei in buffered suspension with a fluorochrome of choice. Despite the recent description of alternative approaches to isolate nuclei, the chopping procedure remains the most widely adopted method, due to its simplicity, rapidity, and effectiveness. In this review article, we discuss the specifics of nuclei isolation buffers and the distorting effects that secondary metabolites may have in nuclear suspensions and how to test them. We also present alternatives to the chopping procedure, options for filtering and fluorochromes, and discuss the applications of these varied approaches. A summary of the best practices regarding the isolation of plant nuclei for the estimation of nuclear DNA content is also provided.

Different from tracheophytes, liverworts commonly have mixed 35S and 5S arrays.

BACKGROUND AND AIMS: Unlike other nuclear genes in eukaryotes, rDNA genes (5S and 35S loci) are present in numerous copies per cell and, when stained, can therefore provide basic information about genome organization. In tracheophytes (vascular plants), they are usually located on separate chromosomes, the so-called S-type organization. An analysis of 1791 species of land plants suggested that S-type arrays might be ancestral in land plants, while linked (L-type) organization may be derived. However, no outgroup and only a handful of ferns and bryophytes were included. METHODS: We analysed genome sizes and the distribution of telomere, 5S and 35S rDNA FISH signals in up to 12 monoicous or dioicous species of liverworts from throughout a phylogeny that includes 287 of the 386 currently recognized genera. We also used the phylogeny to plot chromosome numbers and the occurrence of visibly distinct sex chromosomes. KEY RESULTS: Chromosome numbers are newly reported for the monoicous Lejeunea cavifolia and for females of the dioicous Scapania aequiloba. We detected sex-related differences in the number of rDNA signals in the dioicous Plagiochila asplenioides and Frullania dilatata. In the latter, the presence of two UU chromosomes in females and additional 5S-35S rDNA loci result in a haploid genome 0.2082 pg larger than the male genome; sex-specific genome differences in the other dioicous species were small. Four species have S-type rDNA, while five species have mixed L-S rDNA organization, and transitions may have occurred multiple times, as suggested by rDNA loci not being conserved among closely related species of Pellia. All species shared an Arabidopsis-like telomere motif, and its detection allowed verification of the chromosome number of Radula complanata and chromosome rearrangements in Aneura pinguis and P. asplenioides, the latter also showing sex-specific interstitial telomere repeats. CONCLUSIONS: The S and L rDNA arrangements appear to have evolved repeatedly within liverworts, even in the same species. Evidence for differential accumulation of rDNA between the sexes so far is limited.

Molecular and karyological data confirm that the enigmatic genus Platypholis from Bonin-Islands (SE Japan) is phylogenetically nested within Orobanche (Orobanchaceae).

Molecular phylogenetic studies have greatly improved our understanding of phylogenetic relationships of non-photosynthetic parasitic broomrapes (Orobanche and related genera, Orobanchaceae), but a few genera have remained unstudied. One of those is Platypholis, whose sole species, Platypholis boninsimae, is restricted to the Bonin-Islands (Ogasawara Islands) about 1000 km southeast of Japan. Based on overall morphological similarity, Platypholis has been merged with Orobanche, but this hypothesis has never been tested with molecular data. Employing maximum likelihood and Bayesian analyses on a family-wide data set (two plastid markers, matK and rps2, and three nuclear markers, ITS, phyA and phyB) as well as on an ITS data set focusing on Orobanche s. str., it is shown that P. boninsimae Maxim. is phylogenetically closely linked to or even nested within Orobanche s. str. This position is supported both by morphological evidence and by the newly obtained chromosome number of 2n = 38, which is characteristic for the genus Orobanche s. str.

Evolution of genome size and chromosome number in the carnivorous plant genus Genlisea (Lentibulariaceae), with a new estimate of the minimum genome size in angiosperms.

BACKGROUND AND AIMS: Some species of Genlisea possess ultrasmall nuclear genomes, the smallest known among angiosperms, and some have been found to have chromosomes of diminutive size, which may explain why chromosome numbers and karyotypes are not known for the majority of species of the genus. However, other members of the genus do not possess ultrasmall genomes, nor do most taxa studied in related genera of the family or order. This study therefore examined the evolution of genome sizes and chromosome numbers in Genlisea in a phylogenetic context. The correlations of genome size with chromosome number and size, with the phylogeny of the group and with growth forms and habitats were also examined. METHODS: Nuclear genome sizes were measured from cultivated plant material for a comprehensive sampling of taxa, including nearly half of all species of Genlisea and representing all major lineages. Flow cytometric measurements were conducted in parallel in two laboratories in order to compare the consistency of different methods and controls. Chromosome counts were performed for the majority of taxa, comparing different staining techniques for the ultrasmall chromosomes. KEY RESULTS: Genome sizes of 15 taxa of Genlisea are presented and interpreted in a phylogenetic context. A high degree of congruence was found between genome size distribution and the major phylogenetic lineages. Ultrasmall genomes with 1C values of <100 Mbp were almost exclusively found in a derived lineage of South American species. The ancestral haploid chromosome number was inferred to be n = 8. Chromosome numbers in Genlisea ranged from 2n = 2x = 16 to 2n = 4x = 32. Ascendant dysploid series (2n = 36, 38) are documented for three derived taxa. The different ploidy levels corresponded to the two subgenera, but were not directly correlated to differences in genome size; the three different karyotype ranges mirrored the different sections of the genus. The smallest known plant genomes were not found in G. margaretae, as previously reported, but in G. tuberosa (1C ≈ 61 Mbp) and some strains of G. aurea (1C ≈ 64 Mbp). CONCLUSIONS: Genlisea is an ideal candidate model organism for the understanding of genome reduction as the genus includes species with both relatively large (∼1700 Mbp) and ultrasmall (∼61 Mbp) genomes. This comparative, phylogeny-based analysis of genome sizes and karyotypes in Genlisea provides essential data for selection of suitable species for comparative whole-genome analyses, as well as for further studies on both the molecular and cytogenetic basis of genome reduction in plants.

Chromosomal diversification and karyotype evolution of diploids in the cytologically diverse genus Prospero (Hyacinthaceae).

BACKGROUND: Prospero (Hyacinthaceae) provides a unique system to assess the impact of genome rearrangements on plant diversification and evolution. The genus exhibits remarkable chromosomal variation but very little morphological differentiation. Basic numbers of x = 4, 5, 6 and 7, extensive polyploidy, and numerous polymorphic chromosome variants were described, but only three species are commonly recognized: P. obtusifolium, P. hanburyi, and P. autumnale s.l., the latter comprising four diploid cytotypes. The relationship between evolutionary patterns and chromosomal variation in diploids, the basic modules of the extensive cytological diversity, is presented. RESULTS: Evolutionary inferences were derived from fluorescence in situ hybridization (FISH) with 5S and 35S rDNA, genome size estimations, and phylogenetic analyses of internal transcribed spacer (ITS) of 35S rDNA of 49 diploids in the three species and all cytotypes of P. autumnale s.l. All species and cytotypes possess a single 35S rDNA locus, interstitial except in P. hanburyi where it is sub-terminal, and one or two 5S rDNA loci (occasionally a third in P. obtusifolium) at fixed locations. The localization of the two rDNA types is unique for each species and cytotype. Phylogenetic data in the P. autumnale complex enable tracing of the evolution of rDNA loci, genome size, and direction of chromosomal fusions: mixed descending dysploidy of x = 7 to x = 6 and independently to x = 5, rather than successive descending dysploidy, is proposed. CONCLUSIONS: All diploid cytotypes are recovered as well-defined evolutionary lineages. The cytogenetic and phylogenetic approaches have provided excellent phylogenetic markers to infer the direction of chromosomal change in Prospero. Evolution in Prospero, especially in the P. autumnale complex, has been driven by differentiation of an ancestral karyotype largely unaccompanied by morphological change. These new results provide a framework for detailed analyses of various types of chromosomal rearrangements and karyotypic variation in polyploids.

Next-generation sequencing reveals the impact of repetitive DNA across phylogenetically closely related genomes of Orobanchaceae.

We used next-generation sequencing to characterize the genomes of nine species of Orobanchaceae of known phylogenetic relationships, different life forms, and including a polyploid species. The study species are the autotrophic, nonparasitic Lindenbergia philippensis, the hemiparasitic Schwalbea americana, and seven nonphotosynthetic parasitic species of Orobanche (Orobanche crenata, Orobanche cumana, Orobanche gracilis (tetraploid), and Orobanche pancicii) and Phelipanche (Phelipanche lavandulacea, Phelipanche purpurea, and Phelipanche ramosa). Ty3/Gypsy elements comprise 1.93%-28.34% of the nine genomes and Ty1/Copia elements comprise 8.09%-22.83%. When compared with L. philippensis and S. americana, the nonphotosynthetic species contain higher proportions of repetitive DNA sequences, perhaps reflecting relaxed selection on genome size in parasitic organisms. Among the parasitic species, those in the genus Orobanche have smaller genomes but higher proportions of repetitive DNA than those in Phelipanche, mostly due to a diversification of repeats and an accumulation of Ty3/Gypsy elements. Genome downsizing in the tetraploid O. gracilis probably led to sequence loss across most repeat types.

Molecular phylogenetics of New Caledonian Diospyros (Ebenaceae) using plastid and nuclear markers.

To clarify phylogenetic relationships among New Caledonian species of Diospyros, sequences of four plastid markers (atpB, rbcL, trnK-matK and trnS-trnG) and two low-copy nuclear markers (ncpGS and PHYA) were analysed. New Caledonian Diospyros species fall into three clades, two of which have only a few members (1 or 5 species); the third has 21 closely related species for which relationships among species have been mostly unresolved in a previous study. Although species of the third group (NC clade III) are morphologically distinct and largely occupy different habitats, they exhibit little molecular variability. Diospyros vieillardii is sister to the rest of the NC clade III, followed by D. umbrosa and D. flavocarpa, which are sister to the rest of this clade. Species from coastal habitats of western Grande Terre (D. cherrieri and D. veillonii) and some found on coralline substrates (D. calciphila and D. inexplorata) form two well-supported subgroups. The species of NC clade III have significantly larger genomes than found in diploid species of Diospyros from other parts of the world, but they all appear to be diploids. By applying a molecular clock, we infer that the ancestor of the NC clade III arrived in New Caledonia around 9 million years ago. The oldest species are around 7 million years old and the youngest ones probably much less than 1 million years.

Apomixis is not prevalent in subnival to nival plants of the European Alps.

BACKGROUND AND AIMS: High alpine environments are characterized by short growing seasons, stochastic climatic conditions and fluctuating pollinator visits. These conditions are rather unfavourable for sexual reproduction of flowering plants. Apomixis, asexual reproduction via seed, provides reproductive assurance without the need of pollinators and potentially accelerates seed development. Therefore, apomixis is expected to provide selective advantages in high-alpine biota. Indeed, apomictic species occur frequently in the subalpine to alpine grassland zone of the European Alps, but the mode of reproduction of the subnival to nival flora was largely unknown. METHODS: The mode of reproduction in 14 species belonging to seven families was investigated via flow cytometric seed screen. The sampling comprised 12 species typical for nival to subnival plant communities of the European Alps without any previous information on apomixis (Achillea atrata, Androsace alpina, Arabis caerulea, Erigeron uniflorus, Gnaphalium hoppeanum, Leucanthemopsis alpina, Oxyria digyna, Potentilla frigida, Ranunculus alpestris, R. glacialis, R. pygmaeus and Saxifraga bryoides), and two high-alpine species with apomixis reported from other geographical areas (Leontopodium alpinum and Potentilla crantzii). KEY RESULTS: Flow cytometric data were clearly interpretable for all 46 population samples, confirming the utility of the method for broad screenings on non-model organisms. Formation of endosperm in all species of Asteraceae was documented. Ratios of endosperm : embryo showed pseudogamous apomixis for Potentilla crantzii (ratio approx. 3), but sexual reproduction for all other species (ratios approx. 1·5). CONCLUSIONS: The occurrence of apomixis is not correlated to high altitudes, and cannot be readily explained by selective forces due to environmental conditions. The investigated species have probably other adaptations to high altitudes to maintain reproductive assurance via sexuality. We hypothesize that shifts to apomixis are rather connected to frequencies of polyploidization than to ecological conditions.

Spatial and Ecological Drivers of Genetic Structure in Greek Populations of Alkanna tinctoria (Boraginaceae), a Polyploid Medicinal Herb.

Background and Aims: Quantifying genetic variation is fundamental to understand a species' demographic trajectory and its ability to adapt to future changes. In comparison with diploids, however, genetic variation and factors fostering genetic divergence remain poorly studied in polyploids due to analytical challenges. Here, by employing a ploidy-aware framework, we investigated the genetic structure and its determinants in polyploid Alkanna tinctoria (Boraginaceae), an ancient medicinal herb that is the source of bioactive compounds known as alkannin and shikonin (A/S). From a practical perspective, such investigation can inform biodiversity management strategies. Methods: We collected 14 populations of A. tinctoria within its main distribution range in Greece and genotyped them using restriction site-associated DNA sequencing. In addition, we included two populations of A. sieberi. By using a ploidy-aware genotype calling based on likelihoods, we generated a dataset of 16,107 high-quality SNPs. Classical and model-based analysis was done to characterize the genetic structure within and between the sampled populations, complemented by genome size measurements and chromosomal counts. Finally, to reveal the drivers of genetic structure, we searched for associations between allele frequencies and spatial and climatic variables. Key Results: We found support for a marked regional structure in A. tinctoria along a latitudinal gradient in line with phytogeographic divisions. Several analyses identified interspecific admixture affecting both mainland and island populations. Modeling of spatial and climatic variables further demonstrated a larger contribution of neutral processes and a lesser albeit significant role of selection in shaping the observed genetic structure in A. tinctoria. Conclusion: Current findings provide evidence of strong genetic structure in A. tinctoria mainly driven by neutral processes. The revealed natural genomic variation in Greek Alkanna can be used to further predict variation in A/S production, whereas our bioinformatics approach should prove useful for the study of other non-model polyploid species.

Genome Size and Chromosome Number Evolution in Korean Iris L. Species (Iridaceae Juss.).

Chromosome numbers, karyotypes, and genome sizes of 14 Iris L. (Iridaceae Juss.) species in Korea and their closely related taxon, Sisyrinchium rosulatum, are presented and analyzed in a phylogenetic framework. To date, understanding the chromosomal evolution of Korean irises has been hampered by their high chromosome numbers. Here, we report analyses of chromosome numbers and karyotypes obtained via classic Feulgen staining and genome sizes measured using flow cytometry in Korean irises. More than a two-fold variation in chromosome numbers (2n = 22 to 2n = 50) and over a three-fold genome size variation (2.39 pg to 7.86 pg/1 C) suggest the putative polyploid and/or dysploid origin of some taxa. Our study demonstrates that the patterns of genome size variation and chromosome number changes in Korean irises do not correlate with the phylogenetic relationships and could have been affected by different evolutionary processes involving polyploidy or dysploidy. This study presents the first comprehensive chromosomal and genome size data for Korean Iris species. Further studies involving molecular cytogenetic and phylogenomic analyses are needed to interpret the mechanisms involved in the origin of chromosomal variation in the Iris.

Pleistocene refugia and polytopic replacement of diploids by tetraploids in the Patagonian and Subantarctic plant Hypochaeris incana (Asteraceae, Cichorieae).

We report the phylogeographic pattern of the Patagonian and Subantarctic plant Hypochaeris incana endemic to southeastern South America. We applied amplified fragment length polymorphism (AFLP) and chloroplast DNA (cpDNA) analysis to 28 and 32 populations, respectively, throughout its distributional range and assessed ploidy levels using flow cytometry. While cpDNA data suggest repeated or simultaneous parallel colonization of Patagonia and Tierra del Fuego by several haplotypes and/or hybridization, AFLPs reveal three clusters corresponding to geographic regions. The central and northern Patagonian clusters (approximately 38-51 degrees S), which are closer to the outgroup, contain mainly tetraploid, isolated and highly differentiated populations with low genetic diversity. To the contrary, the southern Patagonian and Fuegian cluster (approximately 51-55 degrees S) contains mainly diploid populations with high genetic diversity and connected by high levels of gene flow. The data suggest that H. incana originated at the diploid level in central or northern Patagonia, from where it migrated south. All three areas, northern, central and southern, have similar levels of rare and private AFLP bands, suggesting that all three served as refugia for H. incana during glacial times. In southern Patagonia and Tierra del Fuego, the species seems to have expanded its populational system in postglacial times, when the climate became warmer and more humid. In central and northern Patagonia, the populations seem to have become restricted to favourable sites with increasing temperature and decreasing moisture and there was a parallel replacement of diploids by tetraploids in local populations.

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.

Multiple Origins and Nested Cycles of Hybridization Result in High Tetraploid Diversity in the Monocot Prospero.

Polyploidy is a major driving force in angiosperm evolution, but our understanding of establishment and early diversification processes following allo- vs. auto-polyploidy is limited. An excellent system to address such questions is the monocot plant Prospero autumnale, as it comprises several genomically and chromosomally distinct diploid cytotypes and their auto- and allotetraploid derivatives. To infer origins and evolutionary trajectories of the tetraploids, we use genome size data, in situ hybridization with parental genomic DNAs and specific probes (satDNA, rDNAs), as well as molecular-phylogenetic analyses. Thus, we demonstrate that an astounding range of allotetraploid lineages has been formed recurrently by chromosomal re-patterning, interactions of chromosomally variable parental genomes and nested cycles of extensive hybridization, whereas autotetraploids have originated at least twice and are cytologically stable. During the recurrent formation and establishment across wide geographic areas hybridization in some populations could have inhibited lineage diversification and nascent speciation of such a hybrid swarm. However, cytotypes that became fixed in populations enhanced the potential for species diversification, possibly exploiting the extended allelic base, and fixed heterozygosity that polyploidy confers. The time required for polyploid cytotype fixation may in part reflect the lag phase reported for polyploids between their formation and species diversification.

Footprints of parasitism in the genome of the parasitic flowering plant Cuscuta campestris.

A parasitic lifestyle, where plants procure some or all of their nutrients from other living plants, has evolved independently in many dicotyledonous plant families and is a major threat for agriculture globally. Nevertheless, no genome sequence of a parasitic plant has been reported to date. Here we describe the genome sequence of the parasitic field dodder, Cuscuta campestris. The genome contains signatures of a fairly recent whole-genome duplication and lacks genes for pathways superfluous to a parasitic lifestyle. Specifically, genes needed for high photosynthetic activity are lost, explaining the low photosynthesis rates displayed by the parasite. Moreover, several genes involved in nutrient uptake processes from the soil are lost. On the other hand, evidence for horizontal gene transfer by way of genomic DNA integration from the parasite's hosts is found. We conclude that the parasitic lifestyle has left characteristic footprints in the C. campestris genome.