Hybridization between trees with weak reproductive barriers can produce individuals with a complex genomic admixture and unclear morphological markers. Hybridization occurs in at least a quarter of plant species and an estimated 70% of all flowering plants have arisen from such events. Oak trees, genus Quercus, are known to have exceptionally high levels of hybridization, and have been referred to as a “worst case scenario for the biological species concept.”
The three most common white oaks found in temperate European forests, Q. petraea, Q. pubescens, and Q. robur, present a challenge for quantifying genomic mixing due to their high levels on ongoing hybridization when they occur in mixed stands. The first step in evaluating these levels is to reliably assign individuals to a given taxon, a task that requires the development of a powerful molecular marker set able to both discriminate between species and assess the degree of mixing between them.
In a recent study published in Annals of Botany, Oliver Reutimann and colleagues set out to develop such a marker set using single-nucleotide polymorphisms (SNPs) from coding regions of the genome, which have been found to perform better than noncoding regions in this instance. The researchers first used a training set of 194 pure reference samples before assessing 633 test individuals using two different statistical approaches. A set of 58 SNPs comprised the marker set for these experiments, though the authors note that this set could probably be trimmed down to fewer than 30 without great loss of discriminatory capacity. “This SNP set was designed with the purpose of analysing samples provided by scientists as well as practitioners and therefore aimed at containing a rather low number of SNPs, hence coming with low genotyping costs,” write the authors.
The marker set was able to correctly assign more than 97% of the individuals in the training set to their correct taxon. When applied to the test individuals, the two approaches had an 84% overlap in their assignments. Mixing among the three species was asymmetrical, with Q. petraea and Q. pubescens showing much higher levels of hybridization between them than with Q. robur. “High levels of admixture could imply an adaptive value for oak trees,” write the authors. “One could address such issues by looking for correlations between levels of admixture and environmental parameters at the level of population and individual trees, or by performing reciprocal transplant experiments using populations with varying degrees of admixture.”