On the origins of extreme mycorrhizal specificity in the Monotropoideae (Ericaceae): performance trade-offs during seed germination and seedling development.
Fungal-induced seed germination is a phenomenon characteristic of mycorrhizal plants that produce dust-like seeds with only minimal nutritional reserves. In such systems, fungi trigger germination and/or subsidize development. We studied mycorrhizal germination in relation to mycorrhizal specificity in the Monotropoideae, a lineage of dust-seeded non-photosynthetic plants that are dependent upon ectomycorrhizal fungi of forest trees. A total of 1695 seed packets, each containing two to five compartments with seeds from different sources, were buried for up to 2 years near known ectomycorrhizal fungi in six different native forest locations. Upon harvest, seedlings were analysed by cultivation-independent molecular methods to identify their mycorrhizal fungi. We report that (i) germination is only induced by the same fungus that associates with mature plants or by closely related congeners; (ii) seedlings associated with the latter fungi develop less than those associated with maternal fungal species in most settings; and (iii) exceptions to this pattern occur in allopatric settings, where novel plant-fungal associations can result in the greatest seedling development. We interpret these results as evidence of performance trade-offs between breadth of host range and rate of development. We propose that in conjunction with host-derived germination cues, performance trade-offs can explain the extreme mycorrhizal specificity observed at maturity. The allopatric exceptions support the idea that performance trade-offs may be based on a coevolutionary arms race and that host range can be broadened most readily when naive fungal hosts are encountered in novel settings.
Bidartondo
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Imperial College London & Royal Botanic Gardens, Kew, TW9 3DS, UK. m.bidartondo@kew.org
Bruns
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TD
D000704:Analysis of Variance; D001483:Base Sequence; D001487:Basidiomycota / Q000235:genetics / Q000502:physiology*; D029791:Ericaceae / Q000254:growth & development* / Q000382:microbiology; D018525:Germination / Q000502:physiology*; D008969:Molecular Sequence Data; D038821:Mycorrhizae / Q000235:genetics / Q000502:physiology*; D012639:Seeds / Q000254:growth & development* / Q000382:microbiology; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D013559:Symbiosis; D014481:United States