Abstract
The use of the distribution patterns of plant natural products-alkaloids, terpenes, phenolics, etc.-is well established as a major tool for investigating population structures, species, and phyletic relationships of genera. Here, it is suggested that the distribution patterns of biogenetically closely related substances should be of considerable value for deducing evolutionary relationships at higher taxonomic levels. Approximately 540 plant taxa (cultivars through orders) have been included in approximately 150 systematic serological publications in the last 25 years. Research has demonstrated that extracts of seeds, pollen, leaves, tubers, and spores of vascular plants can be used if the required extraction procedures are followed. Both quantitative and qualitative immunological techniques have provided complementary data which have proven to be provocative and valuable in the classification of higher plants. The examples presented clearly indicate serology has contributed chemical data which can be-and have been-used with other data to aid in producing systems of classification such as those of Cronquist and Takhtajan. The phylogenetic relationships among 15 species belonging to 12 families of vascular plants based on a comparison of cytochrome c amino acid sequences agree in general outline with morphologically based phylogenetic diagrams. Amino acid sequence data on homologous plant proteins are in too limited a supply to permit other than very preliminary phylogenetic comparisons. Acquisition of more data will require considerable time and work before an impact will be realized. Published protein sequence data have not revolutionized presently accepted phylogenetic diagrams, and it is too soon to hint at the ultimate contribution of sequence data to phylogenetic schemes. The technique of nucleic acid hybridization is, in principle, applicable to chemotaxonomy at all taxonomic levels since it involves the fundamental hereditary material deoxyribonucleic acid (DNA) and its transcribed copy, ribonucleic acid (RNA). In contrast to the relative ease with which meaningful plant natural products distribution patterns are determined, are the difficulties and patience required to carry out nucleic acid hybridization experiments and to interpret the results from them. Thus, it is not surprising that few nucleic acid hybridization data for higher plants are available to meaningfully influence the interpretations of Cronquist and Takhtajan for the evolution of the angiosperms; nevertheless, the method inherently has great potential. With the development of plant natural products chemistry, which deals with a myriad of alkaloids, phenolics, mustard oils, terpenoids, etc. botanists and chemists have revealed that it is possible to employ chemical constituents to help characterize, classify, and describe taxa. Attempts to correlate morphological and chemical characteristics are very old. Greene (1909) indicated that the most remote and primitive of botanical writers, of whatever country, found a botanical The Abstract, General Introduction, and Macromolecules-Systematic Serology were prepared by D. E. Fairbrothers. Micromolecules-Plant Natural Products and Macromolecules -Nucleic Acid Hybridizations were prepared by T. J. Mabry. Macromolecules-Amino Acid Sequences was prepared by R. L. Greshoff (1893) stated several basic tenets. One stated that biochemists and phytochemists had to investigate evolutionary tendencies of metabolic pathways and groups of chemically related plant constituents much more thoroughly before they would achieve an understanding of evolution comparable to that of morphologists. McNair's (1965) book, which is a reprinting of his published papers, considered taxonomy in relation to oils, fats, waxes, oil and starch in seeds, and alkaloids. McNair's 1935 reprinted paper *Angiosperm Phylogeny on a Chemical Basis" included in his book has a "ring" very similar to the present symposium. Gibbs (1974) published a book ( encyclopedic) containing four volumes in which he reported chemical information from a vast amount of literature and chemical tests on numerous flowering plants. Although the concept of employing chemical data in systematic investigations is an old one, a genuine and intensified endeavor to understand possible correlations between plant constituents and classification has been relatively recent. Chemical characteristics were neglected for a long time because information in most plant groups was too scanty and scattered for any individual group. Interest in this type of research has increased as more data have been obtained from biochemical, immunochemical, and organic chemical research. The development of relatively quick and simple analytical techniques has hastened the "coming of age" of chemotaxonomy. The "present age" of chemosystematics or chemotaxonomy commenced in the mid 1950. The oldest of the "present age" plant chemotaxonomic approaches is serotaxonomy and the youngest is amino acid sequencing. Three books (Alston & Turner, 1963; Swain, 1963; Leone, 1964) provided general information and/or reviews about the early chemotaxonomic and serotaxonomic research. Since that time, numerous comprehensive chemotaxonomic reports have been published in journals, symposia, reviews, and books which clearly indicate the mounting interest in this diversified field of research (Bendz & Santesson, 1974; Boulter et al., 1972; Boulter, 1973; Fairbrothers, 1968 Fairbrothers, , 1975;; Harborne, 1967 Harborne, , 1968 Harborne, , 1970;; Harborne & Swain, 1969; Harborne et al, 1971; Hawkes, 1968; Hegnauer, 1962 Hegnauer, -1973;; Heywood, 1971; Hunziker, 1969; Kubitzki, 1969 Kubitzki, , 1972;; Mabry et al., 1968; Runeckles & Mabry, 1973; Runeckles & Tso, 1972; Runeckles & Watkins, 1972; Seikel and Runeckles, 1969; Steelink & Runeckles, 1970; Swain, 1973; Turner, 1969; Vaughan, 1968) . Most chemical approaches to systematic problems can be classified according Systematics-we have witnessed a deluge of chemotaxonomic reports, reviews, volumes, and symposia; much of these data have been painstakingly assembled by Hegnauer (1962 Hegnauer ( -1973) ) and co-workers into six volumes. This burst of activity resulted in part because some sort of structurally precise chemical information can be readily obtained for every plant available for investigation. And the complex chemical structures often represent hundreds of genes. However, despite the wealth of chemical information, only a few systematically meaningful interpretations have emerged. Nevertheless, the future for gaining new insights into angiosperm phylogeny using micromolecular data is bright as more and more future Cronquists and Takhtajans become trained in plant chemistry. Before discussing the extent to which natural products are important for phylogeny at the higher taxonomic categories of angiosperms and the implications of the distributions of these compounds with respect to the Takhtajan and Cronquist systems, certain definitions and general remarks regarding such plant constituents are in order. The expression "plant natural products" is used here to denote the million or so alkaloids, terpenes, phenolics, quinones, ete. which have restricted distributions in plants. It is the "restricted distribution" phenomenon which permits these substances of low molecular weights, usually less than 1000, to be employed as phyletic markers. Each plant species probably produces from about fifty to several hundred natural products for a variety of functions, including metabolism, defense, structure, and energy and material storage. For the most part, these functions have determined which compounds and which structural features within classes of compounds have been either conserved or modified by selection; however, some modifications of the natural products chemistry may have resulted secondarily as selection operated upon the early stages of pathways leading to the natural products. In any case, it is not uncommon to find that a high percentage of a plant's chemistry, sometimes more than 80-90%, has been conserved such that it occurs in a group of closely related species. Although for higher taxonomic categories the percentages become less, it is frequently possible at the level of tribe, family, and order to recognize biogenetically related compounds which reflect the plant's evolutionary history. It is these latter types of chemical patterns, which involve the distribution of biogenetically homogeneous classes of natural products, that form the basis of the present discussion. Anthocyanidins contained as the aglycone in anthocyanins. Pelargonidin: R^"--H; cyanidin: R" = Н, R = OH; peonidin: R""--H, R' = OCH;; delphinidin: R" = H, R^ "-OH; petunidin: Б" = Н, R" = OH, R' = OCH;; malvidin: К" =H, А" drum OCH.. respect to the interpretations of Cronquist (1968) and Takhtajan (1969) for the taxa involved. It should be emphasized that the application of the distribution of a particular class of natural products for phylogeny assumes that the same or similar structures are derived by evolutionally related sets of enzymes. Although this hypothesis is almost certainly true in most instances, especially at the generic level, it should be recognized that the independent origin of some substances apparently does occasionally occur in unrelated taxa.
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