Morphometric and molecular investigations of species limits in Cyclotella meneghiniana (Bacillariophyceae) and closely related species

Abstract

Comparative molecular genetic and morphometric methods were used to clarify long-standing taxonomic uncertainties in one of the most common freshwater diatom species, Cyclotella meneghiniana Kützing. First a single C. meneghiniana population was sampled by isolating clonal cultures. Frustule ultrastructure of the cultures and of field samples was analysed using two different morphometric methodologies. These analyses contributed significantly to clarifying a problem concerning the delimitation of C. meneghiniana from a closely related species, C. scaldensis Muylaert&Sabbe. Next, patterns of ribosomal DNA sequence variation among fifteen cultures displaying the characteristic C. meneghiniana morphology were explored. The patterns observed indicated the presence of genetic structuring in the population sampled, suggesting that C. meneghiniana was either a complex of multiple, cryptic species, or that of strictly clonally or autogamously reproducing lineages. The following step was surveying sequence variation in the D1/D2 regions of the nuclear ribosomal DNA and in a region of the plastid encoded psaA gene in a collection of strains including C. meneghiniana strains with diverse geographic origins and cultures of some closely related species. Strict covariation was observed in the two genomic regions sequenced, further reinforcing the conclusion that C. meneghiniana is a structured species. The hypothesis that this genetic structuring was caused by the complete lack of allogamous sexual reproduction in this morphospecies was tested by quantifying phylogenetic incompatibility present in AFLP (amplified fragment length polymorphism) genotypes of strains from two populations. The results indicated no significant deviation from panmictic expectations, suggesting that Cyclotella meneghiniana is a complex of several, reproductively isolated species, rather than that of strictly autogamous lineages.