Wider Than Expected: Geographic Range and Genetic Diversity of a Commercially Farmed and Traded “Cottonii” Seaweed <i>Kappaphycus malesianus</i>

This study aims to expand the current knowledge on the distribution and genetic diversity of <i>Kappaphycus malesianus</i>, a commercially important carrageenophyte whose cultivation has been limited to the southern Philippines. By broadening the sampling range and using molecular phylogenetic approaches, it seeks to provide a comprehensive overview of the species' genetic structure and diversity in the Philippines. Specimens obtained from four regions across the country were examined for morphological characteristics and subjected to molecular analysis using mitochondrial (<i>COI</i>-5P and <i>cox</i>2-3 spacer), plastid (<i>rbc</i>L), and nuclear (SSU) markers. Genetic diversity and haplotype network analyses, along with Bayesian phylogenetic inference, were conducted to assess genetic variation, population structure, and evolutionary relationships among <i>K. malesianus</i> populations. These analyses revealed new wild <i>K. malesianus</i> populations in Luzon (Sorsogon) and Visayas (Northern Samar, Bohol, and Iloilo), revealing a broader distribution and greater morphological diversity than previously reported. Multiple novel haplotypes were identified, indicating high genetic diversity and a complex population structure shaped by historical gene flow and regional diversification. A key finding is the identification of KMAL-G, a wild haplotype from Sorsogon and Northern Samar, which genetically links the farmed KMAL-A to wild Tawi-Tawi populations, suggesting unrecognized genetic connectivity across these distant regions. Additionally, the close genetic similarity of the unidentified <i>Kappaphycus</i> sp. (GUI1) to <i>K. malesianus</i> suggests potential conspecificity. Similarly, the high genetic affinity between <i>K. malesianus</i> and <i>K. inermis</i> suggests a possible species complex. Overall, these findings demonstrate that <i>K. malesianus</i> is more geographically widespread and genetically diverse than previously known. The discovery of novel haplotypes highlights the importance of continued genetic surveys for aquaculture improvement. These findings not only provide insights on the domestication history of <i>K. malesianus</i> but also advance our understanding of its genetic diversity and phylogeographic distribution in the Philippines.