Paul M. Peterson and Robert J. Soreng
The grasses are the most important plant family for food production. Despite the domestication of Oryza sativa L. (rice), Triticum aestivum L. (wheat), Zea mays L. (corn), Hordeum vulgare L.(barely), Secale cereale L. (rye), Avena sativa L. (oats), Sorghum bicolor (L.) Moench (sorghum), Saacharum officinarum L. (sugar cane), Pennisetum glaucum (L.) R. Br. (pearl millet), Panicum virgatum L. (switchgrass), Eleusine coracana (L.) Gaertn (finger millet.) and Eragrostis tef (Zucc.) Trotter (tef), the family has not been widely studied biogeographically (Bouchenak-Khelladi et al., 2010). Other notable economic uses of grasses include landscaping, construction (primarily bamboos), and biofuel production (Miscanthus x giganteus J.M. Greef & Deuter ex Hodk. & Renvoize, Panicum L., and Zea L.).
The grass family was probably characterized as a distinct entity in many cultures, and the first scientific subdivision of the family was made by Brown (1814), who recognized two different spikelet types between Panicoideae and Pooideae. A synapomorphy for the family is a one-seeded indehiscent fruit (seed coat is fused with the ovary wall), known as a caryopsis or grain (Peterson, 2013). Although in some Chloridoideae and bamboos the seed coat can be free from the pericarp and therefore, technically, is an achene. The grain is rich in endosperm starch, although it contains some protein and small quantities of fats (lipids). The embryo is located on the dorsal side of basal portion of the caryopsis and contains high levels of protein, fats, and vitamins. The ventral side is often sulcate and is marked by a hilar scar that may be basal and small or extend as far as the apex. The stems are referred to as culms and the roots are fibrous, principally adventitious arising from lower portions of the culms. Silica is a conspicuous component of the vegetative epidermis and stored in silica cells. Many grasses have rhizomes (underground stems) or stolons (horizontal above-ground branches) that allow for vegetative reproduction in perennial grasses. Another important feature of grasses is intercalary meristems, which allow growth well below the culm apex, typically near the base of the plant. The leaves are parallel-veined and two-ranked, with the basal portion forming cylindrical sheaths and the upper portions referred to as a blade. A ligule, located on the upper surface at the junction of the blade and sheath, commonly consists of flaps of tissue or hairs. The primary inflorescence is referred to as a spikelet with 1–many, two ranked bracts inserted along the floral axis or rachilla. The lowest two bracts of each spikelet, inserted opposite each other, are called glumes, above which, along the rachilla (axis), are borne paired bracts termed florets. Each floret consists of a lemma (lower bract) and palea (upper bract). Within each palea the highly reduced flowers can be found. Each grass flower usually consists of two or three small scales at the base called lodicules, an ovary with a style and usually two plumose stigmas and 1–6 but more commonly 3 stamens with basifixed anthers that contain single-pored pollen grains. Lodicules function to open the florets during flowering and represent reduced perianth (sepals and petals) segments (Yadav et al., 2007).
The possession of small pollen grains well adapted for wind aided dispersal and intercalary meristems allowing culms to resprout near the base after repeated episodes of fire and/or grazing has enabled the family to be extremely successful in planet-wide radiation and colonization, and grasses are often found in open and frequently disturbed habitats. Two major photosynthetic CO2 assimilation pathways (C3 and C4) are found in the grasses, and there are anatomical, physiological, phytogeographical, and ecological differences between these two types. C3 grasses are well adapted to temperate climates with winter precipitation, whereas C4 grasses are well suited to tropical and desert environments with summer/autumn precipitation. The addition of C4 photosynthesis has allowed the grasses to outcompete other plants in warm, tropical environments by lowering the oxidation levels (photorespiration) of photosynthetic products. These features have led to the family′s ability to occupy 31%–43% of the Earth′s surface in various climatic environments as the dominant component, the grasslands (Gibson, 2009).
The study of biogeography has blossomed into a major discipline of evolutionary biology (Funk et al., 2009; Wen et al., 2013; Wen & Wagner, 2020) and for this special issue we present the current knowledge on the derivation of major lineages that reside in the grass family (also see Welker et al., 2020). Using the fossil record to aid in dating our phylogenetic trees and combining molecular sequence studies of DNA, we reconstruct ancestral states to infer the origin of major clades, i.e., subfamilies, tribes, supersubtribes, subtribes, and selected genera in the grasses. For this special issue, we invited a broadly trained group of 35 scientists to collaborate and present their most recent advances in the biogeography and phylogenetics within the grasses.
Four of the manuscripts in this issue include new dated biogeographical analyses (Gallaher et al., 2022; Peterson et al., 2022b; Soreng et al., 2022a; Zhou et al., 2022); three comprise new phylogenetic analyses with subsequent taxonomic novelties (Da Silva et al., 2022; Peterson et al., 2022a, 2022c), one is an ecological study investigating functional traits in a clade of savanna/wetland grasses (Arthan et al., 2022), one looked at the current distribution of the grasses over the Australian continent to determine migration directionality (Bryceson & Morgan, 2022), and one is an updated classification of the family primarily based on molecular phylogenetic studies (Soreng et al., 2022).
The Poaceae began to diversify in the early–late Cretaceous (crown age of 98.54 Ma) on West Gondwana before the complete split between Africa and South America, and Africa clearly served as the center of origin for much of the early diversification of the lineages within the family (Gallaher et al., 2022). In addition, Gallaher et al. (2022) includes an extensive review of the extant diversity and distribution of species, molecular and morphological evidence supporting the current classification scheme, and a biogeographical history of most lineages. Soreng et al. (2022a) found that the ancestral area of the Alopecurinae, Avenulinae, Coleanthinae, Miliinae, Phleinae, and Poinae (formerly referred to as PPAM clade) was southwestern Asia (including the Caucasus Mountains), originating in the early Miocene (crown mean of 21.81 mya). Muhlenbergia Schreb. apparently originated 9.3 mya in the Sierra Madre (Occidental and Oriental) in Mexico before splitting into six lineages, of which one (M. subg. Muhlenbergia) via long-distance dispersal, colonized Central Asia 1.6–1 mya (Peterson et al., 2021b). Melocanninae originated in the East Himalaya to northern Myanmar in the early Miocene (crown mean of 19.68 mya), and three routes were revealed in forming its present biogeographic pattern: in situ diversification on the Asian mainland, dispersing southwest to Sri Lanka and to the Western Ghats in South India, and spreading southeast to Malesia and Oceania via the Indo-China Peninsula (Zhou et al., 2022).
In their molecular analysis Da Silva et al. (2022) present evidence to support the polyphyly of Chascolytrum Desv. s.l. (Calothecinae), dividing it into nine genera: Boldrinia L.N. Silva, Calotheca Desv., Chascolytrum, Erianthecium Parodi, Lombardochloa B. Rosengurtt & B.R.Arrill., Microbriza Parodi ex Nicora & Rúgolo, Poidium Nees, Rhombolytrum Link, and Rosengurttia L.N. Silva; and describe a new subtribe, Paramochloinae L.N.Silva & Saarela, to include two genera, Laegaardia P.M. Peterson, Soreng, Romasch. & Barberá and Paramochloa P.M. Peterson, Soreng, Romasch. & Barberá. Peterson et al. (2022a), in a molecular study of nine Eleusininae genera, found Coelachyrum Hochst. & Nees polyphyletic and Schoenefeldia Kunth to be paraphyletic, subsequently describing a new genus, Schoenefeldiella P.M. Peterson with a single species and transferring Apochiton burttii C.E. Hubb. to Coelachyrum. In another molecular study, Peterson et al. (2022c) investigated Calamagrostis Adans. (Agrostidinae), hypothesizing the phylogenetic and biogeographical history of seven major species groups and proposing a new genus, Condilorachia P.M. Peterson, Romasch. & Soreng, while subsuming Dichelachne Endl. into Pentapogon R.Br. (Echinopogoninae).
Arthan et al. (2022) investigated the evolution of functional traits of 31 species in the Heteropogon-Themeda clade and found culm height, leaf length, leaf area, awn length, and awn types separated species occupying wetland and grassland/savannas; and that two widespread species, Heteropogon contortus (L.) P. Beauv. ex Roem. & Schult. (South American) and Themeda triandra Forssk. (African) have significantly different bioclimate niches. In their biogeographical survey of Australian grasses, Bryceson & Morgan (2022) suggest Southeast Asia was the gateway for largely one-way dispersal of C4 grasses into Australasia, and that the Paniceae were the earliest arrivals, followed by the Chloridoideae and the Andropogoneae. The worldwide phylogenetic classification in this issue includes 12 subfamilies, seven supertribes, 54 tribes, five supersubtribes, 109 subtribes, and an updated list of the number of species in each of the 787 accepted genera (Soreng et al., 2022b).
We thank Jun Wen for proposing the topic and supporting this special issue, and acknowledge all collaborators for contributing their work. We wish continued success to all agrostologists in uncovering mysteries in the evolutionary history of the grasses.