All in good time: Understanding annual and perennial strategies in plants

One of the most fundamental questions faced by all organisms is when is the best time to reproduce to maximize the survival and success of their off spring. In plants, there are two broad categories of reproductive strategies—annuals that reproduce once and die (monocarpic or semelparous) and perennials that reproduce repeatedly and cycle through vegetative and reproductive phases (used here synonymously with polycarpic or iteroparous) (Fig. 1). Not surprisingly, given the importance of reproduction to fi tness, studies on fl owering time and related life history decisions have been the subject of intense investigations from many angles. Th ese include a rich body of life history theory, detailed molecular mechanisms and genetic architecture of fl owering time in a few model species, studies of resource allocation , trade-off s, and ecological interactions between fl ower-ing plants and their biotic communities. However, these fi elds have largely developed in isolation from one another, and there are few attempts to integrate across disciplines to form a comprehensive understanding of the evolution of life history strategies and reproductive timing. Evolutionary transitions between perenniality and annuality are among the most common transitions in angiosperms and encompass decisions on the allocation to vegetative growth versus fl owering and also the timing of such decisions. Transitions in both directions can be quite frequent, suggesting considerable lability of life cycle, at least among herbaceous species (Barrett et al., 1996). However, compared with other common transitions, such as the evolution of selfi ng from outcrossing, we know very little about the selective mechanisms and genetic changes responsible for this transition. In this perspective piece, we discuss the potential to integrate current knowledge from research on geographical and ecological patterns, with studies of resource allocation and decisions about when to fl ower, with our knowledge of the molecular mechanism underlying the transition to flower. We suggest that by integrating studies across levels we can gain a general and synthetic understanding on the evolution of life history strategies. Ancestral state reconstructions using phylogenetic approaches generally have found that annuals are derived from perennial ancestors, including multiple origins of annuals within a genus Th eoretical models predict that the annual habit evolves as an adaptive response to unpredictable environments , including frequently disturbed habitats and aridity (Stearns, 1992). Empirical data generally supports this idea, with a shift to annual life histories oft en occurring in hot and dry conditions that would …