Haak David C, McGinnis Leslie A, Levey Douglas J, Tewksbury Joshua J
Department of Biology, University of Washington, PO Box 351800, 24 Kincaid Hall, Seattle, WA 98195-1800, USA.
Proc Biol Sci. 2012 May 22;279(1735):2012-7. doi: 10.1098/rspb.2011.2091. Epub 2011 Dec 21.
Evolutionary biologists increasingly recognize that evolution can be constrained by trade-offs, yet our understanding of how and when such constraints are manifested and whether they restrict adaptive divergence in populations remains limited. Here, we show that spatial heterogeneity in moisture maintains a polymorphism for pungency (heat) among natural populations of wild chilies (Capsicum chacoense) because traits influencing water-use efficiency are functionally integrated with traits controlling pungency (the production of capsaicinoids). Pungent and non-pungent chilies occur along a cline in moisture that spans their native range in Bolivia, and the proportion of pungent plants in populations increases with greater moisture availability. In high moisture environments, pungency is beneficial because capsaicinoids protect the fruit from pathogenic fungi, and is not costly because pungent and non-pungent chilies grown in well-watered conditions produce equal numbers of seeds. In low moisture environments, pungency is less beneficial as the risk of fungal infection is lower, and carries a significant cost because, under drought stress, seed production in pungent chilies is reduced by 50 per cent relative to non-pungent plants grown in identical conditions. This large difference in seed production under water-stressed (WS) conditions explains the existence of populations dominated by non-pungent plants, and appears to result from a genetic correlation between pungency and stomatal density: non-pungent plants, segregating from intra-population crosses, exhibit significantly lower stomatal density (p = 0.003), thereby reducing gas exchange under WS conditions. These results demonstrate the importance of trait integration in constraining adaptive divergence among populations.
进化生物学家越来越认识到，进化可能受到权衡的限制，然而我们对于这种限制如何以及何时表现出来，以及它们是否限制种群中的适应性分化的理解仍然有限。在这里，我们表明，水分的空间异质性维持了野生辣椒（查科辣椒）自然种群中辣味（热度）的多态性，因为影响水分利用效率的性状与控制辣味（辣椒素类物质的产生）的性状在功能上是整合在一起的。辛辣和不辛辣的辣椒沿着一条水分梯度分布在它们在玻利维亚的原生范围内，种群中辛辣植物的比例随着水分可利用性的增加而增加。在高湿度环境中，辣味是有益的，因为辣椒素类物质可以保护果实免受致病真菌的侵害，而且没有成本，因为在水分充足条件下种植的辛辣和不辛辣辣椒产生的种子数量相等。在低湿度环境中，辣味的益处较小，因为真菌感染的风险较低，而且会带来显著的成本，因为在干旱胁迫下，辛辣辣椒的种子产量相对于在相同条件下生长的不辛辣植物减少了50%。在水分胁迫（WS）条件下种子产量的这种巨大差异解释了以不辛辣植物为主的种群的存在，这似乎是由于辣味和气孔密度之间的遗传相关性导致的：从种群内杂交中分离出来的不辛辣植物表现出明显较低的气孔密度（p = 0.003），从而在WS条件下减少了气体交换。这些结果证明了性状整合在限制种群间适应性分化中的重要性。
