Genetic diversity of goniomonads: an ancient divergence between marine and freshwater species
Goniomonas is a ubiquitous free-living, phagotrophic zooflagellate genus related to the photosynthetic cryptophytes. Only one freshwater and two marine species have been described. Although the two marine species (G. pacifica and G. amphinema) are morphologically quite distinct from each other, it was unclear until recently whether one of them (G. pacifica) is really separate from the freshwater G. truncata because their morphology and size are so similar. We have isolated six new Goniomonas strains and amplified and sequenced their small subunit ribosomal RNA genes. Our results reveal remarkable genetic diversity within all three nominal species and confirm that G. pacifica is genetically very distinct from G. truncata. Within each morphospecies there is more divergence than between many cryptophyte genera and there are many more substantially different genotypes of Goniomonas than established 'species'. This implies that there are numerous undescribed 'species' of Goniomonas and suggests that goniomonad genetic diversity might equal that of the cryptophytes. Phylogenetic analysis of 10 Goniomonas sequences shows two very robust clades, one consisting only of 'G. truncata' and the other comprising G. pacifica-related sequences and a separate cluster of genotypes that probably represent G. amphinema. Thus, one clade comprises five freshwater strains, which have markedly longer genes, and the other clade comprises five marine strains. The divergence between all freshwater and all marine sequences is several times greater than that within one morphospecies. This deep genetic divergence implies that for several hundred million years, freshwater goniomonads have not been able to colonize marine habitats effectively, and vice versa. Our trees show that the 18S rRNA gene has evolved in freshwater strains faster than in the marine strains and robustly support the holophyly of all goniomonads and their sister relationship with cryptophytes.