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?