Adaptive introgression from S. tashiroi to S.
taiwanensis, S. indica, and S. barbata
Genetic introgression may diminish the species boundary but may transfer
adaptive genes into another species. The multiple introgression events
among Taiwanese Scutellaria species indicate frequent genetic
replacement. Specifically, we found evidence of ancient gene flow
between S. hsiehii and S. tashiroi , S. playfairiiand S. taiwanensis , and the ancestors of S. barbata and
the indica group (Fig. 3B). Although some of the analyses were
incongruent, these findings provide insights into occasional
interbreeding that occurred during the evolutionary history of these
species.
The results revealed adaptive introgression involving three specific
genes (LYC , OPCL1 , and PHYB ) responsible for
antioxidant biosynthesis, pathogen-resistance processes, and
light-promotion responses (Fig. 3A, Table S6). These genes are linked to
the response to environmental stress, which could indirectly affect
species survival, growth, and overall fitness. Notably, the adaptive
introgression of these genes was correlated with BIO2 (mean diurnal
range of temperature) and BIO18 (precipitation of the warmest quarter)
(Table 1). These correlations suggest that variations in temperature and
rainfall impacted populations across altitudes within a narrow latitude
range of the island, creating diverse stress environments. Introducing
genetic material through introgression may have benefited species
survival under these varying stress conditions. Moreover, the rainfall
during the warmest season after spring aligns with the growth season
after germination of these Scutellaria species in Taiwan, which
may influence their growth rate. However, the irregularity of BIO2 and
BIO18 between the source and sink species, along with other associated
bioclimatic variables (BIO4, BIO5, BIO6, and BIO17) (Fig. 3C), implies
that these variables are associated with other genetic elements through
hitchhiking.
This study shows that stress-related genes were adapted and transferred
between species in response to environmental changes, significantly
impacting their survival, growth, and fitness. The correlations of
introgression with temperature and rainfall factors likely aided species
survival across altitude variations. The exchange of stress-related
genes between species probably enabled them to tolerate or resist
adverse conditions, leading to a convergence in adaptive responses to
environmental cues and light/shade tolerance.