School of Biological Sciences, The University of Hong Kong, Hong Kong, China.
South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.
Ann Bot. 2019 Sep 24;124(2):331-342. doi: 10.1093/aob/mcz087.
Mycorrhizal associations in mycoheterotrophic plants are generally more specialized than in autotrophs. Mycoheterotrophs typically bear small, inconspicuous flowers that often self-pollinate to maximize seed set, although some have structurally complex flowers indicative of xenogamy. A trade-off has previously been proposed between specialization in these above- and below-ground symbioses, although empirical data are lacking.
We used next-generation DNA sequencing to compare the mycorrhizal communities from the roots of a mycoheterotrophic species, Thismia tentaculata (Thismiaceae), and its neighbouring autotrophs. We furthermore conducted detailed assessments of floral phenology and pollination ecology, and performed artificial pollination experiments to determine the breeding system.
Thismia tentaculata maintains a symbiotic association with a single arbuscular mycorrhizal Rhizophagus species. The flowers are pollinated by a single species of fungus gnats (Corynoptera, Sciaridae), which are attracted by the yellow pigments and are temporarily restrained within the perianth chamber before departing via apertures between the anthers. The plants are self-compatible but predominantly xenogamous.
Our findings demonstrate that T. tentaculata maintains highly specialized associations with pollinators and mycorrhizal fungi, both of which are widely distributed. We suggest that specialization in multiple symbiotic interactions is possible in mycoheterotrophs if redundant selective pressures are not exerted to further restrict an already constrained suite of life-history traits.
菌根共生在菌异养植物中的联系通常比在自养植物中更加特化。菌异养植物通常具有较小、不显眼的花朵，这些花朵通常通过自花授粉来最大限度地提高种子产量，尽管有些花朵具有结构复杂的特征，表明存在异体受精。先前曾提出过在这些地上和地下共生关系中专业化之间的权衡，尽管缺乏经验数据。
我们使用下一代 DNA 测序技术比较了菌异养植物 Thismia tentaculata（野茉莉科）及其相邻自养植物的根系中的菌根群落。我们还对花的物候学和传粉生态学进行了详细评估，并进行了人工授粉实验以确定繁殖系统。
Thismia tentaculata 与单一丛枝菌根 Rhizophagus 物种维持共生关系。花朵由单一的真菌蝇（Corynoptera，摇蚊科）授粉，真菌蝇被黄色色素吸引，并在离开花药之间的开口之前暂时被束缚在花被室内。植物是自交亲和的，但主要是异体受精的。
我们的研究结果表明，T. tentaculata 与传粉者和菌根真菌保持高度特化的联系，这些传粉者和菌根真菌分布广泛。我们认为，如果没有施加冗余的选择压力来进一步限制已经受到限制的生命史特征组合，那么在菌异养植物中，多种共生相互作用的专业化是可能的。
