Population Genetics And Historical Demography Of Red Seaweed Palmaria Palmata From The North Atlantic

Palmaria palmata，a member of order Palmarials, family Palmariaceae, genusPalmaria, is widely located along both sides of the North Atlantic. In this study, weanalyzed the molecular phylogeography of Palmaria palmata by using themitochondrial cox2-3spacer、the rpl12-rps31-rpl9region of the plastid genome andRAPDs, aiming to explore the population structure, genetic diversity and the effectsof climate (especially the late Pleistocene climatic oscillation) on the distribution andmigration of this typical red alga from the North Atlantic. The main results aresummarized as follows.(1) We examined600individuals of P. palmata from55locations,8countries,almost covered the distribution range in the North Atlantic. We grouped55localitiesinto11geographic populations according to their proximity. The length of cox2-3was378bp, yielding36haplotypes; the length of the rps sequence was569bp, and20haplotypes were identified. Analysis of cox2-3spacer, rps regions and RAPDsrevealed that the English Channel (ENC), Galway Bay (GWB) and the North Sea(NTS) had the top three highest haplotype and nucleotide diversity (cox2-3: h=0.7708-1.0000, π=0.3223-0.7055; rps: h=0.5770-0.6670, π=0.1172-0.1538). Wepropose the existence of glacial refugia and transition zones in ENC, GWB and NTSresulted in distinct genetic diversity there.(2) Based on the topology of the haplotype network and the pattern of geneticdiversity, our results enhanced the hypothesis that there were four potential glacialrefugia on both side of the Atlantic: Hurd deep, Galway Bay, Iceland/Norway andnorthern America, early proposed by Provan et al (2005). Because of the limitation ofsampling sites, we can not confirm the exact location of glacial refugium alongIceland/Norway and northern American coast. Integrated with the haplotypeslocation, STRUCTURE cluster and the information of paleoclimate, we can infer themigration routes of P. palmata after Glaciation. The Hurd Deep refugium did notplay much role in driving postglacial expansion presumably because of habitat change from contact to expansion, from where individuals only expanded into theIrish Sea and the North Sea. The Iberian populations were probably also from theHurd Deep refugium. The southwestern Ireland refugium was the major donation ofthe East Atlantic populations, where the ice sheet was firstly melted. There wereseparate northwards and southwards recolonization routes from the Galway Bay.Populations finally reached the Iceland/Norway coast and the Celtic Sea.Iceland/Norway coast may also have glacial refugia. Furthermore, we propose thatIcelandic refugium was the transit area between the western and the eastern Atlanticpopulations. The haplotype network of the western coast was much simple.Populations close to the refugia had higher population diversity, such as LA/BB fromSt. Lawrence and WH from Fundy Bay.(3) ML, BI and haplotype network analyses consistently indicated three clades forthe East Atlantic populations and shallow genetic structure in the West. The reasonsare as followed. Firstly, the coastline of the East Atlantic is more complicated thanthe West. Populations survived through Glaciation were restricted to severalseparated glacial refugia, where populations harboured different core haplotype andother private haplotypes. For physical barriers to dispersal, gene flow betweendifferent populations was restricted to form present-day population structure.Secondly, populations in the West Atlantic coast had less time for divergenceaccumulation.(4) Population divergence time between the East and West Atlantic populations wasestimated at c.1.25Ma, indicating a survival of many glacial-interglacial cycles forthe West populations. Bayesian skyline plots analysis based on cox2-3spacersequences indicated that the approximate expansion time for all populations of P.palmata was0.455-0.364Ma. Our study presented multiple-angle genetic evidencethat the glaciation before Cromer Interglaciation (0.48-0.34Myr) had a drasticinfluence on the demographic patterns and genetic diversities of P. palmata otherthan the last glacial maximum.