Speaker: Professor Akira Sasaki

Title: Adaptive Spirals in Haploid-Diploid Life Cycles: The Path to Extreme Asymmetry

Affiliation:  Research Center for Integrative Evolutionary Science, The Graduate University for Advanced Studies (SOKENDAI), Hayama, Japan

Hosted by Dieckmann Unit

Venue: L4E48

*Zoom is available:   https://oist.zoom.us/j/91636331541?pwd=Xr3cLr6t8svG9aRrB1oAbwZnYKy9KM.1 

Abstract:

While many multicellular animals have a diploid-dominated life cycle, where mitosis occurs exclusively in diploid cells and haploids are often limited to unicellular gametes, diploid dominance is not an inherent requirement of nuclear phase alternation. Some organisms, including mosses, marine macroalgae, and fungi, exhibit life cycles dominated by haploid stages. Others, such as certain brown seaweed, follow a biphasic (haploid-diploid) life cycle, where both haploid and diploid stages develop into multicellular adults. Thus, although diploid dominance is prevalent in ‘higher’ animals and plants, life cycles across species vary widely, encompassing diplontic, haplontic, and biphasic forms. Despite significant interest in these patterns, our ecological and evolutionary understanding of them remains limited. In this study, we analyze the evolution of life cycles through an evolutionary ecological framework. We explore how life-history traits—such as phase-specific mortalities or average lifespans in haploid and diploid stages—drive the evolution of life cycles. Our model suggests that, given a trade-off in mortality rates between stages, the life cycle is prone to evolve towards extreme asymmetry, resulting in haplontic or diplontic cycles, depending on initial conditions. This trend arises from what we term an “adaptation spiral”: reductions in mortality in one phase increase its evolutionary significance, intensifying selection pressures that further enhances its survivorship. Additionally, we demonstrate that biphasic life cycles can emerge when trade-offs between the phase-specific mortalities are strongly nonlinear, potentially explaining the diversity of life cycles observed in nature.

Biosketch:

Akira Sasaki joined the Department of Evolutionary Studies of Biosystems at SOKENDAI in July 2007 as a Professor of Mathematical Biology. He is working on the stochastic theory of population genetics, host-parasite coevolution, species packing theory, spatially explicit models of ecology and epidemiology, intrahost dynamics of pathogen and immune system, bet-hedging in changing environment, evolution of cooperation, evolution of mutability in fitness landscapes, restriction avoidance and the evolution of word frequency in phage genome, spatial mosaic formation in Müllerian mimicry system, epidemiology and evolution of virulence in small worlds networks, and on other problems in theoretical population biology. Dr. Sasaki received his bachelor degree in biology from Kyoto University and his master's degree and Ph.D. in mathematical biology from Kyushu University. He has worked as assistant professor and associate professor of mathematical biology at Kyushu University. He has been a visiting research fellow at North Carolina State University, the University of Utrecht, University College London Silwood Park, and Oxford University. He has been a member of the Institute for Advanced Study, Princeton, a visiting professor at Harvard University, a research associate at the International Institute for Applied Systems Analysis, Austria, and a Japan Science and Technology Agency PRESTO researcher. He is the co-editor-in-chief of the Journal of Theoretical Biology.