Storm Melissa A, Angilletta Michael J
Department of Ecology and Organismal Biology, Indiana State University, Terre Haute, IN 47809, USA.
J Exp Biol. 2007 Oct;210(Pt 19):3415-21. doi: 10.1242/jeb.005652.
Selection for rapid growth and development in cold environments results in a geographic pattern known as countergradient variation. The eastern fence lizard, Sceloporus undulatus, exhibits countergradient variation in embryonic growth and development along latitudinal clines. To identify the proximate causes of countergradient variation, we compared the energy budgets of embryos from a cold environment (Virginia) and a warm environment (South Carolina) during development at a realistic thermal cycle. The difference in mean egg size between populations was controlled by removing yolk from large eggs and performing a sham manipulation on other eggs. Respiration was measured every 4 days throughout 48 days of incubation. After this period, eggs were dissected and the energy contents of embryos and yolk were determined by calorimetry. As expected from previous experiments, embryos from Virginia reached a more advanced stage of development and deposited more energy within tissues than embryos from South Carolina. The greater absorption of yolk by embryos from Virginia was associated with a higher rate of respiration. Assimilation of yolk by rapidly growing embryos could reduce growth or survival after hatching. Such costs might explain the maintenance of countergradient variation in S. undulatus.
选择在寒冷环境中实现快速生长和发育会导致一种被称为反梯度变异的地理模式。东部围栏蜥蜴（Sceloporus undulatus）在胚胎生长和发育过程中沿纬度梯度呈现反梯度变异。为了确定反梯度变异的直接原因，我们在一个实际的热循环中比较了来自寒冷环境（弗吉尼亚州）和温暖环境（南卡罗来纳州）的胚胎在发育过程中的能量预算。通过从大卵中去除卵黄并对其他卵进行假手术操作，控制了种群间平均卵大小的差异。在整个48天的孵化过程中，每4天测量一次呼吸作用。在此之后，解剖卵并通过量热法测定胚胎和卵黄的能量含量。正如先前实验所预期的那样，来自弗吉尼亚州的胚胎比来自南卡罗来纳州的胚胎发育到更高级阶段，并且在组织内储存了更多能量。来自弗吉尼亚州的胚胎对卵黄的吸收更多，这与更高的呼吸速率相关。快速生长的胚胎对卵黄的同化可能会降低孵化后的生长或存活率。这些代价可能解释了东部围栏蜥蜴反梯度变异的维持。
