Gene flow between subspecies
We wanted to examine the magnitude of among-subspecies gene flow in more detail. The samples collected on the west coast of Australia (population maBB) were morphologically diagnosed as A. m. marina.However, our genetic data show relatively lowFST values between maBB and A. m. eucalyptifolia . FST values between maBB and other A. m. marina populations were comparable with those between maBB and A. m. australasica populations (Supplementary Table 4). The NJ tree also clusters maBB with the two A. m. eucalyptifoliapopulations (Supplementary Figure 1). This contrast between morphological and genetic lines of evidences hints strongly at gene flow between A. m. marina and A. m. eucalyptifolia in Western Australia where the two subspecies grow together. The demographic model assuming that maBB has descended from A. m. marina but exchanges genes with A. m. eucalyptifolia is the most likely in our ABC analyses (model v2, posterior probability 0.933, Table 2 and Figure 4a).
We also used TreeMix to capture more potential gene flow events among populations (Figure 4b). We identified six such events on the population splitting graph. Among the six events, three were between subspecies, with one gene flow event between each pair of the three subspecies. Two gene flow events occurred between pairs of A. m. marinapopulations. The last gene flow event occurred between maBB and the outgroup species A. alba .