Thorogood C J, Rumsey F J, Hiscock S J
School of Biological Sciences, University of Bristol, Woodland Road, Bristol, UK.
Ann Bot. 2009 May;103(7):1005-14. doi: 10.1093/aob/mcp034. Epub 2009 Feb 26.
Orobanche minor is a root-holoparasitic angiosperm that attacks a wide range of host species, including a number of commonly cultivated crops. The extent to which genetic divergence among natural populations of O. minor is influenced by host specificity has not been determined previously. Here, the host specificity of natural populations of O. minor is quantified for the first time, and evidence that this species may comprise distinct physiological races is provided.
A tripartite approach was used to examine the physiological basis for the divergence of populations occurring on different hosts: (1) host-parasite interactions were cultivated in rhizotron bioassays in order to quantify the early stages of the infection and establishment processes; (2) using reciprocal-infection experiments, parasite races were cultivated on their natural and alien hosts, and their fitness determined in terms of biomass; and (3) the anatomy of the host-parasite interface was investigated using histochemical techniques, with a view to comparing the infection process on different hosts.
Races occurring naturally on red clover (Trifolium pratense) and sea carrot (Daucus carota ssp. gummifer) showed distinct patterns of host specificity: parasites cultivated in cross-infection studies showed a higher fitness on their natural hosts, suggesting that races show local adaptation to specific hosts. In addition, histological evidence suggests that clover and carrot roots vary in their responses to infection. Different root anatomy and responses to infection may underpin a physiological basis for host specificity.
It is speculated that host specificity may isolate races of Orobanche on different hosts, accelerating divergence and ultimately speciation in this genus. The rapid life cycle and broad host range of O. minor make this species an ideal model with which to study the interactions of parasitic plants with their host associates.
小列当是一种根全寄生被子植物，可侵染多种寄主物种，包括一些常见的栽培作物。此前尚未确定小列当自然种群间的遗传分化受寄主特异性影响的程度。本文首次对小列当自然种群的寄主特异性进行了量化，并提供了该物种可能包含不同生理小种的证据。
采用三方研究方法来探究不同寄主上种群分化的生理基础：（1）在根箱生物测定中培养寄主 - 寄生植物相互作用，以量化感染和定殖过程的早期阶段；（2）通过互作感染实验，在其自然寄主和外来寄主上培养寄生植物小种，并根据生物量确定其适合度；（3）使用组织化学技术研究寄主 - 寄生植物界面的解剖结构，以比较在不同寄主上的感染过程。
自然发生在红三叶草（Trifolium pratense）和海胡萝卜（Daucus carota ssp. gummifer）上的小种表现出不同的寄主特异性模式：在交叉感染研究中培养的寄生植物在其自然寄主上表现出更高的适合度，这表明小种对特定寄主具有局部适应性。此外，组织学证据表明，三叶草和胡萝卜根对感染的反应不同。不同的根解剖结构和对感染的反应可能是寄主特异性生理基础的支撑。
据推测，寄主特异性可能会隔离不同寄主上的列当小种，加速该属的分化并最终导致物种形成。小列当的快速生命周期和广泛寄主范围使其成为研究寄生植物与其寄主相互作用的理想模型。
