Biere Arjen, Honders Sonja
Department of Plant Population Biology, Netherlands Institute of Ecology, NIOO-CTO, P.O. Box 40, 6666 ZG, Heteren, The Netherlands.
Oecologia. 1996 Aug;107(3):307-320. doi: 10.1007/BF00328447.
It is often assumed that host specialization is promoted by trade-offs in the performance of parasites on different host species, but experimental evidence for such trade-offs is scant. We studied differences in performance among strains of the anther smut fungus Ustilago violacea from two closely related host plant species, Silene alba and S. dioica, on progeny of (1) the host species from which they originated, (2) the alternative host species, and (3) inter-specific hybrids. Significant intra-specific variation in the pathogen was found for both infection success on a range of host genotypes (virulence) and components of spore production per infected host (aggressiveness) (sensu Burdon 1987). Strains did not have overall higher virulence on conspecifics of their host of origin than on strains from the heterospecific host, but they did have a significantly (c. 3 times) higher spore production per infected male host. This finding suggests that host adaptation may have evolved with respect to aggressiveness rather than virulence. The higher aggressiveness of strains on conspecifics of their host of origin resulted both from higher spore production per infected flower (spores are produced in the anthers), and greater ability to stimulate flower production on infected hosts. The latter indicates the presence of adaptive intraspecific variation in the ability of host manipulation. As transmission of the fungus is mediated by insects that are both pollinators of the host and vectors of the disease, we also assessed the effect of strains on host floral traits. Infection resulted in a reduction of inflorescence height, flower size, and nectar production per flower. Strains did not differ in their effect on nectar production, but infection with strains from S. alba resulted in a stronger reduction of inflorescence height and petal size on both host species. Vectors may therefore in principle discriminate among hosts infected by different strains and affect their efficiency of transmission. Contrary to assumptions of recent hypotheses about the role of host hybrids in the evolution of parasites, hybrids were not generally more susceptible than parental hosts. It is therefore unlikely that the rate of evolution of the pathogen on the parental species is slowed down by selection for specialization on the hybrids.
人们常常认为，宿主专一性是由寄生虫在不同宿主物种上的表现权衡所推动的，但这种权衡的实验证据却很少。我们研究了来自两种近缘宿主植物物种白麦瓶草和二型麦瓶草的花药黑粉菌菌株在以下三种子代上的表现差异：（1）它们所起源的宿主物种；（2）替代宿主物种；（3）种间杂种。在一系列宿主基因型上的感染成功率（毒力）以及每个受感染宿主的孢子产生成分（侵袭力）（根据Burdon 1987年的定义）方面，均发现了该病原体显著的种内变异。菌株对其起源宿主的同种个体的总体毒力并不高于来自异种宿主的菌株，但它们在每个受感染雄性宿主上的孢子产量显著更高（约为3倍）。这一发现表明，宿主适应性可能是相对于侵袭力而非毒力进化而来的。菌株对其起源宿主的同种个体具有更高的侵袭力，这既源于每个受感染花朵产生的孢子数量更多（孢子在花药中产生），也源于在受感染宿主上刺激花朵产生的能力更强。后者表明在宿主操控能力方面存在适应性种内变异。由于真菌的传播是由既是宿主传粉者又是疾病传播媒介的昆虫介导的，我们还评估了菌株对宿主花部性状的影响。感染导致花序高度、花朵大小和每朵花的花蜜产量降低。菌株在对花蜜产量的影响上没有差异，但感染来自白麦瓶草的菌株会导致两种宿主物种的花序高度和花瓣大小降幅更大。因此，媒介原则上可能会区分受不同菌株感染的宿主，并影响它们的传播效率。与最近关于宿主杂种在寄生虫进化中作用的假设不同，杂种通常并不比亲本宿主更易感染。因此，病原体在亲本物种上的进化速率不太可能因对杂种的专一化选择而减缓。
