Genetic structure of Populus hybrid zone along the Irtysh River provides insight into plastid-nuclear incompatibility

Abstract

In plants, the maintenance of species integrity despite hybridization has often been explained by the co-adaption of nuclear gene complexes. However, the interaction between plastid and nuclear sub-genomes has been underestimated. Here, we analyzed the genetic structure of a Populus alba and P. tremula hybrid zone along the Irtysh River system in the Altai region, northwest China, using both nuclear microsatellites and plastid DNA sequences. We found high interspecific differentiation, although the hybrid P. × canescens was prevalent. Bayesian inference classified most hybrids into F1, followed by a few back-crosses to P. alba, and fewer F2 hybrids and back-crosses to P. tremula, indicating a few introgressions but preference toward P. alba. When plastid haplotypes in parental species were distinct, P. × canescens carried the haplotypes of both parents, but showed significant linkage between intraspecific haplotype and nuclear genotypes at several microsatellite loci. Selection, rather than migration and assortative mating, might have contributed to such plastid-nuclear disequilibria. By removing later-generated hybrids carrying interspecific combinations of haplotype and nuclear genotypes, plastid-nuclear incompatibility has greatly limited the gene exchange between P. alba and P. tremula via backcrossing with hybrids, demonstrating a significant association between plastid haplotype and the proportion of nuclear admixture.

Hybridization, sex specific genomic architecture and local adaptation

“…Cyto-nuclear incompatibilities are also found in plants where chloroplast driven incompatibilities cause reduced hybrid fitness [72,73], which can be remedied by biparental chloroplast inheritance [74].…”

Spontaneous hybridization of Populus × sibirica and Populus nigra in the city of Novokuznetsk (Kemerovo region)

“…5). Это совпадает с данными, полученными ранее при изучении гибридных зон (Lindtke et al, 2014;Christe et al, 2016;Jiang et al, 2016;Zeng et al, 2016).…”

Using petiole anatomy to identify hybrids between and species of Populus sections Aigeiros and Tacamahaca

“…However, hybrids of the next generations and backcrosses are not always discarded by natural selection and can be quite abundant in populations (Hersch-Green et al, 2014;Roe et al, 2014;Chhatre et al, 2018). Numerous studies in hybridization zones showed that backcrossing between hybrid plants and those of parental species are common (Christe et al, 2016;Hu et al, 2016;Jiang et al, 2016;Zeng et al, 2016;Vasilyeva et al, 2018). Therefore, the observed hybridization is retrogressive, being accompanied by the gradual transfer of genetic material from one taxon to another across inter-species isolation barriers.…”