Low phylogeographic diversity in the calcified green macroalga Halimeda macroloba (Bryopsidales) in Southeast Asia

Abstract

Southeast Asia is an important marine biodiversity hotspot. Revealing the spatial patterns and environmental drivers related to population genetic structure in this region is a prerequisite for conservation biogeography and genetics. In this study, we applied two chloroplast markers (tufA and rpl2–rpl16) to evaluate population genetic variation and phylogeographic structure of the green macroalga Halimeda macroloba (12 populations, 275 individuals) in Southeast Asia. Both markers showed extremely low genetic variation and haplotype diversity in H. macroloba, with no clear phylogeographic separation between both sides of the Thai-Malay Peninsula (TMP). A postglacial founder effect and predominant asexual reproduction by fragmentation in H. macroloba, together with monsoon-driven ocean currents driving stepping-stone dispersal, may account for the observed remarkable phylogeographic homogeneity around the TMP. However, the tufA and rpl2-rpl16 markers congruently detected a phylogeographic break between the TMP and the eastern South China Sea, despite no obvious observable barrier to gene flow. These results raise the importance to take in situ actions to conserve the indicator species Halimeda in an era of ocean acidification and warming.