Outlook
The evolution of extra-tropical tree genera involves dynamic speciation
processes. Fagus species are wind-pollinated but animal-dispersed
and have a narrow ecological niche. Therefore, beech populations are
prone to becoming isolated during phases of area disruption. This can
lead to speciation accompanied by lineage sorting. At the same time,
species boundaries remain permeable: Fagaceae are known for both ancient
and recent gene flow between lineages that diverged tens of millions of
years ago (e.g. McVay et al., 2017; López-Heredia et al., 2020). Phases
of area expansion likely led to secondary contacts and lineage mixing.
Auto- and allopolyploidisation, hybridisation, introgression, and
incomplete lineage sorting may leave complex genetic imprints.
Single-copy or uniparentally inherited genetic markers can only capture
some aspects of this complexity. Biparentally inherited, multi-copy,
multi-locus markers, can instead provide direct evidence for past and
recent reticulation. In this context, the 5S intergenic spacers are a
unique source of information, being the only currently known region that
(i ) is highly divergent, (ii ) can be easily amplified due
to the conservation of the 5S rRNA genes, and (iii ) can be
analysed using High-Throughput Sequencing because of its relatively
short length. In this work, the O- and I/X-types found in F.
japonica point towards a hybrid (allopolyploid) origin, and the
distribution of A- and B-types in the crenata-sylvatica lineage
reflect speciation processes predating the establishment of the modern
species.
Beeches are ideal study objects because of the low species number and
well-documented fossil record. Better understanding the diversity we
observed and the putatively ancestral variants will require material
from East Asian species not included in the present study: F.
engleriana, F. hayatae, F. longipetiolata, and F. lucida , as
well as from the yet poorly studied (eastern) North American F.
grandifolia (but see Galván-Hernández et al., 2020). Similarly, nothing
is known about 5S-IGS gene pools of East Asian and American oaks (and
little about their ITS diversity); nevertheless, phylogenomic data of
North American and French oaks have anticipated the existence of complex
species relationships (Lepoittevin et al., 2015; Hipp et al., 2019).
Non-trivial phylogenetic signals that may be the product of reticulate
evolution also characterize all other Fagaceae (even complete plastomes
cannot resolve the monophyly of oaks) and Fagales in general.
Further applications of 5S-IGS HTS data include taxonomy (co-)informed
by genetics (e.g. Grimm & Denk, 2014; Liede-Schumann et al., 2020) and
the assessment of global biodiversity in addition to the traditional
counting of numbers of species (e.g. Forest et al., 2018). The
species-status of F. sylvatica vs. F. orientalis has long
been discussed but with focus on the wrong populations: while the
western (here: Greek) F. orientalis may be addressed as a
subspecies of F. sylvatica, the Iranian populations deserve the
recognition as a species and represent a most valuable genetic resource.