Homogeneous population of the brown alga Sargassum polycystum in Southeast Asia: possible role of recent expansion and asexual propagation

Abstract

Southeast Asia has been known as one of the biodiversity hotspots in the world. Repeated glacial cycles during Pleistocene were believed to cause isolation of marine taxa in refugia, resulting in diversification among lineages. Recently, ocean current was also found to be another factor affecting gene flow by restricting larval dispersal in animals. Macroalgae are unique in having mode of reproduction that differs from that of animals. Our study on the phylogeographical pattern of the brown macroalga Sargassum polycystum using nuclear Internal Transcribed Spacer 2 (ITS2), plastidal RuBisCO spacer (Rub spacer) and mitochondrial cytochrome oxidase subunit-III (Cox3) as molecular markers revealed genetic homogeneity across 27 sites in Southeast Asia and western Pacific, in sharp contrast to that revealed from most animal studies. Our data suggested that S. polycystum persisted in single refugium during Pleistocene in a panmixia pattern. Expansion occurred more recently after the Last Glacial Maximum and recolonization of the newly flooded Sunda Shelf could have involved asexual propagation of the species. High dispersal ability through floating fronds carrying developing germlings may also contribute to the low genetic diversity of the species.

Figure 1. Haplotype distribution and haplotype network of Sargassum polycystum for ITS2, Rub spacer and Cox3.

Pie chart size is proportional to sample size. Abbreviations for sample sites are given in Table S1 in File S1. Dominant currents are shown in solid lines [13] and seasonally reversing currents in dashed lines [14]. Light gray area shows the coastal outline during Pleistocene maximum low sea level of 120m [1]. SEC: South Equatorial Current, NEC: North Equatorial Current, NECC: North Equatorial Counter Current, NGCC: New Guinea Coastal Current, ME: Mindanao Eddy, HE: Halmahera Eddy.

Figure 2. Subregions of the study area defined for MIGRATE analysis.

1: South China Sea and Gulf of Thailand; 2: West Coast of Malay Peninsula; 3: West Java; 4: Celebes Sea and Flores Sea; 5: Guam; 6: Pacific Islands. Directionality of gene flows between selected subregions based on Cox3 is shown with N_e, the mean number of effective migrants per generation. All values are within the range of 440 to 665. Other details of gene flow among subregions based on all three markers are listed in Tables S6-S8 in File S1 respectively.

Figure 3. Mismatch distribution of Sargassum polycystum using ITS2, Rub spacer and Cox3.

Bar chart and line graph indicate the observed and expected frequency respectively.