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Introduction
The frontal gland is a sac-like organ, which typically opens through the frontal pore (fontanelle). It represents a unique defense
specialization characteristic only for advanced termite families of Rhinotermitidae, Serritermitidae, and Termitidae. Though it is present
in presoldiers (stage preceeding the soldier), soldiers, alate imagoes and artificial intercastes (induced by treatment with juvenile
hormone analogue, JHA), its role has been well described only in soldiers: it manages the biosynthesis, storage, and discharge of
defensive and alarm substances. The aim of our work was to study the composition and potential role of frontal gland secretion in
non-soldier individuals.
Results
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Soldier: Two groups of chemicals were observed to form the secretion: nitroolefins and sesquiterpene
(Z,E)-α-farnesene. Major nitro compound is (E)-1-nitropentadec-1-ene; (E)-1-nitroheptadec-1-ene and
1-nitrotridec-1-ene occur in much lower amounts; 1-nitrotetradec-1-ene, 1-nitrohexadec-1-ene, nitropentadecadiene, and nitrohepta- decadiene
are present in trace amounts (see Table).
| Composition of frontal gland secretion in soldiers, intercastes and swarming imagoes |
| Compound |
Soldier |
Intercaste |
Swarming imago |
| Nitroalkanes |
| 1-nitrotridec-1-ene |
tr |
|
|
| 1-nitrotetradec-1-ene |
tr |
|
|
| (E)-1-nitropentadec-1-ene* |
+++ |
+++ |
+++ |
| 1-nitrohexadec-1-ene |
tr |
|
|
| (E)-1-nitroheptadec-1-ene* |
+ |
++ |
++ |
| Nitroalkadienes |
| nitropentadecadiene
|
tr |
|
|
| nitroheptadecadiene |
tr |
|
|
| Sesquiterpenes |
| α-cedrene |
|
|
tr |
| acora-3,5-diene |
|
|
tr |
| (E)-β-farnesene* |
|
|
+ |
| muurola-4,11-diene |
|
|
tr |
| B-acoradiene |
|
+ |
++ |
| 7αH,10βH-cadina-1(6),4-diene |
|
|
+ |
| γ-curcumene |
|
+ |
+ |
| ar-curcumene |
|
|
tr |
| (3Z,6E)-α-farnesene* |
++ |
+ |
+ |
| zingiberene* |
|
|
+ |
| germacrene A |
|
tr |
+ |
| zingiberene* |
|
|
+ |
| germacrene A |
|
|
+ |
| γ-amorphene |
|
|
+ |
| β-alaskene |
|
|
tr |
| β-bisabolene* |
|
tr |
+ |
| β-curcumene |
|
+ |
+ |
| (Z)-γ-bisabolene |
|
|
tr |
| (E)-γ-bisabolene |
|
tr |
+ |
| γ-cuprenene |
|
|
+ |
| (E)-β-bisabolene |
|
tr |
+ |
| (1E,4Z)-germacrene B |
|
|
tr |
| trans-dauca-4(11),7-diene |
|
|
tr |
+++ = > 5%, ++ = 2 to 5%, + = 0.5 to 2%, tr = < 0.5% of total; * confirmed with standards. |
|
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Alate imagoes: In swarming imagoes, nitro compounds fraction contains only (E)-1-nitropentadec-1-ene and
(E)-1-nitroheptadec-1-ene were (see Table). Rich mixture of sesquiterpenes (21 identified, about 10 unidentified) was found in both
sexes (see Table). No sexual differences in composition of secretion were found except for the ratio between sesquiterpenes and nitro
compounds (see Graph below). In functional dealate reproductives (kings and queens), no compounds present in alates were detected.
Comparison of quantity of (E)-1-nitropentadec-1-ene
and 4 main sesquiterpenes in male and female imagoes
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Presoldiers: In spite of the presence of a secretion inside the frontal gland reservoir, no compounds other than cuticular
hydrocarbonswere detected using GC-MS.
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Artificial intercastes: Although JHA induced intercastes form morphological continuum between pseudergates (false workers)
and soldiers, the composition of the frontal gland secretion strongly resembles that of swarming imagoes. Except nitro compounds,
it is rich in sesquiterpenes, of which 10 (same as in imagoes) were found to occur in the secretion (see Table).
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Conclusions
- Frontal gland is active in soldiers, imagoes and artificial intercastes, but not in presoldiers. It biosynthesizes two classes of
chemicals - nitroolefins and sesquiterpenes (when active).
- Only single sesquiterpene is produced by soldiers, but >30 or 10 different sesquiterpenes were identified in the secretion of
imagoes, and artificial intercastes respectively.
- The function of imaginal frontal gland is restricted to the period of swarming.
- Although JHA induces morphological change towards the soldier appearance, it causes production of imago-like frontal gland secretion
in affected individuals.
Research team
Jan Šobotník, Rafal Piskorski, Robert Hanus, Soňa Vašíčková, Josef Cvačka, Aleš Svatoš, Irena Valterová
Related themes on our web pages
Termites (part II)
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