『稲の日本史』(佐藤洋一郎)②-熱帯ジャポニカと温帯ジャポニカの違い ― 2010/07/22 17:08
そもそも、「熱帯ジャポニカ」と「温帯ジャポニカ」とは、何がどう違うのかという疑問に、一定の答えを出してくれると期待していた本書ですが、少し疑問を感じています。
まず最初に、現存する品種(ジャポニカ種)を二つのグループに分けるところから始まります。
そのグループ分けの指標となるのが、次の三つです。
①胚乳(米粒)のアルカリ溶解度
②イネの種子を真っ暗な部屋で発芽させたときに伸びる中茎と呼ばれる器官の長さ
③籾の形(長さを幅でわった値)
上記の①を横軸に、②を縦軸にグラフを作ると、”何となく二つのグループに分かれそうな傾向が読み取れる”(本書より)そうです。
そして、一つのグループに③の籾が”長い品種”が多く分布し、もう一つのグループに③の籾が”短い品種”が多く分布する、ということです。
結論として、「①溶解、②中茎短、③丸籾」型と、「①非溶解、②中茎長、③細長籾」型の二つに分かれ、それが、ジャポニカの二つのタイプ(熱帯と温帯)になる、ということです。
”何となく曖昧だなぁ”というのが、正直な感想です。
例えば、どちらのタイプに属するのか不明な品種を、上記の三形質から熱帯または温帯ジャポニカと、区別するのは良いとして、それを過去の品種に持ち込むのは、誤った判断の元になるのではないかと思ってしまうのは、素人の考えでしょうか。
【葉緑体のDNA】
二つのジャポニカは、DNAレベルでも区別が可能だと言います。
葉緑体のDNA配列の中で、PS-IDと呼ばれる部分が、二つのジャポニカを区別するのに都合が良いそうです。
そのPS-IDの始めの部分に、四つの塩基のうちCとAが連続して並ぶところがあり、例えばコシヒカリでは、”CCCCCCAAAAAAA”というように、Cが六個、Aが七個(6C7Aと呼んでいる)という配列になるそうです。
さて、そこで熱帯ジャポニカと温帯ジャポニカの区別はどうかと言うと、こうなるそうです。
・熱帯ジャポニカ - 7C6A 又は 6C7A
・温帯ジャポニカ - 6C7A のみ
6C7Aだと、熱帯か温帯かの区別はできないようです。
【核DNA】
紙面の都合で詳しくは述べられていませんが、DNAのある場所では、熱帯ジャポニカにはあって、温帯ジャポニカには無いバンドがあり、その逆になる場所もあるということから、区別ができるそうです。
【根本的な疑問】
DNAによって区別ができるといっても、その根本となるのは、上記の三つの形質による分類から始まっているといって良いでしょう。
形質により分類した上で、「熱帯ジャポニカのDNAはこう」で、「温帯ジャポニカのDNAはこうなっている」と言っているように見えます。
これで良いのでしょうか?
本来は、DNAを分析した上で、その違いから分類するべきなのではないでしょうか?
見ようによっては、形質により分類した二つのジャポニカを、DNAによる違いを見つけることによって、証拠付けを行っていると言えないでしょうか?
葉緑体のDNAの結果からは、二つのジャポニカがあるのではなく、「熱帯ジャポニカの中に、温帯ジャポニカがある」ようにも見えるのですが、それは流石にちょっと短絡的でしょうね。
【古代の米に当てはめて良いのか?】
このように、現存する品種を元に分類された二つのジャポニカですが、それを発掘された古代の米に当てはめて良いものでしょうか?
古代から現在までに、多くの変化をしているのでしょうから、現在の分類を、過去に持ち込んで正しく分類できる気がしません。
直接関係はありませんが、古人骨の「縄文人」と「渡来系弥生人」「在来系弥生人」などの、考え方とどこか似ているようです。
何か根本的なところが間違っている為に、それを元にして研究したさまざまな事が矛盾に包まれていく、そんな雰囲気を感じたのは、私が無知だからでしょうか。
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