Spatially explicit genetic structure in the freshwater sponge Ephydatia fluviatilis (Linnaeus, 1759) within the framework of the monopolisation hypothesis

Abstract

An apparent paradox is known for crustaceans, rotifers and bryozoans living in inland small water bodies: a potential for wide distribution due to the presence of resting stages is coupled with marked genetic differences between nearby water bodies, with enclavedistributions masking clear phylogeographic patterns. According to the monopolisation hypothesis, this is due to the accumulation ofresting stages, monopolising each water body. Freshwater sponges could represent a useful system to assess the generality of the monopolisationhypothesis: these organisms i) live in the same habitats as crustaceans, rotifers and bryozoans, ii) produce resting stagesthat can accumulate, and iii) have indeed a wide distribution. Currently, no studies on spatially explicit genetic differentiation on freshwatersponges are available. The aim of the present study is to provide additional empirical evidence in support of the generality of thescenario for small aquatic animals with resting stages by analysing genetic diversity at different spatial scales for an additional modelsystem, the freshwater sponge ephydatia fluviatilis (Linnaeus, 1759). We expected that system genetic variability would follow enclavedistributions, no clear phylogeographical patterns would be present, and nearby unconnected water bodies would show markedly differentpopulations for this new model too. We analysed the ribosomal internal transcribed spacer regions 5.8S-ITS2-28S, the D3 domainof 28S subunit, the mitochondrial Cytochrome c Oxidase I (COI) and ten specific microsatellite markers of nine Italian and one Hungarianpopulations. Mitochondrial and nuclear sequences showed no or very low genetic polymorphism, whereas high levels of differentiationamong populations and a significant polymorphism were observed using microsatellites. Microsatellite loci also showed ahigh proportion of private alleles for each population and an overall correlation between geographic and genetic distances amongpopulations. All the expectations from the monopolisation hypothesis seemingly were confirmed for the analysed sponge.