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Summary: Grasses (Poaceae) comprise c. 11 800 species and are central to human livelihoods and terrestrial ecosystems. Knowing their relationships and evolutionary history is key to comparative research and crop breeding. Advances in genome‐scale sequencing allow for increased breadth and depth of phylogenomic analyses, making it possible to infer a new reference species tree of the family.We inferred a comprehensive species tree of grasses by combining new and published sequences for 331 nuclear genes from genome, transcriptome, target enrichment and shotgun data. Our 1153‐tip tree covers 79% of grass genera (including 21 genera sequenced for the first time) and all but two small tribes. We compared it to a newly inferred 910‐tip plastome tree.We recovered most of the tribes and subfamilies previously established, despite pervasive incongruence among nuclear gene trees. The early diversification of the PACMAD clade could represent a hard polytomy. Gene tree–species tree reconciliation suggests that reticulation events occurred repeatedly. Nuclear–plastome incongruence is rare, with very few cases of supported conflict.We provide a robust framework for the grass tree of life to support research on grass evolution, including modes of reticulation, and genetic diversity for sustainable agriculture. [ABSTRACT FROM AUTHOR]

In 1986 Clayton and Renvoize (Kew Bull., Addit. Ser. 13: 1-389) assembled 16 genera into the Boutelouinae, one of their four subtribes in tribe Cynodonteae (Gramineae: Chloridoideae). However, evidence we have gathered from morphology (including lemma micromorphology), leaf blade transverse anatomy, cytology, and nuclear and chloroplast DNA sequences indicates that the subtribe, and indeed the tribe, is polyphyletic. Aegopogon and Schaffnerella appear closely related to genera in tribe Eragrostideae-Pereilema and Lycurus, respectively. The affinities of Melanocenchris and Neobouteloua remain uncertain, as is the case for the clade comprised of the sister taxa Hilaria and Pleuraphis (often treated as a single genus, as by Clayton and Renvoize). The remaining 12 Boutelouinae genera form a monophyletic group, but the largest genera-Bouteloua s.s. and Chondrosium-are not monophyletic. Considering our much-improved knowledge of relationships, the features in common to the 12 genera, and the practical issue of identification, we advocate that the group be treated as a single genus (Bouteloua s.1.). The many genetic synonyms are testament to the wide range of variation in inflorescence form-including sexual dimorphism and branch persistence/ abscission-displayed by Bouteloua. The evidence shows that some architectures have evolved in parallel, notably the Chondrosium-like staminate inflorescence of species scattered in several lineages. The long and erroneous affiliation of Aegopogon with Bouteloua, especially the Cathestecum group of species, can be owed to homoplasy. Remarkable is the convergence in inflorescence form between Aegopogon and the Cathestecum group: spikelets three per branch, the lateral two reduced; branches short, pendulous, and deciduous; lemma 3-awned. Although macromorphology has generally served us well in the classification of grasses and other plants, other sources of data are necessary to detect cases of homoplasy and to ensure that the units of classification best reflect phylogeny.

Boutelouinae Stapf (Gramineae: Chloridoideae), composed of 16 genera (Clayton and Renvoize, Genera graminum: Grasses of the World, 1986) and ca. 80 species, are distributed principally in southern North America in xeric habitats. Half of the species have been regarded by most taxonomists as members of Bouteloua Lag., with two subgenera, but Clayton and Renvoize chose to treat the subgenera as genera, Bouteloua sens. str. (ca. 25 spp.) and Chondrosum Desv. (ca. 17 spp.). Noteworthy is the presence of unisexual, dimorphic spikelets in all members of 11 genera (ca. 30 spp.) and some populations of B. chondrosioides (Kunth) Benth. ex S. Watson. An ongoing molecular phylogenetic study of the subtribe was initiated to address circumscription of genera and character evolution, in particular inflorescence branch abscission (the principal character used to separate Bouteloua and Chondrosum) and sexual dimorphism. The internal transcribed spacer region (ITS) of 18S-26S nuclear ribosomal DNA has been sequenced to date for an individual in each of 37 species representing 16 genera (three outgroup). Sequences have yet to be obtained for Melanocenchris Nees (sole genus restricted to the Old World), Neobouteloua Gould (endemic to South America), and Schaffnerella Nash (one sp., endemic to Mexico, possibly extinct). The amount of ITS variation was found to be suitable for both intra- and intergeneric phylogenetic studies. Cladistic parsimony analysis of the data in hand resulted in a pattern of relationships differing in many respects from the one hypothesized by Clayton and Renvoize. Aegopogon Humb. & Bonpl. ex Willd., Hilaria Kunth, and Pleuraphis Torr. are basal with respect to the other genera. Bouteloua, sens. lat. or sens. str., is polyphyletic. Deciduous inflorescence branches and sexually dimorphic spikelets appear to have evolved independently on multiple occasions.

C4 photosynthesis is a series of anatomical and biochemical modifications to the typical C3 pathway that increases the productivity of plants in warm, sunny, and dry conditions. Despite its complexity, it evolved more than 62 times independently in flowering plants. However, C4 origins are absent from most plant lineages and clustered in others, suggesting that some characteristics increase C4 evolvability in certain phylogenetic groups. The C4 trait has evolved 22-24 times in grasses, and all origins occurred within the PACMAD clade, whereas the similarly sized BEP clade contains only C3 taxa. Here, multiple foliar anatomy traits of 157 species from both BEP and PACMAD clades are quantified and analyzed in a phylogenetic framework. Statistical modeling indicates that C4 evolvability strongly increases when the proportion of vascular bundle sheath (BS) tissue is higher than 15%, which results from a combination of short distance between BS and large BS cells. A reduction in the distance between BS occurred before the split of the BEP and PACMAD clades, but a decrease in BS cell size later occurred in BEP taxa. Therefore, when environmental changes promoted C4 evolution, suitable anatomy was present only in members of the PACMAD clade, explaining the clustering of C4 origins in this lineage. These results show that key alterations of foliar anatomy occurring in a C3 context and preceding the emergence of the C4 syndrome by millions of years facilitated the repeated evolution of one of the most successful physiological innovations in angiosperm history. [ABSTRACT FROM AUTHOR]

Only a small percentage of plant species undergo C4 photosynthesis. Despite its rarity, the C4 pathway has evolved numerous times from C3 ancestors, with as many as 18 independent origins in grasses alone. We report non-Kranz (C3) anatomy in Aristida longifolia, a species in a genus of ca. 300 species previously thought to possess only Kranz (C4) anatomy. Leaf blade transections of A. longifolia show widely spaced vascular bundles, nonradiate chlorenchyma, and few or no chloroplasts in cells of the sheaths surrounding the vascular bundle, all features indicative of C3 photosynthesis. Carbon isotope ratios range from -27.68 to -29.71%, likewise indicative of C3 photosynthesis. We also reconstruct the phylogeny of Aristidoideae, comprising Aristida, Sartidia (C3), and Stipagrostis (C4), using a sample of 11 species, including A. longifolia, and DNA sequences of the nuclear ribosomal internal transcribed spacer region and the chioroplast rpl16 intron and trnL-trnF region. Sartidia and Stipagrostis resolve as sisters, and sister to this dade is Aristida. Aristida Ion gifolia resolves as sister to the remaining species in the genus. C3 photosynthesis is hypothesized to be ancestral in Aristidoideae, which means the C4 pathway evolved twice in the subfamily-in Stipagrostis and early in the diversification of the Aristida clade. [ABSTRACT FROM AUTHOR]