FIGURE 7.14. A phylogenetic hypothesis including all Piper species for which there are chemistry data (see References); the tree is based on one published by Jaramillo and Manos (2001). For clarity, the originally published tree was pruned to include only those species for which chemistry data were available.
been examined and may represent an ancestral character, with some instances of loss of amides (Fig. 7.14). There are some common amides that have evolved multiple times in both Old World and New World Piper species (e.g., cepharadione, Fig. 7.14), but most of the common amides that are present in multiple unrelated species have only evolved in Old World species (piperine, pipericide, and guineensine) or only in New World species (8,9-dihydropiplartine and piplartine), which are potentially monophyletic groups (Jaramillo and Manos 2001).
Some species have evolved very high diversities of amides, such as P. amalago, from which 31 different amides have been isolated. In contrast, P. reticulatum does not have any amides (Fig. 7.14). It is interesting to note that there are no apparent trade-offs between N- and C-based defenses. Species with high amide diversity or concentration are not necessarily devoid of terpenes (e.g., P. betle with five amides and eight terpenes), nor are there any striking patterns of evolution of shade-adapted N-based defensive systems versus gap-adapted terpene defensive systems. Despite our findings that the phenotypic plasticity of amide concentrations allows amides to respond to nutrient availability in accordance to predictions of nutrient availability hypotheses (Dyer et al. 2004), it certainly does not appear as if evolutionary predictions based on carbon-nutrient balance (Bryant et al. 1983) will be upheld once enough data are available for phylogenetically independent contrasts. Nor does it appear that herbivores that are susceptible to chemical defense (i.e., mostly generalists) will be influenced greatly by the phylogenetic history of the host plant (sensu Baldwin and Schultz 1988).
The majority of amides have evolved only once (e.g., cenocladamide in P. ceno-cladum; arboreumine in P. arboreum; aduncamide in P. aduncum), but in most instances these contain common moieties (e.g., isobutyl, pyrrolidine, dihydropyridone, piperidine) and only involve slight modifications of amides found in unrelated species. Although it is likely that herbivores and pathogens have provided strong selective pressures that are partly responsible for the diverse secondary chemistry in Piper, no formal macro- or microevolu-tionary hypotheses have been tested. One prediction of coevolutionary theory (Ehrlich and Raven 1964, Cornell and Hawkins 2003) is that generalists should be more susceptible than specialists to toxins such as amides. The one study that directly compares effects of amides on generalists versus that on specialists (Dyer et al. 2003) supports this prediction. The specialist herbivores in that study were two species in the genus Eois. Patterns of Piper-Eois interactions are ideal for studying coevolution, since most Eois species are probably specialized on one or two species of Piper (Dyer and Gentry 2002, unpubl. data) and amides are toxic to generalist herbivores. Experiments comparing performance of Eois on diets with amides from their specific Piper hosts versus amides from related Piper species could reveal whether or not predicted trade-offs occur in specialist herbivores' susceptibility to toxins. In addition, since several good Piper phylogenies exist (Jaramillo and Manos 2002; Chapters 9 and 10), an Eois phylogeny would allow for tests of parallel diversification of the plant and herbivore species (Farrell et al. 1992).
Most Piper chemistry has been conducted to find potential pharmaceuticals or pesticides, and over 90% of the literature focuses on compounds that are cytotoxic, antifungal, antitumor, fragrant, or otherwise useful to humans. For example, safrol occurs in high concentrations in several species of Piper, particularly P hispidinervum. This propenylphe-nol and its derivatives have been used successfully in powerful insecticides, as well as in fragrances, waxes, polishes, soaps, and detergents. Thus, P. hispidinervum has been cultivated for high levels of safrole and could contribute significantly to tropical economies and conservation efforts (Rocha and Ming 1999). A recent example that typifies the search for insecticidal properties of Piper amides is the work of Yang et al. (2002), in which the authors demonstrate the effectiveness of a piperidine amide (pipernonaline, extracted from P. longum infructescence) against Aedes aegypti mosquito larvae. This insecticidal activity has potential human importance because these mosquitoes are vectors for yellow fever. The most extensive review of Piper phytochemistry (Parmar et al. 1997) summarizes the bioac-tivity of Piper chemistry, and most examples are medicinal or pesticidal. The most common uses reported in the Parmar review and subsequent papers are anticarcinogens, insecticides, treatments for respiratory diseases, pain killers, mood enhancers, and treatments for gastrointestinal diseases. Current applied work particularly stresses the insecticidal effects.
The common economic uses of Piper spp. are as a spice (P. nigrum and P guin-eense) and herbal medicine (P. methysticum, or "Kava"). These species have rich histories in economic botany and ethnobotany. For example, Kava has been used for centuries in the Pacific Islands to prepare intoxicating beverages and offerings to gods. Traditional preparations of the beverage involved spitting chewed Kava leaves into a communal bowl for eventual consumption; the salivary enzymes from the chewing putatively helped create a more potent beverage (Cotton 1996). Modern Kava beverages are prepared commercially, without expectoration, and many of the psychoactive constituents of Kava have long been known. Methysticin, one of the more potent physiologically active components of Kava (Parmar et al. 1997), was isolated in 1860 (see Chapter 8). What remains unknown are the specifics of the causes of inebriation or other physiological effects: What are the actual compounds involved, potential synergies, interactions with salivary enzymes, and other aspects of chemistry that are responsible for strong effects on animals?
Aside from the significant economic impact of P. nigrum chemistry throughout its history (e.g., Dove 1997), the actual contributions of Piper chemistry to any country's economy are relatively small. For example P. methysticum sales were only US$ 50 million worldwide in 1998 (Laird 1999). It is unlikely that any aspect of the bioactivity of Piper chemistry will significantly alter the economies of the tropical countries where Piper species are found, and there is always the potential for disaster when Piper species are exploited for economic purposes. One particular concern is that Piper species that are imported for cultivation for chemistry could easily become invasive, such as P. auritum (J. Denslow, pers. comm.) and P. aduncum (Novotny et al. 2003). Another potential problem is overharvesting of sensitive species (Balick et al. 1995), but this is unlikely and can be made even less likely with techniques such as synthesis, tissue culture production (see Chapter 8), and sustainable harvesting (Balick et al. 1996). Finally, there are some notable and important efforts to link economic benefits of sustainable use of Piper chemistry resources to conservation efforts (Laird 1999; other references).
Future research on Piper chemistry should focus on the ecological effects of variation in secondary chemistry, chemical coevolution between Piper species and specialist herbivores, and synergistic interactions between different secondary metabolites. For all of these studies, enhanced isolation and synthesis techniques will be necessary, so these techniques should also be a focus of future research.
Probably the most interesting investigations will be the search for synergy. The relative scarcity of examples of synergistically acting secondary compounds is likely due to the lack of research on this topic or weak statistical methods (Nelson and Kursar 1999), and we have suggested these synergistic effects may be the rule rather than the exception (Dyer et al. 2003). Indeed, synergy may explain the apparent lack of defensive properties that have been indicated for a variety of plant secondary compounds, which have no known function (Harborne 1988, Ayres et al. 1997). This is particularly important for Piper chemistry, because many species have a plethora of defensive compounds that could potentially interact. The large number of research programs that test for pharmaceutical, pesticidal, or other activities of Piper plant secondary metabolites should at least be supplemented with appropriate tests (e.g., Jones 1998, Nelson and Kursar 1999) of pertinent mixtures and whole plant extracts.
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