Zrubek Brandy, Woods H Arthur
University of Texas at Austin, Section of Integrative Biology, C0930 Austin, TX 78712, USA.
Proc Biol Sci. 2006 Apr 7;273(1588):831-4. doi: 10.1098/rspb.2005.3374.
In terrestrial environments, the exchange of respiratory gases exacts a water cost: obtaining oxygen or carbon dioxide requires losing water. Insect eggs should be especially sensitive to this tradeoff-because they are unable to forage for water, have high surface area-to-volume ratios, and experience large temperature-driven changes in oxygen demand. Previous work from our laboratory, on eggs of a common hawk-moth, Manduca sexta, has shown that, during development, metabolic rate and water loss rates rise in parallel. These correlative data suggest that eggshell conductance increases to accommodate increasing metabolic demand. Here, we test this idea experimentally by subjecting eggs of M. sexta to 15, 21 (normoxia) and 35% oxygen for 24h, while measuring rates of metabolism (as carbon dioxide emission) and water loss. Hypoxia depressed egg metabolic rates, but led to pronounced, rapid increases in water loss. By contrast, hyperoxia had no significant effect on metabolism or water loss. These data demonstrate that insect eggs actively participate in balancing oxygen gain and water loss, and that they use tissue oxygen status, or some correlate of it, as a cue for increasing eggshell conductance. Rapid control over conductance may allow eggs to conserve water during an initial period of low metabolic demand, thereby deferring water costs of respiratory gas exchange until late in development.
在陆地环境中，呼吸气体的交换会带来水分消耗：获取氧气或二氧化碳需要损失水分。昆虫卵应该对这种权衡特别敏感——因为它们无法获取水分，表面积与体积之比很高，并且会经历因温度驱动的氧气需求大幅变化。我们实验室之前对一种常见天蛾烟草天蛾的卵所做的研究表明，在发育过程中，代谢率和水分流失率会同步上升。这些相关数据表明，卵壳的导水率会增加以适应不断增加的代谢需求。在这里，我们通过将烟草天蛾的卵置于15%、21%（常氧）和35%的氧气环境中24小时来对这一观点进行实验测试，同时测量代谢率（以二氧化碳排放量表示）和水分流失率。低氧降低了卵的代谢率，但导致水分流失显著且迅速增加。相比之下，高氧对代谢或水分流失没有显著影响。这些数据表明，昆虫卵会积极参与平衡氧气获取和水分流失，并且它们利用组织的氧气状态或与之相关的某种因素作为增加卵壳导水率的信号。对导水率的快速控制可能使卵在代谢需求较低的初始阶段保存水分，从而将呼吸气体交换的水分成本推迟到发育后期。
