Highly differentiated populations of the freshwater diatom Sellaphora capitata suggest limited dispersal and opportunities for allopatric speciation

The diversities and distributions of diatoms are much more complex than ever 3 imagined. To understand the underlying mechanisms, research must focus on evolutionary 4 processes occurring at a population level and employ sufficiently informative molecular 5 markers. Using ten microsatellites, we investigated the genetic structure of populations of the 6 benthic freshwater Sellaphora capitata (until 2004 a cryptic entity within the S. pupula agg. 7 species complex). This is the first time that microsatellites have been used to investigate the 8 genetic structure of any freshwater or benthic microalga. Using an integrated approach 9 (morphology, DNA barcoding, specificity of the microsatellite primers and mating 10 experiments), we verified the identity of 70 S. capitata isolates obtained from lakes in the 11 UK, Belgium and Australia. FST values were very high (>0.4) and in Bayesian analyses, 12 isolates clustered according to their country of origin with limited evidence of admixture. 13 Considering the apparent absence of desiccation-resistant resting stages in most diatoms, we 14 conclude that such levels of differentiation are likely to be a consequence of limited 15 dispersal. With restricted dispersal, previously unacknowledged opportunities for allopatric 16 speciation exist, which may help to explain the huge extant diversity of diatoms.