Abstract
Lichens exhibit varying degrees of specialization with regard to the surfaces they colonize, ranging from substrate generalists to strict substrate specialists. Though long recognized, the causes and consequences of substrate specialization are poorly known. Using a phylogeny of a 150–200 Mya clade of lichen fungi, we asked whether substrate niche is phylogenetically conserved, which substrates are ancestral, whether specialists arise from generalists or vice versa and how specialization affects speciation/extinction processes. We found strong phylogenetic signal for niche conservatism. Specialists evolved into generalists and back again, but transitions from generalism to specialism were more common than the reverse. Our models suggest that for this group of fungi, ‘escape’ from specialization for soil, rock and bark occurred, but specialization for wood foreclosed evolution away from that substrate type. In parallel, speciation models showed positive diversification rates for soil and rock dwellers but not other specialists. Patterns in the studied group suggest that fungal substrate specificity is a key determinant of evolutionary trajectory for the entire lichen symbiosis.
Highlights
Lichens are frequently held up as exemplary environmental indicators owing to their sensitivity to abiotic conditions [1]
Using a phylogeny of a 150–200 Mya clade of lichen fungi, we asked whether substrate niche is phylogenetically conserved, which substrates are ancestral, whether specialists arise from generalists or vice versa and how specialization affects speciation/extinction processes
While a small number of lichen symbioses occur over a wide range of substrates, flourishing on rock, organic soil, tree bark and wood, the large majority of lichens have narrower ranges of substrate use, so much so that substrate has for decades been used as a surrogate for subtle morphological characters to recognize lichens, literally as a key character [4,5]
Summary
Lichens are frequently held up as exemplary environmental indicators owing to their sensitivity to abiotic conditions [1]. We created a plot indicating the number of trees for which a particular ancestral character state was estimated under all possible models for one method. To test whether models prohibiting certain substrate transitions are more likely given our set of trees, we created 30 transition rate matrices describing different scenarios of character change. We compared these constrained models on each of the 100 trees from the BEAST posterior distribution of trees. Owing to the lack of consensus on the performance and suitability of the SSE approach to model evolutionary trends [52], we tested the extent to which the modelled diversification rates respond to the phylogenetic tree alone without a further connection to the distribution of characters on the tree according to the method described in [52]
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