Preface This book is designed to provide an all encompassing vision of the diversity of natural products in the perspective of biodiversity. Both living organisms and fossil remains are taken into account, without any bias for either the sea, land, or extreme environments. Understandably, however, this is not intended to be a comprehensive portrait of natural product diversity, which would be beyond my resources and would demand a multi-volume treatment. My aim was to focus the attention on representative examples - drawing from both my own experiences and the most recent literature - in a balanced coverage of the various niches, considering the local-geographic rather than the global-taxonomic distribution of natural products. To this concern, the attention is focused on the molecular skeletons (grouped in charts for classes of metabolites for each local-geographic environment) as differences in the coding genes are expected to be larger in directing the molecular framework than its detail, i.e. the functional groups. In this perspective, the molecular diversity is semi-quantitatively evaluated by a molecular complexity index. It was encouraging, during this compilation, to see similar strategies applied to the conservation of endangered taxa (Rodriguez 2000) and the comparison ofnatural products with synthetic compounds (Henkel 1999). Then, the natural product diversity is examined at the fimctional level, that is signaling and defensive agents, and how the wild diversity is exploited, modified through biotechnological techniques, and recreated or imitated via total synthesis. To cope with these aspects, the full molecular details are considered. In this analysis, the value of the natural product as a drug is kept within the limits imposed by the advancement in the knowledge of the molecular structure of the targets, which is opening the way to the rational design of drugs. Next, threatening of natural product diversity is examined, either by human activity or mass mortality during the geological eras. Molecular skeletons are shown, for the same reasons above about the natural product diversity from extant organisms. Management and conservation of natural products are also briefly discussed. The major diificulties encountered in gathering the data were for microorganisms on land: papers from the golden period of the antibiotics contain scarce geographic information and this problem has continued to the present with work in industry. On the taxonomic/phylogenetic side, not only the genus and species are given for the eukaryotes, but also the order, which is most significant with respect to the natural product distribution in living organisms. An exception is made for land plants, keeping with tradition to give also the family. Changes in the way living things are taxonomically described are kept to a minimum, preferring to refer to widely accepted positions and names. This has found recently an authoritative support (Lewin 2001). The terminology "higher organisms" or "lower organisms" is used freely in this book for conciseness, without any implication with respect to the rank of the organisms along the biological scale. In particular, "higher" is not intended to mean more evolved, since, provided that the xii ecaferP phylogeny is cleaned from uncertainties (Huelsenbeck 2000) and the evolution si evaluated for lla its possible aspects, the extent of the evolution si related to the age of the group and not to its position along the tree of .efil What is also avoided si any attempt at a revival of the past, lost pristine conditions, since the increased energy demand (which is at the basis of the loss of natural product diversity) was unavoidable with the advances in technology, the prolonged life span in rich countries, and uncontrolled human population growth in the developing world. I believe that the only hope for saving the Earth' s remaining living resources rests largely in new technologies for food, clean energy, and material production. With the exception of a few landmark papers, such as Robinson's total synthesis of tropinone (Robinson 1917) and Blumer' s essay on fossil pigments (Blumer 1965), for reasons ofspace the most recent references are given, preferably review papers; only the first author si given ni the text. This allows us easily to trace back lla previous work, although regrettably the original discoveries are not appropriately acknowledged. To leave space for the titles ofthe papers, which indicate to the reader the importance of reading beyond this book, a smaller font si used for the references. Examples in the tables, and trade names of compounds, were chosen without any commitment or preference for any particular commercial source. Trade names are intended to be registered trade names. The data provided in this book are in good faith, but I make no warranty, expressed or implied, nor assume any legal liability or responsibility for any purpose for which these data are used. I welcome criticism, corrections, and suggestions about gaps or redundancy ni coverage that will help to make a second edition of this book even more useful. Francesco Pietra Rome, October 2001 Definitions of abbreviations for the charts and tables Actinom. = Actinomycetales Dinofl. = DinoflageUatea Ala = alanine diterp. = diterpene(s) or diterpenoid(s) alkal. - alkalo id(s) Echin. = Echinodermata Ang. = Angiospermae or angiosperm(s) Eumyc. = Eumycota Amphib. = Amphibia or amphibian(s) freshw. = freshwater Arach. = Arachnida Glu = glutamic acid Arg = arginine pGlu = pyroglutamic acid = 5-oxoproline Ascid. = Ascidiacea Gly = glycine Ascom. = Ascomycotina GBR = Great Barrier Reef Asn= asparagine Gymn. = Gynmospermae or gymnosperm(s) Asp = aspartic acid Halichon. = Halichondrida Aster. = Asterozoa Hemich. = Hemichordata Axinel. = Axinellida Hexactin. = Hexactinellida AY = Ayensu 1979 Hist = histidine Bact. = bacteria Holoth. = Holothuroidea Basidiomyc. = Basidiomycotina Homoscl. = Homoscleromorpha BC = British Columbia Hyp = hydroxyproline BR = Bruneton 1995 Ins. = Insecta Bryoph. = Bryophyta invertebr. = invertebrate(s) Bryoz. = Bryozoa lie = isoleucine C = Chart isopr. = isoprenoid(s) or isoprene Calc. = Calearea Leu = leucine carboh. = carbohydrate(s) Lys = lysine Chloroph. = Chlorophyta Mamm. = Mammalia Chrysoph. = Chrysophyta meroditerp. = meroditerpene(s) Cilioph. = Ciliophora Met = methionine Cnid. = Cnidaria MI = The Merck Index 12:2 (1998) co smop. = cosmopotitan MoU. = Mollusca Crust. = Crustacea monoterp. = monoterpene(s) or Cyanobact. = cyanobacteria monoterpenoid(s) cyclopept. = cyclopeptide(s) Myxobact. = Myxobacteria Cys = cysteine NC = New Caledonia Demosp. = Demospongiae Nemert. = Nemertea Dendroc. = Dendroceratida Nepheliosp. = Nepheliospongida Dendroph. = Dendrophylliidae Nudibr. = Nudibranchia Deuterom. = Deuteromycotina NZ = New Zealand Diatom. = Diatomae Okin. = Okinawa Dictyoc. = Dictyoceratida Opisthobr. = Opisthobranchia xiv Definitions of abbreviations for eht charts and tables orphan = orphan molecule or molecular Ser = serine skeleton (the producing organism sesquiterp. = sesquiterpene(s) is not known) shikim.- shikimate(s) Osteich. = Osteichthyes T - table pantrop. = pantropical thiopept. = thiopeptide(s) pept. =peptide(s) or cyclopeptide(s), Thr = threonine depsipeptide(s), lipopeptide(s) triterp. = triterpene(s) Phaeoph. = Phaeophyceae Trp = tryptophan Phe = phenylalanine TurbeU. = Turbellaria PNG = Papua New Guinea Tyr = tyrosine Poecil. = Poecilosclerida Ulvoph. = Ulvophyta polyket. = polyketides undeterm. = undetermined po lysacch. = polysaccharide(s) Val = valine Porif. = Porifera (sponges) Vertebr. - Vertebrata Pro I = proline Rhodoph. = Rhodophyceae Chapter 1. Defining biodiversity Biodiversity is difficult both to define and measure. Central to the issue is the concept of species, which has proved most elusive to generations of biologists. Darwin himself was never convinced of the prevailing positions at his time and debates about the concept of species have continued to the present and will continue apace. Although the reader is referred to the biological literature for a thorough illustration of these concepts, a survey of the prevalent positions may serve as an introduction and a basis to our intents. Mayr's concept of biological species as "a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups" (Mayr 1963, 1969) is much encompassing, although Mallet's criticism that "Reproductive isolation is a kind of mystical definition..." may be defended. Other definitions of species, such as ecological species (for groups that occupy different ecological niches), evolutionary species (defining a species as a single lineage of ancestral-descendant populations that remain separate from other such lineages in an evolutionary tree), and geno-species (to account for asexual bacterial strains that allow interchange of genetic material) also serve the scope. In the light that a concept of species that satisfactorily encompasses all organisms can hardly be found, we are now in a position to examine the prevalent definitions ofbiodiversity. 1.1 Biodiversity at species level Biodiversity is commonly understood as the number of different species in a given ecosystem. By doing so, the huge number of insects (estimated from two million to ten million species, Speight 1999) gives the wrong impression that biodiversity on land is far greater than in the oceans. Actually, we will see that the species is a misleading basis on which to compare biodiversity on land and in the sea. Choosing species as a measure of biodiversity is also far from straightforward. The concept of species is poorly defined with microorganisms, particularly for non-sexual or uneulturable ones. When sibling and cryptic species are also accounted for, the difficulty in making an overall estimate of the biodiversity increases, particularly for the sea, where sibling species are plentiful. Accounting for sibling species, a fourfold increase of biodiversity in the sea is expected (Knowlton 1993). Endosymbionts pose even more difficult problems, to the point that there is no agreement whether they should be taken into account in any evaluation ofbiodiversity. In any event, the mere inclusion of microorganisms in the species to be enumerated makes a catalogue of the existing species far from being an affordable enterprise. Dealing with viruses is especially difficult: although they do not live as isolated entities, they are found widespreadly in the oceans, possibly involved in regulating the biodiversity (Fuhrman 1999). The microorganisms deserve much attention, representing the bulk ofphylogenetie diversity, thus contributing enormously to the biological evolution. The high turnover makes also the microorganism Part .I The concept of biodiversity an attractive entity for the laboratory search of correlations between species diversity and their productivity, i.e. the "rate of production of organic matter by a community" (Kassen 2000). 1.2 Biodiversity at higher taxonomic levels Taking higher taxonomic levels as an estimate ofbiodiversity (May 1994), more phyla are found in the oceans than on land. Also, with the only exception of the arthropods, phyla with representatives both on land and in the sea are more speciose in the latter. Of the thirty-three known phyla of extant animals, only one, Onychophora, is exclusive of land, while as many as twenty-one phyla are exclusive of the sea. The latter comprise the brachiopods (lamp shells, about 300 species), chaetognates (arrow worms, about 50 species), ctenophores (comb jellies, over 80 species), echinoderms (about 600 species distributed in the crinoids, i.e. sea lilies, and feather stars, the asterozoans, i.e. starfishes, brittle stars and basket stars, and the echinozoans, i.e. sea urchins, sand dollars and heart urchins), echiurians (spoon worms, about 300 species), gnathostomulids (about 100 species), hemichordates (nearly 100 species, distributed in the enteropneusts - acorn worms - pterobranchs, and planktosphaerids), kinorhynchs (about 100 species), loricifers, phoronids (horsheshoe worms, about 51 species), placozoans (2 species), pogonophors (about 100 species), priapulids (4 species), and rotifers (more than1500 species). The conclusion is that the taxonomic diversity is larger in the sea than on land. 1.3 Biodiversity at genetic level Biodiversity can also be considered within the species or even within the population. This is called genetic diversity, signifying that each individual has its own genetic make-up. This allows the species to adapt to environmental changes and furnishes the seed for speciation, by which biological evolution occurs. Even this definition of biodiversity is not free of pitfalls, however, and it may open the door to ontology. To add to this concern, arbuscular mycorrhizal fungi in the order Glomales are a particularly vexing problem. The mycorrhyzae are asexual organisms, which poses problems in defining biodiversity; they also show different rDNA sequences on different nuclei, which are many, enclosed in a single spore ofthese coenocytic organisms (Hijri 1999; Hosny 1999). It is perhaps this high genetic diversity that has granted success to these organisms for more than 400 million years. 1.4 Biodiversity at ecosystem level Biodiversity can also be examined at ecosystem level. This is of special concern for ecology, because, although attention to ecological problems is increasing, the choice of the appropriate scale to look at the events is problematic (Levin 1999). The spatial and temporal scales needed are often of such large size to make any reliable observation difficult. This is especially true for the sea, due to the lack of barriers which, therefore, makes any subdivision into ecosystems difficult. This is why any quantitative evaluation ofbiodiversity at ecosystem level is far from being an easy task. Chapter 2. The course of biodiversity The discovery of transposons has opened new vistas on the evolutionary events: evolutionary changes, once thought to be small and rare, have been recognized to be large and frequent. Genetically unstable bacterial populations adapt more rapidly than stable populations (Radman 2001). Marine snails in the genus ,sunoC being unusually able to mutate through their third exon, are another good example. Proneness to mutation gives flexibility to the array oftoxic peptides produced by these mollusks, which accounts for their extraordinary success, testified by the existence of about 500 species. Adaptation serves to maintain the efficiency of the peptidic venom for the snarls' harpoons, used in capturing other invertebrates and even fish. Other examples of easy mutation are found with flower plants in the genus Ipomoea, to which moming glory belongs. They are characterized by a reshaping ofthe genome by transposons. Equally flexible are the ciliates: in the formation of a new ,suelcunorca_m the DNA portion comprised between the coding regions is removed, bringing about order in the region. Admittedly, bacteria, Conus, Ipomea, and the ciliates have unusually high mutation ability, but examples of natural genetic engineering are encountered otten, even with the vertebrates. The history ofthe acquisition ofthe immune system is illustrative. Present in the invertebrates as merely an innate immunity by the way of phagocytic cells, the adaptive immune system of vertebrates resulted from the sudden introduction ofa transposon just where a remote forerunner ofthe antibody genes resides. The antibody gene complex could evolve this way, giving rise to B and T lymphocytes. This occurred with sharks, probably when they first appeared in the Ordovician era. Biodiversity may also be fostered locally without any genomic rearrangement. A redistribution of species suffices, in the concept of the refugee. This may be a physical shelter, such as a foreign empty shell for the hermit crab, or a chemical weapon, such as the distasteful metabolites of certain seaweeds that provide a refugee for grazeable seaweeds. Other examples of correlation between the diversity of symbionts and the hosts are provided by mycorrhizal fungi in symbiosis with terrestrial plants (van der Heijden 1998). Leaf-cutting ants in coevolution with plants, and antibiotic-producing actinomycetales (Currie 1999), are examples of associated biodiversity (Swift 1996). All these represent evolutionary adaptations to the changing environment, in the survival of the fittest (Brookfield 2001), which was at the basis of Darwin' s thoughts and has never been disproved. Biodiversity is always in a fluctuating condition. Becoming affirmed is not a condition for species to escape evolution. Natural laws impose a finite length of time to any species, which is more important than a finite length of time imposed to individuals (Sgr6 1999). Then, the course of the evolution brings the species to extinction, providing space and resources to new species. Unstable and evanescent analogical (no gene) life of primordial times (Woolfson 2000) is difficult to imagine ni the perspective of secondary metabolites. Digital (DNA) life has more concrete foundations, dating to at least 3,450 My ago, according to cyanobacterial remains (Mojzsis 1996). The first eukaryotes are latecomers, appeared in the fossil record 2,500 My ago (Schopf 1993), and 6 Part .I The concept ofbiodiversity probably inherited homologues both of tubulin and actin from bacteria (van den Ent 2001). These chronologies are represented in Fig. 2.I, in the perspective of a sea vs land occupancy and mass extinctions. It is seen that the cycads and the ginkgophytes appeared during the end-Permian, following the disappearance ofthe pteridosperms, which had predominated in the carboniferous. This does not mean that the follower descends necessarily ~om the former, since the pteridosperms, rather than the cycads and ginkgos, are considered the ancestors of the angiosperms. The ginkgos and the cycads simply replaced the pteridosperms. The same occurred following the end-Cretaceous mass gymnosperms Cambrian cycads lycopods burst first dezilissof ginkgophytes liverworts modern eukaryotes current angiosperms I t vapslacnutlsa r Cyan~ 1 sedilotamortS it ma~mmals T naimreP-dne f brown T dezilissof( nBc~odrO-dne algae Cyanobacteda) suoecaterC-dne cissairT-dne I I I .... 0 ~ 001 002 003 004 005 057 1400 0052 0543 _L yM erofeb tneserp aisA-aidnI noisuf dna ailartsuA l Pangea dednuorrus yb assalahtnaP tneserp no~sop trats fo aisaruaL noitarapes morf anawdnoG gnimrof( nrehtroN citnaltA )naecO dna acitcratnA noitarapeS fo gnimrof naidnI naecO htuoS aciremA morf ,acirfA gnimrof nrehtuoS citnaltA Www,- First appearance of the indicated organisms >7~:-- ssaM snoitcnitxe eht( worra htgnel si lanoitroporp ot eht )tnetxe --~.- aeS .sv dnal ycnapucco gnirud eht sega Figure 2.1. First appearance of organisms in the perspective of mass extinctions and sea vs land occupancy extinction, when the mammals took the place left vacantly by the reptiles, paving the way to the appearance of the humans. Biodiversity may also be fostered when the ecosystem is unstable, like a pond subjected to alternating periods of dryness and wetness, populated by plants giving aC or 4C metabolites, or shitting to this biosynthetic mode according to the availability of 2OC and the need of saving water (Keeley 1998). Ecological factors may also determine both the phenotype and speciation (Tregenza 1999). At least at the phenotype level, biodiversity may also be fostered by predation; this can be seen in the framework of the evolution of signals, such as with electric fish (Stoddard, P.K. 1999). Threatening ofbiodiversity, and the resistance opposed by the ecosystems, are examined in Part VI. Chapter 3. Taxonomy, phylogeny, and natural products Relationships between the nature of secondary metabolites and the taxonomy of the organismic source have often been sought. This si particularly true in botany, where chemotaxonomy has a long tradition, now supplemented by genomic analysis (Grayner 1999). Since taxonomy and phylogeny go hand in hand, exclusive metabolites may have phylogenetic significance. These ideas have been expanded in an ecological and evolutionary perspective for land plants (Gottlieb 1998). In the animal kingdom, cladograms based on the distribution of natural products have been set up for demosponges (Andersen 1996). Any relationship between secondary metabolites and the taxonomy of the organismic source is subject to rapid revision, however. The analytical techniques for the isolation of natural products present in tiny amounts ni intricate mixtures, and the spectral techniques for structure elucidation, have greatly advanced in the last three decades. This has two major consequences: first, that compounds or compound classes previously thought to be exclusive of certain taxa are more and more frequently found in phylogenetically and ecologically distant organisms (Pietra 1995), and second, that the occurrence of secondary metabolites in families of compounds, differing either in the carbon skeleton or merely in the stereochemistry, has become the norm. Curiously, families of compounds are reported from time to time as a novel observation, in connection with libraries of compounds from combinatorial synthesis or biosynthesis (Brady 2001). The characterization of the metabolites also deserves the greatest care in order that chemotaxonomy remains a valid science. In particular, a close scrutiny ofstereochemical relationships ni a biosynthetic cascade may allow us to relate the metabolite distribution to lineages; this is a task that genetics cannot yet perform (Guella 1997A). Oddly enough, while ecology receives increasing attention, alpha taxonomy, that is the search and description of new species, has been neglected (Winston 2000). Granting agencies and their professional referees have considered alpha taxonomy a jump into the past, which undermines the study of both biodiversity and natural product diversity, if it is true that only 10% of extant species has been describedl This attitude is now changing; signs appear of a renewed interest in alpha taxonomy, relying on bioinformatics (Bisby 2000; Edwards 2000A). Integration of taxonomic, ecological, and biogeographical data from various databases existing around the world is the aim of recem projects (Species 2000). With the coming of programs for personal computers for the treatment of molecular data, taxonomy has largely become digital (DNA) phylogeny. A link with natural products has also been promised (Bisby 2000), but how it could be realized is unclear: large databases of natural products exist indeed but, because ofthe economic interests involved, the access is very expensive, in contrast with free access to taxonomic data. Even without budgetary restrictions, it would be an enormous enterprise - perhaps beyond the human resources - establishing the organismic origin for the huge number of natural products that have been reported. One should have to rely on poor descriptions 10 Part .1I The relationship between biodiversity and natural product diversity in chemical journals, often without the support of a qualified taxonomist. For streptomycin, a famous antibiotic substance, I found no way to trace back where the productive actinomycete, Streptomyces griseus, came from, in spite of gracious help from the specific learned institutions. I could only guess, from the original literature, that the "heavily manured field soil.., or the throat of a chicken", from which the actinomycete was isolated (Schatz 1944), was in a temperate area, probably the US. Lower organisms that face a strong interspecific competition are the best organismic sources. However, no steady relationship has ever been found between the taxonomic rank and the secondary metabolic productivity. Bacteria in a single genus, Streptomyces, have emerged as rich sources of antibiotics. Most microorganisms are not known to produce any unusual metabolite. This is counterbalanced, with productive strains, by the occurrence of secondary metabolites in families, where the members may differ merely in the ftmctional groups, or are more distantly interrelated along a biogenetic cascade, involving rearrangement of the carbon backbone. Biosynthetic studies of triterpenoids of different skeletal type from homogeneously the same species of plant, carried out with chimeric enzymes, revealed that great chemical diversity may stem from tiny differences in the genes coding for the relevant multifimctional enzymes (Chapter 13.1). A few changes in the amino acids at the active site may result in metabolites differing in the carbon skeleton. This suggests that plants have evolved chimeric enzymes acting as multifimctional triterpene synthases (Chapter 14.1 and Chart 14.1: Kushiro 1999). Secondary metabolites that may be taken as biomarkers, and sequences of rRNA, concur in viewing separate groups within both the marine ciliates (Pietra 1997) and the archaeans. For the latter, however, identity of lipid biomarkers was assumed without paying attention to chirality (Hinrichs 1999); if diastereomers are involved, different enzymes for the different archaean groups are implied, while genomic differences may have passed unnoticed because of insufficient resolution.