Affinities of the freshwater red algaAudouinella macrospora(Florideophyceae, Rhodophyta) and related forms based on ssu rrna gene sequence analysis and pit plug ultrastructure

Small subunit rDNA sequencing and transmission electron microscopy were performed to clarify the ordinal affinities ofAudouinella macrospora(Wood) Sheath et Burkholder isolates 3394, 3395, and 3603, as well asChantransiasp. isolate 3585. Culture 3603 is known to produce thalli ofBatrachospermum ‐like morphology under certain culture conditions. Sequence analyses unequivocally placed the threeAudouinella macrosporaisolates in a clade withBatrachospermum macrosporumMontagne of the Batrachospermales, andChantransiasp. was found to have affinities withB. louisianaeSkuja andB. virgato‐decaisneanumSirodot. The pit plugs of theAudouinella macrosporacultures 3394 and 3395 were nearly identical in size and structure, having thickened plug caps and no cap membranes. Both of these features agree with those of the Batrachospermaceae, with the latter feature showing batrachospermacean rather than acrochaetioid affinities. Pit plugs in the chantransia phase of 3603 were similar, but the plug caps were less well developed. TheBatrachospermumphase generated from 3603 had pit plugs that were variable in diameter, according to location in the thallus, thus reflecting the more variable cell size in this phase. Dome‐like outer caps, considered typical ofBatrachospermum, were present between cells of the determinate lateral filaments. The pit plugs ofChantransiasp. had prominent, dome‐like outer caps, but the plug cores were strikingly and consistently smaller in diameter than those of theA. macrosporachantransia cultures, suggesting that plug diameter may be of systematic value in some contexts.