Phylogeography Of Four Marine Bivalves Along China’ Coastline, With Views Into The Evolutionary Processes And Mechanisms

Present genetic patterns of marine species result from the interaction of varioushistorical and contemporary factors on different spatial scales. Phylogeographicalstudies have been widely carried out in some marine hotspots such as the northeasternPacific and the Atlantic-Mediterranean. The vast region of the northwestern Pacific,however, has yet attracted little such attention. Our understanding of historicalphylogeographical processes for this area lags surprisingly far behind that hasachieved for other areas.To fulfill the gap, we conducted phylogeographical studies on four marine bivalvesmainly distributed along China’s coastline. Mitochondrial and nuclear DNAsequences were amplified for the samples and each data set was analyzed to infer thepopulation structure and spatial distribution of genetic diversity based on coalescenttheory. The genetic patterns were discussed to assess the relative effects of historical,contemporary, and ecological factors on shaping present phylogeographical patternsof each species, with views into the interspecific concordance of evolutionaryprocesses.1. Phylogeography of Cyclina sinensisThe phylogeography of bivalve Cyclina sinensis was investigated to test the relativeinfluence of historical glaciations and the Changjiang River outflow on populationstructure. Samples were collected from the three marginal seas. Fragments of twomarkers (mitochondrial COI and nuclear ITS-1) were sequenced for335individuals from21populations. The results suggested significant population structure among thethree seas, and each sea was dominated by a distinct lineage. The divergence timesamong COI lineages were estimated dating from the Pleistocene. Thephylogeographical break revealed is congruent with the historical isolation of seabasins rather than the putative Changjiang River outflow barrier. The large landbridges extending between seas during glaciation allowed accumulation of mutationsand subsequently gave rise to deep divergent lineages. The low-dispersal capacity ofthe clam and current coastal oceanography may facilitate the maintenance of thehistorical patterns for a long time as barriers shift.2. Phylogeography of Tegillarca granosaPresent genetic patterns of marine organisms not only result from historical andcontemporary ecological factors, but also from anthropogenic activities.Disentangling the relative effects of these factors can provide valuable insights intomanagement and protection of exploitable species. The commercially importantmarine clam Tegillarca granosa is representative of species that are translocatedwithin East Asia for coastal aquaculture purposes. We conducted a nucleotidesequence analysis of mtCOI and nuclear ITS-1markers in T. granosa to investigate itsgenetic diversity and distribution in two marginal seas (the East and South China Seas)of the northwestern Pacific. Based on phylogenetic inferences, we identified twoevolutionarily significant units (ESUs) with high genetic distance between them forboth markers. The high genetic distance may be associated with the historicalisolation of the marginal seas during low sea level periods. One ESU was widelydistributed in both seas, whereas the other was restricted to two disjunct localities inthe South China Sea. Based on the isolation by distance analysis and comparison ofpatterns of co-occurring species, this pattern appears to be mostly attributable to thehuman-mediated translocations among coastal waters rather than natural rangeexpansion. Furthermore, from a conservation viewpoint, the southern ESU is nowfacing high extinction risk because of mitochondrial introgression and smaller, fragmented populations; consequently, immediate proper management is required toprotect the endangered populations representing this lineage.3. Comparative phylogeography of Mactra chinensis and M. veneriformisThere is complex interaction between biological evolution and topographic features.Studies carried out in regions with a clear geographical context will benefit the test ofvarious biological hypotheses. The East China Sea, a marginal sea with explicitrefugial information in northwestern Pacific, provides an elegant setting for studyingpostglacial evolution of marine species. Here we examined and compared thephylogeographical structure of two congeneric bivalves (M. chinensis and M.veneriformis) in this region, using both mtCOI and SSR. A star-like genealogy ofmitochondrial haplotypes with shallow genetic divergence was constructed for bothspecies. However, the two clams carried signals of different postglacial evolutionaryhistories: no significant population differentiation was detected for M. veneriformis,whereas substantial population structure was revealed for M. chinensis by both COIand microsatellite data. The habitat distribution, effective population size and the‘sweepstakes’ mode of reproduction and survival are suspected to have driven thediscordance. Three groups were defined for M. chinensis and coalescent approachesrevealed effectively zero migration between most pairwise comparisons, suggestive ofrestricted gene flow despite a pelagic larval duration. The study highlights the role ofpostglacial evolution contributing to present-day marine biodiversity and thespecies-specific evolutionary processes for closely related organisms even undershared environmental factors.