Whole-genome analyses disentangle reticulate evolution of primroses in a biodiversity hotspot

Genomic DNA barcodes provide novel insights into species delimitation in the complex Camellia sect. Thea (Theaceae)

BACKGROUND

Species delimitation within Camellia sect. Thea is taxonomically challenging due to its complex evolutionary history. This study aims to utilize nuclear and chloroplast data as genomic DNA barcodes to delimit species within this economically important group of plants.

RESULTS

Whole genome sequencing (WGS) data were obtained for 98 accessions representing all but one species in C. sect. Thea. Based on 759 high-quality SCNs, 98 whole chloroplast genomes, and by using 2× coverage clean reads from WGS for Skmer analyses, we found that combining the findings from these three data sets resulted in nearly complete species delimitation and resolution of all interspecific relationships within C. sect. Thea. We also found support for the taxonomic elevation of two varieties (C. sinensis var. assamica and C. tachangensis var. remotiserrata) to species status (C. assamica and C. remotiserrata, respectively). Furthermore, we confirmed that C. formosensis represents a distinct species. Gene tree discordances, chloroplast-nuclear conflicts and complex network-like phylogenetic relationships were observed in C. sect. Thea.

CONCLUSION

Compared with the use of single parentally inherited chloroplast data sources, utilizing both uniparentally inherited chloroplast data and biparentally inherited nuclear data improved the species delimitation of taxa within C. sect. Thea. The intricate phylogenetic relationships observed are likely a result of widespread past hybridization and chloroplast capture events among species within this group, which may have blurred the species boundaries. Our novel approach to species delimitation within C. sect. Thea may serve as a blueprint for employing genomic DNA barcodes in other taxa with complex histories, and will significantly contribute to the conservation of cultivated tea plant species and their wild relatives.

Unraveling the extensive phylogenetic discordance and evolutionary history of spurless taxa within the Aquilegia ecalcarata complex

Parallel evolution of the same, or at least very similar, phenotype(s) in different lineages is often interpreted as evidence for the action of natural selection. However, caution is required when inferring parallel evolution based on uncertain or potentially incorrect phylogenetic frameworks. Here, by conducting extensive phylogenomic and population genetic analyses, we aim to clarify the evolutionary history of spurless taxa within the Aquilegia ecalcarata complex. We observed substantial discordance in the phylogenetic patterns across the entire genome, primarily attributed to ancient introgression and incomplete lineage sorting. Additionally, we identified several spurless lineages whose phylogenetic positions were distorted by admixture events. Using a backbone tree and demographic modeling, we determined that these spurless taxa independently originated twice within this group. Intriguingly, our investigation revealed that the spurless taxa experienced population expansion during global cooling, while their spurred sister groups underwent population contraction. The parallel losses of petal spurs, therefore, may be linked to adaptations for low-temperature conditions. These findings emphasize the importance of comprehensive population-level analyses in phylogenetic inference and provide valuable insights into the dynamics of trait loss and its implications for the adaptive strategies.

Unveiling the Genome-Wide Consequences of Range Expansion and Mating System Transitions in Primula vulgaris

Genetic diversity is heterogeneously distributed among populations of the same species, due to the joint effects of multiple demographic processes, including range contractions and expansions, and mating systems shifts. Here, we ask how both processes shape genomic diversity in space and time in the classical Primula vulgaris model. This perennial herb originated in the Caucasus region and was hypothesized to have expanded westward following glacial retreat in the Quaternary. Moreover, this species is a long-standing model for mating system transitions, exemplified by shifts from heterostyly to homostyly. Leveraging a high-quality reference genome of the closely related Primula veris and whole-genome resequencing data from both heterostylous and homostylous individuals from populations encompassing a wide distribution of P. vulgaris, we reconstructed the demographic history of P. vulgaris. Results are compatible with the previously proposed hypothesis of range expansion from the Caucasus region approximately 79,000 years ago and suggest later shifts to homostyly following rather than preceding postglacial colonization of England. Furthermore, in accordance with population genetic theoretical predictions, both processes are associated with reduced genetic diversity, increased linkage disequilibrium, and reduced efficacy of purifying selection. A novel result concerns the contrasting effects of range expansion versus shift to homostyly on transposable elements, for the former, process is associated with changes in transposable element genomic content, while the latter is not. Jointly, our results elucidate how the interactions among range expansion, transitions to selfing, and Quaternary climatic oscillations shape plant evolution.

The Primula edelbergii S-locus is an example of a jumping supergene

Research on supergenes, non-recombining genomic regions housing tightly linked genes that control complex phenotypes, has recently gained prominence in genomics. Heterostyly, a floral heteromorphism promoting outcrossing in several angiosperm families, is controlled by the S-locus supergene. The S-locus has been studied primarily in closely related Primula species and, more recently, in other groups that independently evolved heterostyly. However, it remains unknown whether genetic architecture and composition of the S-locus are maintained among species that share a common origin of heterostyly and subsequently diverged across larger time scales. To address this research gap, we present a chromosome-scale genome assembly of Primula edelbergii, a species that shares the same origin of heterostyly with Primula veris (whose S-locus has been characterized) but diverged from it 18 million years ago. Comparative genomic analyses between these two species allowed us to show, for the first time, that the S-locus can 'jump' (i.e. translocate) between chromosomes maintaining its function in controlling heterostyly. Additionally, we found that four S-locus genes were conserved but reshuffled within the supergene, seemingly without affecting their expression, thus we could not detect changes explaining the lack of self-incompatibility in P. edelbergii. Furthermore, we confirmed that the S-locus is not undergoing genetic degeneration. Finally, we investigated P. edelbergii evolutionary history within Ericales in terms of whole genome duplications and transposable element accumulation. In summary, our work provides a valuable resource for comparative analyses aimed at investigating the genetics of heterostyly and the pivotal role of supergenes in shaping the evolution of complex phenotypes.

Karyotypic stasis and swarming influenced the evolution of viral tolerance in a species-rich bat radiation

The emergence of COVID-19 and severe acute respiratory syndrome (SARS) has prioritized understanding bats' viral tolerance. Myotis bats are exceptionally species rich and have evolved viral tolerance. They also exhibit swarming, a cryptic behavior where large, multi-species assemblages gather for mating, which has been hypothesized to promote interspecific hybridization. To resolve the coevolution of genome architecture and their unusual antiviral tolerance, we undertook a phylogenomic analysis of 60 Old World Myotis genomes. We demonstrate an extensive history of introgressive hybridization that has replaced the species phylogeny across 17%-93% of the genome except for pericentromeric regions of macrochromosomes. Introgression tracts were enriched on microchromosome regions containing key antiviral pathway genes overexpressed during viral challenge experiments. Together, these results suggest that the unusual Myotis karyotype may have evolved to selectively position immune-related genes in high recombining genomic regions prone to introgression of divergent alleles, including a diversity of interleukin loci responsible for the release of pro-inflammatory cytokines.

The genomes of Darwin's primroses reveal chromosome-scale adaptive introgression and differential permeability of species boundaries

Introgression is an important source of genetic variation that can determine species adaptation to environmental conditions. Yet, definitive evidence of the genomic and adaptive implications of introgression in nature remains scarce. The widespread hybrid zones of Darwin's primroses (Primula elatior, Primula veris, and Primula vulgaris) provide a unique natural laboratory for studying introgression in flowering plants and the varying permeability of species boundaries. Through analysis of 650 genomes, we provide evidence of an introgressed genomic region likely to confer adaptive advantage in conditions of soil toxicity. We also document unequivocal evidence of chloroplast introgression, an important precursor to species-wide chloroplast capture. Finally, we provide the first evidence that the S-locus supergene, which controls heterostyly in primroses, does not introgress in this clade. Our results contribute novel insights into the adaptive role of introgression and demonstrate the importance of extensive genomic and geographical sampling for illuminating the complex nature of species boundaries.