Fox Charles W, Caldwell Roy L
Department of Integrative Biology, University of California, 94720, Berkeley, CA, USA.
Oecologia. 1994 Apr;97(3):382-389. doi: 10.1007/BF00317329.
Theoretical models of evolution in a temporally variable environment predict that genotypes with low variance in fitness across generations will be favored. When host use varies temporally and fitness trade-offs exist among hosts, such that an increase in performance on one host results in a correlated decrease on the other, selection for low variance in fitness across generations will favor genotypes which are generalists. Before predictions such as this can be extended to natural herbivore populations, however, it is necessary to understand the extent to which performance trade-offs limit simultaneous adaptation to multiple hosts. The experiment reported here compares two populations of the common milkweed bug, Lygaeus kalmii (Hemiptera: Lygaeidae) which differ in patterns of host usage. One population is largely restricted to milkweed (Asclepias spp.) when milkweed seeds are available, but becomes a scavenger on a large assortment of available seeds when milkweed seeds are unavailable. The second population is restricted largely to dandelion (Taraxacum officinale), without access to milkweed. We examine these populations to test for host-associated genetic trade-offs between specialization on dandelion (Taraxacum) and two species of milkweed, Asclepias fascicularis, which is low in cardiac glycoside content, and A. speciosa, which is high in cardiac glycoside content. Despite the difference in patterns of host use of the two L. kalmii populations, the populations did not differ in their performance on any of the host plants. Within each population, bugs performed nearly as well on each host, except that bugs had significantly lower survivorship on dandelion than on either milkweed species. Trade-offs in performance among hosts were not present in either population: estimated genetic correlations across hosts were strongly positive. The inability of this study to detect host-associated fitness trade-offs is consistent with most published data on this topic.
在一个随时间变化的环境中，进化的理论模型预测，那些在各代适应性方面变异较小的基因型将受到青睐。当寄主利用随时间变化，且在不同寄主之间存在适应性权衡时，即一种寄主上表现的增加会导致另一种寄主上表现的相应降低，那么选择各代适应性变异较小的基因型将有利于那些具有广谱适应性的基因型。然而，在将此类预测扩展到自然食草动物种群之前，有必要了解表现权衡在多大程度上限制了对多种寄主的同时适应。本文报道的实验比较了普通乳草蝽（Lygaeus kalmii，半翅目：长蝽科）的两个种群，它们在寄主利用模式上存在差异。一个种群在有乳草种子时主要以乳草（马利筋属植物）为食，但在没有乳草种子时会成为多种可获得种子的清道夫。第二个种群主要局限于蒲公英（Taraxacum officinale），无法接触到乳草。我们研究这些种群，以测试在专食蒲公英与两种乳草（强心苷含量低的A. fascicularis和强心苷含量高的A. speciosa）之间是否存在与寄主相关的遗传权衡。尽管两个乳草蝽种群的寄主利用模式存在差异，但它们在任何一种寄主植物上的表现并无不同。在每个种群内部，虫子在每种寄主上的表现都差不多，只是在蒲公英上的存活率明显低于在任何一种乳草上的存活率。两个种群中都不存在寄主间表现的权衡：估计的不同寄主间的遗传相关性都很强且为正。本研究未能检测到与寄主相关的适应性权衡，这与关于该主题的大多数已发表数据一致。
