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.