Origin of S. hsiehii through asymmetric hybridization between S. austrotaiwanensis and S. indica
The most plausible explanation for the origin of S. hsiehii is an asymmetric hybridization event between S. austrotaiwanensis andS. indica . In this scenario, S. austrotaiwanensiscontributed a significant proportion (91.2%) of the genetic makeup ofS. hsiehii . The strong asymmetry of the genetic contributions from S. austrotaiwanensis and S. indica supports the second-best hypothesis, namely, that S. hsiehii is derived fromS. austrotaiwanensis , despite the relatively small likelihood difference with the best hybridization scenario.
Although the contribution of S. indica is minor, the STRUCTURE results indicated that it is genetically inseparable from the formation of S. hsiehii . Additionally, the distribution range of S. indica extends across a wide range of altitudes in central Taiwan, up to 2400 m (Hsieh & Huang, 1995), which includes the area where S. hsiehii is found (specifically at 1600 m in central Taiwan (Hsieh, 2013)). This observation is further evidence that S. indica may be a parental species.
An analysis of Grinnellian niches showed that S. hsiehii had different temperature preferences than S. indica and S. austrotaiwanensis . Specifically, S. hsiehii was associated with a higher mean diurnal range (BIO2) and lower maximum temperature of the warmest month (BIO5) and minimum temperature of the coldest month (BIO6) than its parental species (Fig. 3c). This finding suggests a shift in the temperature-related adaptive traits of S. hsiehii compared to its parental species following the hybridization event.
Although we estimate that 91.2% of the genetic composition of S. hsiehii originated from S. austrotaiwanensis , its distribution does not align with that of S. austrotaiwanensis , which is typically found in mountainous areas at altitudes of 400-500 m in southern Taiwan (Hsieh & Huang, 1997). This discrepancy indicates a shift in the distribution of S. hsiehii to higher elevations compared to its parent species S. austrotaiwanensis . A previous study of S. hsiehii (Hsieh, 2013) compared this species with another species of the indica group, S. playfairii , and stated that the floral parts of the two species were morphologically similar; however, this study did not compare S. hsiehii withS. austrotaiwanensis . We found that S. hsiehii most closely resembled S. austrotaiwanensis , although the long-ovate leaved form of S. hsiehii differs greatly from those of the other Taiwanese Scutellaria species (oval to obovate leaves). AlthoughS. austrotaiwanensis is primarily found in the south and is geographically isolated from S. hsiehii in central Taiwan, similar to S. taiwanensis , it is possible that this indicagroup species also retreated to the southern glacial refugium, where the hybridization event may have occurred during or after the glacial period. However, the substantiation of these conclusions requires further evidence.