Deep mitochondrial divergence
Recent molecular-based phylogeography studies in the NWP have uncovered
some general patterns: marine species often harbor genetically divergent
lineages in the SCS and ES (Cheang, Chu, & Ang Jr, 2010 [macroalga:Sargassum hemiphyllum ]; Shen et al., 2011 [flathead grey
mullet: Mugil cephalus ]; Ni et al., 2012 [venus clam:Cyclina sinensis ]; Cheng & Sha, 2017 [Japanese mantis
shrimp: Oratosquilla oratoria ]; Wang, Kong, Chen, Matsukuma, &
Li, 2017 [spotted hard clam: Meretrix petechialis ]). Here,
using phylogenetic and network analyses of mtDNA cox1 sequence
data, we identified two distinct mitochondrial lineages within M.
virgata: a northern lineage found in the cold surface-water temperature
zone in the northern NWP (the Korean coast, the northern part of the
ES/JS and the northern Pacific coast of Japan), and a southern lineage
occupying the warm surface-water temperature zone in the NWP (ECS, SCS
and the southern Pacific coast of Japan). The fixation index
(F st) and AMOVA analysis both support the
conclusion that these lineages are genetically distinct and display a
different biogeographic pattern except for some Japanese populations
(JSH, JSS, JKS, JOI, JKG, JNG; Fig. 1 and Table 1).
Sequence divergence between the two lineages is extremely high
(7.4-9.2%), while within-lineage distance is low (a maximum of 0.6%).
DNA barcoding analyses of other molluscan species using mtDNAcox1 have reported a mean intraspecific variation less than 2%
(Meyer & Paulay, 2005; Zou, Li, & Kong, 2012). However, deeper
intraspecific divergence (>2%) has also been detected in
some Atlantic and Pacific molluscan species (gastropods and bivalve
species); such deep divergence has been attributed to population
subdivision by a biogeographic partition between geographic lineages
(Layton, Martel, & Hebert, 2014). Very high levels of cox1divergence have also been reported in NWP bivalve species (Liu et al.,
2011 [>4.3% in Atrina pectinata ]; Wang et al.,
2017 [5.85% in Meretrix petechialis ]). Such divergence has
been interpreted as molecular evidence of a physical barrier reducing
gene flow. Historical isolation in marginal seas, caused by low sea
level during past glaciations, might be the primary driver for deep
intraspecific genetic splits in the NWP (Layton et al., 2014).