5月4日,據(jù)國(guó)外媒體報(bào)道，美國(guó)科學(xué)家宣布,，經(jīng)由化學(xué)物質(zhì)處理,，一種處于胚胎狀態(tài)的魚類大腦可發(fā)育成近親魚類的大腦。研究同時(shí)發(fā)現(xiàn),，目前被廣泛認(rèn)可的關(guān)于大腦進(jìn)化的“后即大（Late equals large）”理論并不適用于該魚類,。研究論文發(fā)表在5月3日出版的《美國(guó)國(guó)家科學(xué)院院刊》上。
上世紀(jì)90年代中期,，研究大腦進(jìn)化的科學(xué)家提出了“后即大”理論,。他們認(rèn)為，大腦起初只不過(guò)是一塊白板，隨著前體細(xì)胞成熟變成神經(jīng)細(xì)胞,，神經(jīng)開始形成,，前體細(xì)胞變成成熟的神經(jīng)細(xì)胞的時(shí)間越晚，大腦越大,。
但是,，美國(guó)喬治亞理工學(xué)院生物系助理教授陶德·斯垂?fàn)柭I(lǐng)導(dǎo)的研究團(tuán)隊(duì)對(duì)非洲馬拉維湖中的6種棘鰭類熱帶淡水魚的大腦發(fā)育模式進(jìn)行研究后發(fā)現(xiàn)，早在受精48小時(shí)后,，也就是神經(jīng)形成過(guò)程開始之前,，這些魚類大腦的差異就已出現(xiàn)。
這6種魚的基因組幾乎一樣,，但其外形,、行為的差異卻很大，其中3種魚寄居于巖石中,；另3種魚寄居于沙子中,。斯垂?fàn)柭鼒F(tuán)隊(duì)從生態(tài)學(xué)、基因組學(xué)和發(fā)育學(xué)等方面著手,，試圖揭示生物體進(jìn)化出如此多姿多彩形態(tài)的奧秘,。
該研究團(tuán)隊(duì)在魚受精2天到4天之內(nèi)不斷重復(fù)實(shí)驗(yàn)，結(jié)果表明,，寄居于沙子中的魚,，其基因Wnt1（Wnt家族的成員之一，對(duì)神經(jīng)干細(xì)胞的增殖及分化有一定的調(diào)控作用,，Wnt1a蛋白可以促進(jìn)神經(jīng)干細(xì)胞向神經(jīng)細(xì)胞分化）的表達(dá)更加突出,，其丘腦（位于大腦的中心部位，屬于間腦具有重要機(jī)能的部位）也更大,。
斯垂?fàn)柭忉屨f(shuō),，寄居于沙中的魚使用視力搜尋浮游生物，因此其大腦都被用于整合視覺信號(hào),；而寄居于巖石上的魚靠巖石上的水藻為生,，其大腦也更大，可能是為了更好地掌控周圍復(fù)雜的三維環(huán)境,。
斯垂?fàn)柭硎?，其它類似棘鰭類熱帶淡水魚那樣關(guān)系密切的物種也沒有提供支持“后即大”假設(shè)的數(shù)據(jù)。
研究團(tuán)隊(duì)試圖通過(guò)化學(xué)物質(zhì)改變基因表達(dá)的模式以改變胚胎的大腦發(fā)育模式,。在使用氯化鋰對(duì)胚胎進(jìn)行歷時(shí)3小時(shí)到5小時(shí)的處理后,，研究人員將胚胎放入水中，并在魚的不同發(fā)育階段提取樣本進(jìn)行研究,。研究發(fā)現(xiàn),，氯化鋰處理會(huì)讓W(xué)nt信號(hào)更加突出,。處理結(jié)果表明，他們成功地將寄居于巖石中的魚的胚胎的大腦轉(zhuǎn)變成了寄居于沙子中的魚的大腦,。
斯垂?fàn)柭硎?，在大腦發(fā)育和進(jìn)化過(guò)程中，神經(jīng)形成過(guò)程非常重要,。此項(xiàng)研究表明,，物種大腦之間出現(xiàn)差異的時(shí)間要遠(yuǎn)遠(yuǎn)早于人們之前的認(rèn)識(shí)。(生物谷Bioon.com)
生物谷推薦原文出處：
PNAS doi: 10.1073/pnas.1000395107
Brain diversity evolves via differences in patterning
Jonathan B. Sylvestera, Constance A. Richa, Yong-Hwee E. Loha, Moira J. van Staadenb, Gareth J. Fraserc, and J. Todd Streelmana,1
Differences in brain region size among species are thought to arise late in development via adaptive control over neurogenesis, as cells of previously patterned compartments proliferate, die, and/or differentiate into neurons. Here we investigate comparative brain development in ecologically distinct cichlid fishes from Lake Malawi and demonstrate that brains vary among recently evolved lineages because of early patterning. Divergence among rock-dwellers and sand-dwellers in the relative size of the telencephalon versus the thalamus is correlated with gene expression variation in a regulatory circuit (composed of six3, fezf2, shh, irx1b, and wnt1) known from model organisms to specify anterior-posterior (AP) brain polarity and position the shh-positive signaling boundary zona limitans intrathalamica (ZLI) in the forebrain. To confirm that changes in this coexpression network are sufficient to produce the differences we observe, we manipulated WNT signaling in vivo by treating rock-dwelling cichlid embryos with temporally precise doses of LiCl. Chemically treated rock-dwellers develop gene expression patterns, ZLIs, and forebrains distinct from controls and untreated conspecifics, but strongly resembling those of sand-dwellers. Notably, endemic Malawi rock- and sand-dwelling lineages are alternately fixed for an SNP in irx1b, a mediator of WNT signaling required for proper thalamus and ZLI. Together, these natural experiments in neuroanatomy, development, and genomics suggest that evolutionary changes in AP patterning establish ecologically relevant differences in the elaboration of cichlid forebrain compartments. In general, variation in developmental patterning might lay the foundations on which neurogenesis erects diverse brain architectures.
