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.