Pollen structure and development in Nymphaeales: insights into character evolution in an ancient angiosperm lineage.

PREMISE OF THE STUDY: A knowledge of pollen characters in early-diverging angiosperm lineages is essential for understanding pollen evolution and the role of pollen in angiosperm diversification. In this paper, we report and synthesize data on mature pollen and pollen ontogeny from all genera of Nymphaeales within a comparative, phylogenetic context and consider pollen evolution in this early-diverging angiosperm lineage. We describe mature pollen characters for Euryale, Barclaya, and Nymphaea ondinea, taxa for which little to no structural data exist. METHODS: We studied mature pollen for all nymphaealean genera using light, scanning electron, and transmission electron microscopy. We reviewed published reports of nymphaealean pollen to provide a comprehensive discussion of pollen characters in water lilies. KEY RESULTS: Nymphaeales exhibit diversity in key pollen characters, including dispersal unit size, ornamentation, aperture morphology, and tapetum type. All Nymphaeales pollen are tectate-columellate, exhibiting one of two distinct patterns of infratectal ultrastructure-a thick infratectal space with robust columellae or a thin infratectal space with thin columellae. All genera have pollen with a lamellate endexine that becomes compressed in the proximal, but not distal wall. This endexine ultrastructure supports the operculate hypothesis for aperture origin. Nymphaeaceae pollen exhibit a membranous granular layer, which is a synapomorphy of the family. CONCLUSIONS: Variation in pollen characters indicates that significant potential for lability in pollen development was present in Nymphaeales at the time of its divergence from the rest of angiosperms. Structural and ontogenetic data are essential for interpreting pollen characters, such as infratectum and endexine ultrastructure in Nymphaeales.

Coalescent versus concatenation methods and the placement of Amborella as sister to water lilies.

The molecular era has fundamentally reshaped our knowledge of the evolution and diversification of angiosperms. One outstanding question is the phylogenetic placement of Amborella trichopoda Baill., commonly thought to represent the first lineage of extant angiosperms. Here, we leverage publicly available data and provide a broad coalescent-based species tree estimation of 45 seed plants. By incorporating 310 nuclear genes, our coalescent analyses strongly support a clade containing Amborella plus water lilies (i.e., Nymphaeales) that is sister to all other angiosperms across different nucleotide rate partitions. Our results also show that commonly applied concatenation methods produce strongly supported, but incongruent placements of Amborella: slow-evolving nucleotide sites corroborate results from coalescent analyses, whereas fast-evolving sites place Amborella alone as the first lineage of extant angiosperms. We further explored the performance of coalescent versus concatenation methods using nucleotide sequences simulated on (i) the two alternate placements of Amborella with branch lengths and substitution model parameters estimated from each of the 310 nuclear genes and (ii) three hypothetical species trees that are topologically identical except with respect to the degree of deep coalescence and branch lengths. Our results collectively suggest that the Amborella alone placement inferred using concatenation methods is likely misled by fast-evolving sites. This appears to be exacerbated by the combination of long branches in stem group angiosperms, Amborella, and Nymphaeales with the short internal branch separating Amborella and Nymphaeales. In contrast, coalescent methods appear to be more robust to elevated substitution rates.

Compared leaf anatomy of Nymphaea (Nymphaeaceae) species from Brazilian flood plain / Anatomia comparativa da folha de espécies de Nymphaea (Nymphaeaceae) da planície de inundação brasileira

Nymphaea has seven species already catalogued in the flood prone areas of the Brazilian Pantanal. However, some species remain difficult to identify and descriptions of the anatomy of vegetative organs are an important tool for infrageneric separation to aid in group taxonomy. The species collected in the Pantanal and prepared according to the usual techniques for anatomical studies showed similar structural characteristics, and data on the arrangement of vascular bundles in the midrib and petiole, as well as the form and distribution of sclereids, were consistent. Nymphaea oxypetala stands out from the other evaluated species for having a greater number of differential characters, including angular collenchyma and the absence of bicollateral bundles in the petiole. Nymphaea lingulata stands out as the only species to feature bicollateral bundles in the leaf blade. The results, summarised in the dichotomous key, facilitate the identification of species that use the flower as the main differentiation, but are in a vegetative stage.

Nymphaea tem sete espécies catalogadas nas áreas inundáveis do Pantanal brasileiro. No entanto, algumas espécies são de difícil identificação e descrições da anatomia dos órgãos vegetativos são uma ferramenta importante para a separação infragenérica para auxiliar na taxonomia do grupo. As espécies coletadas no Pantanal e preparadas de acordo com as técnicas usuais para estudos anatômicos mostraram as mesmas características estruturais, e os dados de arranjo dos feixes vasculares na nervura central e pecíolo, bem como a forma e distribuição de esclereides, foram consistentes. Nymphaea oxypetala se destaca das outras espécies avaliadas por ter um maior número de caracteres diferenciais, incluindo colênquima angular e ausência de feixes bicolaterais no pecíolo. Nymphaea lingulata se destaca como a única espécie que apresenta feixes bicolaterais no limbo. Os resultados, resumidos em uma chave dicotômica, facilitam a identificação de espécies que utilizam a flor como principal diferenciação quando se encontram em estágio vegetativo.

Insights into the dynamics of genome size and chromosome evolution in the early diverging angiosperm lineage Nymphaeales (water lilies).

Nymphaeales are the most species-rich lineage of the earliest diverging angiosperms known as the ANA grade (Amborellales, Nymphaeales, Austrobaileyales), and they have received considerable attention from morphological, physiological, and ecological perspectives. Although phylogenetic relationships between these three lineages of angiosperms are mainly well resolved, insights at the whole genome level are still limited because of a dearth of information. To address this, genome sizes and chromosome numbers in 34 taxa, comprising 28 species were estimated and analysed together with previously published data to provide an overview of genome size and chromosome diversity in Nymphaeales. Overall, genome sizes were shown to vary 10-fold and chromosome numbers and ploidy levels ranged from 2n = 2x = 18 to 2n = 16x = ∼224. Distinct patterns of genome diversity were apparent, reflecting the differential incidence of polyploidy, changes in repetitive DNA content, and chromosome rearrangements within and between genera. Using model-based approaches, ancestral genome size and basic chromosome numbers were reconstructed to provide insights into the dynamics of genome size and chromosome number evolution. Finally, by combining additional data from Amborellales and Austrobaileyales, a comprehensive overview of genome sizes and chromosome numbers in these early diverging angiosperms is presented.

Compared leaf anatomy of Nymphaea (Nymphaeaceae) species from Brazilian flood plain.

Nymphaea has seven species already catalogued in the flood prone areas of the Brazilian Pantanal. However, some species remain difficult to identify and descriptions of the anatomy of vegetative organs are an important tool for infrageneric separation to aid in group taxonomy. The species collected in the Pantanal and prepared according to the usual techniques for anatomical studies showed similar structural characteristics, and data on the arrangement of vascular bundles in the midrib and petiole, as well as the form and distribution of sclereids, were consistent. Nymphaea oxypetala stands out from the other evaluated species for having a greater number of differential characters, including angular collenchyma and the absence of bicollateral bundles in the petiole. Nymphaea lingulata stands out as the only species to feature bicollateral bundles in the leaf blade. The results, summarised in the dichotomous key, facilitate the identification of species that use the flower as the main differentiation, but are in a vegetative stage.

Hydatellaceae are water lilies with gymnospermous tendencies.

The flowering plant family Hydatellaceae was recently discovered to be allied to the ancient angiosperm lineage Nymphaeales (water lilies). Because of its critical phylogenetic position, members of the Hydatellaceae have the potential to provide insights into the origin and early diversification of angiosperms. Here I report that Hydatella expresses several rare embryological features that, in combination, are found only in members of the Nymphaeales. At maturity, the female gametophyte is four-celled, four-nucleate and will produce a diploid endosperm, as is characteristic of most early divergent angiosperm lineages. As with all members of the Nymphaeales, endosperm in Hydatella is minimally developed and perisperm is the major embryo-nourishing tissue within the seed. Remarkably, Hydatella exhibits a maternal seed-provisioning strategy that is unique among flowering plants, but common to all gymnosperms: pre-fertilization allocation of nutrients to the embryo-nourishing tissue. This exceptional case of pre-fertilization maternal provisioning of a seed in Hydatella may well be an apomorphic feature of Hydatellaceae alone but, given the newly discovered phylogenetic position of this family, potentially represents a plesiomorphic and transitional condition associated with the origin of flowering plants from gymnospermous ancestors.

The chloroplast genome of Nymphaea alba: whole-genome analyses and the problem of identifying the most basal angiosperm.

Angiosperms (flowering plants) dominate contemporary terrestrial flora with roughly 250,000 species, but their origin and early evolution are still poorly understood. In recent years, molecular evidence has accumulated suggesting a dicotyledonous origin of monocots. Phylogenetic reconstructions have suggested that several dicotyledonous groups that include taxa such as Amborella, Austrobaileya, and Nymphaea branch off as the most basal among angiosperms. This has led to the concept of monocots, "eudicots," "basal dicots," and "ANITA" groupings. Here, we present the sequence and phylogenetic analyses of the chloroplast DNA of Nymphaea alba. Phylogenetic analyses of our 14-species data set, consisting of 29,991 aligned nucleotide positions per chloroplast genome, revealed consistent support for Nymphaea being a divergent member of a monophyletic dicot assemblage. Three distinct angiosperm lineages were supported in the majority of our phylogenetic analyses-eudicots, Magnoliopsida, and monocots. However, the monocot lineage leading to the grasses was the deepest branching. Although analyses of only one individual gene alignment (out of 61) is consistent with some recently proposed hypotheses for the paraphyly of dicots, we also report observations that nine genes do not support paraphyly of dicots. Instead, they support the basal monocot-dicot split. Consistent with this finding, we also report observations suggesting that the monocot lineage leading to the grasses has the strongest phylogenetic affinity to gymnosperms. Our findings have general implications for studies of substitution model specification and analyses of concatenated genome data.