Issue 103:3 is online now. The latest Editor’s Choice paper is “A spatially explicit model for flowering time in bamboos: long rhizomes drive the evolution of delayed flowering” by Tachiki et al. Associate Editor – Richard Shefferson – has written a commentary on the paper below.
Bamboos, sex, and the ultimate sacrifice
Nature is a randy thing. The Tree of Life has managed to produce life histories and reproductive modes so diverse that biologists studying reproductive evolution keep finding new surprises all the time. Human reproduction, with its long delay until reproductive maturity, and economic and emotional considerations determining timing, is remarkably boring in relation to the diversity of sex in nature. The commonness of homosexual behavior (Sommer and Vassey 2006), group sex, sexual cannibalism, and even the mass dumping of gametes into the environment as occurs in many marine organisms and wind-dispersed trees, boggles even the most prurient mind.
Enter the monocarpic perennial. These are plants that commonly live more than one year, and so are perennial, but only reproduce once. Following reproduction, they die, and indeed they typically use up all available resources in reproducing, which results in death. Certainly, this mode of reproduction makes these plants sound as though they belong to a cult, long since gone extinct through its misguided puritanism. However, these plants include a diversity of forms and species throughout the plant kingdom, from the famed century plant and other agaves, which can live for many decades before reproducing and dying, to some legumes and many species of bamboo, which typically live for a number of years before reproducing and dying en masse.
But why should any organism have such a bizarre life history strategy? Much of what we know about the evolution of reproductive timing comes from work inspired by Lamont Cole’s citation classic on life history evolution (Cole, 1954). That monograph attempted to explain the evolution of “annual” and “perennial” reproductive modes via a simple evolutionary model, and instead led to the realization that the increased fitness from a perennial strategy was so slight as to be negligible. Since that time, we have come to understand that all sorts of factors can influence reproductive timing, the optimal number of reproductive events, and all aspects of life history. However, models that could account for the observed reproductive timings of long-lived monocarpic perennials such as the century plant remained elusive until the marriage of game theory and demographic modeling, and in particular the development of integral project models (Childs 2003, Rees et al 1999, Rees & Rose 2002)
And this is where this issue’s highlighted paper comes in. Bamboos are truly bizarre species, and include a good number of plant species that reproduce both clonally and sexually. In some species, the combination of reproductive modes yields stands of tall, tree-like bamboos that form veritable forests of genetically different individuals. These individuals are actually all the same age, but cover different areas due to the combination of where their seeds germinated and the growth and spread of their rhizomes. And then, at some point, the entire stand produces flowers, reproduces sexually, and dies. As if this mode of synchronous sexual reproduction wasn’t interesting enough, bamboos actually exhibit a latitudinal cline in flowering intervals, with short intervals in tropical areas and increasingly large intervals moving northward into temperate areas.
Previous ideas about why bamboo reproduction is so odd include herbivore satiation (Janzen 1976), which is a hypothesis used explain masting behavior (synchronous reproduction over large areas), and well as evolution in response to fire intervals (Keeley and Bond 1999). However, models that explain observed flowering intervals have remained elusive. To tackle this problem, Tachiki et al. (2015) created a spatially explicit model to explore the evolution of flowering interval as a function of rhizome growth and seed dispersal distance. They found, among other interesting results, that increasing rhizome growth leads to delayed flowering time, while increasing seed dispersal distance does the opposite. These particular strategies seem to work evolutionarily because of their impacts on patterns of competition. For example, when seeds disperse near the mother plant, kin competition is intensified and sexual reproduction becomes less successful. This yields an advantage to plants that reproduce more clonally, and so favors a longer reproductive interval. Fascinatingly, their model actually seems to account for the noted geographic pattern in flowering interval. This work suggests that the population dynamics of the plant may actually strongly drive these patterns, in contrast to previous hypotheses involving herbivory and environmental stressors.
As someone who studies the life histories of long-lived plants fairly regularly, I must say that it was a pleasure to be a part of the process bringing this paper to the readership of Journal of Ecology. Hopefully it might inspire more interesting work on the subject, and perhaps even inspire more of our young BES members to work in East Asia, a geographic region with a great deal of unexplained biodiversity, and untapped research talent.
Associate Editor, Journal of Ecology
Childs D.Z., Rees M., Rose K.E., Grubb P.J. & Ellner S.P. (2003). Evolution of complex flowering strategies: an age- and size- structured integral projection model. Proceedings of the Royal Society of London Series B-Biological Sciences, 270, 1829-1838.Cole, L. C. (1954) The population consequences of life history phenomena. Quarterly Review of Biology, 29, 103-137.
Janzen, D. H. (1976) Why Bamboos Wait So Long to Flower. Annual Review of Ecology and Systematics, 7, 347-391.
Keeley, J. E. & Bond, W. J. (1999) Mast flowering and semelparity in bamboos: the bamboo fire cycle hypothesis. American Naturalist, 154, 383-391.
Metcalf, C. J. E., Rose, K. E., Childs, D. Z., Sheppard, A. W., Grubb, P. J. & Rees, M. (2008) Evolution of flowering decisions in a stochastic, density-dependent environment. Proceedings of the National Academy of Sciences, 105, 10466-10470.
Metcalf, J. C., Rose, K. E. & Rees, M. (2003) Evolutionary demography of monocarpic perennials. Trends in Ecology and Evolution, 18, 471-480.
Rees M., Childs D.Z., Metcalf J.C., Rose K.E., Sheppard A.W. & Grubb P.J. (2006). Seed dormancy and delayed flowering in monocarpic plants: Selective interactions in a stochastic environment. Am Nat, 168, E53-E71.
Sommer, V. & Vasey, P. L. (2006) Homosexual behaviour in animals: an evolutionary perspective. Cambridge University Press, Cambridge, United Kingdom.
Tachiki, Y., Makita, A., Suyama, Y. & Satake, A. (2015) A spatially explicit model for flowering time in bamboos: long rhizomes drive the evolution of delayed flowering. Journal of Ecology, 103, 585–593.