The molecular signatures of Taxodiaceae / Cupressaceae / Taxaceae (TCT) leaf waxes in modern and ancient samples

Members of the Taxodiaceae/Cupressaceae/Taxaceae (TCT) complex of conifers originated sometime before the late Jurassic. Since that time the group has diverged to fill diverse ecological niches in desert, marsh, tundra, alpine, and coastal habitats and a variety of forest types. 175 species from 35 genera are now found across 6 continents. The aims of this research project are 1) to analyze and describe cuticular isoprenoid and acetogenic lipids from a diverse group of living members of the TCT complex and 2) to begin a search for these compounds and their diagenetic products in geological samples with known contributions from ancient TCT members. Hexane extracts of several hundred modern conifer specimens from more than 25 genera were studied in an attempt to find phylogenetic trends in the distribution and abundance of wax components. Diverse skeletal types of bicyclic, tricyclic, and tetracyclic diterpenes were found throughout the TCT complex. These compounds were found to have the highest absolute and relative abundance in several temperate rainforest and marsh endemics and the lowest relative abundance in desert adapted species. Large scale phylogenetic patterns in the distribution of individual diterpenes were not evident. Some species showed little intraspecific variation in diterpenes, while others showed considerable variability in diterpene products from one tree to another. The waxes of many members of the TCT complex are dominated by uniquely long-chain normal alkanes, with peak abundance at 33 and/or 35 carbons. This character is found within a phylogenetically distinct group of TCT -- including the genera Austrocedrus, Callitris, Calocedrus, Chamaecyparis, Cryptomeria, Cupressus, Diselma, Fitzroya, Juniperus, Libocedrus, Platycladus, Taxodium, Tetraclinis, Thuja, and Thujopsis -- and is seen in plants from extremely different habitats. We postulate that this group within the TCT complex shares a conserved very long chain fatty acid (VLCFA) elongase optimized for the synthesis of C34 and C36 fatty acids. The bimodal distribution of n-alkanes (abundant C25 and C27 and abundant C33 and C35) in some Cupressus species indicates that the expression of this VLCFA elongase may be spatially or temporally limited in some taxa. Examples of fossil TCT leaf waxes have been observed in Pleistocene coastal sediments from California and Washington and in Jurassic coals from the Turpan basin in western China. These wax contributions can be identified by their unique n-alkane and diterpenoid signatures and their relationship with macrofossil and/or microfossil remains tied to members of the TCT complex. The carbon isotopic composition of Pleistocene waxes is consistent with a rainforest or marsh adapted TCT taxon (possibly Thuja plicata), while the isotopic composition of the Jurassic waxes is indicative of a highly water stressed taxon. Unique enzymes for very long chain n-alkane biosynthesis in the core group of TCT taxa listed above may have arisen during the early Mesozoic in a desert or salt marsh adapted species in response to extreme temperatures or water stress.