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Introduction New termite colonies are founded by a pair of imaginal reproductives - queen and king - emerging as winged individuals from their mother colony, often in large numbers, during periodical swarmings. Once their short dispersal flight completed, they shed their wings and start searching for a mate. Sexual communication in termites is mediated by sexual pheromones secreted by exocrine glands: tergal glands, posterior sternal glands or sternal glands. They are present in females, rarely in both sexes (1), females usually release a long range pheromone to attract conspecific males. Despite the variety of pheromonal sources the chemical diversity of termite sexual pheromones is limited: only four compounds have been identified as termite sexual pheromones so far, occurring without any clear phylogenetic trend across the whole order, often they act also in other contexts, such as trail following (2). Here we present the results of our investigation of sexual chemical communication in Prorhinotermes simplex acquired during swarming of two laboratory colonies (from Cuba and Florida) in winter 2006/7.
Exocrine glands Prorhinotermes simplex imagoes possess abdominal exocrine glands absent in all other castes - tergal glands (present in both sexes) and posterior sternal glands (in males only). They consist of secretory cells class I and II, and differ in ultrastructure between sexes. The posterior sternal glands in males are similar in structure to their tergal glands (3).
Sexual behaviour Dealated males are attracted by females and actively look for them. When confronted with females they follow them in a tandem run: they touch with their antennae and mouthparts the posterior dorsal region of the female's abdomen. In absence of males females were observed to perform a calling behaviour by exposing their tergal glands.
Behavioural tests Males, removed from the periphery of the colony, were dealated and exposed to olphactory and control stimuli in the following setup:
Electrophysiology Simultaneously with behavioural tests the electrophysiological essays were conducted with identical stimuli. Isolated male antennae were submitted to GC-EAD measurements with using the female tergal gland extracts together with hydrocarbon standards (C8 - C22). Subsequently the standards of candidate compounds were tested and compared with the response acquired with glandular extracts. Following temperature program was used: 2 minutes at 50 °C, then 30 °C/min, and 10 minutes at 270 °C.
Glandular extracts
 Unfortunately, despite a careful chromatographic analysis using GC×GC MS-TOF Leco we did not find in the glandular extracts any candidate compound with corresponding chromatographic behaviour. Thus, given the limited chemical diversity of termite pheromones, we decided to test standards of termite sexual and trail pheromones with resembling chemical characteristics: dodecatrienol and dodecadienol (4).
Chemical characteristics 2D chromatogram of the standard of (3Z,6Z,8E)-dodeca-3,6,8-trien-1-ol (see 5 for synthesis) in coelution with hydrocarbons. The calculated Kováts index for dodecatrienol was 1529. Right: mass spectrum.
Conclusions Electrophysiological and behavioural experiments as well as chromatographic analysis of a standard congruently indicate an important biological role of dodecatrienol in imagoes of P. simplex. The observed behavioural repertoire of the two sexes and the origin of the active compound in tergal glands, exclusive for imagoes, clearly evidence that the compound acts in a sexual context: as an only (or one of several) constituent(s) of female sexual pheromone. In the current state of our investigation we lack an important direct proof: the identification of the compound in the extract of female tergal glands. Nevertheless, as can be seen from behavioural experiments, even a dose of units of picograms of dodecatrienol elicits the typical male searching behaviour, an amount which is below the detection limit of our equipment. The same indicate other studies: a detectable amount of dodecatrienol had to be extracted from thousands individual termites in Reticulitermes santonensis (4). Such a number of individuals cannot be obtained during a swarming of a laboratory colony but can be occasionally collected in nature. (3Z,6Z,8E)-dodeca-3,6,8-trien-1-ol occurs as a termite releaser pheromone in various contexts (sexual or trail pheromone) in several species including the representants of the family Rhinotermitidae (2), but also in species phylogenetically distant. Moreover, in different species the compound is secreted by different glands: tergal (posterior sternal) or sternal glands. Thus it is an excellent example of parsimony in use of semiochemicals in phylogenetic context.
References
Research team Rober Hanus, Blanka Kalinová, Anna Luxová, Jan Šobotník, Pavel Jiroš, Irena Valterová, Rafal Piskorski, Christian Bordereau
Michal Hoskovec © 8.X.2007
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