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.