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1. Disentangling Complex Histories of Hybridisation: The Genomic Consequences of Ancient and Recent Introgression in Channel Island Monkeyflowers

   https://doi.org/10.1111/mec.17778  

   Short, Aidan_W; Streisfeld, Matthew_A (April 2025, Molecular Ecology)

   ABSTRACT Hybridisation is a common feature of evolutionary radiations, but its genomic consequences vary depending on when it occurs. Since reproductive isolation takes time to accumulate, hybridisation can occur at multiple points during divergence. Previous studies suggested that the taxonomic diversity in evolutionary radiations can help infer the timing of past gene flow events. Here, we assess the power of these approaches for revealing when gene flow occurred between two monkeyflower taxa (Mimulus aurantiacus) endemic to the Channel Islands of California. Coalescent simulations reveal that conventional four‐taxon tests may not be capable of fully distinguishing between recent and ancient introgression, but genome‐wide patterns of phylogenetic discordance vary predictably with different histories of hybridisation. Using whole‐genome sequencing and phylogenetic tests for introgression across theM. aurantiacusradiation, we identify signals of both ancient and recent hybridisation that occurred between the island taxa and their ancestors. In addition, we find widespread selection against introgressed ancestry, consistent with polygenic barriers to gene flow. However, we also identify localised signals across the genome that may indicate adaptive introgression. This study highlights the power and challenges of trying to disentangle complex histories of hybridisation. More broadly, our results illustrate the multiple roles that gene flow can play in evolutionary radiations: hybridisation can expose genetic incompatibilities that contribute to reproductive isolation while also likely facilitating adaptation by transferring beneficial alleles between taxa. These findings underscore the dynamic interplay between the timing of hybridisation and natural selection in shaping evolutionary trajectories within radiations. 

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2. Ancient hybridization leads to the repeated evolution of red flowers across a monkeyflower radiation

   https://doi.org/10.1093/evlett/qrad024  

   Short, Aidan W.; Streisfeld, Matthew A. (June 2023, Evolution Letters)

   Abstract The reuse of old genetic variation can promote rapid diversification in evolutionary radiations, but in most cases, the historical events underlying this divergence are not known. For example, ancient hybridization can generate new combinations of alleles that sort into descendant lineages, potentially providing the raw material to initiate divergence. In the Mimulus aurantiacus species complex, there is evidence for widespread gene flow among members of this radiation. In addition, allelic variation in the MaMyb2 gene is responsible for differences in flower color between the closely related ecotypes of subspecies puniceus, contributing to reproductive isolation by pollinators. Previous work suggested that MaMyb2 was introgressed into the red-flowered ecotype of puniceus. However, additional taxa within the radiation have independently evolved red flowers from their yellow-flowered ancestors, raising the possibility that this introgression had a more ancient origin. In this study, we used repeated tests of admixture from whole-genome sequence data across this diverse radiation to demonstrate that there has been both ancient and recurrent hybridization in this group. However, most of the signal of this ancient introgression has been removed due to selection, suggesting that widespread barriers to gene flow are in place between taxa. Yet, a roughly 30 kb region that contains the MaMyb2 gene is currently shared only among the red-flowered taxa. Patterns of admixture, sequence divergence, and extended haplotype homozygosity across this region confirm a history of ancient hybridization, where functional variants have been preserved due to positive selection in red-flowered taxa but lost in their yellow-flowered counterparts. The results of this study reveal that selection against gene flow can reduce genomic signatures of ancient hybridization, but that historical introgression can provide essential genetic variation that facilitates the repeated evolution of phenotypic traits between lineages. 

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3. Geographic variation in phenotypic divergence between two hybridizing field cricket species

   https://doi.org/10.3897/jor.32.90713  

   Byerly, Amy R.; Jenck, Clara; Goetz, Alexander R.; Weissman, David B.; Gray, David A.; Ross, Charles L.; Maroja, Luana S.; Larson, Erica L. (September 2023, Journal of Orthoptera Research)

   Patterns of morphological divergence across species’ ranges can provide insight into local adaptation and speciation. In this study, we compared phenotypic divergence among 4,221 crickets from 337 populations of two closely related species of field cricket,Gryllus firmusandG. pennsylvanicus, and their hybrids. We found that these species differ across their geographic range in key morphological traits, such as body size and ovipositor length, and we directly compared phenotype with genotype for a subset of crickets to demonstrate nuclear genetic introgression, phenotypic intermediacy of hybrids, and essentially unidirectional mitochondrial introgression. We discuss how these morphological traits relate to life history differences between the two species. Our comparisons across geographic areas support prior research suggesting that cryptic variation withinG. firmusmay represent different species. Our study highlights how variable morphology can be across wide-ranging species and the importance of studying reproductive barriers in more than one or two transects of a hybrid zone. 

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4. Species Delimitation, Phylogenomics, and Biogeography of Sulawesi Flying Lizards: A Diversification History Complicated by Ancient Hybridization, Cryptic Species, and Arrested Speciation

   https://doi.org/10.1093/sysbio/syad020  

   Mcguire, Jimmy A.; Huang, Xiaoting; Reilly, Sean B.; Iskandar, Djoko T.; Wang-Claypool, Cynthia Y.; Werning, Sarah; Chong, Rebecca A.; Lawalata, Shobi Z. S.; Stubbs, Alexander L.; Frederick, Jeffrey H.; et al (April 2023, Systematic Biology)

   Abstract The biota of Sulawesi is noted for its high degree of endemism and for its substantial levels of in situ biological diversification. While the island’s long period of isolation and dynamic tectonic history have been implicated as drivers of the regional diversification, this has rarely been tested in the context of an explicit geological framework. Here, we provide a tectonically informed biogeographical framework that we use to explore the diversification history of Sulawesi flying lizards (the Draco lineatus Group), a radiation that is endemic to Sulawesi and its surrounding islands. We employ a framework for inferring cryptic speciation that involves phylogeographic and genetic clustering analyses as a means of identifying potential species followed by population demographic assessment of divergence-timing and rates of bi-directional migration as means of confirming lineage independence (and thus species status). Using this approach, phylogenetic and population genetic analyses of mitochondrial sequence data obtained for 613 samples, a 50-SNP data set for 370 samples, and a 1249-locus exon-capture data set for 106 samples indicate that the current taxonomy substantially understates the true number of Sulawesi Draco species, that both cryptic and arrested speciations have taken place, and that ancient hybridization confounds phylogenetic analyses that do not explicitly account for reticulation. The Draco lineatus Group appears to comprise 15 species—9 on Sulawesi proper and 6 on peripheral islands. The common ancestor of this group colonized Sulawesi ~11 Ma when proto-Sulawesi was likely composed of two ancestral islands, and began to radiate ~6 Ma as new islands formed and were colonized via overwater dispersal. The enlargement and amalgamation of many of these proto-islands into modern Sulawesi, especially during the past 3 Ma, set in motion dynamic species interactions as once-isolated lineages came into secondary contact, some of which resulted in lineage merger, and others surviving to the present. [Genomics; Indonesia; introgression; mitochondria; phylogenetics; phylogeography; population genetics; reptiles.] 

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5. Machine Learning Inference of Gene Regulatory Networks in Developing Mimulus Seeds

   https://doi.org/10.3390/plants13233297  

   Tucci, Albert; Flores-Vergara, Miguel A; Franks, Robert G (December 2024, Plants)

   The angiosperm seed represents a critical evolutionary breakthrough that has been shown to propel the reproductive success and radiation of flowering plants. Seeds promote the rapid diversification of angiosperms by establishing postzygotic reproductive barriers, such as hybrid seed inviability. While prezygotic barriers to reproduction tend to be transient, postzygotic barriers are often permanent and therefore can play a pivotal role in facilitating speciation. This property of the angiosperm seed is exemplified in the Mimulus genus. In order to further the understanding of the gene regulatory mechanisms important in the Mimulus seed, we performed gene regulatory network (GRN) inference analysis by using time-series RNA-seq data from developing hybrid seeds from a viable cross between Mimulus guttatus and Mimulus pardalis. GRN inference has the capacity to identify active regulatory mechanisms in a sample and highlight genes of potential biological importance. In our case, GRN inference also provided the opportunity to uncover active regulatory relationships and generate a reference set of putative gene regulations. We deployed two GRN inference algorithms—RTP-STAR and KBoost—on three different subsets of our transcriptomic dataset. While the two algorithms yielded GRNs with different regulations and topologies when working with the same data subset, there was still significant overlap in the specific gene regulations they inferred, and they both identified potential novel regulatory mechanisms that warrant further investigation. 

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