Yoh Iwasa
@kyudai.jp
Department of Biology
Kyushu University
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
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Rena Hayashi, Akane Hara, and Yoh Iwasa
Elsevier BV
Kosei Matsuo, Rena Hayashi, and Yoh Iwasa
Elsevier BV
Rena Hayashi and Yoh Iwasa
Springer Science and Business Media LLC
AbstractA high mutation rate of the RNA virus results in the emergence of novel mutants that may escape the immunity activated by the original (wild-type) strain. However, many of them go extinct because of the stochasticity due to the small initial number of infected cells. In a previous paper, we studied the probability of escaping stochastic extinction when the novel mutant has a faster rate of infection and when it is resistant to a drug that suppresses the wild-type virus. In this study, we examine the effect of escaping the immune reaction of the host. Based on a continuous-time branching process with time-dependent rates, we conclude the chance for a mutant strain to be established $$p\\left(t\\right)$$ p t decreases with time $$t$$ t since the wild-type infection when the mutant is produced. The number of novel mutants that can escape extinction risk has a peak soon after the wild-type infection. The number of novel escape mutations produced per patient in the early phase of host infection is small both for very strong and very weak immune responses, and it attains its maximum value when immune activity is of an intermediate strength.
Yoh Iwasa
Wiley
We review several mathematical models and concepts in developmental biology that have been established over the last decade. [1] Feedback vertex set: Ascidian embryos contain cells of seven types, and cell fate is controlled by approximately 100 interacting genes. "Feedback vertex set" of the directed graph of gene regulatory network consists of a small number of genes. By experimentally manipulating them, we can differentiate cells into any cell type. [2] Tissue deformation: Describing morphological changes in tissues and relating them to gene expression and other cellular processes is a key in understanding morphogenesis. Expansion and anisotropy of the tissue are described by "deformation tensor" at each location. A study on chick limb bud formation revealed that both the volume growth rate and anisotropy in deformation differed significantly between locations and stages. [3] Mechanobiology: Forces operating on each cell may alter cell shape and gene expression, which may subsequently exert forces on their surroundings. Measurements of force, tissue shape, and gene expression help us understand autonomous tissue deformation. [4] Adaptive design of development: An optimal growth schedule in fluctuating environments explains the growth response to starvation in Drosophila larvae. Adaptive placement of morphogen sources makes development robust to noises. This article is protected by copyright. All rights reserved.
Yoh Iwasa, Sou Tomimoto, and Akiko Satake
Oxford University Press (OUP)
Abstract Genomic sequencing revealed that somatic mutations cause a genetic differentiation of the cells in a single tree. We studied a mathematical model for stem cell proliferation in the shoot apical meristem (SAM). We evaluated the phylogenetic distance between cells sampled from different portions of a shoot, indicating their genetic difference due to mutations accumulated during shoot elongation. The plant tissue has cell walls that suppress the exchange of location between cells. This leads to the genetic differentiation of cells according to the angle around the shoot and a larger genetic variance among cells in the body. The assumptions are as follows: stem cells in the SAM normally undergo asymmetric cell division, producing successor stem cells and differentiated cells. Occasionally, a stem cell fails to leave its successor stem cell and the vacancy is filled by the duplication of one of the nearest neighbor stem cells. A mathematical analysis revealed the following: the genetic diversity of cells sampled at the same position along the shoot increases with the distance from the base of the shoot. Stem cells hold a larger variation if they are replaced only by the nearest neighbors. The coalescent length between two cells increases not only with the difference in the position along the shoot but also in the angle around the shoot axis. The dynamics of stem cells at the SAM determine the genetic pattern of the entire shoot.
Yoh Iwasa and Rena Hayashi
Elsevier BV
Yoh Iwasa and Sachi Yamaguchi
The Royal Society
Marine animals show diverse and flexible sexual systems. Here, we review several advancements of theoretical studies made in the last decade. (i) Sex change in coral fishes is often accompanied by a long break in reproductive activity. The delay can be shortened by retaining the inactive gonad for the opposite sex. (ii) Barnacles adopt diverse sexual patterns. The game model was analysed assuming that newly settled larvae choose either growth or immediate reproduction and large individuals adjust male–female investments. (iii) Some parasitic barnacles produce larvae with sexual size dimorphism and others produce larvae with the sex determined after settlement on hosts. (iv) In some fish and many reptiles, sex is determined by the temperature experienced as eggs. The dynamics of sex hormones were studied when the enzymatic reaction rates were followed by the Arrhenius equation. The FMF pattern (male at intermediates temperature; female both at high and low temperatures) required some reactions with enhanced temperature dependence at higher temperatures. The game model provides a useful framework for understanding diverse sexual patterns if we incorporate various constraints, such as unpredictability, cost of trait change and social situations. For further developments, we need to consider constraints imposed by physiological and molecular mechanisms.
Yoh Iwasa and Sachi Yamaguchi
Springer Science and Business Media LLC
Abstract In most sex-changing fishes in coral reefs, a dominant male and multiple females form a mating group (harem). In a few species, the subordinates are simultaneous hermaphrodites that may act as sneakers. In this paper, we ask whether the subordinates in most sex changers choose to be female or whether they are forced to give up their male function to avoid eviction by the harem holder. We consider a game model in which (1) the dominant male evicts some hermaphroditic subordinates if the risk of sperm competition in regard to fertilizing eggs is high, and (2) each subordinate individual chooses its own sex allocation considering the risk of being evicted. In the evolutionarily stable state, the dominant male evicts subordinates only when the subordinates vary greatly in their reproductive resources. All the subordinate individuals are female if the summed male function of the subordinates is smaller than that of the dominant male. Otherwise, all the subordinates are hermaphrodites, and the large individuals have the same male investment but a greatly different female investment, while small individuals have a reduced male investment to avoid eviction risk. We conclude that situations in which the sex allocation of subordinates is affected by the possibility of eviction by the harem holder are rather limited Significance statement We studied the role of eviction in social evolution. In most sex-changing fishes in coral reefs, a dominant male and multiple females form a mating group. In a few species, subordinates are simultaneous hermaphrodites. We asked whether the subordinates are forced to give up their male function to avoid eviction by the harem holder. We examined a game model in which the dominant male evicts hermaphroditic subordinates with a high risk of sperm competition, and each subordinate chooses its own sex allocation considering the eviction risk. We derived mathematical conditions for when subordinates are females or hermaphrodites in the ESS. The model demonstrated that the control by the dominant over subordinate reproductive decisions is rather limited.
Rena Hayashi, Shingo Iwami, and Yoh Iwasa
Elsevier BV
Yuka Uchiyama, Yoh Iwasa, and Sachi Yamaguchi
Elsevier BV
Yoh Iwasa, Yoichi Yusa, and Sachi Yamaguchi
Elsevier BV
Yoh Iwasa and Sachi Yamaguchi
Elsevier BV
Yoh Iwasa, Akane Hara, and Shihomi Ozone
Elsevier BV
Jason Olejarz, Yoh Iwasa, Andrew H. Knoll, and Martin A. Nowak
Springer Science and Business Media LLC
AbstractThe Great Oxygenation Event (GOE), ca. 2.4 billion years ago, transformed life and environments on Earth. Its causes, however, are debated. We mathematically analyze the GOE in terms of ecological dynamics coupled with a changing Earth. Anoxygenic photosynthetic bacteria initially dominate over cyanobacteria, but their success depends on the availability of suitable electron donors that are vulnerable to oxidation. The GOE is triggered when the difference between the influxes of relevant reductants and phosphate falls below a critical value that is an increasing function of the reproductive rate of cyanobacteria. The transition can be either gradual and reversible or sudden and irreversible, depending on sources and sinks of oxygen. Increasing sources and decreasing sinks of oxygen can also trigger the GOE, but this possibility depends strongly on migration of cyanobacteria from privileged sites. Our model links ecological dynamics to planetary change, with geophysical evolution determining the relevant time scales.
Sachi Yamaguchi, Yoichi Yusa, and Yoh Iwasa
Elsevier BV
Many sea slugs of Sacoglossa (Mollusca: Heterobranchia) are sometimes called "solar-powered sea slugs" because they keep chloroplasts obtained from their food algae and receive photosynthetic products (termed kleptoplasty). Some species show life cycle dimorphism, in which a single species has some individuals with a complex life cycle (the mother produces planktotrophic larvae, which later settle in the adult habitat) and others with a simple life cycle (mothers produce benthic offspring by direct development or short-term nonfeeding larvae in which feeding planktonic stages are skipped). Life cycle dimorphism is not common among marine species. In this paper, we ask whether some aspects of the ecology of solar-powered sea slugs have promoted the evolution of life cycle dimorphism in them. We study the population dynamics of the two life-cycle types that differ in summer (one with planktonic life and the other with benthic life), but both have benthic life in other seasons. We obtain the conditions in which two types with different life cycles coexist stably or a single type generating offspring with different life cycles evolves. We conclude that the stable coexistence of two life cycles can evolve if benthic individuals in summer experience strongly density-dependent processes or if the between-year fluctuation of biomass growth in summer is very large. We discuss whether these results match the life cycles of solar-powered sea slugs with life cycle dimorphism.
Shintaro Hishida and Yoh Iwasa
Elsevier BV
We studied the spatial pattern of two microbial strains along the intestinal duct. Probiotic bacteria acidify the environment and suppress their competitors, non-probiotic bacteria. Food resources are supplied from the proximal end, and there exists a flow from the proximal end to the distal end. In the steady state, we observed three major patterns. In the "standard" pattern (ST), the abundance of probiotic bacteria was high in the proximal end, and it decreased toward the distal end; in contrast, the abundance of non-probiotic bacteria was low in the proximal end, and it increased toward the distal end. In the "proximal reversion" pattern (PR), non-probiotic bacteria were dominant and probiotic bacteria were suppressed in the proximal portion of the duct. Subsequently, the abundance values of the two competitors switched, followed by a spatial pattern similar to ST. In the "distal suppression" pattern (DS), the pattern was similar to ST in the proximal portion; however, toward the distal end, the abundance of probiotic bacteria remained at an intermediate level and suppressed the abundance of non-probiotic bacteria, resulting in a peak abundance of non-probiotic bacteria in the middle portion of the duct. We additionally discuss the nonmonotonic increase in the abundance of non-probiotic bacteria in ST and the transition of the spatial pattern from one type to another due to changes in the resource abundance in the influx.
Kazuto Kiriyama and Yoh Iwasa
Wiley
Junnosuke Horita, Yoh Iwasa, and Yuuya Tachiki
Springer Science and Business Media LLC
AbstractThe enhanced or reduced growth of juvenile masu salmon (Oncorhynchus masou masou) may result from climate changes to their environment and thus impact on the eco-evolutionary dynamics of their life-history choices. Male juveniles with status, i.e., if their body size is larger than a threshold, stay in the stream and become resident males reproducing for multiple years, while those with smaller status, i.e., their body size is below the threshold, migrate to the ocean and return to the stream one year later to reproduce only once. Since juvenile growth is suppressed by the density of resident males, the fraction of resident males may stay in equilibrium or fluctuate wildly over a 2-year period. When the threshold value evolves, the convergence stable strategy may generate either an equilibrium or large fluctuations of male residents. If environmental changes occur faster than the rate of evolutionary adaptation, the eco-evolutionary dynamics exhibit a qualitative shift in the population dynamics. We also investigated the relative assessment models, in which individual life-history choices are made based on the individual’s relative status within the juvenile population. The eco-evolutionary dynamics are very different from the absolute assessment model, demonstrating the importance of understanding the mechanisms of life history choices when predicting the impacts of climate change.
Ryo Yamaguchi, Yoh Iwasa, and Yuuya Tachiki
The Royal Society
In an archipelagic system, species diversity is maintained and determined by the balance among speciation, extinction and migration. As the number of species increases, the average population size of each species decreases, and the extinction likelihood of any given species grows. By contrast, the role of reduced population size in geographic speciation has received comparatively less research attention. Here, to study the rate of recurrent speciation, we adopted a simple multi-species two-island model and considered symmetric interspecific competition on each island. As the number of species increases on an island, the competition intensifies, and the size of the resident population decreases. By contrast, the number of migrants is likely to exhibit a weaker than proportional relationship with the size of the source population due to rare oceanic dispersal. If this is the case, as the number of species on the recipient island increases, the impact of migration strengthens and decelerates the occurrence of further speciation events. According to our analyses, the number of species can be stabilized at a finite level, even in the absence of extinction.
Sachi Yamaguchi and Yoh Iwasa
Elsevier BV
Androdioecy, the coexistence of hermaphrodites and males, is very rare in vertebrates but occurs in mangrove killifish living in ephemeral or unstable habitats. Hermaphrodites reproduce both by outcrossing with males and by selfing. Outbreeding is advantageous because of inbreeding depression, but it requires encounters with males. The advantages of a propensity for outcrossing among hermaphrodites and the production of males affect each other very strongly. To study the evolutionary coupling of these two aspects, we here analyze a simple evolutionary game for a population composed of three phenotypes: outcrossing-oriented hermaphrodites, selfing-oriented hermaphrodites, and males. Outcrossing-oriented hermaphrodites first attempt to search for males and perform outcrossing if they encounter males. If they fail to encounter males, they reproduce via selfing. Selfing-oriented hermaphrodites simply reproduce by selfing. The replicator dynamics may show bistability, in which both the androdioecious population (with outcrossing-oriented hermaphrodites and males) and the pure hermaphroditic population are locally stable. The model shows the fraction of males is either zero or relatively high (more than 25%), which is not consistent with the observed low fraction of males (less than 5%). To explain this discrepancy, we studied several models including immigration and enforced copulation. We concluded that the observed pattern can be most likely explained by a population dominated by selfing-oriented hermaphrodites receiving immigration of males.
Ryota Kobayashi, Sachi Yamaguchi, and Yoh Iwasa
Elsevier BV
Legumes produce root nodules containing symbiotic rhizobial bacteria that convert atmospheric molecular nitrogen into ammonia or related nitrogenous compounds. The host plant supplies photosynthetic products to root nodules forming a mutualistic system. Legumes have physiological mechanisms for regulating nodule production with chemical signals produced in leaves, called the autoregulation of nodulation. In this paper, we discuss the optimal number of root nodules that maximizes the performance of the host plant. Here, we study two models. In the stationary plant model, the acquired photosynthetic products minus cost and loss are used for reproduction. In the growing plant model, the excess material is invested to produce leaves, roots, and root nodules, resulting in the exponential growth of the whole plant. The analysis shows that having root nodules is beneficial to the plant for a high leaf nitrogen content, faster plant growth rate, a short leaf longevity, a low root/shoot ratio, and low soil nutrient concentration. We discuss the long-distance control of nodulation-autoregulation and dependence on the environmental conditions of terrestrial plants considering these results.
Yuki Kanayama and Yoh Iwasa
Elsevier BV
Dinosaurs are known for their large body size. Sauropod dinosaurs (Sauropodomorpha) had an especially large body size; some species reached 30 m long and 50 tons. Many hypotheses have been proposed to explain this phenomenon. In this study we examined this question using the life history theory. We constructed a simple model of life history with the following assumptions: the body size of immature individuals increases following a logistic equation. A higher quality and availability of food plants make the initial growth rate faster and the final saturating size larger. The increase in body size stops once reproduction starts. Fertility increases with adult body size and food-plant quality. Mortality due to predation is mitigated by a larger body size. We calculated the optimal body size at maturity that would maximize the lifetime reproductive success or fitness. The analysis showed that adult body size increased with food-plant quality and availability but decreased with higher mortality due to predators and other factors. This conclusion is consistent with geological studies that suggest a high quality and availability of food plants in the Mesozoic era, efficient air-sac breathing, and the lightweight bones of sauropod dinosaurs, allowing rapid growth of small individuals.
Moeko Tanaka, Sachi Yamaguchi, and Yoh Iwasa
Springer Science and Business Media LLC
Abstract Cancer is caused by the lifetime accumulation of multiple somatic deformations of the genome and epigenome. At a very low rate, mistakes occur during genomic replication (e.g., mutations or modified epigenetic marks). Long-lived species, such as elephants, are suggested to have evolved mechanisms to slow down the cancer progression. Recently, the life span of companion dogs has increased considerably than before, owing to the improvement of their environment, which has led to an increase in the fraction of companion dogs developing cancer. These findings suggest that short-term responses of cancer risk to longevity differ from long-term responses. In this study, to clarify the situation, we used a simple multi-step model for cancer. The rates of events leading to malignant cancer are assumed to be proportional to those of genomic replication error. Perfect removal of replication error requires a large cost, resulting in the evolution of a positive rate of genomic replication error. The analysis of the model revealed: that, when the environment suddenly becomes benign, the relative importance of cancer enhances, although the age-dependent cancer risk remains unchanged. However, in the long run, the genomic error rate evolves to become smaller and mitigates the cancer risk.
Junnosuke Horita, Yoh Iwasa, and Yuuya Tachiki
University of Chicago Press
Competitive interaction among individuals of a single population may result in the differentiation of two or more distinct life-history tactics. For example, although they exhibit unimodal size distribution, male juveniles of salmonids differentiate into those going down to the ocean to grow and returning to the natal stream after several years to reproduce (migratory tactic) and those staying in the stream and reproducing for multiple years (resident tactic). In this study, we developed a simple mathematical model for the positive feedback between hormonal and behavioral dynamics, with the expectation of establishing multiple discrete clusters of hormone levels leading to differentiation of life-history tactics. The assumptions were that probability of winning in fighting depends both on the body size and hormone level of the two contestants. An individual with a higher hormone level would be more likely to win the competition, which further enhanced hormone production, forming a positive feedback loop between hormone level and fighting ability. If the positive feedback was strong but not excessive, discrete clusters of hormone levels emerged from a continuous distribution. In contrast, no clear clustering structure appeared in the distribution of hormone levels if the probability of winning in fighting was controlled by the body size.
Yoh Iwasa and Sachi Yamaguchi
Springer Science and Business Media LLC
AbstractSome ant species have multiple worker castes that differ in body size; workers in one caste remain in the colony and those in the other forage outside the colony (caste polyethism). In other species, all workers engage in both tasks, but the younger workers remain in the colony and the older workers forage (age polyethism). Here, we ask which of these two is the most efficient for colony level performance when foragers suffer a higher daily mortality than workers in the colony and when the optimal worker size differs between two tasks. We studied two models: in the stationary colony model, the colony size and composition remain constant, and the amount of excess resources that can be used for producing reproductive individuals is maximized; in the growing colony model, all of the resources obtained are used for producing new workers, and the rate of the colony growth is maximized. In both models, we observed similar results: caste polyethism is more advantageous than age polyethism if the difference in mortality between the two tasks is small and the difference in the optimal size is large. In the opposite situation, the age polyethism is more advantageous.