Substantial genetic divergence among subspecies
In this study we comprehensively sampled A. marina populations
across their geographical range, assembled an extensive SNP data set,
and used it to quantify the genetic differentiation among the three
morphologically recognized subspecies. Our study finds a robust genetic
split of A. marina into three groups, noting that this divergence
was observed both in the genetic distance DXYmatrix and in PCA clustering based on a SNP frequency matrix. The
genetic grouping pattern is generally consistent with the morphological
classification of the three subspecies, marina ,eucalyptifolia , and australasica . The levels of
within-subspecies genetic diversity are found to differ among
subspecies, implying that gene pools of the three subspecies are
separated to some degree.
Genetic differentiation among populations, usually attributed to
isolation by distance or isolation by geographic barriers, have been
documented in many mangrove species, such as the deep genetic
differentiation between populations on the opposite sides of the Malay
Peninsula in Rhizophora (Guo et al., 2016; Wee et al.,
2015), Ceriops (Tan et al., 2005) , Lumnitzera (J. Li et
al., 2016), and Xylocarpus (Guo et al., 2018b). Like
previous findings, the differentiation on the two sides of Malay
Peninsula is also observed in populations of the subspeciesmarina and is attributed to isolation of the Malay Peninsula
currently and the whole Sundaland historically. Although
propagules of A. marina are buoyant on sea water and disperse
over via currents (Steinke & Ward, 2003), they are reported to be
relatively weak in dispersal (Clarke, Kerrigan, & Westphal, 2001; Duke
et al., 1998). Our estimates of differentiation among A. marinasubspecies exceed those based on geographical isolation. In addition,
there is no geographical barrier inferred between the regions inhabited
by australasica and eucalyptifolia. There must be some
other factors causing their substantial genetic divergence. These
findings indicate that the subspecies designation in A. marinaindeed represents a stage beyond structured populations on the
speciation continuum.
Despite substantial divergence, these subspecies are not completely
isolated. Genetics is not in concordance with morphology in some
populations where two subspecies occur in coexistence or adjacently. The
individuals from Bunbury, Australia, (BB) are morphologically diagnosed
as marina, but genetically closer to eucalyptifolia as
shown by the neighbor-joining tree and MDS clustering ofDXY . A recently published study focusing on theA. marina population on the west coast of Australia revealed
genetic differentiation across geographical distance but not between
subspecies, though the samples likely contain both marina andeucalyptifolia (Binks et al., 2019). This implies that constraint
on gene flow between subspecies appears to be relaxed once geographical
isolation (by distance or barrier) is removed. This is compatible with a
subspecies designation because full species are less likely to allow
gene flow, although accidental introgression via hybridization cannot be
completely ruled out.