Diverse ecological functions and the convergent evolution of grass awns

Abstract

The awn of grasses is a long, conspicuous outgrowth of the floral bracts in a grass spikelet. It is known to impact agricultural yield, but we know little about its broader ecological function, nor the selective forces that lead to its evolution. Grass awns are phenotypically diverse across the extant ~12,000 species of Poaceae. Awns have been lost and gained repeatedly over evolutionary time, between and within lineages, suggesting that they could be under selection and might provide adaptive benefit in some environments. Despite the phylogenetic context, we know of no studies that have tested whether the origin of awns correlates with putative selective forces on their form and function. Presence or absence of awns is not plastic; rather, heritability is high. The awns of grasses often are suggested as adaptations for dispersal, and most experimental work has been aimed at testing this hypothesis. Proposed dispersal functions include soil burial, epizoochory, and aerial orientation. Awns may also protect the seed from drought, herbivores, or fire by helping it become buried in soil. We do not fully understand the fitness or nutrient costs of awn production, but in some species awns function in photosynthesis, providing carbon to the seed. Here we show that awns likely provide an adaptive advantage, but argue that studies on awn function have lacked critical phylogenetic information to demonstrate adaptive convergent evolution, are taxonomically biased, and often lack clear alternative hypotheses.

Multifunctionality of angiosperm floral bracts: a review

“…In Poaceae, awns can also serve multiple functions in seed dispersal. They can respond to changes in moisture and move diaspores into or across the soil, position the diaspore in the air to enhance germination upon landing, and also attach to animals (epizoochory) (reviewed by Petersen & Kellogg, 2022). It should be noted that these functions are not mutually exclusive and are not universal.…”

“…In addition to increasing the time herbivores spend handling the seeds, as is commonly observed in other physical defences (e.g. spines) (Titulaer et al ., 2018), the awns (the outgrowths of glumes or lemmas, or occasionally paleas) of grasses can also prevent post‐dispersal herbivory via propelling seeds deeply into the soil (reviewed by Petersen & Kellogg, 2022). For example, hygroscopic awns in Stipa tenacissima (Poaceae) can allow diaspores to move or actively bury themselves before ants have the chance to consume them (Schöning et al ., 2004).…”

“…Although pre‐dispersal herbivory on grains is substantial and well documented in grasses (e.g. Parsons & Munkvold, 2010; Awuni et al ., 2015), compared to post‐dispersal herbivory defence, the role of awns in preventing pre‐dispersal herbivory is rarely studied in natural populations (Petersen & Kellogg, 2022).…”

“…However, awns do not seem to guarantee a germination advantage after a fire. For instance, preceding the occurrence of fire in Australia, there was a significantly higher abundance of awned species compared to those without awns; however, following the fire, the seedling abundance of awned species experienced a substantial decline and was replaced by unawned species (Petersen & Kellogg, 2022). Therefore, investigating the relationship between the length of awns and fire intensity across a geographic gradient is an intriguing endeavour (Garnier & Dajoz, 2001).…”

“…In some cases, depending on their position in the inflorescence, subtending bracts may be called involucral bracts or spathes (basal most) or involucellar bracts (distal to primary involucre). An involucel (or glumes/lemmas/paleas) is a secondary involucre at the base of a single branch in a compound inflorescence (Beentje, 2016;Petersen & Kellogg, 2022).…”