The three subspecies are distinct in morphology, genetic
divergence and demography
In this study we comprehensively sampled A. marina populations
across their geographical range, assembled an extensive SNP data set and
used it to test the genetic basis for distinguishing three
morphologically recognized subspecies. Our study indeed confirms a
robust genetic split of A. marina into three varietal groups,
noting that this divergence was observed both in the genetic distanceDXY matrix and in PCA clustering based on a SNP
frequency matrix. Given the almost mutually exclusive distribution of
these three varieties (Figure 1), the genetic divergence between
subspecies is expected to be shaped to a large extent by geographical
barriers.
The boundary of current distributions of A. m.eucalyptifolia and A. m. marina in Asia roughly aligns
with Weber’s Line. Similar to the cases identified in other mangrove
trees, such as Rhizophora apiculata (Zixiao Guo et al., 2016),Sonneratia alba (Yang et al., 2017), Ceriops tagal (Y.
Huang et al., 2012) and Xylocarpus granatum (Zixiao Guo et al.,
2018), we presumes that the Indonesia-through flow in the Wallacea Zone
was the geographic barrier isolating A. m. marina in Asia andA. m. eucalyptifolia in Australia and New Guinea (Gordon, 2005;
Hall, 2009). On the east coast of Australia, the occurrence of A.
m. eucalyptifolia grades into A. m. australasica between
Rockhampton and Brisbane, where the North Caledonian Jet bifurcates into
the North Queensland Current and the East Australian Current (Ganachaud
et al., 2007; Schiller et al., 2008). Hence, we also infer that this
bifurcating ocean current may underlie the divergence of A. m.
eucalyptifolia and A. m. australasica . Although geographical
isolation is no longer considered as the only factor driving speciation,
ocean currents seem to have played an important role in the driving
divergence of these three subspecies.
Sister subspecies are postulated to be separate evolutionary lineages,
although some gene flow can be accommodated if they come into contact.
Genetic structure between populations can result from reduction in gene
flow, but evidence from multiple aspects is necessary to establish that
such events occur. The very different levels of genetic diversity among
subspecies may provide such evidence. We find that the model that treats
the three subspecies as separate lineages, with gene flow betweenA. m. marina and A. m. eucalyptifolia, fits our data best.
A previous study estimated that A. m. australasica diverged from
the other two subspecies at about 2.7 MYA, while A. m.eucalyptifolia diverged from A. m. marina at about
1.8 MYA (X. Li et al., 2016). The split between A. m.marina and A. m. eucalyptifolia coincides with the
beginning of Pleistocene, when sea level significantly dropped. The sea
level drop emerged shallow seas into lands in the Indo-Australian
Archipelago region, possibly leading to separation of nascent subspecies
ranges.