Limitations and prospects: the case of Q. canariensis and Q. faginea
In the reference dataset, the best-sampled, species of sectionQuercus is Q. canariensis (Denk and Grimm 2010), one species of subsection Galliferae (Tschan and Denk 2012). Denk and Grimm (2010) highlighted the high variation occurring in Q. canariensis at both the ITS and the 5S IGS markers, compared to other species of the same section, and the large extent of identical variants shared with Q. faginea . In this work, we identified 5S-IGS variants apparently unique to Q. canariensis and Q. pyrenaica (genotype ‘Q1’; Fig. S3-4 in Supplementary file S3) that attract hits in all HTS samples comprising material of either Q. faginea (pure sample H1), or Q. canariensis (sample E5 and F2) (Fig. 4). This finding could indicate that the investigated Q. faginea population is in fact a Q. canariensis relict, morphologically mimicking the particular steppe-, dry-climate adapted morphotype of Q. faginea locally known as ‘Q. lusitanica ’. According to Tschan and Denk (2012), the exclusive presence of theQ. lusitanica morphotype in highly disturbed habitats, where it typically attains a shrubby habitus, and its morphology (based on a large dataset of micro- and macroscopic characters), point towards its consideration as an extreme adaptation of Q. faginea . The two species are also not readily diagnosable morphologically. It is therefore conceivable that particularly severe ecological conditions trigger Q. canariensis approaching morphologically Q. faginea/lusitanica , especially in the contact zone of both species. Asymmetrical introgression could enforce this pattern (e.g. Neophytou et al. 2010). However, it may also indicate that the reference data is unrepresentative for the genetic diversity in Q. faginea. In any case, it remains to explain the connection with Q. pyrenaica, a sympatric (but not belonging to Galliferae ) species, only found in (south-)western Europe and (mainly) Northwestern Africa. It could be due to kinship with the only other fully mesic Iberian member of sectionQuercus (Q. canariensis ) included in the reference dataset, thus indicating in situ differentiation of a regional mesic lineage. In the RAD-seq work of Hipp et al. (2019b), Q. faginea , Q. canariensis , Q. pyrenaica and Q. lusitanica , all represented by 1–2 individuals, were distributed on all four subclades produced in the section, thus indicating an early split of these western Mediterranean species. Lepais et al. (2013) and Leroy et al. (2017) showed that Q. pyrenaica seems to be not fully incorporated in the Q. petraea-robur-pubescens syngameon and concluded that Q. pyrenaica may represent a more anciently diverged species. The possible identification of ancestral sectional traits is therefore plausible. Interestingly, we identified a geographical partitioning in the HTS reads assigned to Galliferae(and Q. pyrenaica ) in the three samples comprising western Mediterranean species of this subsection (H1, F2, E5), and sample E4, where the only eastern Mediterranean species of this subsection (Q. infectoria ) was mixed with several other members of the section. The canariensis-pyrenaica shared 5S-IGS variants (and the higher intra-genomic 5S-IGS diversity) could then be a witness of this antiquity: obtained before the Galliferae were isolated from the Q. petraea-robur species complex and retained in the two least evolved, and/or earliest diverged, species of each lineage as intra-genomic variation while being lost in all other species due to concerted evolution. Future studies with denser samplings including a single taxon per sample may therefore be interesting to see whether such variants show coherent distributional patterns or relate to morphological gradients.