XVI International Botanical Congess

Finding a generally useful correlation between molecular data (extent of sequence divergence) and taxonomic level has been elusive. We offer a new possibility, based on a particular type of change in rDNA repeat regions present in all eukaryotes, compared to a universal facet of the eukaryote life cycle, fertilization. All eukaryotes require successful completion of two activities, gamete recognition (with adherance and subsequent activation) and gamete fusion for zygote formation. Organisms capable of carrying out these two steps we define as belonging to the same Z SPECIES (z = zygote). We have observed empirically a characteristic of DNA sequence divergence that has predictive value for the level of Z species in different groups of eukaryotes. If two organisms differ, in their rDNA ITS-2 sequences, by a double-sided compensating base change (CBC) in any of the relatively conserved nucleotide positions involved in forming the helices necessary to the RNA transcript secondary structure, then these two organisms do not belong to the same Z species- there is no possibility of interfertility. This is true for several groups of green algae (Volvocales, Desmids), for brown algae (Laminariales), for trematodes and very probably for angiosperms, offering a new approach to comparisons of taxonomic levels.