Ancient and recent introgression shape the evolutionary history of pollinator adaptation and speciation in a model monkeyflower radiation (MimulussectionErythranthe)
Authors: Nelson Thomas C. , Stathos Angela M. , Vanderpool Daniel D. , Finseth Findley R. , Yuan Yao-wu , Fishman Lila 
Published: September 2020

Inferences about past processes of adaptation and speciation require a gene-scale and genome-wide understanding of the evolutionary history of diverging taxa. In this study, we use genome-wide capture of nuclear gene sequences, plus skimming of organellar sequences, to investigate the phylogenomics of monkeyflowers inMimulussectionErythranthe(27 accessions from seven species). Taxa withinErythranthe, particularly the parapatric and putatively sister speciesM. lewisii(bee-pollinated) andM. cardinalis(hummingbird-pollinated), have been a model system for investigating the ecological genetics of speciation and adaptation for over five decades. Across >8000 nuclear loci, multiple methods resolve a predominant species tree in whichM. cardinalisgroups with other hummingbird-pollinated taxa (37% of gene trees), rather than being sister toM. lewisii(32% of gene trees). We independently corroborate a single evolution of hummingbird pollination syndrome inErythrantheby demonstrating functional redundancy in genetic complementation tests of floral traits in hybrids; together, these analyses overturn a textbook case of pollination-syndrome convergence. Strong asymmetries in allele-sharing (Patterson’s D-statistic and related tests) indicate that gene-tree discordance reflects ancient and recent introgression rather than incomplete lineage sorting. Consistent with abundant introgression blurring the history of divergence, low-recombination and adaptation-associated regions support the new species tree, while high-recombination regions generate phylogenetic evidence for sister status forM. lewisiiandM. cardinalis. Population-level sampling of core taxa also revealed two instances of chloroplast capture, with SierranM. lewisiiand Southern CalifornianM. parishiieach carrying organelle genomes nested within respective sympatricM. cardinalisclades. A recent organellar transfer fromM. cardinalis, an outcrosser where selfish cytonuclear dynamics are more likely, may account for the unexpected cytoplasmic male sterility effects of selferM. parishiiorganelles in hybrids withM. lewisii. Overall, our phylogenomic results reveal extensive reticulation throughout the evolutionary history of a classic monkeyflower radiation, suggesting that natural selection (re-)assembles and maintains species-diagnostic traits and barriers in the face of gene flow. Our findings further underline the challenges, even in reproductively isolated species, in distinguishing re-use of adaptive alleles from true convergence and emphasize the value of a phylogenomic framework for reconstructing the evolutionary genetics of adaptation and speciation.

Author Summary

Adaptive radiations, which involve both divergent evolution of new traits and recurrent trait evolution, provide insight into the processes that generate and maintain organismal diversity. However, rapid radiations also generate particular challenges for inferring the evolutionary history and mechanistic basis of adaptation and speciation, as multiple processes can cause different parts of the genome to have distinct phylogenetic trees. Thus, inferences about the mode and timing of divergence and the causes of parallel trait evolution require a fine-grained understanding of the flow of genomic variation through time. In this study, we used genome-wide sampling of thousands of genes to re-construct the evolutionary histories of a model plant radiation, the monkeyflowers ofMimulussectionErythranthe. Work over the past half-century has established the parapatric and putatively sister speciesM. lewisii(bee-pollinated) andM. cardinalis(hummingbird-pollinated, as are three other species in the section) as textbook examples of both rapid speciation via shifts in pollination syndrome and convergent evolution of floral syndromes. Our phylogenomic analyses re-write both of these stories, placingM. cardinalisin a clade with other hummingbird-pollinated taxa and demonstrating that abundant introgression between ancestral lineages as well as in areas of current sympatry contributes to the real (but misleading) affinities betweenM. cardinalisandM. lewisii. This work illustrates the pervasive influence of gene flow and introgression during adaptive radiation and speciation, and underlines the necessity of a gene-scale and genome-wide phylogenomics framework for understanding trait divergence, even among well-established species.