Geographical barriers played a fundamental role in shaping genetic structuring of coastal plant species
In addition to open oceans that are too wide for propagules to pass through, land barriers and ocean currents are commonly found to hinder the dispersal of coastal plant species that disperse propagules via seawater (Kadereit, Arafeh, Somogyi, & Westberg, 2005; H. Yang, Lu, Wu, & Zhang, 2012). Mangrove species, which are the most typical coastal plants, strictly inhabit the intertidal zones and disperse via buoyant propagules such as fruits, seeds, and hypocotyls. The propagules of some species can float at sea for months before reaching a new intertidal habitat (Tomlinson, 2016). However, the phylogeographical pattern of mangrove species had been fundamentally shaped by land barriers and ocean currents, such as the Central American Isthmus to Rhizophora mangle or R. racemosa (Takayama, Tamura, Tateishi, Webb, & Kajita, 2013) and the ocean currents at the northeastern extremity of South America to R. mangle , Avicennia germinans orAvicennia schaueriana (Mori, Zucchi, & Souza, 2015; Pil et al., 2011).
The Indo-West Pacific region, particularly Southeast Asia, is the hotspot of mangrove species diversity (Duke, 2017), thus most of the studies discussing phylogeography of mangrove species concentrated on this region. The Malay Peninsula (or the Sundaland when sea level was low) was identified to be the most dominant land barrier for mangrove species. Genetic discontinuity between the Indian ocean and Pacific Ocean sides of Sundaland has been observed in R. apiculata (Z. Guo et al., 2016), R. mucronata (Yan, Duke, & Sun, 2016), Avicennia marina (Wang et al., 2021), Sonneratia alba (Y. Yang et al., 2017), S. caseolaris (Y. Yang et al., 2016), Ceriops tagal (Huang et al., 2012), C. decandra (Huang et al., 2008), Xylocarpusspecies (Z. Guo, Guo, et al., 2018), Lumnitzera racemosa (J. Li et al., 2016), Bruguiera gymnorhiza (Minobe et al., 2010; Urashi et al., 2013), Excoecaria agallocha (W. Guo et al., 2018),Heritiera littoralis (Banerjee et al., 2020), Scaevola taccada (Banerjee et al., 2021) and Acanthus ilicifolus (W. Guo et al., 2020). In this study, the Malay Peninsula was again identified to isolate populations of A. corniculatum . The isolation of Sundaland corresponds to the population differentiation between the subgroup “Gulf of Bengal” and “n-SCS”, as well as “Bali” and “n-SCS”. Despite its dominant role in mangrove species, the barrier effect of the Malay Peninsula receded largely in some coastal plants that inhabit more inland, such as Canavalia rosea (T. He et al., 2021) and Pluchea indica (Lin et al., 2020).
Genetic discontinuities in a lot of mangrove species also occur in the Wallacea region where the Indonesian-through flow functions as a major barrier hindering the dispersal of propagules. This barrier generates a population structure that populations in Southeast Asia are divergent from those in Australasia. Such pattern has been observed in the species of R. apiculata (Z. Guo et al., 2016), R. stylosa (Yan et al., 2016), S. alba (Y. Yang et al., 2017), S. caseolaris(Y. Yang et al., 2016), Avicennia marina (Wang et al., 2021),C. tagal (Huang et al., 2008, 2012), C. decandra (Huang et al., 2008), X. granatum (Z. Guo, Guo, et al., 2018), and L. racemosa (J. Li et al., 2016). This water barrier appears less dominant that populations of several species are not isolated, e.g.Acanthus ilicifolus (W. Guo et al., 2020), H. littoralis(Banerjee et al., 2020), Scaevola taccada (Banerjee et al., 2021), and X. mucronata (Z. Guo, Guo, et al., 2018). The difference in the ability to disperse across the sea currents may underlie the different population structures, even in congeneric species like X. granatum and X. mucronata (Z. Guo, Guo, et al., 2018). The populations of A. corniculatum showed a slight genetic break aligning with the Wallacea barrier. The hypocotyl of A. corniculatum is reported to have a relatively strong long-distance dispersal ability (Clarke, 1995).
IBD might have played a role in differentiating populations of coastal plants. Our mantel test revealed a significant positive correlation between genetic distance and geographic distance. For example, the long distance between “Bali” and “Gulf of Bengal” is likely the factor underlying their slight genetic differentiation. Taking consideration of the large geographical scale of its distribution range and the limited ability of dispersal, it’s expectable to observe the IBD effect inA. corniculatum .
The patterns discussed above are generally consistent with previous knowledge. However, the striking genetic break between “s-SCS” and “n-SCS” is quite intriguing, some unaddressed factors should have been in function. We address this issue in the next section.