領域 名 環境生態機能学
教員氏名 安井 行雄
研究分野 進化生態学
研究キーワード：進化，適応，性選択，配偶行動
最近の研究課題
１．生物の適応進化を解明する
生物はそれぞれの生息環境に適応した性質を持っている。生物の個体群には遺伝的な変異があり、生息環境に適した形質をコードする遺伝子は世代を経るごとに集団中に増えていく。これがチャールズ・ダーウィンによって提唱された自然選択である。安井研究室では昆虫をはじめとする小型の節足動物を材料として、生物の適応戦略を解き明かすことを目指している。昆虫の色や形は彼らの生存繁殖の上でどのような機能を持つのかその美しさの秘密を探るとともに、世界の昆虫の多様性や里山の環境問題にも関心を持っている。
２．雌の多回交尾の進化に関する研究
大きくて栄養豊富な卵に比べて精子は小さくほとんど栄養を含まないので大量に生産することができる。そのため雄は精子数に余裕があり、多くの雌と交尾すればするほど子供の数（適応度）を増やすことができるが、雌は多数の雄と交尾しても子供の父親が入れ替わるだけで子供の数自体を増やすことはできない。このことから雄は無差別に多くの雌と交尾するように、一方雌は少数の高品質の雄を選んで大事に子供を育てるように進化していると考えられる。しかし多くの動物ではこの予測に反して、雌は複数の雄と交尾（多回交尾）をしている。雌の多回交尾の進化の探究は単に動物の適応戦略の解明にとどまらない。ヒトの男女にも国籍や民族によらずこのような基本的な性質があるので、それは文化や宗教によって後天的に形成されたものではなく人間になる以前の動物の段階で生じ受け継がれたものだと考えることができる。雌の多回交尾は人間性の起源にもせまる大きな謎として世界の研究者を魅了してきた。Yasui (1997)が提唱したGood sperm仮説（雌が多回交尾を通じて雄間の精子競争を誘発し、競争能力の高い優秀な遺伝子を子供に受け継がせる）とYasui and Garcia-Gonzalez (2016)により再構築された多回交尾bet-hedging仮説（複数の雄と交尾することで雄原因の交尾失敗や不妊のリスクを分散する）は、雌の多回交尾の進化を説明する基礎理論として世界に知られている。
安井研究室ではフタホシコオロギなどの昆虫を用いてこの問題を研究している。雌が多回交尾したときにどの雄が受精するかを視覚的に判定できる白眼突然変異系統(黒眼野生型系統に対して潜性遺伝)を利用して、これらの仮説を検証中である。2019年度の課題研究に基づくYasui and Yamamoto (2021)は日本動物行動学会誌Journal of Ethology Editor’s Choice 2021に選定された。紹介動画をご覧ください。卒論でもこのレベルの研究ができます。
３．進化的bet-hedging理論の研究
生物は予測できない変動環境で生きている。例えば一年生植物は翌年の生育環境、暑い夏になるかそれとも冷夏か、を知らずに種子を生産しなければならないし、種子も雨が降るかどうか遅霜が下りないかどうかを知らずに発芽しなければならない。蝶はその葉が明日牛に食われるかどうか知ることなく産卵しなければならない。このような不確実な状況では、リスクを回避または分散するbet-hedging（両賭け）戦略は長期的な持続可能性を達成する上で有効である（長期存続の基礎理論でありSDGｓとも深く関連する）。例えば猛暑と冷夏の中間温度に適した子供を産んで、どちらの環境になっても（最適ではないが）そこそこやっていけるようにする「保守的な両賭け」戦略や寄主植物の異なる株に分散して産卵することでどれかは食われてもどれかは生き残らせる「多様化した両賭け」戦略などが進化する。私は雌の多回交尾研究の過程でbet-hedging理論に出合い、長年この理論に取り組むうちに交尾以外の生活史戦略へと興味を拡大した結果、ついに生物学の根幹原理に到達した。難解な数式を使わずに概念を再構築したライフワーク論文1および全訳1をお読みください。この論文は日本生態学会Ecological Research誌のTop Cited Article 2022-2023に選定され出版元Wiley社から表彰を受けた。結論は、全ての生物はbet-hedgingを行っており、それゆえbet-hedgingは生物学における普遍万有の法則であるというものである。生命の根本原理を私と一緒に考えてみませんか。
４．有性生殖の進化理論
全ての高等生物は雄と雌の間で有性生殖（異型配偶子生殖anisogamy）を行う。なぜ性が必要なのかそして生物学的な性別(交配して子供ができる組み合わせ)はなぜ雄と雌の２つだけなのか。（こんな根源的なことを考えたことがありますか。当たり前のように見えることに疑問を持つ者が科学を進展させるのです。）異型配偶子生殖は減数分裂を経るため子供にゲノムを半分しか伝えられず、子供の半数は自ら子を産まない雄になるため、無性生殖（産雌性単為生殖thelytoky）に比べて半分の増殖率しか持たない。この「性の2倍のコスト」にもかかわらず有性生殖が多数派を占めていることは進化生物学最大の謎とされる。古代ギリシアの哲人アリストテレス（BC325頃）も『動物発生論』のなかで性別の存在意義について論じておりこれは人類永遠のテーマである。20世紀以降John Maynard-Smith, Geoffrey A. Parker, William D. Hamilton等数々の天才たちがこの「性の2倍のコスト」問題に挑んだが説明は不十分である。無性生殖は遺伝的多様性が乏しいため環境変動、特に病原体や寄生者に対抗できずに滅ぶ（「赤の女王」仮説）というのが現在の定説だが、コストは世代あたり2倍なので10世代後には個体数1024倍の差となり、無性集団が長期的に滅びる運命にあるとしても、その前に有性集団は短期的競争に負けて絶滅することになる（すなわち「赤の女王」でも遅すぎるのである）。私はもう一つのライフワークとして長年この問題を考究し、ついに答えを出した。研究は二編の論文にまとめられ、無性生殖から同型配偶子生殖isogamyへの進化（最初の有性個体はどうやって配偶者を得たのか）、同型配偶子生殖から異型配偶子生殖への進化（接合子を栄養不足で死なせることなく、なぜ雄が配偶子サイズを減らすことが可能だったのか）を扱った第一論文（論文2および全訳2をお読みください）は世界を驚かせJournal of Ethology Editor’s Choice Award 2022（論文賞）を受賞および同誌の歴代最多ダウンロード数を達成したばかりでなくSpringer-Nature社の“Research Highlights 2022 – Evolutionary Biology”に選定された。同社の、Natureを筆頭とする学術雑誌は数百に上り、その年に公表された進化生物学の論文は1万編を超える中で最もインパクトのある論文9編中に入った。10世代で1024倍有利な無性生殖の侵入を有性生殖がいかに食い止めることができたかを扱う第二論文を世に出すべく現在奮闘中です。香川にいても世界トップレベルで戦うことができる。雄と雌、すなわち男と女の存在理由について私と一緒に考えてみませんか。
昆虫と蝶を愛する人へ
大屋崇・安井行雄(2025, Feb., 16)『トリバネアゲハデジタル大図鑑』無償公開中。全世界が絶賛。
代表的な研究業績
Yasui, Y. 1997. A “good-sperm” model can explain the evolution of costly multiple mating by females. The American
Naturalist 149: 573-584. http://www.jstor.org/stable/2463384?seq=1#page_scan_tab_contents
Yasui, Y. 1998. The ‘genetic benefits’ of female multiple mating reconsidered. Trends in Ecology and Evolution 13:
246-250. http://www.sciencedirect.com/science/article/pii/S0169534798013834
Yasui, Y. 2001. Female multiple mating as a genetic bet-hedging strategy when mate choice criteria are unreliable.
Ecological Research 16: 605-616. http://onlinelibrary.wiley.com/doi/10.1046/j.1440-1703.2001.00423.x/full
Garcia-Gonzalez, F., Yasui, Y. and Evans, J. P. 2015. Mating portfolios: bet-hedging, sexual selection and female
multiple mating. Proceedings of the Royal Society of London. Series B. Biological Sciences. vol.282
http://dx.doi.org/10.1098/rspb.2014.1525
Yasui, Y. and Garcia-Gonzalez, F. 2016. Bet-hedging as a mechanism for the evolution of polyandry, revisited.
Evolution 70: 385-397. http://onlinelibrary.wiley.com/doi/10.1111/evo.12847/abstract
Yasui, Y. and Yoshimura, J. 2018. Bet-hedging against male-caused reproductive failures may explain ubiquitous cuckoldry
in female birds. Journal of Theoretical Biology, 437: 214-221, https://doi.org/10.1016/j.jtbi.2017.10.029.
Yasui, Y. and Yamamoto, Y. 2021. An empirical test of bet-hedging polyandry hypothesis in the field cricket Gryllus
bimaculatus. Journal of Ethology 39: 329-342. https://doi.org/10.1007/s10164-021-00707-0
同誌のEditor’s Choice 2021に選定され紹介動画が作られました。
Yasui, Y. 2022. Life-history traits of the fairy shrimp Branchinella kugenumaensis are highly variable between
neighboring rice paddies in Japan. Ecological Research. 37: 344-354. https://doi.org/10.1111/1440-1703.12296
Yasui, Y. 2022. Evolutionary bet-hedging reconsidered: What is the mean–variance trade-off of fitness?
Ecological Research, 37: 406-420.https://doi.org/10.1111/1440-1703.12303
Yasui, Y. and Hasegawa, E. 2022. The origination events of gametic sexual reproduction and anisogamy.
Journal of Ethology. 40: 273-284. https://doi.org/10.1007/s10164-022-00760-3
同誌のEditor’s Choice Award 2022（論文賞）を受賞（紹介動画も作られる予定）。
Springer-Nature 2022 Research Highlights – Evolutionary Biologyに選定。https://www.ag.kagawa-u.ac.jp/?p=31001
Yasui, Y. 2023. Mite dilemma: molting to acquire sexual maturity or not molting to ensure durability
and dispersal ability in Phorytocarpais fimetorum (Parasitiformes; Gamasida; Parasitidae).
Journal of Ethology. 41:177–184. https://doi.org/10.1007/s10164-023-00783-4
同誌のEditor’s Choice 2023に選定（紹介動画も作られる予定）。
Yamamoto, Y. and Yasui, Y. 2024. Polyandry works as bet-hedging in the field cricket Gryllus bimaculatus,
even after eliminating females in poor condition that cannot accept remating.
Journal of Ethology, 42.1: 61-69. DOI: 10.1007/s10164-023-00803-3
Research Area: Environmental Science
Research Specialization: Evolutionary Ecology
Name: YASUI, Yukio
Keywords: evolution, adaptation, sexual selection, mating behavior
Recent Researches
1. Adaptive significance of insect characters
Organisms have evolved many adaptive characters to fit their environments. Genetic variation exists in any characters in species population and the genes coding adaptive characters will increase in frequency in the population through generations. This is natural selection, the evolutionary process advanced by Charles Darwin (1859). In Yasui Laboratory, we study adaptive strategies of insects and other arthropods. In particular, we are exploring adaptive significance and evolutionary function of insect coloration. Why are butterflies so beautiful? – It cannot be easily explained unlike bird ornaments because female butterflies do not choose mates based on male ornaments. We are also interested in the conservation of world insect faunas and biodiversity of Japanese “Satoyama” environments.
2. The evolution of female multiple mating or polyandry
Compared to the large, nutrient-rich eggs, sperm are small and contain little nutrition and can be produced in large quantities. Therefore, males can increase the number of offspring (fitness) by mating with a large number of females, but females cannot increase the number of offspring themselves by mating with many males, only changing the fathers of the offspring. This suggests that males have evolved to mate indiscriminately with many females, while females select a few high-quality males to carefully rear their offspring. In many animals, however, this prediction is not true and females mate with multiple males (polyandry). The exploration of the evolution of female multiple mating is not limited to the elucidation of an animal’s adaptive strategy. Since human males and females, regardless of nationality or ethnicity, have this basic trait, it can be assumed that it was not acquired through culture or religion, but arose and was inherited during the pre-human animal stage. Female polyandry has fascinated researchers around the world as a great mystery that also implicates the origin of human sexuality. The good sperm hypothesis proposed by Yasui (1997) (females induce sperm competition among males through multiple mating and pass on superior genes with high competitive ability to their offspring) and the multiple-mating bet-hedging hypothesis reformulated by Yasui and Garcia-Gonzalez (2016) (i.e., mating with multiple males to spread the risk of male-caused mating failure and infertility) is known worldwide as the basic theory explaining the evolution of multiple mating in females.
Yasui laboratory is studying this issue using insects such as the Field crickets Gryllus bimaculatus. They are testing these hypotheses using white-eyed mutant strains (inherited recessively relative to black-eyed wild-type strains) that can visually determine which males are fertilized when females mate multiple times. Yasui and Yamamoto (2021), based on the graduation thesis of Yamamoto in 2019, was selected as the Editor’s Choice 2021 in Journal of Ethology published from the Japan Ethological Society and Springer-Nature. Please see the introductory video. You can do this level of research for your thesis.
3. Theoretical studies of evolutionary bet-hedging
Organisms live in unpredictable and variable environments. For example, annual plants must produce seeds without knowing whether the next summer will be hot or cool, and seeds must germinate without knowing whether there will be rain or a late frost. Butterflies must lay eggs without knowing whether the leaves will be eaten by cows tomorrow. In such uncertain situations, bet-hedging strategies that avoid or spread risk can be effective in achieving long-term sustainability (a basic theory of long-term survival and closely related to the SDGs). For example, “conservative bet-hedging” strategies evolve, such as producing offspring suited to intermediate temperatures between extreme heat and cool summer so that they can do reasonably well (though not optimally) in either environment, or “diversified bet-hedging” strategies that allow some to survive while others are eaten by dispersing their eggs among different stocks of host plants. I came across the bet-hedging theory in the course of my research on multiple mating in females, and as I worked on this theory for many years, I expanded my interest to life history strategies other than mating, finally arriving at the fundamental principles of biology. Please read my life work paper1 and Japanese translation1 in which I reconstructed the concept of bet-hedging without using esoteric mathematical formulas. This paper was selected as Top Cited Article 2022-2023 in Ecological Research, published by Ecological Society of Japan, and received an award from the publisher, Wiley. The conclusion is that all organisms are bet-hedging, and therefore bet-hedging is a universal law of biology. Join me in considering the fundamental principles of life.
4. evolutionary theory of sexual reproduction
All higher organisms reproduce sexually between males and females (anisogamy). Why is sex necessary and why are there only two biological sexes (i.e., a combination that produces children through crossbreeding), male and female? (Have you ever thought about these fundamental questions? It is those who question the seemingly obvious that advance science.) Anisogamy has only half the reproductive rate of asexual reproduction (parthenogenetic thelytoky) because only half of the genome is transmitted to the offspring through meiosis and half of the offspring become males who do not produce offspring themselves. The fact that sexual reproduction is the majority despite the “twofold cost of sex” is considered one of the greatest mysteries of evolutionary biology. The ancient Greek philosopher Aristotle (c. 325 BC) also discussed the significance of sex in his De Generatione Animalium, and this is an eternal theme for humanity. Since the 20th century, numerous geniuses such as John Maynard-Smith, Geoffrey A. Parker and William D. Hamilton have attempted to solve this “twofold cost of sex” problem, but their explanations are insufficient. The current most dominant theory is that asexual reproduction will perish due to lack of genetic diversity and inability to resist environmental changes, especially pathogens and parasites (the “Red Queen” hypothesis), but the cost is twice as high per generation, so after 10 generations the difference will be 1024 times the number of individuals, and even if asexual populations are destined to perish in the long term, before the sexual population will become extinct due to short-term competition (i.e., even the “Red Queen” will be too slow). I have been pondering this question for many years as another life’s work and have finally come up with an answer. The research was summarized in two papers, dealing with the evolution from asexual reproduction to isogamy (how did the first sexual individual obtain partner?) and from isogamy to anisogamy (how were males able to reduce own gamete size without causing zygotes to die from lack of nutrition?) The first paper (read paper 2 and Japanese translation 2), which surprised the world and won the Journal of Ethology Editor’s Choice Award 2022 (Best Paper Award) and becoming the most downloaded paper in the journal’s history. Among the hundreds of journals published by Springer-Nature, headed by Nature, and more than 10,000 papers in evolutionary biology published that year, our paper was selected as one of the nine most impactful papers. I am currently working on a second paper on how sexual reproduction was able to stop the invasion of asexual reproduction, which was 1024 times more advantageous in 10 generations. We are currently struggling. Even in Kagawa, I can compete at the world’s top level. Why don’t you join me in thinking about the reason for the existence of males and females (men and women)?
For lovers of insects and butterflies
T. Ohya and Y. Yasui (2025, Feb., 16) “Digital Encyclopedia Birdwing Butterflies” now available free of charge. Worldwide acclaim.
Publications
Yasui, Y. 1997. A “good-sperm” model can explain the evolution of costly multiple mating by females. The American
Naturalist 149: 573-584. http://www.jstor.org/stable/2463384?seq=1#page_scan_tab_contents
Yasui, Y. 1998. The ‘genetic benefits’ of female multiple mating reconsidered. Trends in Ecology and Evolution 13:
246-250. http://www.sciencedirect.com/science/article/pii/S0169534798013834
Yasui, Y. 2001. Female multiple mating as a genetic bet-hedging strategy when mate choice criteria are unreliable.
Ecological Research 16: 605-616. http://onlinelibrary.wiley.com/doi/10.1046/j.1440-1703.2001.00423.x/full
Garcia-Gonzalez, F., Yasui, Y. and Evans, J. P. 2015. Mating portfolios: bet-hedging, sexual selection and female
multiple mating. Proceedings of the Royal Society of London. Series B. Biological Sciences. vol.282
http://dx.doi.org/10.1098/rspb.2014.1525
Yasui, Y. and Garcia-Gonzalez, F. 2016. Bet-hedging as a mechanism for the evolution of polyandry, revisited.
Evolution 70: 385-397. http://onlinelibrary.wiley.com/doi/10.1111/evo.12847/abstract
Yasui, Y. and Yoshimura, J. 2018. Bet-hedging against male-caused reproductive failures may explain ubiquitous cuckoldry
in female birds. Journal of Theoretical Biology, 437: 214-221, https://doi.org/10.1016/j.jtbi.2017.10.029.
Yasui, Y. and Yamamoto, Y. 2021. An empirical test of bet-hedging polyandry hypothesis in the field cricket Gryllus
bimaculatus. Journal of Ethology 39: 329-342. https://doi.org/10.1007/s10164-021-00707-0
同誌のEditor’s Choice 2021に選定され紹介動画が作られました。
Yasui, Y. 2022. Life-history traits of the fairy shrimp Branchinella kugenumaensis are highly variable between
neighboring rice paddies in Japan. Ecological Research. 37: 344-354. https://doi.org/10.1111/1440-1703.12296
Yasui, Y. 2022. Evolutionary bet-hedging reconsidered: What is the mean–variance trade-off of fitness?
Ecological Research, 37: 406-420.https://doi.org/10.1111/1440-1703.12303
Yasui, Y. and Hasegawa, E. 2022. The origination events of gametic sexual reproduction and anisogamy.
Journal of Ethology. 40: 273-284. https://doi.org/10.1007/s10164-022-00760-3
同誌のEditor’s Choice Award 2022（論文賞）を受賞（紹介動画も作られる予定）。
Springer-Nature 2022 Research Highlights – Evolutionary Biologyに選定。https://www.ag.kagawa-u.ac.jp/?p=31001
Yasui, Y. 2023. Mite dilemma: molting to acquire sexual maturity or not molting to ensure durability
and dispersal abilityin Phorytocarpais fimetorum (Parasitiformes; Gamasida; Parasitidae).
Journal of Ethology. 41:177–184. https://doi.org/10.1007/s10164-023-00783-4
同誌のEditor’s Choice 2023に選定（紹介動画も作られる予定）。
Yamamoto, Y. and Yasui, Y. 2024. Polyandry works as bet-hedging in the field cricket Gryllus bimaculatus,
even after eliminating females in poor condition that cannot accept remating.
Journal of Ethology, 42.1: 61-69. DOI: 10.1007/s10164-023-00803-3
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