Biology subjects

Engle-Wrye, N. J.

Publications and source records attributed to Engle-Wrye, N. J..

6 recordsLinked to original sources

bioRxiv2026

"Fuzzy specificity" shapes diazotroph diversity and composition in nodulating plants of the Southeastern USA

Nitrogen-fixing symbioses, particularly those occurring in root nodules, are among the most consequential mutualisms in natural and agricultural systems and represent a globally important source of bioavailable nitrogen. Despite their importance, patterns of diversity and composition among diazotrophic symbionts--and the processes structuring those patterns in natural systems--remain poorly resolved, with competing hypotheses emphasizing ecological, or phylogenetic constraints on host-symbiont associations. Here, using a broad survey of nodulating plants from the southeastern United States, we examine how diazotrophic symbiont communities vary across host plant phylogeny, habitat context, and geographic origin. We find that host phylogeny is the primary determinant of symbiont composition, outweighing effects of fine-scale taxonomic identity. Symbiont associations are therefore structured mainly at deeper phylogenetic levels, consistent with phylogenetically constrained, or "fuzzy," host specificity. Likewise, nodule community diversity--potentially reflecting variation in host control over infection--differs primarily among higher-level clades rather than among closely related taxa. Habitat context also shapes nodule communities, but its influence is secondary and most evident in undisturbed environments. As well, nonnative legumes harbor distinct symbiont assemblages despite occupying similar habitats, whereas distantly related legume clades share symbionts across habitats, highlighting interactions among phylogeny, ecology, and geographic history. Overall, our results show that host phylogeny exerts the strongest influence on nodule microbial communities, likely reflecting evolutionary divergence in symbiotic function across major host lineages.

ecology↗

bioRxiv2026

THE GEOGRAPHIC STRUCTRUE OF CHLOROPLAST CAPTURE IN A HYBRID ZONE

The frequency of hybridization differs strongly across different taxa and areas of Earth, yet the environmental drivers of this variation remain poorly understood. Areas strongly influenced by Pleistocene glaciation have long been hypothesized to be rich in hybrid taxa through forcing previously allopatric species into contact and interrupting ecological barriers. Here, we revisit naturally occurring Heuchera americana x H. richardsonii in North America using deep population level sampling of nuclear and plastid genomes to re-evaluate the long-standing hypothesis that Pleistocene climate dynamics promoted hybridization in this system. An extensive population-level sampling scheme involving nuclear and plastid genetic data (729 individuals across 455 populations) reveals a more complex hybridization history than implied by previous representative phylogenetic sampling. Our analyses identify multiple independent chloroplast capture events since diversification within this complex, rather than a previously hypothesized single ancestral event. Strong geographic structure at higher-level plastid clades but little structure within subclades, together with the rarity of ancestral plastid genomes, suggests a unidirectional inheritance of the introgressed plastid genome rather than a simple geographic distribution of plastid haplotypes. Ancestral niche projections indicate that Pleistocene glacial maxima created a shared narrow eastern refugium east of the Mississippi River, enabling localized hybridization, while western refugia were larger and taxa probably allopatric. This contrast led to an east-west gradient in cytonuclear discord. Our results corroborate and refine a 90-year-old hypothesis of climate-mediated hybridization by demonstrating that historical gene flow was spatially restricted but hybrid genotypes now command a larger geographic area. Our study highlights the importance of deep population-level sampling for accurately reconstructing hybridization histories and underscores the catalytic role of climatic instability in general as a driver of hybridization in Heuchera, and likely many other taxa across the tree of life.

plant biology↗

bioRxiv2026

SPECIES DELIMITATION IN AN INTRACTABLE SYNGAMEON: BRINGING ORDER TO THE POLYPHYLETIC HEUCHERA AMERICANA GROUP

Species are the fundamental analytical units of evolutionary processes; thus evidence-based species delimitation is a crucial step for understanding species radiations. However, the task of delimiting species is particularly challenging in the context of a syngameon--a group of distinct, but closely related species that have incomplete reproductive isolation and frequently hybridize in nature. This problem is further exacerbated by the presence of cryptic species--species that are phenotypically distinct, though difficult to distinguish with gross morphology alone. Heuchera subsect. Heuchera comprises both clear and cryptic species within a syngameon that has seen study from morphological, experimental, and phylogenetic aspects. This group has long been recognized for its taxonomic complexity, namely two recognized hybrid zones with extreme morphological variation and persistent non-monophyly among parental populations. Here, we reassess species limits within Heuchera subsect. Heuchera, focusing on the hybrid complex between H. americana and H. richardsonii and adjacent H. americana populations. We use a multipronged approach with deep population-level sampling to 1) assess the genetic structure of 655 individuals across the geographic range of the H. americana group to identify genetic lineages and 2) assess the phenotypic diagnosability of these lineages. Despite extensive admixture and gene tree conflict, we find multiple cohesive lineages with diagnosable phenotypes. We recognize five species and three varieties within the H. americana group, one new and four resurrected. Our results demonstrate that even highly reticulate syngameons can be partitioned into meaningful taxonomic units with multiple lines of evidence.

plant biology↗

bioRxiv2026

Investigating the landscape of plant-pollinator interactions in a hybrid zone

Little is known about environmental drivers of opportunities for hybridization, but its phylogenetic distribution across species and areas is heterogeneous, suggesting that ecological traits may play an important role in concert with postzygotic isolation. Because plant-pollinator interactions are responsible for gene flow in most plant species, differences in the mosaic landscape of plant-pollinator interactions could explain why some plants are particularly prone to hybridization. Prezygotic isolation is mediated by sometimes complex pollen presentation; conversely, conserved pollination strategies would lead to evolutionary constraints on pollinator assemblage divergence in the speciation process and therefore predict higher opportunities for gene flow, although this hypothesis has yet to be tested. The plant taxonomic tribe Heuchereae (Saxifragaceae) is a well-characterized system for pollinator interactions and particularly for floral scent, the primary pollinator attractant in the group. Floral volatile organic compounds (VOCs) in this clade are hypervariable at the population level and are thought to be responsible for pollination selectivity, leading to divergent pollinator assemblages. Observing a contrast of hybridizing and non-hybridizing species, the levels of attractant divergence may therefore predict levels of hybridization. We investigated pollination biology in the plant genus Heuchera, notable for frequent interspecific gene flow compared to tribal relatives, asking whether high rates of hybridization may be associated with low interspecific divergence of VOCs and the pollinator assemblages they shape, using as our system the hybrid zone between H. americana var. americana and H. richardsonii in the midwestern USA. We optimized a closed-space collection and GC-MS (gas chromatography-mass spectrometry) protocol to characterize VOCs in Heuchera flowers. To identify floral visitation and effective pollinators we conducted pollination observations at 40 Heuchera populations over the span of two field seasons. GC-MS data from 89 Heuchera specimens representing 69 populations suggests that classes of VOCs, and to a large extent individual compounds, are shared within the hybrid complex while other Heuchera that are not thought to hybridize with these species have distinct species-specific compounds. Pollination observations and metabarcoding of pollinator pollen loads confirm shared effective pollinators in the hybrid zone and between adjacent parental populations. Attractant and visitation data considered together suggest that conservatism of pollinator interactions may be a typical feature associated with frequent hybridizers, perhaps arising from developmental or biochemical constraints on prezygotic isolation, and more broadly that the macroevolution of isolation mechanisms may be predictive of natural hybridization rate.

plant biology↗

bioRxiv2023

Phylogenetic and spatial determinants of leaf endophyte microbiomes in the flowering plant genus Heuchera (Saxifragaceae)

Premise: Endophytic plant-microbe interactions range from mutualistic relationships that confer important ecological and agricultural traits to neutral or quasi-parasitic relationships. In contrast to root-associated endophytes, the role of environmental and host-related factors for acquiring leaf endophyte communities remains relatively unexplored. Here we assess leaf endophyte diversity to test the hypothesis that membership of these microbial communities is driven primarily by abiotic environment and host phylogeny. Methods: We used a broad geographic coverage of North America in the genus, Heuchera (Saxifragaceae). Bacterial and fungal communities were characterized with 16S and ITS amplicon sequencing, using QIIME2 to call operational taxonomic units and calculate species richness, Shannon diversity, and phylogenetic diversity. We assembled environmental predictors for microbial diversity at collection sites including latitude, elevation, temperature, precipitation, and soil parameters. Results: We find differing assembly patterns for bacterial and fungal endophytes; we found that only host phylogeny is significantly associated with bacteria, while geographic distance alone was the best predictor of fungal community composition. Species richness and phylogenetic diversity are very similar across sites and species, with only fungi showing a response to aridity and precipitation for some metrics. Unlike what has been observed with root-associated microbial communities, in this system microbes show no relationship with pH or other soil factors. Conclusions: Host phylogeny and geographic distance independently influence different microbial communities, while aridity and precipitation determine fungal diversity within leaves of Heuchera. Our results indicate the importance of detailed clade-based investigation of microbiomes and the complexity of microbiome assembly within specific plant organs.

ecology↗

bioRxiv2022

Identifying climatic drivers of hybridization in Heuchereae (Saxifragaceae)

Applications of molecular phylogenetic approaches have uncovered evidence of hybridization across numerous clades of life, yet the environmental factors responsible for driving opportunities for hybridization remain obscure. Verbal models implicating geographic range shifts that brought species together during the Pleistocene have often been invoked, but quantitative tests using paleoclimatic data are needed to validate these models. Here, we produce a phylogeny for Heuchereae, a clade of 15 genera and 83 species in Saxifragaceae, with complete sampling of recognized species, using 277 nuclear loci and nearly complete chloroplast genomes. We then employ an improved framework with a coalescent simulation approach to test and ultimately confirm previous hybridization hypotheses and identify one new intergeneric hybridization event. Focusing on the North American distribution of Heuchereae, we introduce and implement a newly developed approach to reconstruct potential past distributions for ancestral lineages across all species in the clade and across a paleoclimatic record extending from the late Pliocene. Time calibration based on both nuclear and chloroplast trees recovers a mid- to late-Pleistocene date for most inferred hybridization events, a timeframe concomitant with repeated geographic range restriction into overlapping refugia. Our results indicate an important role for past episodes of climate change, and the contrasting responses of species with differing ecological strategies, in generating novel patterns of range contact among plant communities and therefore new opportunities for hybridization.

evolutionary biology↗