A re-classification of the Acrochaetiales based on molecular and morphological data, and establishment of the Colaconematales ord. nov. (Florideophyceae, Rhodophyta)

Abstract

Systematics of the red algal order Acrochaetiales and related taxa was investigated using combined small- and large-subunit nuclear ribosomal DNA (SSU and LSU rDNA, respectively) sequence data. These data were subjected to distance, parsimony and maximum likelihood analyses. The resulting phylogenies were congruent with previously published SSU results in that all included orders (Balbianiales, Batrachospermales, Nemaliales, Palmariales and Thoreales) were resolved as monophyletic except the Acrochaetiales, which consisted of two lineages (Acrochaetiales I and II). The Batrachospermales and Thoreales occupied equivocal positions as early diverging lineages, while the Balbianiales generally resolved as sister to an Acrochaetiales–Nemaliales–Palmariales (ANP) complex. Relationships among the four lineages of the ANP complex were not completely resolved, but detailed analyses weakly positioned Acrochaetiales II as sister to the Nemaliales, whereas Acrochaetiales I displayed a moderate to strong affiliation with the Palmariales. Acrochaetiales I included representatives of the genera Acrochaetium,Audouinella and Rhodochorton, whereas Acrochaetiales II had a number of acrochaetioid species including a representative of the genus Colaconema. Compared with published SSU phylogenies, bootstrap values within the two Acrochaetiales lineages increased substantially in combined SSU/LSU analyses. Based on these results, emended generic descriptions are provided for Acrochaetium, Audouinella, Colaconema and Rhodochorton, and a new family of acrochaetioid algae is described, the Colaconemataceae J. T. Harper et G. W. Saunders. The Acrochaetiaceae now includes the genera Acrochaetium, Audouinella and Rhodochorton, while the Colaconemataceae is considered monogeneric at this time. It is quite likely that additional genera will be recognized within the Colaconemataceae pending further investigation. Acrochaetiaceae is retained as the sole family of the Acrochaetiales, although the tenuous recognition of the Palmariales as distinct from this order is discussed, whereas Colaconemataceae is transferred to the new order Colaconematales J. T. Harper et G. W. Saunders.