MOLECULAR SYSTEMATICS OF THE GELIDIALES - INFERENCES FROM SEPARATE AND COMBINED ANALYSES OF PLASTID RBCL AND NUCLEAR SSU GENE-SEQUENCES

Nucleotide gene sequences for both the nuclear-encoded small-subunit R NA (SSU) and plastid-encoded large subunit of RuBisCO (rbcL) were dete rmined for 16 species classified in the red algal order Gelidiales. Se quence comparisons indicate that rbcL is evolving at a faster rate tha n SSU in these species and that there is a more even distribution of c hanges across the length of the rbcL gene compared with that observed for the SSU gene. The rbcL sequences also showed a marked anti-GC bias at synonymous third-codon base positions. Parsimony and maximum-likel ihood analyses were used to generate phylogenetic trees from both sepa rate and combined analyses of the two sequence data sets. Relationship s among most taxa are resolved robustly; however, the relative order o f branching for the Capreolia and Ptilophora clades remains uncertain. The molecular data provide unambiguous, independent support for recog nition of the newly established genus Pterocladiella. Although the pos ition of Pterocladiella within the Gelidiales was not resolved clearly by the rbcL data alone, analyses of both the SSU and combined data ma trices indicate that this genus is one of the three earliest-diverging lineages within the order. These data also suggest that Gelidium as c urrently circumscribed is not monophyletic. Molecular and morphologica l evidence suggests that the origin and, perhaps, diversification of t he major lineages of gelidialean algae is correlated with distinct, in dependently evolved nutritional strategies for the developing carpospo rophyte.