Understanding the evolution of pyrenoid-bearing taxa in brown algae through molecular phylogenies
Résumé
In brown algae (Phaeophyceae), only a few taxa are reported to have plastids containing pyrenoids and this character has
often been used for systematic delineation. Members of the Ectocarpales display large, stalked and exserted pyrenoids in their
plastids, whereas the Scytothamnales, as well as the genera Asterocladon, Asteronema and Bachelotia, exhibit plastids arranged
in a stellate configuration with pyrenoids embedded in the plastid stroma. The evolution of plastid characters has remained
difficult to establish, especially because of a persistent lack of resolution in brown algal molecular phylogenies. Here, we
generated a seven-locus dataset for a species-rich taxon sampling, to reassess phylogenetic relationships of pyrenoid-bearing
brown algal taxa. Within the Ectocarpales, the relationships among families are clarified. The minute chordariacean genus
Herponema is assigned to the family Acinetosporaceae. The systematic positions of two enigmatic genera are revised:
Chordariopsis is assigned to the Adenocystaceae, and Spongonema is transferred back to the Ectocarpaceae. The new order
Asterocladales, which is sister to the Ectocarpales, is proposed to accommodate species of the genus Asterocladon. The order
Scytothamnales is shown to include the genus Bachelotia in a new family Bachelotiaceae, as well as another new family
Asteronemataceae, accommodating the genus Asteronema. These new taxonomic insights also shed light on the evolution of
plastid morphology and arrangement in brown algae. Stellate plastid configurations have evolved independently in
Asterocladales and Scytothamnales. Evolutionary scenarios underlying the diversity of stellate configurations in the
Scytothamnales and the Asterocladales–Ectocarpales lineage are discussed.