Abstract
Semelparous annual plants flower a single time during their 1-yr life cycle, investing much of their energy into rapid reproduction. By contrast, iteroparous perennial plants flower multiple times over several years, and partition their resources between reproduction and persistence. To which extent evolutionary transitions between life-cycle strategies are internally constrained at the developmental, genetic and phylogenetic level is unknown. Here we study the evolution of life-cycle strategies in the grass subfamily Pooideae and test if transitions between them are facilitated by evolutionary precursors. We integrate ecological, life-cycle strategy and growth data in a phylogenetic framework. We investigate if growth traits are candidates for a precursor. Species in certain Pooideae clades are predisposed to evolve annuality from perenniality, potentially due to the shared inheritance of specific evolutionary precursors. Seasonal dry climates, which have been linked to annuality, were only able to select for transitions to annuality when the precursor was present. Allocation of more resources to above-ground rather than below-ground growth is a candidate for the precursor. Our findings support the hypothesis that only certain lineages can respond quickly to changing external conditions by switching their life-cycle strategy, likely due to the presence of evolutionary precursors.
Highlights
As sessile organisms, plants are unable to quickly move away from stressful environments that can adversely affect fitness
To test the hypothesis that transitions between life history strategies are enabled by one or more evolutionary precursors, we modelled evolutionary transitions between perennial and annual life-cycle strategies in a phylogeny of 388 Pooideae species, and found that transitions from perenniality to annuality were clustered nonrandomly, consistent with them being facilitated by one or more precursors
The results indicated that major increases in RGR and standardised growth rate (SGR) did not coincide with the Pooideae precursor node inferred from our reconstruction of life-cycle strategy evolution (Fig. 1)
Summary
Plants are unable to quickly move away from stressful environments that can adversely affect fitness. Iteroparous perennial (hereafter perennial) plants flower more than once and avoid limiting conditions through the induction of stress-resistant organs such as rhizomes and stolons to escape limiting conditions via a period of quiescence or to tolerate them (Palmer et al, 2017) Both life history strategies can be effective at maximising fitness in harsh environments, annuals tend to be more abundant than perennials in environments characterised by seasonally high temperatures, drought, frost and grazing (Whyte, 1977; Pettit et al, 1995; Tofts, 2004; Evans et al, 2005; Kadereit et al, 2006; Cruz-Mazo et al, 2009). The latter relationship suggests that annuals may be better adapted than perennials to maximise fitness in extreme, but variable, environments
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