Evolution of the host range in spider mites

叶螨属的植食性螨类是高度多食性的，存在于各种植物物种上。然而，以存活率和产卵率表示的寄主适应性在不同植物之间差异很大，有些物种如菊花对它们非常不利。本研究阐明了决定叶螨寄主范围的机理，特别是二斑叶螨，这是最重要的农业害虫之一。研究结果总结如下。麻黄病菌的寄主适应性存在种内遗传变异，而在农业生态系统中也可能发生的人工选择能迅速提高对不利寄主的生态表现。然而，螨类能否在不合适的宿主上停留和定居，在很大程度上取决于它们以前通过取食经历在宿主上的经历，也受到植物排放的恶臭的经历的影响。即使只经历了Odur，它们对不利寄主的接受性也大大增加了。因此，这些螨类在适应新寄主方面具有很高的行为可塑性。未交配的雌性比交配的雌性对不利于寄主的可塑性要高得多。用随机扩增多态性DNA方法分析表明，不同寄主种类间的遗传分化不太可能发生在多食性蜘蛛体内。我们的结论是，蠕形螨的寄主可接受性被非遗传的行为变化所增强，而这种可接受性又被作用于该种群的遗传选择压力所加速，该寄主的扩散能力很低。

Phytophagous mites of the genus Tetranychus are highly polyphagous and occurs on a variety of plant species. However, the host adaptability of the mites, as expressed by their survival and oviposition rates, varies greatly among the plant species, and some species such as chrysanthemums are very unfavorable to them. This study elucidated the mechnisms that determine the host range of the mites, with special emphasis on the two-spotted spider mite, T.urticae, one of the most important agricultural pests. The results of the study are summarized as follows. Genetic, intraspecific variations exist in the host adaptability of T.urticae, and the ecological performance on unfavorable hosts is enhanced very rapidly by an artificial selection that may occur also in agricultural ecosystems. However, whether or not the mites can stay and settle on an unsuitable host is largely affected by their previous experiences on the host through feeding experience and also by the experience of the odur that is emitted by the plant. Even if the mites experienced only the odur, their acceptability of the unfavorable host was greatly increased. Thus, the mites have high behavioral plasticity in adapting to new hosts. Unmated females have much higher plasticity than mated ones, to unfavorable hosts. The analysis using RAPD method suggested that genetic differentiation is not unlikely to occur among different host species in polyphagous spider mites, such as T.urticae. We conclude that the host acceptability of T.urticae, which has a very low dispersal ability, is enhanced by the non-genetic, behavioral change of the mites, and it is then accerelated by genetic selection pressure that will act on the mite population.