Abstract
BackgroundFactors promoting diversification in lichen symbioses remain largely unexplored. While Pleistocene events have been important for driving diversification and affecting distributions in many groups, recent estimates suggest that major radiations within some genera in the largest clade of macrolichens (Parmeliaceae, Ascomycota) vastly predate the Pleistocene. To better understand the temporal placement and sequence of diversification events in lichens, we estimated divergence times in a common lichen-forming fungal genus, Melanohalea, in the Northern Hemisphere. Divergence times were estimated using both concatenated gene tree and coalescent-based multilocus species tree approaches to assess the temporal context of major radiation events within Melanohalea. In order to complement our understanding of processes impacting genetic differentiation, we also evaluated the effects of Pleistocene glacial cycles on population demographics of distinct Melanohalea lineages, differing in reproductive strategies.ResultsWe found that divergence estimates, from both concatenated gene tree and coalescent-based multilocus species tree approaches, suggest that diversification within Melanohalea occurred predominantly during the Miocene and Pliocene, although estimated of divergence times differed by up to 8.3 million years between the two methods. These results indicate that, in some cases, taxonomically diagnostic characters may be maintained among divergent lineages for millions of years. In other cases, similar phenotypic characters among non-sister taxa, including reproductive strategies, suggest the potential for convergent evolution due to similar selective pressures among distinct lineages. Our analyses provide evidence of population expansions predating the last glacial maximum in the sampled lineages. These results suggest that Pleistocene glaciations were not inherently unfavorable or restrictive for some Melanohalea species, albeit with apparently different demographic histories between sexually and vegetatively reproducing lineages.ConclusionsOur results contribute to the understanding of how major changes during the Miocene and Pliocene have been important in promoting diversification within common lichen-forming fungi in the northern Hemisphere. Additionally, we provide evidence that glacial oscillations have influenced current population structure of broadly distributed lichenized fungal species throughout the Holarctic.
Highlights
Factors promoting diversification in lichen symbioses remain largely unexplored
In this study we provide a comparison of dated divergence time estimates between concatenated gene trees and a calibrated multilocus species tree for the lichenforming fungal genus Melanohalea
Our results showed a consistent pattern of more recent divergence times estimated from the coalescent-based species tree approach, relative to the concatenated gene tree analysis
Summary
Factors promoting diversification in lichen symbioses remain largely unexplored. While Pleistocene events have been important for driving diversification and affecting distributions in many groups, recent estimates suggest that major radiations within some genera in the largest clade of macrolichens (Parmeliaceae, Ascomycota) vastly predate the Pleistocene. In spite of the recent advancements in recognizing diversity in lichen-forming fungi, assessing diversification within a temporal context remains largely unexplored in most groups of these important fungal symbionts, (exceptions include [12,13,14]). This is largely due to the poor fossil record for lichenized fungi, and fungi in general, and uncertainties in the interpretation of the few known fossil records [15,16,17]. The timing of speciation events plays a valuable role, complementary to discovering and describing diversity, by addressing biogeographical, climatic, ecological, and other factors associated with diversification and extinction within a temporal context, (e.g. [12,18,19,20])
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