Gene flow between diploid and tetraploid junipers - two contrasting evolutionary pathways in two Juniperus populations

Interspecific transfer of genetic information through polyploid bridges

Hybridization blurs species boundaries and leads to intertwined lineages resulting in reticulate evolution. Polyploidy, the outcome of whole genome duplication (WGD), has more recently been implicated in promoting and facilitating hybridization between polyploid species, potentially leading to adaptive introgression. However, because polyploid lineages are usually ephemeral states in the evolutionary history of life it is unclear whether WGD-potentiated hybridization has any appreciable effect on their diploid counterparts. Here, we develop a model of cytotype dynamics within mixed-ploidy populations to demonstrate that polyploidy can in fact serve as a bridge for gene flow between diploid lineages, where introgression is fully or partially hampered by the species barrier. Polyploid bridges emerge in the presence of triploid organisms, which despite critically low levels of fitness, can still allow the transfer of alleles between diploid states of independently evolving mixed-ploidy species. Notably, while marked genetic divergence prevents polyploid-mediated interspecific gene flow, we show that increased recombination rates can offset these evolutionary constraints, allowing a more efficient sorting of alleles at higher-ploidy levels before introgression into diploid gene pools. Additionally, we derive an analytical approximation for the rate of gene flow at the tetraploid level necessary to supersede introgression between diploids with nonzero introgression rates, which is especially relevant for plant species complexes, where interspecific gene flow is ubiquitous. Altogether, our results illustrate the potential impact of polyploid bridges on the (re)distribution of genetic material across ecological communities during evolution, representing a potential force behind reticulation.

Allopolyploidy: An Underestimated Driver in Juniperus Evolution

Allopolyploidy is considered as a principal driver that shaped angiosperms' evolution in terms of diversification and speciation. Despite the unexpected high frequency of polyploidy that was recently discovered in the coniferous genus Juniperus, little is known about the origin of these polyploid taxa. Here, we conducted the first study devoted to deciphering the origin of the only hexaploid taxon in Juniperus along with four of its closely related tetraploid taxa using AFLP markers with four primers combinations. Phylogenetic analysis revealed that the 10 studied species belong to 2 major clusters. J. foetidissima appeared to be more related to J. thurifera, J. sabina, and J. chinensis. The Bayesian clustering analysis showing a slight variation in genetic admixture between the studied populations of J. foetidissima, suggesting an allopolyploid origin of this species involving J. thurifera and J. sabina lineages, although a purely autopolyploidy origin of both J. thurifera and J. foetidissima cannot be ruled out. The admixed genetic pattern revealed for J. seravschanica showed that the tetraploid cytotypes of this species originated from allopolyploidy, whereas no clear evidence of hybridization in the origin of the tetraploid J. thurifera and J. chinensis was detected. This study provides first insights into the polyploidy origin of the Sabina section and highlights the potential implication of allopolyploidy in the evolution of the genus Juniperus. Further analyses are needed for a more in-depth understanding of the evolutionary scenarios that produced the observed genetic patterns.

Molecular and Phytochemical Variability of Endemic Juniperus sabina var. balkanensis from Its Natural Range

Juniperus sabina L. var. balkanensis R.P. Adams & Tashev is a recently described endemic variety from the Balkan Peninsula. Its strong sprouting ability and fast vegetative propagation, on one hand, and fragmented distribution, on the other, can lead to lower genetic diversity in local populations and to the differentiation of populations. As there has been no detailed investigation of this variety, we studied Balkan natural populations using phytochemical and molecular markers. Leaf essential oils (EOs) were chosen based on their proven usability in the population studies of Juniperus taxa, while ISSRs (Inter Simple Sequence Repeats) have been used due to their high resolution. In addition, since this variety is best described using molecular markers, the chloroplast trnS-trnG region was amplified from individuals from different populations having different chemotypes. Based on the essential oil profile, three chemotypes could be identified with a difference in their distribution. The analysis of molecular variance showed moderate differentiation of populations and regions, attesting to the start of the separation of three regions in the Balkans: west, east and south. The bioclimatic and environmental parameters and sex of the individual did not influence the EO profile, although some of the compounds present in low-to-medium concentrations showed strong correlation with several bioclimatic parameters.