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.