Craig Timothy P, Itami Joanne K, Horner John D
Department of Biology, University of Minnesota-Duluth, Duluth, Minnesota 55803, USA.
Evolution. 2007 May;61(5):1137-52. doi: 10.1111/j.1558-5646.2007.00099.x.
The geographic mosaic theory of coevolution predicts that geographic variation in species interactions will lead to differing selective pressures on interacting species, producing geographic variation in the traits of interacting species (Thompson 2005). We supported this hypothesis in a study of the geographic variation in the interactions among Eurosta solidaginis and its natural enemies. Eurosta solidaginis is a fly (Diptera: Tephritidae) that induces galls on subspecies of tall goldenrod, Solidago altissima altissima and S. a. gilvocanescens. We measured selection on E. solidaginis gall size and shape in the prairie and forest biomes in Minnesota and North Dakota over an 11-year period. Galls were larger and more spherical in the prairie than in the forest. We supported the hypothesis that the divergence in gall morphology in the two biomes is due to different selection regimes exerted by natural enemies of E. solidaginis. Each natural enemy exerted similar selection on gall diameter in both biomes, but differences in the frequency of natural enemy attack created strong differences in overall selection between the prairie and forest. Bird predation increased with gall diameter, creating selection for smaller-diameter galls. A parasitic wasp, Eurytoma gigantea, and Mordellistena convicta, an inquiline beetle, both caused higher E. solidaginis mortality in smaller galls, exerting selection for increased gall diameter. In the forest there was stabilizing selection on gall diameter due to a combination of bird predation on larvae in large galls, and M. convicta- and E. gigantea-induced mortality on larvae in small galls. In the prairie there was directional selection for larger galls due to M. convicta and E. gigantea mortality on larvae in small galls. Mordellistena convicta-induced mortality was consistently higher in the prairie than in the forest, whereas there was no significant difference in E. gigantea-induced mortality between biomes. Bird predation was nonexistent in the prairie so the selection against large galls found in the forest was absent. We supported the hypothesis that natural enemies of E. solidaginis exerted selection for spherical galls in both biomes. In the prairie M. convicta exerts stabilizing selection to maintain spherical galls. In the forest there was directional selection for more spherical galls. Eurytoma gigantea exerted selection on gall shape in the forest in a complex manner that varied among years. We also supported the hypothesis that E. gigantea is coevolving with E. solidaginis. The parasitoid had significantly longer ovipositors in the prairie than in the forest, indicating the possibility that it has evolved in response to selection to reach larvae in the larger-diameter prairie galls.
协同进化的地理镶嵌理论预测，物种间相互作用的地理差异将导致对相互作用物种的不同选择压力，从而使相互作用物种的性状产生地理差异（汤普森，2005年）。在一项关于欧洲金蝇及其天敌相互作用的地理差异研究中，我们支持了这一假设。欧洲金蝇是一种蝇类（双翅目：实蝇科），它会在高杆一枝黄花的亚种高杆一枝黄花和金黄一枝黄花上诱导形成虫瘿。我们在明尼苏达州和北达科他州的草原和森林生物群落中，历时11年测量了欧洲金蝇虫瘿大小和形状的选择情况。草原上的虫瘿比森林中的更大且更接近球形。我们支持这样的假设，即两个生物群落中虫瘿形态的差异是由欧洲金蝇天敌施加的不同选择机制导致的。每种天敌在两个生物群落中对虫瘿直径的选择作用相似，但天敌攻击频率的差异使得草原和森林在总体选择上产生了强烈差异。鸟类捕食随着虫瘿直径的增加而增加，从而产生了对较小直径虫瘿的选择。一种寄生蜂，巨瘤姬蜂，以及一种寄居甲虫，定罪隐翅虫，在较小的虫瘿中都会导致欧洲金蝇更高的死亡率，从而产生了对更大虫瘿直径的选择。在森林中，由于大型虫瘿中的幼虫受到鸟类捕食，以及小型虫瘿中的幼虫受到定罪隐翅虫和巨瘤姬蜂诱导的死亡，对虫瘿直径存在稳定选择。在草原上，由于小型虫瘿中的幼虫受到定罪隐翅虫和巨瘤姬蜂的死亡影响，对更大的虫瘿存在定向选择。定罪隐翅虫诱导的死亡率在草原上始终高于森林，而巨瘤姬蜂诱导的死亡率在两个生物群落之间没有显著差异。草原上不存在鸟类捕食，因此森林中对大型虫瘿的选择不存在。我们支持这样的假设，即欧洲金蝇的天敌在两个生物群落中都对球形虫瘿进行了选择。在草原上，定罪隐翅虫施加稳定选择以维持球形虫瘿。在森林中，对更接近球形的虫瘿存在定向选择。巨瘤姬蜂以一种复杂的方式在森林中对虫瘿形状进行选择，这种方式在不同年份有所变化。我们还支持巨瘤姬蜂与欧洲金蝇正在协同进化的假设。这种寄生蜂在草原上的产卵器明显比在森林中的长，这表明它有可能是为了应对在更大直径的草原虫瘿中接触幼虫的选择而进化的。
