来自美国能源部Argonne国家实验室的科学家最近的新发现能解释为何数百万年前的大型昆虫现在都不复存在。在古生代,当大气中的氧浓度达到历史最高值时,一些昆虫进化成巨型昆虫。而当氧浓度恢复后,这些巨型昆虫灭绝了。
原因可能在于昆虫的呼吸系统。和脊椎动物不同,昆虫通过封闭末端的气管系统输送氧气。随着昆虫变大,这种氧气输送方式变得效率很低。但是一旦大气氧浓度升高,更长的气管就能发挥作用,这使得巨型昆虫得以进化生成。
在最近的Proceedings of the National Academy of Science上的文章中确认了气管系统确实限制了昆虫的体积。Argonne APS,Midwestern大学和Arizona州立大学小组分析了甲虫的气管系统变化如何造成体积的增加。小组使用APS得到的X射线图像检查了4种甲虫气管,它们的体重各不相同。结果发现,大型甲虫身体的更大一部分用于气管系统。
研究小组主要关注从身体的核心部位通向头部和腿部的出入口。他们认为这些小口可能是气管系统的瓶颈,并决定了有多少的氧气可以被输送到身体末端。接着小组测量了甲虫的气管以确定现存的甲虫的可能最大体积。从头部数据得到的体积很不切实际,然而从腿部数据则很好的符合现存的实际情况。
Argonne的生物学家Jake Socha认为:“这是迈向了解昆虫体积限制因素的第一步,但对于甲虫外的其它昆虫,还需要进一步的研究。”
原文链接:http://www.physorg.com/news105891955.html
最新的一项研究帮助我们解释,为什么昆虫的体形曾经要比现在大得多。美国中西大学生理学系生理学博士亚历山大·凯撒就是这一研究报告的主要作者。
凯撒博士说,针对昆虫体形变小的问题,人们提出数百种理论,但是他们都不能够得到证实。有一种理论认为,昆虫的呼吸系统限制了它的大小。为了试验这一的理论的正确性,他和他的同事利用甲虫和果蝇进行了全面研究。
这些研究大部分是在伊利诺伊州的阿尔贡国立实验室中进行的,对各式各样的昆虫的呼吸系统进行了检测。他们利用了新型X射线技术帮助确定昆虫是如何进行呼吸的。
研究成果表明凯撒博士和他的同事在他们建立理论中选对了方向。昆虫呼吸是通过一种直接将氧气送到细胞的网络。这些气管,特别是在腿部,占用了大型甲虫的很多空间。
凯撒说,在三亿年前,空气中的氧气占到百分之三十一到三十五。这意味着昆虫的呼吸系统能够变得较小并依旧可以让足够的氧气满足他们的需要,并让这此昆虫长得更大。
凯撒博士和他的团队计划将来还要进行类似研究,目标将定在更古老的物种,比如蜻蜓,因为甲虫和果蝇比较起来相对“较新”。(教育部科技发展中心)
原始出处:
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