Diatom abundance in the polar oceans is predicted by genome size.

A principal goal in ecology is to identify the determinants of species abundances in nature. Body size has emerged as a fundamental and repeatable predictor of abundance, with smaller organisms occurring in greater numbers than larger ones. A biogeographic component, known as Bergmann's rule, describes the preponderance, across taxonomic groups, of larger-bodied organisms in colder areas. Although undeniably important, the extent to which body size is the key trait underlying these patterns is unclear. We explored these questions in diatoms, unicellular algae of global importance for their roles in carbon fixation and energy flow through marine food webs. Using a phylogenomic dataset from a single lineage with worldwide distribution, we found that body size (cell volume) was strongly correlated with genome size, which varied by 50-fold across species and was driven by differences in the amount of repetitive DNA. However, directional models identified temperature and genome size, not cell size, as having the greatest influence on maximum population growth rate. A global metabarcoding dataset further identified genome size as a strong predictor of species abundance in the ocean, but only in colder regions at high and low latitudes where diatoms with large genomes dominated, a pattern consistent with Bergmann's rule. Although species abundances are shaped by myriad interacting abiotic and biotic factors, genome size alone was a remarkably strong predictor of abundance. Taken together, these results highlight the cascading cellular and ecological consequences of macroevolutionary changes in an emergent trait, genome size, one of the most fundamental and irreducible properties of an organism.

The dynamic trophic architecture of open-ocean protist communities revealed through machine-guided metatranscriptomics.

Intricate networks of single-celled eukaryotes (protists) dominate carbon flow in the ocean. Their growth, demise, and interactions with other microorganisms drive the fluxes of biogeochemical elements through marine ecosystems. Mixotrophic protists are capable of both photosynthesis and ingestion of prey and are dominant components of open-ocean planktonic communities. Yet the role of mixotrophs in elemental cycling is obscured by their capacity to act as primary producers or heterotrophic consumers depending on factors that remain largely uncharacterized. Here, we develop and apply a machine learning model that predicts the in situ trophic mode of aquatic protists based on their patterns of gene expression. This approach leverages a public collection of protist transcriptomes as a training set to identify a subset of gene families whose transcriptional profiles predict trophic mode. We applied our model to nearly 100 metatranscriptomes obtained during two oceanographic cruises in the North Pacific and found community-level and population-specific evidence that abundant open-ocean mixotrophic populations shift their predominant mode of nutrient and carbon acquisition in response to natural gradients in nutrient supply and sea surface temperature. Metatranscriptomic data from ship-board incubation experiments revealed that abundant mixotrophic prymnesiophytes from the oligotrophic North Pacific subtropical gyre rapidly remodeled their transcriptome to enhance photosynthesis when supplied with limiting nutrients. Coupling this approach with experiments designed to reveal the mechanisms driving mixotroph physiology provides an avenue toward understanding the ecology of mixotrophy in the natural environment.

Resolving Marine-Freshwater Transitions by Diatoms Through a Fog of Gene Tree Discordance.

Despite the obstacles facing marine colonists, most lineages of aquatic organisms have colonized and diversified in freshwaters repeatedly. These transitions can trigger rapid morphological or physiological change and, on longer timescales, lead to increased rates of speciation and extinction. Diatoms are a lineage of ancestrally marine microalgae that have diversified throughout freshwater habitats worldwide. We generated a phylogenomic data set of genomes and transcriptomes for 59 diatom taxa to resolve freshwater transitions in one lineage, the Thalassiosirales. Although most parts of the species tree were consistently resolved with strong support, we had difficulties resolving a Paleocene radiation, which affected the placement of one freshwater lineage. This and other parts of the tree were characterized by high levels of gene tree discordance caused by incomplete lineage sorting and low phylogenetic signal. Despite differences in species trees inferred from concatenation versus summary methods and codons versus amino acids, traditional methods of ancestral state reconstruction supported six transitions into freshwaters, two of which led to subsequent species diversification. Evidence from gene trees, protein alignments, and diatom life history together suggest that habitat transitions were largely the product of homoplasy rather than hemiplasy, a condition where transitions occur on branches in gene trees not shared with the species tree. Nevertheless, we identified a set of putatively hemiplasious genes, many of which have been associated with shifts to low salinity, indicating that hemiplasy played a small but potentially important role in freshwater adaptation. Accounting for differences in evolutionary outcomes, in which some taxa became locked into freshwaters while others were able to return to the ocean or become salinity generalists, might help further distinguish different sources of adaptive mutation in freshwater diatoms.

The Cretaceous Diatom Database: A tool for investigating early diatom evolution.

The Cretaceous period is the time of the first appearance of the diatoms in the fossil record. These fossils give us direct evidence of the age and early evolution of the diatom lineage. The fossil record, however, is incomplete and therefore often extrapolated through time-calibrated phylogenies. These two approaches offer different perspectives on the early evolution of diatoms, which is still poorly understood. We compiled the first comprehensive Cretaceous Diatom Database, a tool to investigate the taxonomy, diversity, and occurrence of the earliest known diatom lineages. To further aid the integration and use of the oldest diatom fossils in molecular clock analyses, we present a set of well-documented Cretaceous fossils that can be placed onto molecular phylogenetic trees of extant and extinct species, making them ideal candidates for the calibration of molecular clocks. The analysis of the fossil record and the Cretaceous Diatom Database revealed Cretaceous diversity is substantially greater than previously thought, yet considerable taxonomic work is still needed. The Cretaceous Diatom Database and the list of Cretaceous fossils for calibrating molecular clocks represent valuable resources for future evolutionary and taxonomic studies of modern and fossil diatoms.

The dynamic response to hypo-osmotic stress reveals distinct stages of freshwater acclimation by a euryhaline diatom.

The salinity gradient separating marine and freshwater environments is a major ecological divide, and the mechanisms by which most organisms adapt to new salinity environments are poorly understood. Diatoms are a lineage of ancestrally marine microalgae that have repeatedly colonized and diversified in freshwaters. Cyclotella cryptica is a euryhaline diatom found in salinities ranging from fully freshwater to fully marine, thus providing a powerful system for understanding the genomic mechanisms for mitigating and acclimating to low salinity. To understand how diatoms mitigate acute hypo-osmotic stress, we abruptly shifted C. cryptica from seawater to freshwater and performed transcriptional profiling during the first 10 h. Freshwater shock dramatically remodelled the transcriptome, with ~50% of the genome differentially expressed in at least one time point. The peak response occurred within 1 h, with strong repression of genes involved in cell growth and osmolyte production, and strong induction of specific stress defence genes. Transcripts largely returned to baseline levels within 4-10 h, with growth resuming shortly thereafter, suggesting that gene expression dynamics may be useful for predicting acclimation. Moreover, comparison to a transcriptomics study of C. cryptica following months-long acclimation to freshwater revealed little overlap between the genes and processes differentially expressed in cells exposed to acute stress versus fully acclimated conditions. Altogether, this study highlights the power of time-resolved transcriptomics to reveal fundamental insights into how cells dynamically respond to an acute environmental shift and provides new insights into how diatoms mitigate natural salinity fluctuations and have successfully diversified across freshwater habitats worldwide.

Phylotranscriptomics reveals the reticulate evolutionary history of a widespread diatom species complex.

In contrast to surveys based on a few genes that often provide limited taxonomic resolution, transcriptomes provide a wealth of genomic loci that can resolve relationships among taxonomically challenging lineages. Diatoms are a diverse group of aquatic microalgae that includes important bioindicator species and many such lineages. One example is Nitzschia palea, a widespread species complex with several morphologically defined taxonomic varieties, some of which are critical pollution indicators. Morphological differences among the varieties are subtle and phylogenetic studies based on a few genes fail to resolve their evolutionary relationships. We conducted morphometric and transcriptome analyses of 10 Nitzschia palea strains to resolve the relationships among strains and taxonomic varieties. Nitzschia palea was resolved into three clades, one of which corresponds to a group of strains with narrow linear-lanceolate valves. The other morphological group recovered in the shape outline analysis was not monophyletic and consisted of two clades. Gene-tree concordance analyses and phylogenetic network estimations revealed patterns of incomplete lineage sorting and gene flow between intraspecific lineages. We detected reticulated evolutionary patterns among lineages with different morphologies, resulting in a putative recent hybrid. Our study shows that phylogenomic analyses of unlinked nuclear loci, complemented with morphometrics, can resolve complex evolutionary histories of recently diverged species complexes.

The genome of a nonphotosynthetic diatom provides insights into the metabolic shift to heterotrophy and constraints on the loss of photosynthesis.

Although most of the tens of thousands of diatom species are photoautotrophs, a small number of heterotrophic species no longer photosynthesize. We sequenced the genome of a nonphotosynthetic diatom, Nitzschia Nitz4, to determine how carbon metabolism was altered in the wake of this trophic shift. Nitzschia Nitz4 has retained its plastid and plastid genome, but changes associated with the transition to heterotrophy were cellular-wide and included losses of photosynthesis-related genes from the nuclear and plastid genomes, elimination of isoprenoid biosynthesis in the plastid, and remodeling of mitochondrial glycolysis to maximize adenosine triphosphte (ATP) yield. The genome contains a ß-ketoadipate pathway that may allow Nitzschia Nitz4 to metabolize lignin-derived compounds. Diatom plastids lack an oxidative pentose phosphate pathway (oPPP), leaving photosynthesis as the primary source of NADPH to support essential biosynthetic pathways in the plastid and, by extension, limiting available sources of NADPH in nonphotosynthetic plastids. The genome revealed similarities between nonphotosynthetic diatoms and apicomplexan parasites for provisioning NADPH in their plastids and highlighted the ancestral absence of a plastid oPPP as a potentially important constraint on loss of photosynthesis, a hypothesis supported by the higher frequency of transitions to parasitism or heterotrophy in lineages that have a plastid oPPP.

Transcriptional Response of Osmolyte Synthetic Pathways and Membrane Transporters in a Euryhaline Diatom During Long-term Acclimation to a Salinity Gradient.

How diatoms respond to fluctuations in osmotic pressure is important from both ecological and applied perspectives. It is well known that osmotic stress affects photosynthesis and can result in the accumulation of compounds desirable in pharmaceutical and alternative fuel industries. Gene expression responses to osmotic stress have been studied in short-term trials, but it is unclear whether the same mechanisms are recruited during long-term acclimation. We used RNA-seq to study the genome-wide transcription patterns in the euryhaline diatom, Cyclotella cryptica, following long-term acclimation to salinity that spanned the natural range of fresh to oceanic water. Long-term acclimated C. cryptica exhibited induced synthesis or repressed degradation of the osmolytes glycine betaine, taurine and dimethylsulfoniopropionate (DMSP). Although changes in proline concentration is one of the main responses in short-term osmotic stress, we did not detect a transcriptional change in proline biosynthetic pathways in our long-term experiment. Expression of membrane transporters showed a general tendency for increased import of potassium and export of sodium, consistent with the electrochemical gradients and dependence on co-transported molecules. Our results show substantial between-genotype differences in growth and gene expression reaction norms and suggest that the regulation of proline synthesis important in short-term osmotic stress might not be maintained in long-term acclimation. Further examination using time-course gene expression experiments, metabolomics and genetic validation of gene functions would reinforce patterns inferred from RNA-seq data.

Signal, Uncertainty, and Conflict in Phylogenomic Data for a Diverse Lineage of Microbial Eukaryotes (Diatoms, Bacillariophyta).

Diatoms (Bacillariophyta) are a species-rich group of eukaryotic microbes diverse in morphology, ecology, and metabolism. Previous reconstructions of the diatom phylogeny based on one or a few genes have resulted in inconsistent resolution or low support for critical nodes. We applied phylogenetic paralog pruning techniques to a data set of 94 diatom genomes and transcriptomes to infer perennially difficult species relationships, using concatenation and summary-coalescent methods to reconstruct species trees from data sets spanning a wide range of thresholds for taxon and column occupancy in gene alignments. Conflicts between gene and species trees decreased with both increasing taxon occupancy and bootstrap cutoffs applied to gene trees. Concordance between gene and species trees was lowest for short internodes and increased logarithmically with increasing edge length, suggesting that incomplete lineage sorting disproportionately affects species tree inference at short internodes, which are a common feature of the diatom phylogeny. Although species tree topologies were largely consistent across many data treatments, concatenation methods appeared to outperform summary-coalescent methods for sparse alignments. Our results underscore that approaches to species-tree inference based on few loci are likely to be misled by unrepresentative sampling of gene histories, particularly in lineages that may have diversified rapidly. In addition, phylogenomic studies of diatoms, and potentially other hyperdiverse groups, should maximize the number of gene trees with high taxon occupancy, though there is clearly a limit to how many of these genes will be available.

A single loss of photosynthesis in the diatom order Bacillariales (Bacillariophyta).

PREMISE OF THE STUDY: Loss of photosynthesis is a common and often repeated trajectory in nearly all major groups of photosynthetic eukaryotes. One small subset of "apochloritic" diatoms in the genus Nitzschia have lost their ability to photosynthesize and require extracellular carbon for growth. Similar to other secondarily nonphotosynthetic taxa, apochloritic diatoms maintain colorless plastids with highly reduced plastid genomes. Although the narrow taxonomic breadth of apochloritic Nitzschia suggests a single loss of photosynthesis in their common ancestor, previous phylogenetic analyses suggested that photosynthesis was lost multiple times. METHODS: We analyzed genes from the nuclear, plastid, and mitochondrial genomes for a broad set of taxa to test whether photosynthesis was lost one or multiple times in Bacillariales. We also sequenced and characterized the plastid genome of a nonphotosynthetic Nitzschia species. KEY RESULTS: Phylogenetic analyses showed that genes from all three genetic compartments either supported or failed to reject monophyly of apochloritic Nitzschia species, consistent with a single loss of photosynthesis in this group. The plastid genomes of two apochloritic Nitzschia are highly similar in all respects, indicating streamlining of the plastid genome before the split of these two species. CONCLUSIONS: A better understanding of the phylogeny and ecology of apochloritic Nitzschia, together with emerging genomic resources, will help identify the factors that have driven and maintained the loss of photosynthesis in this group of diatoms. Finally, some habitats host diverse communities of co-occurring nonphotosynthetic diatoms, reflecting resource abundance or resource partitioning in ecologically favorable habitats.

Accelerated diversification is related to life history and locomotion in a hyperdiverse lineage of microbial eukaryotes (Diatoms, Bacillariophyta).

Patterns of species richness are commonly linked to life history strategies. In diatoms, an exceptionally diverse lineage of photosynthetic heterokonts important for global photosynthesis and burial of atmospheric carbon, lineages with different locomotory and reproductive traits differ dramatically in species richness, but any potential association between life history strategy and diversification has not been tested in a phylogenetic framework. We constructed a time-calibrated, 11-gene, 1151-taxon phylogeny of diatoms - the most inclusive diatom species tree to date. We used this phylogeny, together with a comprehensive inventory of first-last occurrences of Cenozoic fossil diatoms, to estimate ranges of expected species richness, diversification and its variation through time and across lineages. Diversification rates varied with life history traits. Although anisogamous lineages diversified faster than oogamous ones, this increase was restricted to a nested clade with active motility in the vegetative cells. We propose that the evolution of motility in vegetative cells, following an earlier transition from oogamy to anisogamy, facilitated outcrossing and improved utilization of habitat complexity, ultimately leading to enhanced opportunity for adaptive divergence across a variety of novel habitats. Together, these contributed to a species radiation that gave rise to the majority of present-day diatom diversity.

Phylogenomics reveals an extensive history of genome duplication in diatoms (Bacillariophyta).

PREMISE OF THE STUDY: Diatoms are one of the most species-rich lineages of microbial eukaryotes. Similarities in clade age, species richness, and primary productivity motivate comparisons to angiosperms, whose genomes have been inordinately shaped by whole-genome duplication (WGD). WGDs have been linked to speciation, increased rates of lineage diversification, and identified as a principal driver of angiosperm evolution. We synthesized a large but scattered body of evidence that suggests polyploidy may be common in diatoms as well. METHODS: We used gene counts, gene trees, and distributions of synonymous divergence to carry out a phylogenomic analysis of WGD across a diverse set of 37 diatom species. KEY RESULTS: Several methods identified WGDs of varying age across diatoms. Determining the occurrence, exact number, and placement of events was greatly impacted by uncertainty in gene trees. WGDs inferred from synonymous divergence of paralogs varied depending on how redundancy in transcriptomes was assessed, gene families were assembled, and synonymous distances (Ks) were calculated. Our results highlighted a need for systematic evaluation of key methodological aspects of Ks-based approaches to WGD inference. Gene tree reconciliations supported allopolyploidy as the predominant mode of polyploid formation, with strong evidence for ancient allopolyploid events in the thalassiosiroid and pennate diatom clades. CONCLUSIONS: Our results suggest that WGD has played a major role in the evolution of diatom genomes. We outline challenges in reconstructing paleopolyploid events in diatoms that, together with these results, offer a framework for understanding the impact of genome duplication in a group that likely harbors substantial genomic diversity.

Hoarding and horizontal transfer led to an expanded gene and intron repertoire in the plastid genome of the diatom, Toxarium undulatum (Bacillariophyta).

Although the plastid genomes of diatoms maintain a conserved architecture and core gene set, considerable variation about this core theme exists and can be traced to several different processes. Gene duplication, pseudogenization, and loss, as well as intracellular transfer of genes to the nuclear genome, have all contributed to variation in gene content among diatom species. In addition, some noncoding sequences have highly restricted phylogenetic distributions that suggest a recent foreign origin. We sequenced the plastid genome of the marine diatom, Toxarium undulatum, and found that the genome contains three genes (chlB, chlL, and chlN) involved in light-independent chlorophyll a biosynthesis that were not previously known from diatoms. Phylogenetic and syntenic data suggest that these genes were differentially retained in this one lineage as they were repeatedly lost from most other diatoms. Unique among diatoms and other heterokont algae sequenced so far, the genome also contains a large group II intron within an otherwise intact psaA gene. Although the intron is most similar to one in the plastid-encoded psaA gene of some green algae, high sequence divergence between the diatom and green algal introns rules out recent shared ancestry. We conclude that the psaA intron was likely introduced into the plastid genome of T. undulatum, or some earlier ancestor, by horizontal transfer from an unknown donor. This genome further highlights the myriad processes driving variation in gene and intron content in the plastid genomes of diatoms, one of the world's foremost primary producers.

Phylogeny, ecology, morphological evolution, and reclassification of the diatom orders Surirellales and Rhopalodiales.

The Surirellales and Rhopalodiales are large, widespread, and morphologically diverse groups of raphid pennate diatoms (Bacillariphyta) whose raphe, a structure that facilitates active motility, opens internally into a siliceous canal. We collected 202 representatives of the lineage and sequenced genes from the nuclear, plastid, and mitochondrial genomes to infer phylogenetic relationships as a basis for comparative study of ecology and morphological evolution as well as reclassification. The lineage was ancestrally marine, and we report the first evidence for a 'stepping stone' model of marine-freshwater transitions in which freshwater invasions were preceded by adaptation to intermediate brackish habitats. Phylogenetic comparative analyses also showed that the shift from an apical (e.g., Entomoneis) to transapical major axis of development (e.g., Surirella) did not have to proceed through subcircular intermediate forms (i.e., Campylodiscus). Rather, subcircular forms evolved both within lineages with longer apical axis or longer transapical axis. We also used the inferred phylogeny as a basis for genus-level reclassification of the lineage. Campylodiscus now includes the fastuosoid members of Surirella and Campylodiscus, but excludes other marine Campylodiscus which are now classified as Coronia. Surirella includes the Surirella striatula clade, Surirella Pinnatae group, and species formerly classified as Cymatopleura. We resurrected the genus Iconella to accommodate Stenopterobia and the robustoid members of Surirella and Campylodiscus. We broadened Epithemia to include members of the paraphyletic genus Rhopalodia. Finally, we discuss the challenges of constructing a classification that best leverages available phylogenetic data, while minimizing disruption to the research community and recognizing practical considerations stemming from the slow rate of progress on systematic studies of understudied organisms.

Unique role for translation initiation factor 3 in the light color regulation of photosynthetic gene expression.

Light-harvesting antennae are critical for collecting energy from sunlight and providing it to photosynthetic reaction centers. Their abundance and composition are tightly regulated to maintain efficient photosynthesis in changing light conditions. Many cyanobacteria alter their light-harvesting antennae in response to changes in ambient light-color conditions through the process of chromatic acclimation. The control of green light induction (Cgi) pathway is a light-color-sensing system that controls the expression of photosynthetic genes during chromatic acclimation, and while some evidence suggests that it operates via transcription attenuation, the components of this pathway have not been identified. We provide evidence that translation initiation factor 3 (IF3), an essential component of the prokaryotic translation initiation machinery that binds the 30S subunit and blocks premature association with the 50S subunit, is part of the control of green light induction pathway. Light regulation of gene expression has not been previously described for any translation initiation factor. Surprisingly, deletion of the IF3-encoding gene infCa was not lethal in the filamentous cyanobacterium Fremyella diplosiphon, and its genome was found to contain a second, redundant, highly divergent infC gene which, when deleted, had no effect on photosynthetic gene expression. Either gene could complement an Escherichia coli infC mutant and thus both encode bona fide IF3s. Analysis of prokaryotic and eukaryotic genome databases established that multiple infC genes are present in the genomes of diverse groups of bacteria and land plants, most of which do not undergo chromatic acclimation. This suggests that IF3 may have repeatedly evolved important roles in the regulation of gene expression in both prokaryotes and eukaryotes.

Rapid evolution of enormous, multichromosomal genomes in flowering plant mitochondria with exceptionally high mutation rates.

Genome size and complexity vary tremendously among eukaryotic species and their organelles. Comparisons across deeply divergent eukaryotic lineages have suggested that variation in mutation rates may explain this diversity, with increased mutational burdens favoring reduced genome size and complexity. The discovery that mitochondrial mutation rates can differ by orders of magnitude among closely related angiosperm species presents a unique opportunity to test this hypothesis. We sequenced the mitochondrial genomes from two species in the angiosperm genus Silene with recent and dramatic accelerations in their mitochondrial mutation rates. Contrary to theoretical predictions, these genomes have experienced a massive proliferation of noncoding content. At 6.7 and 11.3 Mb, they are by far the largest known mitochondrial genomes, larger than most bacterial genomes and even some nuclear genomes. In contrast, two slowly evolving Silene mitochondrial genomes are smaller than average for angiosperms. Consequently, this genus captures approximately 98% of known variation in organelle genome size. The expanded genomes reveal several architectural changes, including the evolution of complex multichromosomal structures (with 59 and 128 circular-mapping chromosomes, ranging in size from 44 to 192 kb). They also exhibit a substantial reduction in recombination and gene conversion activity as measured by the relative frequency of alternative genome conformations and the level of sequence divergence between repeat copies. The evolution of mutation rate, genome size, and chromosome structure can therefore be extremely rapid and interrelated in ways not predicted by current evolutionary theories. Our results raise the hypothesis that changes in recombinational processes, including gene conversion, may be a central force driving the evolution of both mutation rate and genome structure.

Origins and recombination of the bacterial-sized multichromosomal mitochondrial genome of cucumber.

Members of the flowering plant family Cucurbitaceae harbor the largest known mitochondrial genomes. Here, we report the 1685-kb mitochondrial genome of cucumber (Cucumis sativus). We help solve a 30-year mystery about the origins of its large size by showing that it mainly reflects the proliferation of dispersed repeats, expansions of existing introns, and the acquisition of sequences from diverse sources, including the cucumber nuclear and chloroplast genomes, viruses, and bacteria. The cucumber genome has a novel structure for plant mitochondria, mapping as three entirely or largely autonomous circular chromosomes (lengths 1556, 84, and 45 kb) that vary in relative abundance over a twofold range. These properties suggest that the three chromosomes replicate independently of one another. The two smaller chromosomes are devoid of known functional genes but nonetheless contain diagnostic mitochondrial features. Paired-end sequencing conflicts reveal differences in recombination dynamics among chromosomes, for which an explanatory model is developed, as well as a large pool of low-frequency genome conformations, many of which may result from asymmetric recombination across intermediate-sized and sometimes highly divergent repeats. These findings highlight the promise of genome sequencing for elucidating the recombinational dynamics of plant mitochondrial genomes.

Transcriptomic Insights into the Life History of Bolidophytes, the Sister Lineage to Diatoms.

Diatoms are perhaps the most diverse lineage of eukaryotic algae, with their siliceous cell wall and diplontic life history often considered to have played important roles in their extraordinary diversification. The characteristic diminution of the diatom cell wall over the course of vegetative growth provides a reliable, intrinsic trigger for sexual reproduction, establishing a direct link between the evolution of their cell-wall and life-history features. It is unclear, however, whether the diplontic life cycle of diatoms represents an ancestral or derived trait. This uncertainty is based in part on our lack of understanding of the life cycle of the sister lineage to diatoms, which includes a mix of two free-living and separately classified forms: naked biflagellate unicells in the genus Bolidomonas and silicified forms in the order Parmales. These two forms might represent different life-history stages, although directly establishing such links can be difficult. We sequenced transcriptomes for Bolidomonas and two diatoms and found that ~0.1% of the coding regions in the two diploid diatoms are heterozygous, whereas Bolidomonas is virtually devoid of heterozygous alleles, consistent with expectations for a haploid genome. These results suggest that Bolidomonas is haploid and predict that parmaleans represent the diploid phase of a haplodiplontic life cycle. These data fill an important gap in our understanding of the origin of the diplontic life history of diatoms, which may represent an evolutionarily derived, adaptive feature.

The "fossilized" mitochondrial genome of Liriodendron tulipifera: ancestral gene content and order, ancestral editing sites, and extraordinarily low mutation rate.

BACKGROUND: The mitochondrial genomes of flowering plants vary greatly in size, gene content, gene order, mutation rate and level of RNA editing. However, the narrow phylogenetic breadth of available genomic data has limited our ability to reconstruct these traits in the ancestral flowering plant and, therefore, to infer subsequent patterns of evolution across angiosperms. RESULTS: We sequenced the mitochondrial genome of Liriodendron tulipifera, the first from outside the monocots or eudicots. This 553,721 bp mitochondrial genome has evolved remarkably slowly in virtually all respects, with an extraordinarily low genome-wide silent substitution rate, retention of genes frequently lost in other angiosperm lineages, and conservation of ancestral gene clusters. The mitochondrial protein genes in Liriodendron are the most heavily edited of any angiosperm characterized to date. Most of these sites are also edited in various other lineages, which allowed us to polarize losses of editing sites in other parts of the angiosperm phylogeny. Finally, we added comprehensive gene sequence data for two other magnoliids, Magnolia stellata and the more distantly related Calycanthus floridus, to measure rates of sequence evolution in Liriodendron with greater accuracy. The Magnolia genome has evolved at an even lower rate, revealing a roughly 5,000-fold range of synonymous-site divergence among angiosperms whose mitochondrial gene space has been comprehensively sequenced. CONCLUSIONS: Using Liriodendron as a guide, we estimate that the ancestral flowering plant mitochondrial genome contained 41 protein genes, 14 tRNA genes of mitochondrial origin, as many as 7 tRNA genes of chloroplast origin, >700 sites of RNA editing, and some 14 colinear gene clusters. Many of these gene clusters, genes and RNA editing sites have been variously lost in different lineages over the course of the ensuing ∽200 million years of angiosperm evolution.