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The diversities and distributions of diatoms are much more complex than was ever imagined. To understand the underlying mechanisms, research must focus on evolutionary processes occurring at a population level and employ sufficiently informative molecular markers. Using ten microsatellites and ITS rDNA sequence data, we investigated the genetic structure of populations of the benthic freshwater diatom Sellaphora capitata (until 2004 a cryptic entity within the S. pupula agg. species complex). This is the first time that microsatellites have been used to investigate the genetic structure of any freshwater or benthic microalga. Using an integrated approach (morphology, DNA barcoding and specificity of the microsatellite primers), we verified the identity of 70 S. capitata isolates obtained from lakes in the UK, Belgium and Australia. Standardized F'(ST) values were very high (>0.4) and in Bayesian analyses, isolates clustered according to their country of origin, with limited evidence of admixture. However, selected isolates from all countries were sexually compatible, a result consistent with limited ITS divergence. Considering the apparent absence of desiccation-resistant resting stages in most diatoms, we conclude that such levels of differentiation are likely to be a consequence of limited dispersal. With restricted dispersal, previously unacknowledged opportunities for allopatric speciation exist, which may help to explain the huge extant diversity of diatoms.

The study of reproductive isolation between populations, combined with estimates of genetic divergence, provides important insights into mechanisms of speciation. In this study, sixteen morphologically heterogeneous sympatric clones of Eunotia bilunaris sensu lato (Bacillariophyta) were brought into culture to study their phylogenetic relationships and pre- and postzygotic reproductive barriers. An ITS rDNA phylogeny was congruent with morphology and divided the clones into three groups ('slender', 'robust' and 'labile'), pointing to the presence of several species in E. bilunaris. Whereas most strains had a heterothallic mating system, four 'labile' clones displayed apomictic behaviour. A further 'labile' clone had a heterothallic mating behaviour, however, suggesting a very recent origin for apomixis. Despite high sequence divergence, hybridization occurred between clones belonging to different groups, but was 20-400 times less frequent than in intra-group matings. F1 hybrids had an intermediate morphology and were almost completely sterile; gamete formation was generally arrested in the early stages of meiosis I. The ITS divergence of 11.5-12.3% between the 'robust' and 'slender' clones seems to represent an upper limit of divergence in which cell pairing, gamete formation and auxosporulation are still possible but heavily reduced, and where hybrid sterility has already evolved.