1 INTRODUCTION
Geographical isolation refers to a situation where populations become
separated by physical barriers such as islands and mountains, and has
long been considered as a key driving force in speciation (MacArthur &
Wilson, 1967; Li et al., 2011; Robin et al., 2015). Geographical
barriers can fragment distribution ranges and gene flow dynamics between
different sides of the barriers, thereby allowing the increase in
population differentiation. As a result, populations become fragmented
and genetically isolated and, in some cases, new species may evolve
(Crimmins et al., 2011; Li et al., 2011; Robin et al., 2015; Demenou et
al., 2016). The knowledge of such processes is crucial for gaining
insight into the origins and evolution of biodiversity, but also useful
for formulating biodiversity conservation strategies. Consequently, the
identification of geographic barriers using genetic data is increasingly
emphasized in modern biodiversity conservation studies (Li et
al. , 2011; Robin et al. , 2015; Tan et al. , 2020).
Hainan Island in south China harbors c. 4800 seed plant taxa in
total and about 500 endemics (Francisco-Ortega et al. , 2010a, b;
Yang, 2013), and is a distinctive part of the globally important
Indo-Burma biodiversity hotspot (Myers et al. , 2000). Notably,
the richest biodiversity and 80% of the endemic species occur in the
south-central mountains of the island (Yu et al. , 2001;
Francisco-Ortega et al. , 2010a, b), which are largely located
within Hainan Tropical Rainforest National Park. Home to rich tropical
plant species, this Park is an important natural gene pool for the
entire globe. However, several large rivers, mountains and valleys cause
a pronounced geographical fragmentation in this region (Ling et al.,
2017a, b; Li et al., 2020), with potentially negative effects on
ecosystem integrity and the biodiversity conservation of the Park. Such
patterns offer opportunities to study the mechanisms underlying
evolutionary species diversification in the island, which involves both
isolation by the sea and by riverine and mountainous barriers.
The family Gesneriaceae in Hainan Island is notable for its extremely
high species endemism (Wei, 2010; Ling et al., 2017a, 2017b). Fourteen
genera and 25 species of Gesneriaceae are currently found on the island,
including two endemic genera and nine endemic species (Li & Wang, 2005;
Yang, 2013; Ling et al., 2017a, 2020b). The species endemism proportion
of Hainan Gesneriaceae is 30%, a great value similar to that of the
nearby Guangxi Province (Ling et al., 2017b), long believed to be the
species distribution and endemism centre of Gesneriaceae in SW China due
to its widespread limestone landscapes (Li & Wang, 2005; Wei et al.,
2004; Wei, 2010). All these Hainan endemics are found on crown lineages
(Ling et al., 2017b), suggesting a recent origin (Ling et al., 2017b,
2020a). However, the mechanisms and processes underlying speciation
patterns of these narrow endemics have received little attention.
In this investigation, we examine the influence of rivers and mountains
on the genetic divergence and phylogeographic patterns in the
Hainan-endemic Primulina heterotricha (Merr.) Y.Dong & Yin
Z.Wang (Gesneriaceae) using both nuclear and chloroplast DNA sequences.
This study addresses three specific questions to test if the
geographical isolation of populations by rivers and valleys on the
island were determinant to the speciation, distribution and genetic
diversification of P. heterotricha : (1) What is the genetic and
phylogeographic structure of P. heterotricha populations? (2) Can
these patterns be associated with any geographic barriers? (3) Did these
barriers play a role in the speciation and adaptation history of this
montane plant?