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.