Demographic size dynamics such as bottleneck may be a general mechanism to shape the genetic structure of coastal plants
Consistent with our hypothesis, we observed that the bottleneck event caused by past sea-level changes had promoted population subdivision inA. corniculatum , resulting in the genetic break between populations in the southern and northern South China Sea. This mechanism is not likely unique to A. corniculatum , but generally to other species. The intraspecific genetic diversity of Senecio rodriguezii was found to be highly structured in that cpDNA haplotypes were not shared between the Mediterranean islands and a high proportion of haplotypes were restricted to small geographical areas within the islands (Molins, Mayol, & Rosselló, 2009). The population history of this species was supposed to have been dominated by both expansion and contraction events in the Quaternary sea-level changes. The three major lineages of Nigella arvensis in the Aegean Archipelago were found to evolve from multiple fragmentation events of a widespread ancestral stock, in which genetic drift appears to have played a significant role (Bittkau & Comes, 2005).
The mechanism of bottleneck events promoting population subdivision is highly likely applicable to coastal plants, which are common to have buoyant propagules, even in species growing on coastal dunes which are rarely inundated by seawater (Kadereit et al., 2005; H. Yang et al., 2012). Due to the nature of dispersing via seawater, genetic discontinuities in coastal plants were usually correlated with geographical barriers of land masses and ocean currents. It has known that biological properties of a species, such as breeding system, dispersal ability, and life history, would have an influence on how the geographic forces shape genetic variation within its distribution range (Nybom & Bartish, 2000; Wee et al., 2020; Westberg & Kadereit, 2009). We showed that the high dynamics of the habitats of coast plants may introduce additional complexity to their phylogeography.
The demographic histories of some coastal plants in the IWP region have been investigated. The effective population size (Ne ) of R. apiculata populations in the Malacca Strait (Z. Guo et al., 2016) and L. racemosa populations in the Malacca Strait and Hainan island were found to have been drastically reduced (J. Li et al., 2016). In contrast, the S. alba populations in the Malacca strait have largerNe than the surrounding populations, due to population admixture (Y. Yang et al., 2017). The mismatch distribution analysis and neutrality tests revealed some signals of population expansion in A. ilicifolius (W. Guo et al., 2020) while the BOTTLENECK analysis revealed no evidence of genetic bottleneck inCanavalia rosea (T. He et al., 2021). Interestingly, the possible correlation between genetic structure and historic demographic size change was not explored in these studies. Even if the mechanism we proposed before is general in the natural world, a reliable investigation into the bottleneck event must precede correlating it to any observed genetic break.
Lastly, attention to the demographic size change of a species is of particular importance for conservation. A large population is usually the fundament for plants to provide ecological services. The ecological importance of mangrove species has never been overly emphasized. Investigating their historic effective population sizes is an efficient approach to retrospect their past and prospect their future response to the current global change (Z. Guo, Li, et al., 2018). The A. corniculatum populations in the southern South China Sea call for conservation priority due to the reduced genetic diversity.