XVI International Botanical Congess

Agar-type phycocolloids occur in orders as diverse as the Bangiales, Corallinales, Ahnfeltiales, Gelidiales, Gracilariales, and Ceramiales. The distribution of these orders in phylogenetic trees derived from nucleotide sequence data indicates production of agar-type galactans is an ancestral character. By contrast, the carrageenophytes are a monophyletic group confined to the Gigartinales. Some representatives of the Halymeniales, Rhodymeniales, and Gigartinales produce intermediate" polysaccharides with both agar and carrageenan structure but the implications of this are moot. Some intraordinal phylogenetic hypotheses for the Gigartinales can be inferred from the distribution of non-glycosyl substitution patterns in carrageenans.