Department of Entomology, Michigan State University, 288 Farm Lane, Room 243, East Lansing, MI 48824, USA.
Natural History Museum, University Museum of Bergen, University of Bergen, NO-5007 Bergen, Norway.
Mol Phylogenet Evol. 2021 Jun;159:107107. doi: 10.1016/j.ympev.2021.107107. Epub 2021 Feb 17.
The influence of plants in the diversification of herbivorous insects, specifically those that utilize moribund and dead hosts, is little explored. Host shifts are expected because the effectiveness of toxic secondary chemicals is lessened by decay of dead plants. Feeding on dead plants also releases herbivorous insect lineages from diversifying within a particular plant lineage. Thus, phylogenetic constraints on the herbivorous insect lineage imposed by the host plants are diminished and repeated patterns of species diversification in an association with unrelated host trees is hypothesized (i.e., taxon cycle). Scolytini, a diverse weevil tribe, specialize on many different dead and moribund plant taxa as a source of food. These species and their hosts offer an opportunity to examine the association between dead host plants and the extent of phylogenetic constraints. A phylogeny of the Scolytini was reconstructed with likelihood and Bayesian analyses of DNA sequence data from nuclear (28S, CAD, ArgK) and mitochondrial (COI) genes. Ancestral host usage and geography was reconstructed using likelihood criteria and conservation of host use was tested. Results supported a monophyletic Scolytini, Ceratolepis, Loganius, and a paraphyletic Scolytus, Camptocerus and Cnemonyx. Diversification of the Scolytini generally occurred well after their host taxa diversified and suggests a sequential evolution of host use. In this scenario the beetle imposes little selection pressure on the tree but the tree provides a platform for beetle evolution. Major changes in host tree use occurred during periods of global cooling associated with changes in beetle biogeography. Diversification of beetles occurred on common and widespread hosts and there was likely a single origination of conifer-feeding from angiosperm-feeding species during the early Pliocene and a radiation of beetle species from the Palearctic to the Nearctic. Overall, the observed patterns of Scolytini host use are conserved and are similar to those expected in a taxon pulse diversification. That is, after a host switch to an unrelated tree, the beetles diversify within the host plant lineage. The need to locate an ephemeral food resource, i.e., a dying tree, likely maintains host specificity once a host shift occurs. These findings suggest that characteristics of dead and moribund host plants (e.g. secondary chemicals) influence the diversification of these saproxlic weevils despite the reduction of selection pressures.
植物对食草昆虫多样性的影响，特别是那些利用垂死和死亡宿主的昆虫，研究甚少。由于死亡植物的有毒次生化学物质的有效性降低，预计会发生宿主转移。以死亡植物为食也使食草昆虫谱系摆脱了在特定植物谱系内多样化的束缚。因此，宿主植物对食草昆虫谱系的系统发育约束被减弱，并且与无关的宿主树木相关联的物种多样化的重复模式被假设（即分类群循环）。象甲科是一个多样化的象甲部落，专门以许多不同的死亡和垂死的植物类群为食源。这些物种及其宿主为研究死亡宿主植物与系统发育约束程度之间的关系提供了机会。利用核（28S、CAD、ArgK）和线粒体（COI）基因的 DNA 序列数据进行似然和贝叶斯分析，重建了象甲科的系统发育。利用似然标准重建了祖先的宿主使用和地理分布，并测试了宿主使用的保守性。结果支持象甲科、Ceratolepis、Loganius 的单系性，以及 Scaptocerus、Camptocerus 和 Cnemonyx 的并系性。象甲科的多样化通常发生在其宿主类群多样化之后，这表明宿主使用的顺序进化。在这种情况下，甲虫对树施加的选择压力很小，但树为甲虫的进化提供了一个平台。宿主树使用的重大变化发生在与甲虫生物地理学变化相关的全球变冷时期。甲虫在常见和广泛的宿主上发生多样化，并且可能在早更新世期间从被子植物食性物种单一起源出针叶树食性物种，并且在古北区到近北区辐射出甲虫物种。总体而言，观察到的象甲科宿主使用模式是保守的，与分类群脉冲多样化中预期的模式相似。也就是说，在与无关的树发生宿主转移后，甲虫在宿主植物谱系内多样化。一旦发生宿主转移，找到一个短暂的食物资源（即垂死的树）的需求可能会保持宿主特异性。这些发现表明，尽管选择压力降低，但垂死和垂死宿主植物的特征（例如次生化学物质）会影响这些腐木象甲的多样化。
