Chemical defense secretions of some species of Malaysian rhinotermitidae (Isoptera, Rhinotermitidae)

Chemistry of the Secondary Metabolites of Termites

Isolation, structure determination, synthesis, and biochemistry of the low-molecular-weight compounds of the secretion of exocrine glands of termites are described, with an emphasis on pheromones and defensive compounds.

Co-option of the sphingolipid metabolism for the production of nitroalkene defensive chemicals in termite soldiers

The aliphatic nitroalkene (E)-1-nitropentadec-1-ene (NPD), reported in early seventies in soldiers of the termite genus Prorhinotermes, was the first documented nitro compound produced by insects. Yet, its biosynthetic origin has long remained unknown. Here, we investigated in detail the biosynthesis of NPD in P. simplex soldiers. First, we track the dynamics in major metabolic pathways during soldier ontogeny, with emphasis on likely NPD precursors and intermediates. Second, we propose a hypothesis of NPD formation and verify its individual steps using in vivo incubations of putative precursors and intermediates. Third, we use a de novo assembled RNA-Seq profiles of workers and soldiers to identify putative enzymes underlying NPD formation. And fourth, we describe the caste- and age-specific expression dynamics of candidate initial genes of the proposed biosynthetic pathway. Our observations provide a strong support to the following biosynthetic scenario of NPD formation, representing an analogy of the sphingolipid pathway starting with the condensation of tetradecanoic acid with l-serine and leading to the formation of a C₁₆ sphinganine. The C₁₆ sphinganine is then oxidized at the terminal carbon to give rise to 2-amino-3-hydroxyhexadecanoic acid, further oxidized to 2-amino-3-oxohexadecanoic acid. Subsequent decarboxylation yields 1-aminopentadecan-2-one, which then proceeds through six-electron oxidation of the amino moiety to give rise to 1-nitropentadecan-2-one. Keto group reduction and hydroxyl moiety elimination lead to NPD. The proposed biosynthetic sequence has been constructed from age-related quantitative dynamics of individual intermediates and confirmed by the detection of labeled products downstream of the administered labeled intermediates. Comparative RNA-Seq analyses followed by qRT-PCR validation identified orthologs of serine palmitoyltransferase and 3-ketodihydrosphingosine reductase genes as highly expressed in the NPD production site, i.e. the frontal gland of soldiers. A dramatic onset of expression of the two genes in the first days of soldier's life coincides with the start of NPD biosynthesis, giving further support to the proposed biosynthetic hypothesis.

Chemical alarm in the termite Termitogeton planus (Rhinotermitidae)

Effective defense is a common characteristic of insect societies. Indeed, the occurrence of specialized defenders, soldiers, has been the first step toward eusociality in several independent lineages, including termites. Among the multitude of defensive strategies used by termite soldiers, defense by chemicals plays a crucial role. It has evolved with complexity in advanced isopteran lineages, whose soldiers are equipped with a unique defensive organ, the frontal gland. Besides direct defense against predators, competitors, and pathogens, the chemicals emitted by soldiers from the frontal gland are used as signals of alarm. In this study, we investigated the chemical composition of the defensive secretion produced by soldiers of the termite Termitogeton planus (Isoptera: Rhinotermitidae), from West Papua, and the effects of this secretion on the behavior of termite groups. Detailed two-dimensional gas chromatography/mass spectrometry analyses of the soldier defensive secretion revealed the presence of four linear and nine monoterpene hydrocarbons. Soldier head extracts, as well as synthetic mixtures of the monoterpenes found in these extracts, elicited alarm behavior in both soldiers and pseudergates. Our results suggest that the alarm is not triggered by a single monoterpene from the defensive blend, but by a multi-component signal combining quantitatively major and minor compounds.

Biodiversity of termite (Insecta: Isoptera) in tropical peat land cultivated with oil palms

Termites are the major decomposers in tropical region but yet their occurrences in oil palm plantation especially in peat soil are generally treated as pest. Study of termite species in peat land was conducted in selected oil palm plantations in North Sarawak with 5-7 years old palms and South Sarawak with 13-15 years old palms with two sites in each area. Results of quadrate (25 x 25 x 30 cm) sampling showed termite was significantly higher in relative density with increasing depth of soil (0-10 = 21.23, 10-20 = 42.52 and 20-30 cm = 81.12%) which could be advantaged from being predated by ants (Hymenoptera: Formicidae) which were higher in density from soil surface to 10 cm soil depth with relative density of 31.84%. Modified transect sampling (50x6 m) had successfully sampled 18 species of termites from 2 families (Rhinotermitidae and Termitidae), 5 subfamilies (Rhinotermitinae, Coptotermitinae, Termitinae, Macrotermitinae and Nasutitermitinae) and 11 genera (Coptotermes, Schedorhinotermes, Termes, Macrotermes, Nasutitermes, Globitermes, Amitermes, Parrhinotermes, Pericapritermes, Havilanditermes and Prohamitermes). Both plantation sites have termite dominantly feeding on rotten wood as a result of abundant dead woods. However, Coptotermes curvignathus Holmgren was identified to feed on the living tissues of oil palm causing damage or death of the tree. Study showed higher encounter of soil-feeding termite in longer established plantation. It indicates the gradually shifting of soil condition towards a stabilized environment which favors the successful settlement of soil feeder termite species. Termite control should be more targets specific to avoid harming beneficial termites.

Chemical warfare in termites

The rapid development of analytical methods in the last four decades has led to the discovery of a fascinating diversity of defensive chemicals used by termites. The last exhaustive review on termite defensive chemicals was published by G.D. Prestwich in 1984. In this text, we aim to fill the gap of the past 25 years and overview all of the relevant primary sources about the chemistry of termite defense (126 original papers, see Fig. 1 and online supplementary material) along with related biological aspects, such as the anatomy of defensive glands and their functional mechanisms, alarm communication, and the evolutionary significance of these defensive elements.

Nitroalkenes and sesquiterpene hydrocarbons from the frontal gland of three prorhinotermes termite species

Frontal gland contents of soldiers of three Prorhinotermes species, Prorhinotermes canalifrons, Prorhinotermes inopinatus, and Prorhinotermes simplex, consisted of two groups of compounds: nitroalkenes and sesquiterpene hydrocarbons. Analysis by gas chromatography-mass spectrometry revealed (E)-1-nitropentadec-1-ene as the major component of the glands with mean values of 152, 207, and 293 microg/individual for P. canalifrons, P. inopinatus, and P. simplex, respectively. Four other 1-nitroalkenes (C13, C14, C16, and C17), and two nitrodienes (C15 and C17) were also detected in the three species. The C17:1 nitroalkene was identified as (E)-1-nitroheptadec-1-ene. The sesquiterpene composition of the gland was species-specific: P. simplex contained (3Z,6E)-alpha-farnesene (mean of 39 microg/individual), while P. canalifrons and P. inopinatus contained the same compound (means of 0.5 and 1.5 microg/individual, respectively) as well as the (3E,6E) isomer (means of 1.8 and 0.7 microg/individual, respectively). Two other sesquiterpenes, trans-beta-bergamotene and (Z)-gamma-bisabolene, were also found in low quantities in the frontal gland of P. canalifrons.

Interspecific variation in defense secretions of Malaysian termites from the genus Bulbitermes

The chemicals of the defense secretions of Malaysian Bulbitermes, B. singaporensis, B. germanus, B. sarawakensis, and Bulbitermes sp. B, show that B. singaporensis is distinct from the other species, which are themselves closely related; the genetic distance between B. singaporensis and B. germanus is 0.71. B. singaporensis contains tetracyclic kempane, and B. germanus and B. sarawakensis contain tricyclic trinervitene; Bulbitermes sp. B contains a mixture of kempane and trinervitene. The mono- and diterpenoid compositions are species-specific.