Yoh Iwasa is a professor emeritus at Kyushu University, Japan. He received PhD from Kyoto University (Theoretical Biophysics) in 1980. After postdoctoral studies at Stanford and Cornell, he joined the faculty of Department of Biology, Kyushu University in 1985. Dr Yoh Iwasa started his carrier in the theoretical study of ecology, evolution, and animal behavior, including the evolution of mate preference, the dynamics of tropical forests, and social-ecological coupled dynamics for ecosystem management. More recently he has also been working on biological rhythm, cancer, development, and immune system, as well as cultural/social studies. He has repeatedly found that the same mathematical and computational methods are applicable to diverse branches of biology, and similar concepts are able to give insights in different subfields of life sciences. Director, Institute of Advanced Study Kyushu University (since 2010). FHM of American Academy of Arts and Sciences (since 2006).
EDUCATION
1975 B Sc Kyoto University, Japan
1980 Ph.D Kyoto University, Japan
RESEARCH INTERESTS
mathematical biology
378
Scopus Publications
45779
Scholar Citations
84
Scholar h-index
288
Scholar i10-index
Scopus Publications
Is Thymic Involution Truly a Deterioration or an Adaptation? Yoh Iwasa, Rena Hayashi, Akane Hara, Kosei Matsuo Bulletin of Mathematical Biology, 2026 In mammals, the immune system recognizes and combats pathogens while retaining a memory of prior encounters. In the thymus, naïve T cells capable of recognizing specific antigens are generated through random gene rearrangement, ensuring a diverse immune repertoire. However, the production rate of naïve T cells declines with age, typically following an exponential or power-law function—a phenomenon known as thymic involution, which is often regarded as a deterioration of biological function (immunosenescence). In this paper, we propose a novel theory suggesting that thymic involution may represent an adaptive strategy. As individuals age, repeated exposure to diverse pathogens leads to the accumulation of memory T cells, thereby reducing the need for newly generated naïve T cells to combat infections. Moreover, naïve T cells can persist in the periphery and retain the capacity to initiate immune responses against novel antigens. Using Pontryagin’s Maximum Principle, we calculate the optimal schedule of naïve T cell production. The results show that the production rate peaks during a brief period shortly after birth, followed by an exponential decline throughout life, eventually reaching a phase in which naïve T cell production ceases. If peripheral naïve T cells decay very slowly, the optimal strategy may consist of producing all cohorts at birth, with no subsequent production.
Human papillomavirus driving cervical cancer: A mathematical model with persistent infection, cancer progression, and spontaneous remission Rena Hayashi, Akane Hara, Yoh Iwasa Journal of Theoretical Biology, 2026 Human papillomavirus (HPV), a DNA virus, causes cervical cancer, which is the most common cancer among Japanese women in their forties. Upon infection, HPV temporarily proliferates but is usually eliminated by the immune system. However, if the virus enters the nuclei of epithelial cells, it can evade immune detection and establish a persistent infection. In this state, HPV inhibits apoptosis and allows genomic mutations to accumulate. Over many years, this can lead to dysplasia, genetic abnormalities, and eventually, invasive cancer with metastasis. While many individuals with persistent HPV infections experience spontaneous remission, a small proportion develop cervical cancer. In this study, we aim to understand the sharp contrast between cervical cancer and other solid tumors (cancers of epithelial tissues). We analyze a mathematical model for stochastic transitions between infection states, where the likelihood of persistent infection is proportional to the cumulative viral load, influenced by viral dynamics, immune effectors, and immune memory. We derive formulas for total cancer incidence, mean age at diagnosis, and age variance. Key parameters were estimated from data using the MCMC method. We conclude that major characteristics of cervical cancer arise from the strong age-dependence of viral genome incorporated into the epithelial tissue - shaped by the human sexual behavior - and from the very high rate of spontaneous remission.
Branching architecture affects genetic diversity within an individual tree Sou Tomimoto, Yoh Iwasa, Akiko Satake Journal of Theoretical Biology, 2025 While a tree grows over many years, somatic mutations accumulate and form genetic variation among branches within an individual. Trees can transmit such mutations to subsequent generations, potentially enhancing the genetic diversity of the population. We study a mathematical model to understand the relationship between within-individual genetic variation and branching architecture. We generate branching architecture by repeatedly adding two new branches (main and lateral daughter branches) to each terminal branch (mother branch). The architecture is characterized by two key parameters: main-lateral ratio (ML) and daughter-mother ratio (DM). During branch elongation, somatic mutations accumulate in the stem cells of a shoot apical meristem (SAM) at the tip of each branch. In branching, all the stem cells are passed on from the mother to the main daughter branch, but only one stem cell is chosen for the lateral daughter branch. We evaluate genetic variation by Z¯, the mean genetic differences between all pairs of branches of a tree, and examine how Z¯ varies with DM and ML while keeping the total branch length constant. As a result, (1) Z¯ increases monotonically with ML; (2) Z¯ attains the maximum for an intermediate DM, when stem cells in a SAM are genetically homogeneous; (3) Z¯ decreases monotonically with DM when stem cells are heterogeneous. The effect of branching architecture varies significantly depending on the genetic heterogeneity within a SAM, which results from the behavior of stem cells during growth. Our study sheds light on the overlooked role of branching architecture in storing genetic diversity.
Age-dependence of food allergy due to decreased supply of naïve T cells Yuna Kotsubo, Akane Hara, Rena Hayashi, Yoh Iwasa Journal of Theoretical Biology, 2025 • We study the dynamics of helper T cells (Th2) and regulatory T cells (iTreg). • Both differentiate from naïve T cells, supplied at a rate that decreases with age. • Th2 cells promote allergies, while iTreg cells help reduce them. • Consequently, food allergies may develop at one age and resolve later. • Allergies to proteins resembling self components start and resolve earlier in life. Food allergies to eggs and cow’s milk are common during infancy but often undergo desensitization during childhood. To investigate the age dependence of food allergies, we develop a simple mathematical model focusing on T helper 2 cells (Th2) causing allergies and induced regulatory T cells (iTreg) suppressing them. We assume as follows: Both types of cells differentiate from naïve T cells reactive to specific food allergens, with the rate of supply from the thymus decreasing with age. Naïve T cells are activated by allergens in peripheral tissues, differentiating into both Th2 and iTreg cells. The activation rate of Th2 cells is reduced by iTreg cells. Th2 cells promote allergies while iTreg cells help mitigate them. Analyses show that food allergies may develop at one age and resolve at a later age. Negative selection in the thymus reduces the number of naïve T cells that react to proteins resembling components of the body. As a result, allergies to these substances tend to start and resolve earlier in life than those to dissimilar materials. Food allergy starting at an older age tends to have a longer duration if the rate of naïve T cell supply decreases according to a hyperbolic (instead of exponential) function of age.
Modeling Innate Immunity Causing Chronic Inflammation and Tissue Damage Kosei Matsuo, Yoh Iwasa Bulletin of Mathematical Biology, 2025 Mathematical models of immune responses have traditionally focused on adaptive immunity and pathogen-immune dynamics. However, recent advances in immunology have highlighted the critical role of innate immunity. In response to physical damage or pathogen attacks, innate immune cells circulating throughout the body rapidly migrate from blood vessels and accumulate at the site of injury, triggering inflammation. These cells engulf, break down, and eliminate pathogens. This innate immune response occurs much faster than adaptive immune responses, which require time for cell activation and proliferation. While inflammation helps eliminate pathogens, it can sometimes lead to chronic inflammation by triggering excessive immune responses, ultimately causing tissue damage. In this study, we examine a simple dynamical model of innate immunity. The analysis indicates that when an infection occurs, it triggers inflammation, which activates the innate immune system and initiates the activation cycle. Consequently, pathogens may be eradicated, leaving behind persistent chronic inflammation. Alternatively, the pathogens may not be eradicated, with their abundance either stabilizing at a positive level or oscillating indefinitely. The dynamics exhibit both transcritical and Hopf bifurcations. When innate immunity is activated in the absence of inflammation, pathogens are eradicated more easily, and the likelihood of oscillations in inflammation, immune responses, and pathogen abundance is reduced.
Optimal seasonal schedule for producing biogenic volatile organic compounds for tree defense Yoh Iwasa, Rena Hayashi, Akiko Satake Journal of Theoretical Biology, 2025 • Biogenic volatile organic compounds (BVOCs) protect leaves from various threats. • The seasonal production schedule optimal for the tree was analyzed with PMP. • BVOC production is advantageous for slow decay rate and fast photosynthetic rate. • More BVOC is produced for slower decay rate and faster photosynthetic rate. • BVOC production does not increase monotonically with the threat level. • BVOCs are produced in season earlier than the peak period of the threat. The leaves of many trees emit biogenic volatile organic compounds (BVOCs) that protect them from various threats, including herbivory, pathogens, and heat stress. In a previous study, we analyzed the optimal seasonal schedule for producing isoprene, a highly volatile BVOC, in leaves to mitigate heat damage and maximize net carbon gain. In this paper, we investigate the seasonal production schedule of BVOCs stored in leaves, such as monoterpenes and sesquiterpenes, which decay slowly. When the leaves are bitten, these chemicals are emitted and help to prevent further herbivory. The optimal seasonal schedule, analyzed using Pontryagin’s maximum principle, includes a period of singular control. Producing BVOCs for defense is advantageous if their decay rate is slow and the photosynthetic rate is fast. The amount of BVOCs produced increases with slower decay rate and faster photosynthetic rate. But it does not increase monotonically with the magnitude of the threat. BVOCs are produced earlier than the peak period of the threat for which the chemicals are intended. Based on the results of the model, we discuss the reported variations in BVOC production among different chemical species and tree species, as well as the seasonal patterns of gene expression in different pathways for BVOC production.
Genetic diversity within a tree and alternative indexes for different evolutionary effects Yoh Iwasa, Sou Tomimoto, Akiko Satake Quantitative Plant Biology, 2024 Trees, living for centuries, accumulate somatic mutations in their growing trunks and branches, causing genetic divergence within a single tree. Stem cell lineages in a shoot apical meristem accumulate mutations independently and diverge from each other. In plants, somatic mutations can alter the genetic composition of reproductive organs and gametes, impacting future generations. To evaluate the genetic variation among a tree’s reproductive organs, we consider three indexes: mean pairwise phylogenetic distance ( $\\overline{D}$ ), phylogenetic diversity ( $PD$ ; sum of branch lengths in molecular phylogeny) and parent-offspring phylogenetic distance ( ${D}_{PO}$ ). The tissue architecture of trees facilitated the accumulation of somatic mutations, which have various evolutionary effects, including enhancing fitness under strong sib competition and intense host-pathogen interactions, efficiently eliminating deleterious mutations through epistasis and increasing genetic variance in the population. Choosing appropriate indexes for the genetic diversity of somatic mutations depends on the specific aspect of evolutionary influence being assessed.
Optimal seasonal schedule for the production of isoprene, a highly volatile biogenic VOC Yoh Iwasa, Rena Hayashi, Akiko Satake Scientific Reports, 2024 The leaves of many trees emit volatile organic compounds (abbreviated as BVOCs), which protect them from various damages, such as herbivory, pathogens, and heat stress. For example, isoprene is highly volatile and is known to enhance the resistance to heat stress. In this study, we analyze the optimal seasonal schedule for producing isoprene in leaves to mitigate damage. We assume that photosynthetic rate, heat stress, and the stress-suppressing effect of isoprene may vary throughout the season. We seek the seasonal schedule of isoprene production that maximizes the total net photosynthesis using Pontryagin’s maximum principle. The isoprene production rate is determined by the changing balance between the cost and benefit of enhanced leaf protection over time. If heat stress peaks in midsummer, isoprene production can reach its highest levels during the summer. However, if a large portion of leaves is lost due to heat stress in a short period, the optimal schedule involves peaking isoprene production after the peak of heat stress. Both high photosynthetic rate and high isoprene volatility in midsummer make the peak of isoprene production in spring. These results can be clearly understood by distinguishing immediate impacts and the impacts of future expectations.
Human movement avoidance decisions during Coronavirus disease 2019 in Japan Ryosuke Omori, Koichi Ito, Shunsuke Kanemitsu, Ryusuke Kimura, Yoh Iwasa Journal of Theoretical Biology, 2024 Understanding host behavioral change in response to epidemics is important to forecast the disease dynamics. To predict the behavioral change relevant to the epidemic situation (e.g., the number of reported cases), we need to know the epidemic situation at the moment of decision, which is difficult to identify from the records of actually performed human mobility. In this study, the largest travel accommodation reservation data covering half of the existed accommodations in Japan was analyzed to observe decision-making timings and how it responded to the changing epidemic situation during Japan's Coronavirus Disease 2019 until February 2023. To this end, we measured mobility avoidance index proposed in Ito et al., 2022 to indicate people's decision of mobility avoidance and quantified it using the time-series of the accommodation booking/cancellation data. We observed matches of the peak dates of the mobility avoidance and the number of reported cases, and mobility avoidance changed proportional to the logarithmic number of reported cases. We also found that the slope of mobility avoidance against the change of the logarithmic number of reported cases were similar among the epidemic waves, while the intercept of that was much reduced as the first epidemic wave passed by. People measure the intensity of epidemic by logarithm of the number of reported cases. The sensitivity of their response is established during the first wave and the people's response became weakened after the first experience, as if the number of reported cases were multiplied by a constant small factor.
Advantages of sexual reproduction resulting from sibling diversity: Effects of selection intensity, environmental variance, and reduced genetic diversity Evolutionary Ecology Research, 2017
Evolution of the density response of juvenile hormone concentration in the determination of wing type Evolutionary Ecology Research, 2017
The evolution of altruism by costly punishment in lattice-structured populations: Score-dependent viability versus score-dependent fertility Evolutionary Ecology Research, 2005
Dynamics of chronic myeloid leukaemia Franziska Michor, Timothy P. Hughes, Yoh Iwasa, Susan Branford, Neil P. Shah, Charles L. Sawyers, Martin A. Nowak Nature, 2005
Erratum: Optimal growth pattern of defensive organs: The diversity of shell growth among mollusks (The American Naturalist 165 (238-249)) American Naturalist, 2005
The linear process of somatic evolution Martin A. Nowak, Franziska Michor, Yoh Iwasa Proceedings of the National Academy of Sciences of the United States of America, 2003
Stochastic elimination of cancer cells Franziska Michor, Martin A. Nowak, Steven A. Frank, Yoh Iwasa Proceedings of the Royal Society B Biological Sciences, 2003
Mathematical simulation of plane and multi-layer flows in a large and deep lake. In IAHR Preprint Proc 21st Congress Volume 1 Theme A Subsurface Flow Pollutant Transport and Salinity, 1985
Is Thymic Involution Truly a Deterioration or an Adaptation? Y Iwasa, R Hayashi, A Hara, K Matsuo Bulletin of Mathematical Biology 88 (2), 28 , 2026 2026
Human papillomavirus driving cervical cancer: A mathematical model with persistent infection, cancer progression, and spontaneous remission R Hayashi, A Hara, Y Iwasa Journal of Theoretical Biology 617 (112289), doi.org/10.1016/j.jtbi.2025.112289 , 2026 2026 Citations: 2
Seasonal scheduling of chemical defense in trees: An optimal-control approach: Y. Iwasa et al. Y Iwasa, R Hayashi, A Satake Japan Journal of Industrial and Applied Mathematics 42 (4), 1291-1307 , 2025 2025
Branching architecture affects genetic diversity within an individual tree S Tomimoto, Y Iwasa, A Satake Journal of Theoretical Biology 605 (112093), doi org/10.1016/j.jtbi.2025.112093 , 2025 2025 Citations: 5
Age-dependence of food allergy due to decreased supply of naive T cells Y Kotsubo, A Hara, R Hayashi, Y Iwasa Journal of Theoretical Biology 602 (112060), doi.org/10.1016/j.jtbi.2025.112060 , 2025 2025 Citations: 3
Optimal seasonal schedule for producing biogenic volatile organic compounds for tree defense Y Iwasa, R Hayashi, A Satake Journal of Theoretical Biology 596 (111986), doi.org/10.1016/j.jtbi.2024.111986 , 2025 2025 Citations: 4
Modeling innate immunity causing chronic inflammation and tissue damage K Matsuo, Y Iwasa Bulletin of Mathematical Biology 87 (art 34), doi.org/10.1007/s11538-024-01410-0 , 2025 2025 Citations: 5
Thymic involution as an adaptive schedule for combating diverse pathogens Y Iwasa, K Matsuo BioRxiv, 2024.12. 21.629868 , 2024 2024 Citations: 1
Genetic diversity within a tree and alternative indexes for different evolutionary effects Y Iwasa, S Tomimoto, A Satake Quantitative Plant Biology 5 (e11, 1-9.), https://dx.doi.org/10.1017/qpb.2024.9 , 2024 2024 Citations: 5
Optimal seasonal schedule for the production of isoprene, a highly volatile biogenic VOC Y Iwasa, R Hayashi, A Satake Scientific Reports 14 (12311), doi.org/10.1038/s41598-024-62975-3 , 2024 2024 Citations: 8
Human movement avoidance decisions during coronavirus disease 2019 in Japan R Omori, K Ito, S Kanemitsu, R Kimura, Y Iwasa Journal of Theoretical Biology 585, 111795 , 2024 2024 Citations: 3
Viral rebound occurrence immediately after drug discontinuation involving neither drug resistance nor latent reservoir R Hayashi, A Hara, Y Iwasa Journal of Theoretical Biology 582, 111767 , 2024 2024 Citations: 4
Multiple colonies of cancer involved in mutual suppression with the immune system K Matsuo, R Hayashi, Y Iwasa Journal of Theoretical Biology 572, 111577 , 2023 2023 Citations: 8
Temporal pattern of the emergence of a mutant virus escaping cross-immunity and stochastic extinction within a host. R Hayashi, Y Iwasa Bulletin of Mathematical Biology 85, 81 (doi: 10.1007/s11538-023-01184) , 2023 2023 Citations: 1
Mathematical modeling for developmental processes Y Iwasa Development Growth and Differentiation 2023, 1-10. doi: 10.1111/dgd.12856 , 2023 2023 Citations: 2
The genetic structure within a single tree is determined by the behavior of the stem cells in the meristem Y Iwasa, S Tomimoto, A Satake Genetics 223, iyad020 doi:10.1093/genetics/iyad020 , 2023 2023 Citations: 15
Waves of infection emerging from coupled social and epidemiological dynamics Y Iwasa, R Hayashi Journal of Theoretical Biology 558, 111366 , 2023 2023 Citations: 7
Theoretical studies of diverse sexual patterns in marine animals Y Iwasa, S Yamaguchi Proceedings of the Royal Society B: Biological Sciences 290 (1990) , 2023 2023 Citations: 1
On the role of eviction in group living sex changers Y Iwasa, S Yamaguchi Behavioral Ecology and Sociobiology 76 (4), 49 , 2022 2022 Citations: 3
Escaping stochastic extinction of mutant virus: temporal pattern of emergence of drug resistance within a host R Hayashi, S Iwami, Y Iwasa Journal of Theoretical Biology 537, 111029 , 2022 2022 Citations: 3
MOST CITED SCHOLAR PUBLICATIONS
Sexual selection M Andersson, Y Iwasa Trends in ecology & evolution 11 (2), 53-58 , 1996 1996 Citations: 17471
Influence of nonlinear incidence rates upon the behavior of SIRS epidemiological models W Liu, SA Levin, Y Iwasa Journal of mathematical biology 23 (2), 187-204 , 1986 1986 Citations: 1161
Dynamics of chronic myeloid leukaemia F Michor, TP Hughes, Y Iwasa, S Branford, NP Shah, CL Sawyers, ... Nature 435 (7046), 1267-1270 , 2005 2005 Citations: 1026
The evolution of costly mate preferences II. The “handicap” principle Y Iwasa, A Pomiankowski, S Nee Evolution 45 (6), 1431-1442 , 1991 1991 Citations: 979
Demographic theory for an open marine population with space‐limited recruitment J Roughgarden, Y Iwasa, C Baxter Ecology 66 (1), 54-67 , 1985 1985 Citations: 707
Prey distribution as a factor determining the choice of optimal foraging strategy Y Iwasa, M Higashi, N Yamamura The American Naturalist 117 (5), 710-723 , 1981 1981 Citations: 700
Dynamics of cancer progression F Michor, Y Iwasa, MA Nowak Nature reviews cancer 4 (3), 197-205 , 2004 2004 Citations: 679
How should we define goodness?—reputation dynamics in indirect reciprocity H Ohtsuki, Y Iwasa Journal of theoretical biology 231 (1), 107-120 , 2004 2004 Citations: 617
The leading eight: social norms that can maintain cooperation by indirect reciprocity H Ohtsuki, Y Iwasa Journal of theoretical biology 239 (4), 435-444 , 2006 2006 Citations: 612
The evolution of costly mate preferences I. Fisher and biased mutation A Pomiankowski, Y Iwasa, S Nee Evolution 45 (6), 1422-1430 , 1991 1991 Citations: 517
Indirect reciprocity provides only a narrow margin of efficiency for costly punishment H Ohtsuki, Y Iwasa, MA Nowak Nature 457 (7225), 79-82 , 2009 2009 Citations: 430
The evolution of cooperation in a lattice-structured population M Nakamaru, H Matsuda, Y Iwasa Journal of theoretical Biology 184 (1), 65-81 , 1997 1997 Citations: 401
Onymity promotes cooperation in social dilemma experiments Z Wang, M Jusup, RW Wang, L Shi, Y Iwasa, Y Moreno, J Kurths Science advances 3 (3), e1601444 , 2017 2017 Citations: 386
Theory of oviposition strategy of parasitoids. I. Effect of mortality and limited egg number Y Iwasa, Y Suzuki, H Matsuda Theoretical population biology 26 (2), 205-227 , 1984 1984 Citations: 364
The evolution of mate preferences for multiple sexual ornaments Y Iwasa, A Pomiankowski Evolution 48 (3), 853-867 , 1994 1994 Citations: 363
Shoot/root balance of plants: optimal growth of a system with many vegetative organs Y Iwasa, J Roughgarden Theoretical population biology 25 (1), 78-105 , 1984 1984 Citations: 341
Exploiting a cognitive bias promotes cooperation in social dilemma experiments Z Wang, M Jusup, L Shi, JH Lee, Y Iwasa, S Boccaletti Nature communications 9 (1), 2954 , 2018 2018 Citations: 339
Aggregation in model ecosystems. I. Perfect aggregation Y Iwasa, V Andreasen, S Levin Ecological Modelling 37 (3-4), 287-302 , 1987 1987 Citations: 323
Evolution of resistance during clonal expansion Y Iwasa, MA Nowak, F Michor Genetics 172 (4), 2557-2566 , 2006 2006 Citations: 306
Stochastic tunnels in evolutionary dynamics Y Iwasa, F Michor, MA Nowak Genetics 166 (3), 1571-1579 , 2004 2004 Citations: 306
