Summary Pairs of angiosperm species putatively separated by continental drift following the break-up of Gondwanaland have been used to derive linear regressions relating macromolecular differences to time. Differences have been derived from N-terminal amino acid sequences of the small subunit of rubisco from members of Proteaceae, Winteraceae, Nothofagus and Solanum. Alternative sets of times have been taken from the literature on plate tectonics and biogeography. Botanical criteria have been used to discriminate tentatively between the two regression lines. The rate of evolution corresponding to the preferred regression is one inferred nucleotide difference between two evolutionary lineages in seven million years. An estimate of the variance about the regression lines has been obtained and has allowed suggestions about which species disjunctions are likely to have originated more recently than the continental separations.

A molecular evolutionary clock for angiosperms

Angiosperm phylogeny studied using sequences of five macromolecules

Assembly of Rubisco from native subunits.

Partial sequences of 18s rRNA were obtained for 2 gymnosperms and 12 angiosperms from a wide range of families and these were analyzed with 5 other published sequences to form a phylogenetic tree. Using 16 published sequences of the large subunit of rubisco (rbcL), also from a wide range of angiosperm families, another phylogenetic tree was derived and the two approaches were compared. Both phylogenetic trees gave good grouping within families but in neither case was there resolution of the branching order of major taxa. Superficially the long rbcL sequences (whose base composition was homogeneous among all species) seemed very promising, but analysis showed that a large proportion of the variation did not affect the amino acid sequence. Although silent substitution contained some phylogenetic information, at the level required to order major taxa, much of it was random and obfuscating. It was concluded that neither macromolecule alone was likely to yield a solution to the problem of angiosperm phylogeny and therefore that studies of both, at least, will be required. For this reason, a method was described for obtaining both DNA and RNA of good quality from the same preparation and which had been used successfully with a wide range of species including many with pungent leaves.

A comparison of 18s ribosomal RNA and rubisco large subunit sequences for studying angiosperm phylogeny

A phylogenetic tree for some monocotyledons and gymnosperms derived from protein sequences

Ribulose bisphosphate carboxylase has been prepared from 50 species of angiosperms from 16 diverse families. In 35 preparations, well known 'bland leaf' methods were used but 15 species had 'pungent leaves' and for these a new preparative method is described. Automatic methods have been used to obtain N-terminal sequences (40 amino acids) of the small subunit (SSU) from all 50 species and the pattern of variability is discussed: 26 of 40 positions are variable to a degree similar to that found in plastocyanin and plant cytochrome c, i.e, an average of 3.7 different amino acids per variable site. These results, and the fact that sufficient protein can be obtained from 100 g of leaves, make a widespread phylogenetic survey of angiosperm SSU feasible and it is claimed that the method is at least as practicable as nucleic acid sequencing. A limited amount of sequencing has been carried out on the large subunit (LSU) but its low variability discourages a protein sequencing survey. Implications for gene structure and function are discussed and evidence is given that active LSU is derived from a precursor with 14 additional amino acids at the N-terminus. In SSU, variability of the two N- terminal amino acids suggests that they are not involved in the signals for removal of either the transit peptide or, in the RNA, of the intron, excision of one end of which depends on the codons for the invariable amino acids at positions 3 and 4. Evidence is also given that if the N-terminus of SSU is methionine, as is common, then it is modified and associated with a 'frayed' N-terminus.

The study of plant phylogeny using amino acid sequences of Ribulose-1,5-Bisphosphate carboxylase. I. Patterns of variability

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The study of plant phylogeny using amino acid sequences of Ribulose-1,5-Bisphosphate carboxylase. I. Patterns of variability