Andrzej Witkowski was right: Tryblionella compressa is not a Tryblionella

Abstract

Tryblionella punctata was described by W. Smith in 1853, transferred to Nitzschia by Grunow in 1880, and returned to Tryblionella in 1990 by Round et al., when the genus was restored and 41 species transferred into it from Nitzschia. Meanwhile, the species was claimed to be a synonym of Pyxidicula compressa by Boyer, who made the new combination Nitzschia compressa, though most authors continued to use N. punctata. The 1990 restoration of Tryblionella was not accepted by some, including Andrzej Witkowski et al. in their key work on coastal marine diatoms [2000, Diatom Flora of Marine Coasts], but others did accept it and M. Poulin accordingly made the new combination T. compressa. Although we demonstrate that T. compressa (as currently understood in the literature and exemplified by isolates from a coastal lagoon) has wide valves, an almost marginal raphe system, a strongly undulate valve face and a marginal ridge, like most species assigned to Tryblionella, molecular evidence indicates that it is not related to the group containing the type of Tryblionella, T. acuminata. Instead, it belongs to a clade (Bacillariaceae clade 8B) containing lanceolate Nitzschia species such as N. amphibia, N. inconspicua and N. reskoi, as well as Denticula kuetzingii. Some morphological characters support the same relationship, notably the structure of the girdle, which has alternating wide and narrow bands in the epitheca. Morphology suggests further that T. lanceola is a close relative. Both species need to be removed from Tryblionella but Bacillariaceae clade 8B does not correspond to any single accepted, named genus and there is no named monophyletic group to which they can yet be transferred. Until a consensus is reached about generic circumscriptions in the Bacillariaceae, we suggest that the classification of the two species should revert to the pre-1990 state (i.e. assignment to Nitzschia), to maximize continuity with older literature and maintain the current naming of isolate CCMP561, which is a strain widely used in genomic and biochemical studies. This also solves the problem that, on the basis of Bailey’s description and illustrations, ‘compressa’ cannot be used for this species. In contrast, authentic Smith material and illustrations of T. punctata agree well morphologically, though not perfectly, with the three genetically characterized isolates, allowing them all to be assigned for the moment to N. punctata sensu lato. Witkowski et al.’s caution about the 1990 changes was abundantly justified.