MOLECULAR-BIOLOGY STUDY OF ANGIOSPERMS OR IGIN

Nucleotide sequence of 23S-5S chloroplast rDNA spacer region including 4.5S rRNA gene of several dozens of seed plants was determined. The d ata obtained were used to construct phylogenetic trees and to compare them with the analogous data from literature. Topologies of trees cons tructed for various types of macromolecules and by different methods d emonstrate obvious similarities although they are not identical. Some clue stages of seed plants evolution still remain obscure. Critical an alysis of all the available information allows to come to several more or less definite conclusions. All the data say that angiosperms are a monophyletic group which diversified far before their fossils are def initely registered, i.e., before lower Cretaceous. Ancestral angiosper ms were not genealogically related to modern woody Magnoliales but wer e represented by <<paleoherbs>>, i.e. herbaceous and semiherbaceous ma gnoliids and monocots. Monocots originated at the earliest stages of a ngiosperms evolution and are not, probably, monophyletic. Woody Magnol iales and eudicots with tricolpate pollen seem to appear later in evol ution. The conclusion that Gnetales is a sister group to angiosperms d oes not find enough support in molecular studies. Summing up, it looks as if a long period of existence of angiophytes preceeded the pre-Cre taceous angiosperms irradiation. This line of development originated s imultaneously with phylogenetic lineages of modern gymnosperms.