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An important synapomorphic character of all harvestmen is the presence of so-called scent glands, which constitute the largest exocrine system of this arachnid order. Scent gland secretions are considered to be mainly for defense (e.g., Martens 1978; Holmberg 1986; Eisner et al. 2004: Machado et al. 2005), and their chemistry has been investigated since the 1950s, revealing a large array of unusual natural compounds, including rare quinones, ketones, and nitrogen-containing compounds (Gnaspini and Hara 2007; Raspotnig 2012). Most interestingly, compound profiles from scent glands are taxon specific and have emerged as a tool to trace the evolutionary history of secretion chemistry across Opiliones (e.g., Raspotnig et al. 2015, 2017).
The chemistry of two cyphophthalmid species of family Sironidae was initially investigated in 2005 and was found to be surprisingly complex, comprising more than 20 compounds, all of which belonged to the chemical classes of naphthoquinones and methyl ketones (Raspotnig et al. 2005). Among naphthoquinones, unusual chloro-naphthoquinones, at this time unique in the animal kingdom, were reported. Since then, only two further investigations, one on the chemistry of Stylocellidae (Jones et al. 2009) and a second one on the chemistry of Pettallidae (Raspotnig et al. 2012) have been conducted, both studies confirming the basic naphthoquinone/methyl ketone-organization of cyphophthalmid secretions, though in species-specific patterns. In total, the secretions of four species from three families, Sironidae, Stylocellidae, and Pettallidae, have been analyzed. According to current views on the internal systematics of Cyphophthalmi, three further families are recognized: Troglosironidae (restricted to New Caledonia), Ogoveidae (Africa), and Neogoveidae (South America, eastern North America and West Africa) (e.g., Giribet et al. 2012; Fernandez et al. 2017). The scent gland chemistry of these three families is still enigmatic.
We here report on the scent glands of a first representative of Neogoveidae, Metasiro savannahensis. Metasiro is the northern most neogoveid genus and is distributed in the southeastern United States.
Regarding the biological significance of opilionid secretions, a general defensive function has been postulated (see introduction), and indeed experimentally been proven for a few species (Eisner et al. 2004). With respect to leg dabbing against offenders (Juberthie 1961), chemical defense by cyphophthalmid scent glands is obvious; interestingly, in species of Cyphophthalmus, the secretion also spreads all over the body, hence impregnating the body surface, a phenomenon that has been observed in Metasiro as well. Naphthoquinones have been ascribed antimicrobial properties for many decades (e.g., Thomson 1971), and thus it is likely that the secretion of Metasiro additionally provides broad protection against bacteria and fungi. This latter function may be of uppermost importance when inhabiting a microorganism-rich habitat such as the deep layers of leaf litter.
Metasiro savannahensisis the fifth species of Cyphophthalmi and the first of the family Neogoveide that has chemically been investigated. In total, 33 components have so far been identified from cyphophthalmid secretions. The secretion of Metasiro resembles the chemistry known from Sironidae, Stylocellidae and Pettalidae with respect to its general organization into naphthoquinones and methyl ketones. However, the naphthoquinone content in Metasiro is relatively high, clearly dominating the secretion. In contrast to Sironidae, but like in Pettalidae and Stylocellidae, chloro-naphthoquinones do not contribute much to this naphthoquinone-fraction (Raspotnig et al. 2005, 2012; Jones et al. 2009). A comparison of secretions from all five species hitherto analyzed is shown in Table 2.
To establish successful control program for these exotic wisterias, it will be important to understand the biology of these plants, especially their reproductive biology. Chinese and japanese wisterias are known for their fragrant flowers, a trait of important aesthetic value. The strong floral scent of wisterias may also be important for their reproduction. For many plants, floral scent plays a critical role in attracting pollinators for pollination (Wright and Schiestl, 2009). Wisteria flowers have been observed to be visited by bees, flies, and hummingbirds. Characterizing the volatile chemistry of wisteria flowers will be important for us to understand the role of floral scent of these species for their reproductive success, which may lead to new strategies for the control of these plants as exotic weeds. To this end, the first objective of this study was to identify the volatile compounds emitted from the flowers of chinese wisteria and japanese wisteria.
The composition and amounts of floral volatiles may change during anthesis. Such changes can be regulated by various internal and external factors, including light, flowering stages, diurnal pattern, and visitation by pollinators (Andrews et al., 2007). It is advantageous for the plants to have their scent output at maximal levels only when their potential pollinators are active so as to conserve energy (Dudareva and Pichersky, 2000). The rhythmic pattern of floral volatile emission has been demonstrated in many herbaceous species (Altenburger and Matile, 1988; Jiang et al., 2011; Lee et al., 2010). Its occurrence in woody species is much less studied. Therefore, the second objective of this study was to determine whether the emission of floral volatiles from wisteria displays a diurnal pattern and, if so, whether the diurnal pattern is regulated by light, circadian clock, or both.
In this article, we report the complete chemical profile of floral scents of two wisteria species. Flowers of both chinese wisteria and japanese wisteria emit a complex mixture of volatiles (Table 1). Similar to many other plants (Knudsen et al., 1993, 2006), the floral volatiles of the two species are dominated by compounds from four chemical classes: fatty acid derivative, benzenoids/phenylpropanoids, terpenoids, and nitrogen-containing compounds. Linalool was the most abundant compound emitted from both species, accounting for approximately half of the total emission rate, which is consistent with the results of previous studies of wisteria floral volatiles using different methods (Joulain, 1987; Joulain and Tabacchi, 1994). Despite strong similarities, significant variations were observed. For example, chinese wisteria flowers emit only one sesquiterpene, whereas japanese wisteria flowers emit six sesquiterpenes (Table 1). In addition, although the number of benzenoids/phenylpropanoids from japanese wisteria flowers was much less than that from chinese wisteria flowers, benzyl alcohol was emitted from japanese wisteria at a high rate, but this compound was not detected from chinese wisteria flowers at all. These suggest that different biochemical pathways that lead to different floral chemistry may have evolved and may provide fitness benefits to the specific wisteria species.
The knowledge of floral chemistry of two wisteria species may kindle the interests of several research directions. One direction is to study the biosynthesis of wisteria floral volatiles, which may lead to novel biochemistry. Especially interesting is the molecular basis for the production of methyl esters, which are unusually rich in the floral volatiles of chinese wisteria. These metabolites include methyl octanoate, methyl decanoate, methyl pentanoate, methyl hexadecanoate, methyl salicylate, methyl benzoate, methyl eugenol, and methyl chavicol. Based on the known biochemistry, we can hypothesize that these metabolites are formed by the action of methyltransferases. Clustering analysis suggests that three types of methyltransferases may be involved in the production of methyl esters in chinese wisteria, which is supported by the current knowledge about plant methyltransferases. Methyl benzoate and methyl salicylate are synthesized by the action of carboxyl methyltransferase called benzoic acid/salicylic acid carboxyl methyltransferase that belongs to the SABATH family (Zhao et al., 2007, 2008, 2010). Methyl isoeugenol and methyl chavicol are formed by the action of O-methyltransferase, which belongs to a protein family different from the SABATH family (Gang et al., 2002). Fatty acid methyl esters may be formed by the action of a distinct family of methyltransferases yet to be identified. Therefore, chinese wisteria presents a useful model for studying the biosynthesis and regulation of the floral volatiles at the molecular and biochemical levels. With the advent of new technology in molecular biology and genomics such as next-generation sequencing, it will be relatively easy to identify and isolate responsible genes.
The second direction is to study the biological/ecological significance of floral volatiles on the reproductive biology of wisteria species. Because the flowers of wisterias are visited by various organisms, many of which can serve as pollinators, it is tempting to speculate that floral scent plays a critical role in the reproductive success of wisterias. Interestingly, a recent study showed that the majority of invasive wisteria plants in the southeastern United States analyzed are hybrids between chinese wisteria and japanese wisteria (Trusty et al., 2007b). New wisteria produced by interspecific or intraspecific hybridization gains more vigor to compete with native species. Therefore, hybridization may play an important role in the invasiveness of wisterias. The creation of a hybrid invasive wisteria species is likely promoted by pollinators guided by strong floral scent. In addition to their role in attracting pollinators, some compounds of floral volatiles may function as repellents against herbivores (Dudareva and Pichersky, 2000; Shrivastava et al., 2010). For example, benzyl cyanide and benzyl isocyanide, which were emitted from chinese wisteria and japanese wisteria, respectively, are toxic to many organisms including florivores. The knowledge of floral volatile chemistry of wisterias will help elucidate the roles of these compounds in pollination and defense. Manipulation of the key volatile compounds of wisteria species through breeding or genetic engineering may lead to novel wisteria ornamental plants that are less invasive. 2b1af7f3a8