秋天，吉爾吉斯斯坦的野生杏樹(shù)和蘋(píng)果樹(shù)生出了明亮的紅色,，這或許將幫助它們抵御害蟲(chóng)。
圖片提供：Marco Archetti/Harvard University
每到秋季,，綠油油的樹(shù)葉都會(huì)變成深紅色,，這是為什么呢？一個(gè)世紀(jì)以來(lái),，科學(xué)家一直在苦苦思索這種轉(zhuǎn)換的背后機(jī)制是什么。如今,，一項(xiàng)有關(guān)蚜蟲(chóng)和蘋(píng)果樹(shù)的新研究支持了這樣一種理論,，即紅色能夠?qū)δ切┮詷?shù)葉為食或在此安家的害蟲(chóng)提出警告。
長(zhǎng)期以來(lái),，這一由已故英國(guó)進(jìn)化生物學(xué)家William Hamilton最先提出的理論一直存在巨大的爭(zhēng)論,。這一概念認(rèn)為，樹(shù)葉中的紅色素——被化學(xué)家稱(chēng)為花青素——能夠警告昆蟲(chóng)：眼前的這棵樹(shù)并不適于食用或筑巢,。美國(guó)哈佛大學(xué)的進(jìn)化生物學(xué)家Marco Archetti指出：“這是一種警報(bào)信號(hào),，就像有毒的蛙類(lèi)或蝴蝶身上艷麗的顏色一樣。”然而有一些科學(xué)家卻相信，紅色素能夠幫助樹(shù)葉免受太陽(yáng)光的傷害,。這是因?yàn)殡S著生理機(jī)能上的變化,，樹(shù)葉對(duì)于寒冷秋季中的這種傷害變得尤為敏感。
由于注意到與果園中的蘋(píng)果樹(shù)相比,，野生蘋(píng)果樹(shù)上的紅葉子更為普遍,，Archetti因而開(kāi)始了新的研究。蘋(píng)果樹(shù)起源于中亞,，一直到現(xiàn)在,，吉爾吉斯斯坦和哈薩克斯坦境內(nèi)的野生蘋(píng)果樹(shù)依然很常見(jiàn)。Archetti表示：“如果你在秋季去這些國(guó)家,，你會(huì)看到一棵棵鮮紅的樹(shù)木——但是你在歐洲種植的蘋(píng)果樹(shù)中絕對(duì)看不到這種現(xiàn)象,。”Archetti推測(cè)，經(jīng)過(guò)馴化的蘋(píng)果樹(shù)已經(jīng)喪失了它們?cè)谇锛镜?ldquo;警告色”,，這是因?yàn)閿?shù)百年來(lái),，果農(nóng)一直將這些果樹(shù)與果實(shí)更大、更好吃的樹(shù)木進(jìn)行雜交,，卻很少選擇更抗蟲(chóng)害的果樹(shù)進(jìn)行雜交,。
Archetti將野生蘋(píng)果樹(shù)與果園中的果樹(shù)進(jìn)行了比較，結(jié)果顯示,，62.2%的中亞野生蘋(píng)果樹(shù)的樹(shù)葉在秋季會(huì)變紅,，而這一數(shù)字在人工種植的英國(guó)蘋(píng)果樹(shù)中僅為2.8%。為了驗(yàn)證這一假設(shè),，即紅色的樹(shù)葉給昆蟲(chóng)造成了麻煩,，Archetti于2007年秋季分別在紅樹(shù)葉蘋(píng)果樹(shù)和綠樹(shù)葉蘋(píng)果樹(shù)上安置了蚜蟲(chóng)窩。在接下來(lái)的春季中,，60%在綠樹(shù)葉蘋(píng)果樹(shù)上安家的蚜蟲(chóng)都活了下來(lái),，相比之下，在紅樹(shù)葉蘋(píng)果樹(shù)上安家的蚜蟲(chóng)僅有29%幸存,。Archetti在4月15日出版的英國(guó)《皇家學(xué)會(huì)學(xué)報(bào)B》報(bào)告了這一研究成果,。盡管造成這一差異的背后機(jī)制至今尚未搞清，但Archetti和其他的研究表明,，對(duì)于幼小的蚜蟲(chóng)而言,，紅色的樹(shù)葉不是含有有毒的化學(xué)物質(zhì)，就是攜帶了更少的養(yǎng)分,。
英國(guó)利物浦約翰·摩爾斯大學(xué)的環(huán)境科學(xué)家David Wilkinson稱(chēng)Archetti的研究“富有想象力”,，并預(yù)言它將在這場(chǎng)有關(guān)顏色的爭(zhēng)論中再次引發(fā)人們的興趣。然而新的研究成果似乎不足以終結(jié)這場(chǎng)爭(zhēng)論,。英國(guó)頓提大學(xué)的植物遺傳學(xué)家Andrew Flavell表示：“這項(xiàng)研究很有趣,，但是卻需要‘再加點(diǎn)兒鹽’,。”他認(rèn)為，紅色樹(shù)葉的減少可能還有其他的原因,。Flavell說(shuō),，例如，葉子的顏色與果實(shí)的味道之間便很有可能存在著遺傳聯(lián)系,。（生物谷Bioon.com）
生物谷推薦原始出處：
Proceedings of the Royal Society B April 15, 2009, doi: 10.1098/rspb.2009.0355
Evidence from the domestication of apple for the maintenance of autumn colours by coevolution
Marco Archetti*
Department of Zoology, University of Oxford South Parks Road, OX1 3PS Oxford, UK
Abstract
The adaptive value of autumn colours is still a puzzle for evolutionary biology. It has been suggested that autumn colours are a warning signal to insects that use the trees as a host. I show that aphids (Dysaphis plantaginea) avoid apple trees (Malus pumila) with red leaves in autumn and that their fitness in spring is lower on these trees, which suggests that red leaves are an honest signal of the quality of the tree as a host. Autumn colours are common in wild populations but not among cultivated apple varieties, which are no longer under natural selection against insects. I show that autumn colours remain only in the varieties that are very susceptible to the effects of a common insect-borne disease, fire blight, and therefore are more in need of avoiding insects. Moreover, varieties with red leaves have smaller fruits, which shows that they have been under less effective artificial selection. This suggests a possible trade off between fruit size, leaf colour and resistance to parasites. These results are consistent with the hypothesis that autumn colours are a warning signal to insects, but not with other hypotheses.
