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.