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

<i>Sonneratia alba</i> Sm. and <i>Sonneratia caseolaris</i> (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 <i>S. alba</i> and 131 <i>S. caseolaris</i> 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: <i>S. alba</i> formed three clusters (including one admixed group), whereas <i>S. caseolaris</i> 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 (<i>F</i>_ST = 0.364 in <i>S. alba</i> and 0.321 in <i>S. caseolaris</i>). Genetic differentiation increased with geographic distance in <i>S. caseolaris</i>, whereas no such relationship was detected in <i>S. alba</i>. Both species exhibited low levels of genetic diversity (H_o = 0.173; H_e = 0.223 in <i>S. alba</i> and H_o = 0.290; H_e = 0.406 in <i>S. caseolaris</i>). 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.