Discussion
The present examination of 51 nuclear genes shows that frequent
hybridization and introgression events were important for S.
taiwanensis origination and in situ speciation of theindica group through increased genetic diversity. Standing
variation is crucial during the early stages of plant speciation and
governs the repeatability of speciation processes (Bock et al., 2023).
Incorporating standing variation through hybridization helps hybrid
organisms survive and reach new adaptive peaks. Hybrids that acquire new
adaptive genes and subsequent mutations that function later in the
speciation process (Bock et al., 2023) can widen their ecological
potential, adapt to new habitats, and reduce vulnerability to climate
change (Brauer et al., 2023).
Chiang, Huang, and Liao (2012) established that S. barbata is the
most divergent species (and a heterotypic synonym of S.
taipeiensis (Chao et al., 2020; Hsiung et al., 2017)) in the phylogeny,
followed by S. taiwanensis and the ”indica group” (which
includes S. indica , S. tashiroi , S. playfairii ,S. austrotaiwanensis (Chiang, Huang, & Liao, 2012), and the new
species S. hsiehii (Hsieh, 2013)). Hybridization has occurred not
only within the indica group but also between the indicagroup and S. barbata and S. taiwanensis , implying a lack
of reproductive isolation between species. We focused on the two endemic
species with genetic admixture, S. taiwanensis and S.
hsiehii, which both have hybrid origins. Hybridization of the
progenitor of S. taiwanensis or an extinct species (i.e., ghost)
with other in situ diverged species brought S. taiwanensiscloser to the indica group in the phylogeny. The speciation time
for sympatric speciation (i.e., the indica group) was relatively
recent, leading to weak reproductive barriers with frequent
introgressions. These frequent hybridizations and introgressions widened
the ecological potential of Scutellaria species to adapt to the
highly heterogeneous habitats of Taiwan Island by increasing genomic
complexity, consistent with the hybrid model, which showed that hybrid
genome instability enhances adaptation to wild environments (Porretta &
Canestrelli, 2023; Stelkens & Bendixsen, 2022). In this study, we used
the Scutellaria species limited to Taiwan Island as a model
system to explore the relationships of frequent hybridization with
origin and adaptation.