Induction of a Reproductive-Specific Cuticular Hydrocarbon Profile by A
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Chemoecology (2016) 26:195–203 DOI 10.1007/s00049-016-0219-8 CHEMOECOLOGY ORIGINAL ARTICLE Induction of a reproductive-specific cuticular hydrocarbon profile by a juvenile hormone analog in the termite Zootermopsis nevadensis Colin S. Brent1 • Clint A. Penick2,3 • Beth Trobaugh2 • Dani Moore2 • Ju¨rgen Liebig2 Received: 26 February 2016 / Accepted: 27 July 2016 / Published online: 6 August 2016 Ó Springer International Publishing (Outside the USA) 2016 Abstract Establishment and maintenance of the reproduc- linked to its gonadal state. Rather than directly driving the tive division of labor within social insect colonies relies on expression of reproductive-specific hydrocarbons, the clear communication between nestmates. Fertile members gonads may act indirectly through their influence on circu- convey their status to prevent others from becoming repro- lating JH. ductively active. Recent findings in some basal termites indicate that cuticular hydrocarbon profiles may indicate Keywords Cuticular hydrocarbons Á Fertility signaling Á reproductive state, but there is little evidence to show a Gonads Á Juvenile hormone Á Termites direct link between reproductive status and hydrocarbon production—a prerequisite for an ‘‘honest’’ fertility signal. Here, we report that the putative signaling mechanism is Introduction influenced by juvenile hormone (JH), a primary regulator of gonadal development and activity in insects. Topical appli- Within the eusocial insects, signals of fertility status are cation of a JH-analog (pyriproxyfen) to reproductively used to maintain a reproductive division of labor, ensuring inactive alates of the basal dampwood termite Zootermopsis that only one or a few individuals in a colony reproduce, nevadensis induced both females and males to express sig- while the rest serve as a functionally sterile workforce nificantly more of a reproductive-specific hydrocarbon (Wilson 1971; Gadagkar 1994; Sherman et al. 1995). The (6,9,17-tritriacontatriene). However, the JH-analog did not current model suggests that functional reproductives pro- significantly enhance gonadal development or activity in duce a chemical signature linked with their fertility (Heinze treated termites beyond what is usually observed in maturing 2004; Monnin 2006; Le Conte and Hefetz 2008; Peeters alates released from the inhibitory stimuli of their natal nest. and Liebig 2009; Liebig 2010; Van Oystaeyen et al. 2014; These results suggest that a rise in JH following disinhibition Bagne`res and Hanus 2015), and that this chemical signa- drives the expression of reproductive-specific hydrocarbons, ture prevents other colony members from reproducing via but that an individual’s hydrocarbon profile is not directly self-regulated inhibition (Seeley 1979, 1985; Keller and Nonacs 1993). The means by which the production of such fertility signals is coordinated with an individual’s repro- Handling Editor: Stephen J. Martin. ductive status remains to be fully elucidated, but elucidation of this mechanism is crucial to understanding & Colin S. Brent how a stable reproductive division of labor might have [email protected] evolved in social species. 1 US Department of Agriculture, Arid-Land Agricultural For basal termites, cuticular hydrocarbons appear to be a Research Center, Maricopa, AZ 85138, USA major conduit of information needed for colony organiza- 2 School of Life Sciences, Arizona State University, Tempe, tion. At a rudimentary level, hydrocarbon profiles appear to AZ 85287, USA encode species and colony identity (Howard et al. 1982; 3 Department of Biology, North Carolina State University, Haverty et al. 1988; Howard and Blomquist 2005). Within Raleigh, NC 27695-7617, USA the colony, individual profiles appear to convey caste- 123 196 C. S. Brent et al. (Sevala et al. 2000; Klochkov et al. 2005; Darrouzet et al. mate, JH production drops sharply and remains low for 2014) and gender-specific (Bordereau et al., 2010) infor- more than a month (Brent et al. 2005). Subsequently, JH mation. Recently, reproductive-specific hydrocarbons have release increases, corresponding with an increase in been identified in Cryptotermes secundus (Weil et al. 2009) gonadal activity (Brent and Traniello 2001a, b). During this and Zootermopsis nevadensis (Liebig et al. 2009, 2012). transition, females and males begin to express reproduc- These compounds may play a role as primer pheromones, tive-specific cuticular hydrocarbons, namely alkadienes ensuring that nestmates do not develop into supernumerary and alkatrienes (Liebig et al. 2009, 2012). In transcriptomic reproductives in the presence of active kings and queens. analyses, mRNA reads of an elongase and a desaturase, Although there is not yet clear evidence of their impact on both needed for the production of long-chained alkadienes conspecific development in termites, cuticular hydrocarbons or alkatrienes, were most expressed in reproductive males are used for reproductive regulation through fertility sig- and females (Terrapon et al. 2014). Of the four such naling in a number of ant species (Endler et al. 2004; Smith compounds identified, 6,9,17-tritriacontatriene has the et al. 2009; Holman et al. 2010; Smith et al. 2013; Van most pronounced peak in the cuticular hydrocarbon profile. Oystaeyen et al. 2014). Similar patterns in other eusocial The expression level of these compounds was found to be Hymenoptera suggest a widespread use of such hydrocar- positively associated with gonadal development, at least for bon-based signals (Monnin 2006; Le Conte and Hefetz 2008; males (Brent et al. 2005). Collectively, these data suggest Peeters and Liebig 2009; Liebig 2010; Van Oystaeyen et al. that there is at least a correlational relationship between JH 2014). There is also evidence that removal of individuals titer, gonadal activation and hydrocarbon expression. bearing reproductive-specific hydrocarbons quickly evokes However, further evidence is needed to establish a clear a pronounced behavioral response in Z. nevadensis larval regulatory link between reproductive state and odorant helpers, suggesting that they are attuned to the chemical profile before any conclusions can be drawn about the role signatures of their nestmates (Penick et al. 2012). of these hydrocarbons as a fertility signaling mechanism. To establish that the identified hydrocarbons signal Accordingly, the current study was undertaken to investi- reproductive status, it is necessary to show a mechanistic gate the role that JH may have in linking the expression of connection between signal production and reproductive the reproductive-specific cuticular hydrocarbon with activity. Crucial to both insect reproduction and the gonadal development in females and males of Z. expression of cuticular hydrocarbons is the activity of the nevadensis. neuroendocrine system, with juvenile hormone (JH) playing a central role. In termites, JH is involved in determining an individual’s reproductive fate (Lu¨scher 1972; Wanyonyi Methods and materials 1974; Yin and Gillot 1975;Lenz1976; Okot-Kotber 1982; Okot-Kotber and Prestwich 1991a, b; Miura et al. 2003; Animals Scharf et al. 2005a; Cornette et al. 2008; Elliott and Stay 2008; Korb et al. 2009a; Leniaud et al. 2011), and this hor- Colonies of Zootermopsis nevadensis were collected in mone also appears to be associated with gonadal activity Pebble Beach near Monterey, California in 2009. Collec- (Greenberg and Tobe 1985; Brent et al. 2005, 2006; Scharf tion location and the cuticular hydrocarbon profiles of the et al. 2005a; Elliott and Stay 2007; Saiki et al. 2015). colonies indicate that they belong to the subspecies nuttingi Cockroaches, which belong to the same group as termites (Haverty and Thorne 1989). Colonies were extracted from (Kambhampati 1995; Thorne 1997; Inward et al. 2007), also wood logs in the laboratory and transferred into nests use JH to regulate gonadal activity (Engelmann 1959; Stay consisting of several pre-cavitated 5-mm-thick layers of et al. 1980; Koeppe 1981; Koeppe et al. 1985; Schal et al. moistened Spruce Pine (Pinus glabra) sheets that were 1997), and to stimulate hydrocarbon synthesis, storage and bolted together. This allowed ready access to the termites transport to the cuticle (Schal et al. 1994; Fan et al. 2002; by disassembling and subsequently reassembling the wood reviewed in Tillman et al. 1999; Schal et al. 2003). Although nest. Nests were periodically sprayed with ddH2Oto the specific relationship between JH, gonadal activity, and maintain moisture, and were kept in transparent plastic hydrocarbon expression remains to be elucidated in termites, boxes under a 12L:12D light cycle at 25.2 °C. we anticipate that the association follows a pattern similar to that of the cockroaches. Hormone treatment and experimental conditions In Zootermopsis alates, the winged imagoes that found new colonies, JH titers remain high so long as alates Alate females and males were collected from 16 laboratory remain in their natal nest exposed to stimuli inhibiting stock colonies. Because alates turn dark brown as their reproductive maturation. A few days after an alate has been cuticle becomes sclerotized following their imaginal molt, separated from the inhibitory stimuli and paired with a choosing alates with similarly dark pigmentation 123 Induction of a reproductive-specific cuticular hydrocarbon profile by a juvenile hormone… 197 minimized pretreatment developmental differences. 1979; Brent and Traniello 2001a, b, c). At the time of Zootermopsis alates maintained in their natal nest have sampling, neither