Abstract
Relationships among the apical pore field-bearing diatom genera Brebissonia, Cymbella, Gomphonema, Gomphoneis, Didymosphenia, Gomphocymbella, and Reimeria and their close ally Encyonema were evaluated using cladistics. These biraphid genera are a polythetic group defined by chloroplast number and type, mode of sexual reproduction, valve orientation and presence of apical pore fields. Character states of valve as well as non-siliceous features were described and then polarized using the outgroup method, with the naviculoid genera Anomoeoneis and Placoneis serving as outgroups. The cladistic analysis suggests two groups of terminal taxa, corresponding to to a predominantly cymbelloid lineage and a gomphonemoid lineage. Didymosphenia, previously thought to be closely allied to gomphonemoid diatoms, is shown to be more closely related to Cymbella. Gomphonema appears to be without distinguishing characteristics. Relationships of the other genera are described. The systematic position of small, doubly-punctate Gomphonema species is discussed.
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