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1. Differences in flowering time maintain species boundaries in a continental radiation of Viburnum

   https://doi.org/10.1002/ajb2.1292  

   Spriggs, Elizabeth L. ; Schlutius, Caroline ; Eaton, Deren A. ; Park, Brian ; Sweeney, Patrick W. ; Edwards, Erika J. ; Donoghue, Michael J. ( May 2019 , American Journal of Botany)

   We take an integrative approach in assessing how introgression and Pleistocene climate fluctuations have shaped the diversification of the coreLentagoclade ofViburnum, a group of five interfertile species with broad areas of sympatry. We specifically tested whether flowering time plays a role in maintaining species isolation.

   RAD‐seq data for 103 individuals were used to infer the species relationships and the genetic structure within each species. Flowering times were compared among species on the basis of historical flowering dates documented by herbarium specimens.

   Within each species, we found a strong relationship between flowering date and latitude, such that southern populations flower earlier than northern ones. In areas of sympatry, the species flower in sequence rather than simultaneously, with flowering dates offset by ≥9 d for all species pairs. In two cases it appears that the offset in flowering times is an incidental consequence of adaptation to differing climates, but in the recently diverged sister speciesV. prunifoliumandV. rufidulum, we find evidence that reinforcement led to reproductive character displacement. Long‐term trends suggest that the two northern‐most species are flowering earlier in response to recent climate change.

   We argue that speciation in theLentagoclade has primarily occurred through ecological divergence of allopatric populations, but differences in flowering time were essential to maintain separation of incipient species when they came into secondary contact. This combination of factors may underlie diversification in many other plant clades.

    

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2. Active pollination drives selection for reduced pollen‐ovule ratios

   https://doi.org/10.1002/ajb2.1412  

   Pellmyr, Olle ; Kjellberg, Finn ; Herre, Edward Allen ; Kawakita, Atsushi ; Hembry, David H. ; Holland, J. Nathaniel ; Terrazas, Teresa ; Clement, Wendy ; Segraves, Kari A. ; Althoff, David M. ( December 2019 , American Journal of Botany)

   Variation in pollen‐ovule ratios is thought to reflect the degree of pollen transfer efficiency—the more efficient the process, the fewer pollen grains needed. Few studies have directly examined the relationship between pollen‐ovule ratio and pollen transfer efficiency. For active pollination in the pollination brood mutualisms of yuccas and yucca moths, figs and fig wasps, senita and senita moths, and leafflowers and leafflower moths, pollinators purposefully collect pollen and place it directly on the stigmatic surface of conspecific flowers. The tight coupling of insect reproductive interests with pollination of the flowers in which larvae develop ensures that pollination is highly efficient.

   We used the multiple evolutionary transitions between passive pollination and more efficient active pollination to test if increased pollen transfer efficiency leads to reduced pollen‐ovule ratios. We collected pollen and ovule data from a suite of plant species from each of the pollination brood mutualisms and used phylogenetically controlled tests and sister‐group comparisons to examine whether the shift to active pollination resulted in reduced pollen‐ovule ratios.

   Across all transitions between passive and active pollination in plants, actively pollinated plants had significantly lower pollen‐ovule ratios than closely related passively pollinated taxa. Phylogenetically corrected comparisons demonstrated that actively pollinated plant species had an average 76% reduction in the pollen‐ovule ratio.

   The results for active pollination systems support the general utility of pollen‐ovule ratios as indicators of pollination efficiency and the central importance of pollen transfer efficiency in the evolution of pollen‐ovule ratio.

    

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3. Molecular signatures of long‐distance oceanic dispersal and the colonization of Pacific islands in Lycium carolinianum

   https://doi.org/10.1002/ajb2.1626  

   Levin, Rachel A. ; Miller, Jill S. ( April 2021 , American Journal of Botany)

   Long‐distance dispersal has been important in explaining the present distributions of many plant species. Despite being infrequent, such dispersal events have considerable evolutionary consequences, because bottlenecks during colonization can result in reduced genetic diversity. We examined the phylogeographic history ofLycium carolinianum, a widespread taxon that ranges from southeastern North America to several Pacific islands, with intraspecific diversity in sexual and mating systems.

   We used Bayesian, likelihood, and coalescent approaches with nuclear and plastid sequence data and genome‐wide single nucleotide polymorphisms to reconstruct the dispersal history of this species. We also compared patterns of genetic variation in mainland and island populations using single nucleotide polymorphisms and allelic diversity at theS‐RNasemating system gene.

   Lycium carolinianumis monophyletic and dispersed once from the North American mainland, colonizing the Pacific islands ca. 40,100 years ago. This dispersal was accompanied by a loss of genetic diversity in SNPs and theS‐RNaselocus due to a colonization bottleneck and the loss of self‐incompatibility. Additionally, we documented at least two independent transitions to gynodioecy: once following the colonization of the Hawaiian Islands and loss of self‐incompatibility, and a second time associated with polyploidy in the Yucatán region of Mexico.

   Long‐distance dispersal via fleshy, bird dispersed fruits best explains the unusually widespread distribution ofL. carolinianum. The collapse of diversity at theS‐RNaselocus in island populations suggests that self‐fertilization may have facilitated the subsequent colonization of Pacific islands following a single dispersal from mainland North America.

    

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4. Enabling evolutionary studies at multiple scales in Apocynaceae through Hyb‐Seq

   https://doi.org/10.1002/aps3.11400  

   Straub, Shannon C. K. ; Boutte, Julien ; Fishbein, Mark ; Livshultz, Tatyana ( November 2020 , Applications in Plant Sciences)

   Apocynaceae is the 10th largest flowering plant family and a focus for study of plant–insect interactions, especially as mediated by secondary metabolites. However, it has few genomic resources relative to its size. Target capture sequencing is a powerful approach for genome reduction that facilitates studies requiring data from the nuclear genome in non‐model taxa, such as Apocynaceae.

   Transcriptomes were used to design probes for targeted sequencing of putatively single‐copy nuclear genes across Apocynaceae. The sequences obtained were used to assess the success of the probe design, the intrageneric and intraspecific variation in the targeted genes, and the utility of the genes for inferring phylogeny.

   From 853 candidate nuclear genes, 835 were consistently recovered in single copy and were variable enough for phylogenomics. The inferred gene trees were useful for coalescent‐based species tree analysis, which showed all subfamilies of Apocynaceae as monophyletic, while also resolving relationships among species within the genusApocynum. Intraspecific comparison ofElytropus chilensisindividuals revealed numerous single‐nucleotide polymorphisms with potential for use in population‐level studies.

   Community use of this Hyb‐Seq probe set will facilitate and promote progress in the study of Apocynaceae across scales from population genomics to phylogenomics.

    

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5. Incomplete reproductive isolation and low genetic differentiation despite floral divergence across varying geographic scales in Castilleja

   https://doi.org/10.1002/ajb2.1700  

   Wenzell, Katherine E. ; McDonnell, Angela J. ; Wickett, Norman J. ; Fant, Jeremie B. ; Skogen, Krissa A. ( July 2021 , American Journal of Botany)

   Divergence depends on the strength of selection and frequency of gene flow between taxa, while reproductive isolation relies on mating barriers and geographic distance. Less is known about how these processes interact at early stages of speciation. Here, we compared population‐level differentiation in floral phenotype and genetic sequence variation among recently divergedCastillejato explore patterns of diversification under different scenarios of reproductive isolation.

   Using target enrichment enabled by the Angiosperms353 probe set, we assessed genetic distance among 50 populations of fourCastillejaspecies. We investigated whether patterns of genetic divergence are explained by floral trait variation or geographic distance in two focal groups: the widespreadC. sessilifloraand the more restrictedC. purpureaspecies complex.

   We document thatC. sessilifloraand theC. purpureacomplex are characterized by high diversity in floral color across varying geographic scales. Despite phenotypic divergence, groups were not well supported in phylogenetic analyses, and little genetic differentiation was found across targeted Angiosperms353 loci. Nonetheless, a principal coordinate analysis of single nucleotide polymorphisms revealed differentiation withinC. sessilifloraacross floral morphs and geography and less differentiation among species of theC. purpureacomplex.

   Patterns of genetic distance inC. sessiliflorasuggest species cohesion maintained over long distances despite variation in floral traits. In theC. purpureacomplex, divergence in floral color across narrow geographic clines may be driven by recent selection on floral color. These contrasting patterns of floral and genetic differentiation reveal that divergence can arise via multiple eco‐evolutionary paths.

    

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