Phylogeography of Batrachospermum gelatinosum (Batrachospermales, Rhodophyta) shows postglacial expansion in Europe

The freshwater red alga Batrachospermum gelatinosum inhabits streams of Europe and North America and has been collected frequently on both continents. A study of this species in North America showed evidence of a glacial refugium in the southeastern United States with little genetic variation throughout its more northern range in eastern North America. The present study was initiated to investigate its phylogeography throughout Europe and to compare these results with those obtained for North America. Specimens were collected from Belgium, Estonia, Finland, France, Great Britain, Italy, Latvia, Lithuania, Poland and Spain. Both the mitochondrial cytochrome c oxidase subunit I (COI) barcode region and the nuclear rDNA internal transcribed spacer (ITS 1 and 2) regions were analysed. Of the 70 individuals analysed, 12 COI haplotypes were uncovered. In addition, ITS variation of 67 individuals was surveyed and showed 21 haplotypes. The haplotype network of COI data showed a large number of the individuals distributed among three common haplotypes. The other nine haplotypes differed from the common ones by only one to two base pairs and were represented by only one to six individuals. The ITS network had a star appearance with a common haplotype (17 individuals) and many closely related haplotypes with few individuals per haplotype. Compared to North America, there were more COI haplotypes present in Europe (12 vs 5), and the relationship among haplotypes was more complex. The geographic distribution of haplotypes did not appear to follow a glaciation pattern; rather, the common haplotypes were widespread, suggesting a recent expansion.