來(lái)自美國(guó)能源部Argonne國(guó)家實(shí)驗(yàn)室的科學(xué)家最近的新發(fā)現(xiàn)能解釋為何數(shù)百萬(wàn)年前的大型昆蟲(chóng)現(xiàn)在都不復(fù)存在。在古生代,，當(dāng)大氣中的氧濃度達(dá)到歷史最高值時(shí)，一些昆蟲(chóng)進(jìn)化成巨型昆蟲(chóng),。而當(dāng)氧濃度恢復(fù)后,，這些巨型昆蟲(chóng)滅絕了,。
原因可能在于昆蟲(chóng)的呼吸系統(tǒng),。和脊椎動(dòng)物不同,，昆蟲(chóng)通過(guò)封閉末端的氣管系統(tǒng)輸送氧氣,。隨著昆蟲(chóng)變大，這種氧氣輸送方式變得效率很低,。但是一旦大氣氧濃度升高,，更長(zhǎng)的氣管就能發(fā)揮作用,，這使得巨型昆蟲(chóng)得以進(jìn)化生成,。
在最近的Proceedings of the National Academy of Science上的文章中確認(rèn)了氣管系統(tǒng)確實(shí)限制了昆蟲(chóng)的體積,。Argonne APS，Midwestern大學(xué)和Arizona州立大學(xué)小組分析了甲蟲(chóng)的氣管系統(tǒng)變化如何造成體積的增加,。小組使用APS得到的X射線圖像檢查了4種甲蟲(chóng)氣管,，它們的體重各不相同,。結(jié)果發(fā)現(xiàn),，大型甲蟲(chóng)身體的更大一部分用于氣管系統(tǒng)。
研究小組主要關(guān)注從身體的核心部位通向頭部和腿部的出入口,。他們認(rèn)為這些小口可能是氣管系統(tǒng)的瓶頸,，并決定了有多少的氧氣可以被輸送到身體末端。接著小組測(cè)量了甲蟲(chóng)的氣管以確定現(xiàn)存的甲蟲(chóng)的可能最大體積,。從頭部數(shù)據(jù)得到的體積很不切實(shí)際,，然而從腿部數(shù)據(jù)則很好的符合現(xiàn)存的實(shí)際情況。
Argonne的生物學(xué)家Jake Socha認(rèn)為：“這是邁向了解昆蟲(chóng)體積限制因素的第一步,，但對(duì)于甲蟲(chóng)外的其它昆蟲(chóng),，還需要進(jìn)一步的研究。”
原文鏈接：http://www.physorg.com/news105891955.html
最新的一項(xiàng)研究幫助我們解釋,，為什么昆蟲(chóng)的體形曾經(jīng)要比現(xiàn)在大得多,。美國(guó)中西大學(xué)生理學(xué)系生理學(xué)博士亞歷山大·凱撒就是這一研究報(bào)告的主要作者,。
凱撒博士說(shuō)，針對(duì)昆蟲(chóng)體形變小的問(wèn)題,，人們提出數(shù)百種理論,，但是他們都不能夠得到證實(shí)。有一種理論認(rèn)為,，昆蟲(chóng)的呼吸系統(tǒng)限制了它的大小,。為了試驗(yàn)這一的理論的正確性，他和他的同事利用甲蟲(chóng)和果蠅進(jìn)行了全面研究,。
這些研究大部分是在伊利諾伊州的阿爾貢國(guó)立實(shí)驗(yàn)室中進(jìn)行的,，對(duì)各式各樣的昆蟲(chóng)的呼吸系統(tǒng)進(jìn)行了檢測(cè)。他們利用了新型X射線技術(shù)幫助確定昆蟲(chóng)是如何進(jìn)行呼吸的,。
研究成果表明凱撒博士和他的同事在他們建立理論中選對(duì)了方向,。昆蟲(chóng)呼吸是通過(guò)一種直接將氧氣送到細(xì)胞的網(wǎng)絡(luò)。這些氣管,，特別是在腿部,，占用了大型甲蟲(chóng)的很多空間。
凱撒說(shuō),，在三億年前,，空氣中的氧氣占到百分之三十一到三十五。這意味著昆蟲(chóng)的呼吸系統(tǒng)能夠變得較小并依舊可以讓足夠的氧氣滿足他們的需要,，并讓這此昆蟲(chóng)長(zhǎng)得更大,。
凱撒博士和他的團(tuán)隊(duì)計(jì)劃將來(lái)還要進(jìn)行類似研究，目標(biāo)將定在更古老的物種,，比如蜻蜓,，因?yàn)榧紫x(chóng)和果蠅比較起來(lái)相對(duì)“較新”。（教育部科技發(fā)展中心）
原始出處:
Published online before print July 31, 2007, 10.1073/pnas.0611544104
PNAS | August 7, 2007 | vol. 104 | no. 32 | 13198-13203
BIOLOGICAL SCIENCES / PHYSIOLOGY
Increase in tracheal investment with beetle size supports hypothesis of oxygen limitation on insect gigantism
Alexander Kaiser*,, C. Jaco Klok, John J. Socha, Wah-Keat Lee, Michael C. Quinlan*, and Jon F. Harrison
*Department of Basic Sciences, Midwestern University, Glendale, AZ 85308; Section of Organismal, Integrative and Systems Biology, School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501; and X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439
Edited by May R. Berenbaum, University of Illinois at Urbana–Champaign, Urbana, IL, and approved July 5, 2007 (received for review December 24, 2006)
Recent studies have suggested that Paleozoic hyperoxia enabled animal gigantism, and the subsequent hypoxia drove a reduction in animal size. This evolutionary hypothesis depends on the argument that gas exchange in many invertebrates and skin-breathing vertebrates becomes compromised at large sizes because of distance effects on diffusion. In contrast to vertebrates, which use respiratory and circulatory systems in series, gas exchange in insects is almost exclusively determined by the tracheal system, providing a particularly suitable model to investigate possible limitations of oxygen delivery on size. In this study, we used synchrotron x-ray phase–contrast imaging to visualize the tracheal system and quantify its dimensions in four species of darkling beetles varying in mass by 3 orders of magnitude. We document that, in striking contrast to the pattern observed in vertebrates, larger insects devote a greater fraction of their body to the respiratory system, as tracheal volume scaled with mass1.29. The trend is greatest in the legs; the cross-sectional area of the trachea penetrating the leg orifice scaled with mass1.02, whereas the cross-sectional area of the leg orifice scaled with mass0.77. These trends suggest the space available for tracheae within the leg may ultimately limit the maximum size of extant beetles. Because the size of the tracheal system can be reduced when oxygen supply is increased, hyperoxia, as occurred during late Carboniferous and early Permian, may have facilitated the evolution of giant insects by allowing limbs to reach larger sizes before the tracheal system became limited by spatial constraints.
allometric scaling | hyperoxia | Tenebrionidae | tracheal system
