自家不和合性の意味は?メリット・デメリットとは

「自家不和合性」という言葉を聞いたことありますでしょうか。
植物が受粉する時の話で、簡単に説明すると、
「自分自身の花粉では受粉せず、同じ種類の別の固体からの花粉でのみ受粉する」というものです。

具体的な植物としてはスモモや梨、栗などがあげられるのですが、私たち人間から見ると
「わざわざ別の固体から花粉を待たなくてはいけないなんて不便だな」
なんて思ってしまいます。

植物にとって、この「自家不和合性」という性質にメリットはあるのでしょうか。
また、どんなデメリットがあるのでしょうか。
今日はそんな、自家不和合性のメリット・デメリットについて詳しく見ていきます。

「自家不和合性」について何となくは知ってはいるものの、詳細までは分からないという方は是非参考にしてみてください。

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「自家不和合性」とは

自家不和合性とは、植物や菌類に見られる性質で、同じ個体や同じ遺伝子型の個体との受精を防ぐことです。
自家不和合性は、遺伝的に多様性を保つために有利な進化的なメカニズムと考えられています。

自家不和合性は、植物と菌類では異なるメカニズムで起こります。
植物の場合は、S遺伝子座と呼ばれる一つまたは複数の遺伝子座で制御されるタンパク質の相互作用によって、
花粉の発芽や花粉管の伸長や受精が阻害されます。

植物の自家不和合性は、配偶体型と胞子体型、または同形花型と異形花型に分けられます。
配偶体型では、花粉の表現型は花粉自体の半数性遺伝子型で決まります。
胞子体型では、花粉の表現型は花粉を作った葯の二倍体遺伝子型で決まります。

同形花型では、雌雄同株で両性花をつけます。異形花型では、雌雄異株で雌花と雄花を別々につけます。

菌類の場合は、特定の他個体や他系統の株とでなければ有性生殖が成立しないことです。
菌類の自家不和合性は、+-やA1A2などの記号で表される配偶子型によって決まります。
菌類の自家不和合性は、配偶子のう接合やアメーバ接合などの有性生殖過程に影響します。

 

同形花型とは

同形花型の植物とは、雌雄同株で両性花をつける植物のことです。
自家不和合性の同形花型の植物は、自分と同じ遺伝子型の花粉と受精しないように、
花粉の発芽や花粉管の伸長や受精を阻害するメカニズムを持っています。

同形花型の植物の例としては、ナシやリンゴ(バラ科)、ヒナゲシ(ケシ科)、コスモス(キク科)などが挙げられます。

 

異形花型とは

異形花型の植物とは、雌雄同株で単性花をつける植物のことです。
異形花型の植物では、花柱と雄蕊の長さが相対的に異なる個体があり、自家不和合性を示します。
自家不和合性とは、自分と同じ遺伝子型の花粉と受精しないように、花粉の発芽や花粉管の伸長や受精を阻害するメカニズムのことです。

異形花型の植物の例としては、
ソバやサクラソウ(サクラソウ科)、ミソハギやカタバミ(カタバミ科)、トモエソウ(オトギリソウ科)などが挙げられます。

 

自家不和合性のメリット

自家不和合性とは、自分と同じ遺伝子型の花粉と受精しないように、花粉の発芽や花粉管の伸長や受精を阻害するメカニズムのことです。
自家不和合性は、被子植物において自殖(自家生殖)を防ぐ手段であり、新しい遺伝子型を作成し、
地球上に被子植物が広がった成功の要因の一つであると考えられています。

自家不和合性のメリットは、主に以下の2点です。

 

近交弱勢の回避:

自殖をすると、同じ遺伝子型の個体が増えてしまい、遺伝的多様性が失われます。
これは、有害な突然変異や隠れ性状が現れやすくなり、生存力や繁殖力が低下する現象で、近交弱勢と呼ばれます。
自家不和合性は、自殖を抑制することで近交弱勢を回避し、健全な個体を維持することができます。

 

他殖の促進:

他殖とは、異なる遺伝子型の花粉と受精することです。
他殖をすると、新しい組み合わせの遺伝子型が生まれて、遺伝的多様性が広がります。

これは、環境の変化に対応できる個体が出現しやすくなり、進化の速度や可能性が高まることを意味します。
自家不和合性は、他殖を促進することで進化に有利な形質を獲得することができます。

 

自家不和合性のデメリット

自家不和合性とは、自分と同じ遺伝子型の花粉と受精しないように、花粉の発芽や花粉管の伸長や受精を阻害するメカニズムのことです。
自家不和合性は、他個体との交雑を促進し、遺伝的多様性を広げるメリットがありますが、以下のようなデメリットもあります。

 

受精の確率が低下する:

自家不和合性の植物は、自分と異なる遺伝子型の花粉としか受精できません。
しかし、被子植物の受粉は虫や風などに頼っているため、必ずしも適切な花粉が届くとは限りません。

特に、同じ種の個体が少ない場合や、悪天候や虫が花粉を運んでくれない場合には、受精が行われない可能性が高くなります。
これは、種子の形成や子孫の維持に影響を与えるデメリットです。

 

近交強勢の利用ができない:

近交強勢とは、近交によって有利な形質がホモ接合体になり、生存力や繁殖力が向上する現象です。
近交強勢は、近交弱勢とは逆に、有害な突然変異や隠れ性状が現れにくくなることを意味します。

自家不和合性の植物は、自殖を抑制することで近交弱勢を回避しますが、同時に近交強勢を利用することもできません。
これは、環境に適応した形質を固定しやすくする近交強勢のメリットを享受できないことを意味します。

 

主な「自家不和合性」の植物

 

野菜・果物

 

ナシ:

ナス科の植物で、配偶体型自家不和合性を持ちます。
雌性決定要素はS-RNaseというRNA分解酵素で、雄性決定要素はSLFというF-ボックスタンパク質です。
自己の花粉と非自己の花粉を識別し、非自己の花粉だけを受け入れる仕組みになっています。

 

ダイコン:

アブラナ科の植物で、胞子体型自家不和合性を持ちます。
雌性決定要素はSRKという受容体キナーゼで、雄性決定要素はSP11/SCRという小さなタンパク質です。
自己の花粉と非自己の花粉を識別し、自己の花粉を排除する仕組みになっています。

 

ハクサイ:

アブラナ科の植物で、胞子体型自家不和合性を持ちます。
雌性決定要素はSRKという受容体キナーゼで、雄性決定要素はSP11/SCRという小さなタンパク質です。
自己の花粉と非自己の花粉を識別し、自己の花粉を排除する仕組みになっています。

 

サツマイモ:

ヒルガオ科の植物で、配偶体型自家不和合性を持ちます。
雌性決定要素はS-リボヌクレアーゼというRNA分解酵素で、雄性決定要素はS-ローカスF-ボックスタンパク質というタンパク質です。
自己の花粉と非自己の花粉を識別し、非自己の花粉だけを受け入れる仕組みになっています。

 

ソバ:

タデ科の植物で、異形花型自家不和合性を持ちます。
胞子体型であり、花粉を作る植物が持つ2つの対立遺伝子が、花粉の自家不和合性反応を決定します。
自家不和合性遺伝子座は、花粉でも雌蕊でも一方が他方に対して優性を示す対立遺伝子2種類のみを常に含みます。

 

木・草

 

ヒナゲシ:

ケシ科の植物で、配偶体型自家不和合性を持ちます。
雌性決定要素は低分子シグナル伝達物質で、雄性決定要素は細胞膜局在レセプターと推定されています。
自己の花粉と非自己の花粉を識別し、非自己の花粉だけを受け入れる仕組みになっています。

 

サクラソウ:

サクラソウ科の植物で、異形花型自家不和合性を持ちます。
胞子体型であり、花粉を作る植物が持つ2つの対立遺伝子が、花粉の自家不和合性反応を決定します。
自家不和合性遺伝子座は、花粉でも雌蕊でも一方が他方に対して優性を示す対立遺伝子2種類のみを常に含みます。

 

カタバミ:

カタバミ科の植物で、異形花型自家不和合性を持ちます。
胞子体型であり、花粉を作る植物が持つ2つの対立遺伝子が、花粉の自家不和合性反応を決定します。
自家不和合性遺伝子座は、花粉でも雌蕊でも一方が他方に対して優性を示す対立遺伝子2種類のみを常に含みます。

 

自家和合性(自家受粉)する植物

 

トマト:

ナス科の植物で、同じ花の中で雄蕊と雌蕊が成熟する同花受粉を行います。
花は下向きに咲き、風や振動によって花粉が柱頭に落ちる仕組みになっています。

 

ピーマン:

ナス科の植物で、トマトと同様に同花受粉を行います。
花は上向きに咲き、雄蕊が雌蕊を囲むように配置されています。風や振動で花粉が柱頭に付着すると受粉が成立します。

 

ナス:

ナス科の植物で、トマトやピーマンと同じく同花受粉を行います。
花は下向きに咲き、雄蕊が雌蕊を包むように配置されています。風や振動で花粉が柱頭に落ちると受粉が成立します。

 

インゲン:

マメ科の植物で、自動自家受粉を行います。
花は蝶形花と呼ばれる特殊な形をしており、雄蕊と雌蕊が密着しています。
花弁が開く前に内部で受粉が完了するため、外部からの干渉を受けません。

 

ライマメ:

マメ科の植物で、インゲンと同様に自動自家受粉を行います。
花は蝶形花で、雄蕊と雌蕊が密着しています。
花弁が開く前に内部で受粉が完了するため、外部からの干渉を受けません。

 

スイートピー:

マメ科の植物で、インゲンやライマメと同様に自動自家受粉を行います。
花は蝶形花で、雄蕊と雌蕊が密着しています。
花弁が開く前に内部で受粉が完了するため、外部からの干渉を受けません。

 

ピーナッツ:

マメ科の植物で、自動自家受粉を行います。
花は黄色い筒状の形をしており、雄蕊と雌蕊が密着しています。
花弁が開く前に内部で受粉が完了するため、外部からの干渉を受けません。

 

コムギ:

イネ科の植物で、風媒自家受粉を行います。
穂は小穂と呼ばれる単位に分かれており、各小穂には3つの小花があります。
小花は雄しべと雌しべだけからなり、風によって花粉が柱頭に飛び移ります。

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「自家不和合性の意味は?メリット・デメリットとは」についてのまとめ

「自家不和合性」とは、自分自身の花粉では受粉しないものを指す言葉で、主に植物に使われます。
メリットとしては、別の遺伝子を取り入れることで進化の可能性を広げたり、環境の変化への対応をしやすくしているということでしたね。

また、自家受粉できる植物も多くあり、その種類を見てみると人間が農作物として進化させ、上手く利用してきたことが分かります。

植物の種類や性質は、調べれば調べるほど奥が深く、理解できた気になってもまた疑問がわいてくるようなものです。
色々見てみて、自分自身の納得できる解を見つけてみてはいかがでしょうか。

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