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.