THE GIARDIA-LAMBLIA ACTIN GENE AND THE PHYLOGENY OF EUKARYOTES

The single-copy actin gene of Giardia lamblia lia lacks introns; it ha s an average of 58% amino acid identity with the actin of other specie s; and 49 of its amino acids can be aligned with the amino acids of a consensus sequence of heat shock protein 70. Analysis of the potential RNA secondary structure in the transcribed region of the G. lamblia a ctin gene and of the single-copy actin gene of nine other species did not reveal any conserved structures, The C. lamblia actin sequence was used to root the phylogenetic trees based on 65 actin protein sequenc es from 43 species. This tree is congruent with small-subunit rRNA tre es in that it shows that oomycetes are not related to higher fungi; th at kineto-platid protozoans, green plants, fungi and animals are monop hyletic groups; and that the animal and fungal lineages share a more r ecent common ancestor than either does with the plant lineage. In cont rast to small-subunit rRNA trees, this tree shows that slime molds div erged after the plant lineage. The slower rate of evolution of actin g enes of slime molds relative to those of plants, fungi, and animals sp ecies might be responsible for this incongruent branching.