# Genetic Diversity and Population Structure in Two Mangrove Species (Sonneratia alba and Sonneratia caseolaris) Across Coastal Areas of Thailand

## TL;DR

This study examines the genetic diversity and population structure of two mangrove species in Thailand, revealing strong geographic clustering and low genetic diversity.

## Contribution

The study provides genome-wide insights into the spatial genetic structure of two Thai mangrove species using RAD-seq data.

## Key findings

- Both Sonneratia alba and Sonneratia caseolaris show low genetic diversity and high genetic differentiation between coastal regions.
- Population structure is strongly associated with geography, with distinct genetic clusters in the Andaman Sea and Gulf of Thailand.
- Genetic differentiation increases with geographic distance in S. caseolaris but not in S. alba.

## Abstract

To assess the genetic diversity and population structure of Sonneratia alba and Sonneratia caseolaris in Thailand, we used restriction site-associated DNA sequencing (RAD-seq) to genotype individuals from natural populations along the Thai coast. Based on SNPs identified from the S. alba and S. caseolaris genome sequences, we found low genetic diversity and high genetic differentiation among 107 S. alba and 131 S. caseolaris individuals. Population structure was strongly associated with geography: individuals formed two main genetic clusters corresponding to the Andaman Sea and Gulf of Thailand coasts, as consistently supported by population structure, principal component and phylogenetic analyses.

Sonneratia alba Sm. and Sonneratia caseolaris (L.) Engl. are two ecologically important components of mangrove communities in Thailand. However, their population genetic patterns in Thailand remain poorly understood. Here, we assessed the genetic diversity and population structure of 107 S. alba and 131 S. caseolaris individuals sampled across their full coastal range in Thailand using single-nucleotide polymorphism (SNP) markers. Population structure analyses consistently revealed strong genetic subdivision associated with geography: S. alba formed three clusters (including one admixed group), whereas S. caseolaris formed two clusters. In both species, populations were clearly separated between the Andaman Sea and the Gulf of Thailand, reflecting the isolating influence of the Malay Peninsula. Genetic differentiation between clusters was high (FST = 0.364 in S. alba and 0.321 in S. caseolaris). Genetic differentiation increased with geographic distance in S. caseolaris, whereas no such relationship was detected in S. alba. Both species exhibited low levels of genetic diversity (Ho = 0.173; He = 0.223 in S. alba and Ho = 0.290; He = 0.406 in S. caseolaris). Together, these results reveal pronounced spatial genetic structure and limited evolutionary connectivity between coastal regions, providing genome-wide insights into mangrove population differentiation with important implications for conservation and restoration.