The roles of Kra Isthmus in shaping the genetic structure of <i>Paphiopedilum godefroyae</i> (Orchidaceae)

Abstract Paphiopedilum godefroyae is an orchid species endemic to southern Thailand, known for its diverse flowers. It is spread across the Kra Isthmus and has three recognized varieties: P. godefroyae var. ang-thong and P. godefroyae var. godefroyae on the eastern side, and P. godefroyae var. leucochilum on the western side. In this study, we investigated how the Kra Isthmus acts as a barrier, affecting the genetic makeup of these orchid populations. We used double-digest restriction site-associated DNA sequencing (ddRAD-seq) to analyse genetic diversity and structure among 111 samples. The analyses revealed two distinct genetic groups corresponding to the east and west coasts. The eastern populations showed higher genetic diversity and less gene flow, while the western population displayed signs of a historical bottleneck and reduced genetic diversity due to natural and human disturbances over the past 1000 years. This study emphasized that the Kra Isthmus significantly limits gene flow between the eastern and western populations. The limited dispersal ability of hoverfly pollinators and wind-dispersed seeds, influenced by seasonal monsoons, further affects gene flow. By contrast, the eastern varieties had more stable demographic histories. The reduced genetic diversity in the western population, resulting from recent demographic bottlenecks and the geographical barrier restricting gene flow, underscores the need for targeted conservation efforts. Given the low genetic diversity in this population, strategies such as genetic rescue and habitat restoration are essential to enhance genetic variability and ensure the long-term survival of the species.