The xanthorioid taxa Xanthomendoza borealis and Xanthoria mawsonii are revised using morphological, anatomical, secondary chemical, and molecular characters. We conclude that most specimens determined as X. mawsonii from the Antarctic are conspecific with specimens labelled X. borealis from the Northern Hemisphere. Some specimens named X. mawsonii from the Antarctic are wrongly determined specimens of Xanthoria candelaria (L.) Th. Fr, including the holotype of Xanthoria mawsonii C. W. Dodge. Accordingly, Xanthoria mawsonii C. W. Dodge is a synonym to Xanthoria candelaria (L.) Th. Fr. The only available correct name for the bipolar species, which comprises material previously determined as X. borealis and X. mawsonii, and is distinct from the more widespread taxon X. candelaria, is Xanthomendoza borealis (R. Sant. & Poelt) Søchting, Kärnefelt & S. Kondratyuk.

When generating kelp hybrid crosses in vitro, small blades are frequently seen in single-sex cultures of microscopic gametophytes as well as in putative hybrid crosses. We demonstrate a polymerase chain reaction (PCR)-based method that quickly determines the presence/absence of possible parental genomes in the observed blades. ITS1 nrDNA fragments that varied in length in a species-specific way were amplified. Amplification products from possible hybrid blades, apogamic blades and parental gametophytes were compared on agarose gels. Using these methods, we were able to identify a number of apogamic blades (i.e. formed directly from gametophytic cells) as well as a true hybrid sporophyte blade in crosses between Alaria marginata (Alariaceae) and Lessoniopsis littoralis (Lessoniaceae), members of two separate families of the Laminariales. To our knowledge this is the first genotypic confirmation of a hybrid kelp.