Department of Biology, Duke University, Durham, NC, 27708, USA.
Department of Cell & Systems Biology, University of Toronto, 25 Willcocks St., Toronto, ON, M5S 3B2, Canada.
New Phytol. 2013 Apr;198(2):557-566. doi: 10.1111/nph.12165. Epub 2013 Feb 13.
Coevolutionary theory predicts that the distribution of obligately symbiotic organisms will be determined by the dispersal ability and ecological range of both partners. We examined this prediction for lichen-forming fungi that form obligate symbioses with cyanobacteria. We compared genotypes of both partners of 250 lichens collected at multiple spatial scales in British Columbia, Canada. Multilocus sequence data collected from a subset of 128 of the specimens were used to determine the degree of recombination within the cyanobacterial populations. We found that six distinct clusters of cyanobacterial genotypes are distributed throughout the known global phylogeny of the genus Nostoc, and that each appears to be evolving clonally. Fungal specialization is high, with each species associating with either one or two of the cyanobacterial clusters, while cyanobacterial specialization varies, with clusters associating with between one and 12 different fungal species. Specialization also varies geographically, with some combinations restricted to a single site despite the availability of both partners elsewhere. Photobiont association patterns are determined by a combination of genetically based specificity, spatial population structure, and ecological factors and cannot be easily predicted by photobiont dispersal syndromes.
协同进化理论预测，专性共生生物的分布将由两个共生伙伴的扩散能力和生态范围决定。我们检验了这一预测在与蓝细菌形成专性共生关系的地衣真菌中的适用性。我们比较了在加拿大不列颠哥伦比亚省多个空间尺度采集的 250 种地衣中两个共生伙伴的基因型。从 128 个样本中的一个子集收集的多基因序列数据用于确定蓝细菌种群内重组的程度。我们发现，六个不同的蓝细菌基因型簇分布在整个 Nostoc 属的已知全球系统发育中，每个基因型簇似乎都在以克隆的方式进化。真菌的专化性很高，每个物种都与一个或两个蓝细菌簇相关，而蓝细菌的专化性则不同，每个蓝细菌簇与 1 到 12 种不同的真菌物种相关联。专化性也存在地理差异，尽管在其他地方也有两个共生伙伴，但某些组合仅局限于一个单一的地点。生境藻的共生模式是由基于遗传的特异性、空间种群结构和生态因素共同决定的，不能简单地根据生境藻的扩散模式来预测。
