The renaissance of comparative biochemistry

The study of chemical diversity has a long history in the field of plant systematics. Methods for separating and identifying chemical compounds, such as chromatography and mass spectrometry, were developed in the early 1900s (reviewed by Marston, 2007), with plant pigments such as chlorophylls and carotenoids among the first targets of analysis (Twsett, 1906). These technological advances gave way to tailored approaches designed to survey major classes of plant metabolites (e.g., phenolics, terpenoids, sugars) across diverse taxa (Harborne, 1973). The techniques developed by Harborne and others could often be implemented with relatively simple equip­ ment and materials, opening the door for many botanists to begin exploring chemical diversity in their own systems. Indeed, by the 1950s, studies in this area had emerged as a distinct discipline, “bio­ chemical systematics” or “chemosystematics” (Alston and Turner, 1963; Gibbs, 1963, 1974). With a sudden wealth of new comparative data, the field of chemosystematics put forward new phylogenetic hypotheses, many of which have come to be strongly supported with other sources of information (Grayer et al., 1999). One classic example is that of the Gesneriaceae, in which early phytochemical surveys supported the division of the Old World Cyrtandroideae and the mostly New World Gesnerioideae (including the South America/ South Pacific disjunct Coronanthereae). The former subfamily pro­ duces anthocyanins and chalcones or aurones, while the latter pro­ duces novel 3­ deoxyanthocyanins and flavones (Harborne, 1966, 1967). Subsequent molecular systematics studies have supported the monophyly of Gesnerioideae (Smith et al., 1997; Woo et al., 2011), consistent with a single origin of 3­ deoxyanthocyanins from anthocyanin­ producing ancestors. As comparative biochemical studies expanded, many classes of plant metabolites were found to exhibit complex evolutionary histories, with multiple gains and losses across the phylogeny. For instance, mustard oils, the glucosinolate­ derived compounds in­ volved in defense against herbivores, are found in 15 plant families and appear to have evolved twice, once in the Malpighiales and once in the Brassicales, with a secondary loss in one lineage (Ettlinger and Kjaer, 1968; Rodman et al., 1998). The evolutionary pattern presented by betalain pigments is even more intriguing, wherein following a single origin near the base of the Caryophyllales, mul­ tiple lineages have lost these nitrogenous compounds and reverted to the ancestral state of producing anthocyanin pigments, seem­ ingly after millions of years without them (Brockington et al., 2015; COMMENTARY