Chemical constraints on the evolution of olfactory communication channels of moths

Innate recognition of pheromone and food odors in moths: a common mechanism in the antennal lobe?

The survival of an animal often depends on an innate response to a particular sensory stimulus. For an adult male moth, two categories of odors are innately attractive: pheromone released by conspecific females, and the floral scents of certain, often co-evolved, plants. These odors consist of multiple volatiles in characteristic mixtures. Here, we review evidence that both categories of odors are processed as sensory objects, and we suggest a mechanism in the primary olfactory center, the antennal lobe (AL), that encodes the configuration of these mixtures and may underlie recognition of innately attractive odors. In the pheromone system, mixtures of two or three volatiles elicit upwind flight. Peripheral changes are associated with behavioral changes in speciation, and suggest the existence of a pattern recognition mechanism for pheromone mixtures in the AL. Moths are similarly innately attracted to certain floral scents. Though floral scents consist of multiple volatiles that activate a broad array of receptor neurons, only a smaller subset, numerically comparable to pheromone mixtures, is necessary and sufficient to elicit behavior. Both pheromone and floral scent mixtures that produce attraction to the odor source elicit synchronous action potentials in particular populations of output (projection) neurons (PNs) in the AL. We propose a model in which the synchronous output of a population of PNs encodes the configuration of an innately attractive mixture, and thus comprises an innate mechanism for releasing odor-tracking behavior. The particular example of olfaction in moths may inform the general question of how sensory objects trigger innate responses.

Female sex pheromone of the Yunnan pine caterpillar moth Dendrolimus houi: first (E,Z)-isomers in pheromone components of Dendrolimus spp

The Yunnan pine caterpillar Dendrolimus houi Lajonquière is a serious defoliator of coniferous forests in southwestern China. Gas chromatography-electroantennography (GC-EAG) analyses of extracts of female sex pheromone glands of D. houi moths revealed the presence of three compounds eliciting antennal responses. These were identified as (5E,7Z)-5,7-dodecadien-1-ol (E5,Z7-12:OH), (5E,7Z)-5,7-dodecadien-1-yl acetate (E5,Z7-12:OAc), and (5E,7Z)-5,7-dodecadienal (E5,Z7-12:Ald) by comparison of their GC retention indices, mass spectra, and EAG activities with those of synthetic standards. Average amounts of E5,Z7-12:OH, E5,Z7-12:OAc, and E5,Z7-12:Ald per calling virgin D. houi female were 14.7 +/- 12.9 ng (+/- SD), 5.8 +/- 5.4 ng, and 0.8+1.4 ng, respectively, in a ratio of 100:39.7:5.6. These three components were also collected from the headspace of calling virgin female moths by solid-phase microextraction (SPME). In addition, trace quantities of (Z)-5-dodecen-1-ol (Z5-12:OH), (5Z,7E)-5,7-dodecadien-1-ol (Z5,E7-12:OH), (5E,7E)-5, 7-dodecadien-1-ol (E5,E7-12:OH), (5Z,7E)-5,7-dodecadien-1-yl acetate (Z5,E7-12:OAc), (5Z,7Z)-5,7-dodecadien-1-yl acetate (Z5,Z7-12:OAc), and (5E,7E)-5,7-dodecadien-1-yl acetate (E5,E7-12:OAc) were tentatively identified in female pheromone gland extracts by selected ion monitoring GC-MS. Field trapping experiments showed that E5,Z7-12:OH, E5,Z7-12:OAc, and E5,Z7-12:Ald were essential for attraction of male D. houi moths. Traps baited with a 20:1:1 blend (alcohol/acetate/aldehyde) loaded on gray rubber septa were as effective as traps baited with virgin female moths. The optimum ratio of acetate to aldehyde was 1:1, and this ratio was more critical than the ratio of either compound to the alcohol. This represents the first example of (E,Z)-isomers in pheromone blends of Dendrolimus species.

Bidirectional selection for novel pheromone blend ratios in the almond moth, Cadra cautella

The sex pheromone of the almond moth, Cadra cautella, is a blend of (Z,E)-9,12-tetradecadienyl acetate (Z9,E12-14:Ac, the major component capable of inducing attraction alone) and (Z)-9-tetradecenyl acetate (Z9-14:Ac, the minor component, which is unattractive alone but augments attraction of the major component). In this study, the ratio of the two components responded to artificial directional selection in five of six selected lines, whereas no change was observed in the three control lines. The mean ratio (+/-SE) of Z9,E12-14:Ac to Z9-14:Ac went from 13.72 +/- 1.02:1 to 20.13 +/- 0.68:1 in high line 1, an increase of 47%. In the second high-selected line, the mean ratio (+/-SE) increased from 9.87 +/- 0.54:1 to 15.89 +/- 0.85:1, an increase of 61%. In low line 1, the mean ratio (+/-SE) in the parental generation was 10.74 +/- 0.78:1 and 7.35 +/- 0.41:1 in the last selected generation, a decrease of 32%. The response to selection was greater in low line 2, as the mean ratio (+/-SE) decreased from 10.11 +/- 0.66:1 to 5.65 +/- 0.55:1 in the last generation, a decrease of 44%. In low line 3, the mean ratio (+/-SE) in the parental generation was 13.63 +/- 0.82:1 and 6.47 +/- 0.26:1 in the last generation, a decrease of 53%. The response to selection was approximately symmetrical with a mean increase of 54% and a mean decrease of 43%. The increases in ratio observed in the high lines were caused by an increase in the titer of the Z9,E12-14:Ac component with no concurrent change in the titer of the component Z9-14:Ac. Among the low selected lines, the titers of both components increased; however, there was a greater relative increase in the titer of the component Z9-14:Ac. The absolute and relative titers of the sex pheromone components had decreased significantly in the F10 generation in some of the selected lines, five generations after the discontinuation of selection.

Assortative mating between European corn borer pheromone races: beyond assortative meeting

Background

Sex pheromone communication systems may be a major force driving moth speciation by causing behavioral reproductive isolation via assortative meeting of conspecific individuals. The ‘E’ and ‘Z’ pheromone races of the European corn borer (ECB) are a textbook example in this respect. ‘Z’ females produce and ‘Z’ males preferentially respond to a ‘Z’ pheromone blend, while the ‘E’ race communicates via an ‘E’ blend. Both races do not freely hybridize in nature and their populations are genetically differentiated. A straightforward explanation would be that their reproductive isolation is a mere consequence of “assortative meeting” resulting from their different pheromones specifically attracting males towards same-race females at long range. However, previous laboratory experiments and those performed here show that even when moths are paired in a small box – i.e., when the meeting between sexual partners is forced – inter-race couples still have a lower mating success than intra-race ones. Hence, either the difference in attractivity of E vs. Z pheromones for males of either race still holds at short distance or the reproductive isolation between E and Z moths may not only be favoured by assortative meeting, but must also result from an additional mechanism ensuring significant assortative mating at close range. Here, we test whether this close-range mechanism is linked to the E/Z female sex pheromone communication system.

Methodology/Principal Findings

Using crosses and backcrosses of E and Z strains, we found no difference in mating success between full-sisters emitting different sex pheromones. Conversely, the mating success of females with identical pheromone types but different coefficients of relatedness to the two parental strains was significantly different, and was higher when their genetic background was closer to that of their male partner's pheromone race.

Conclusions/Significance

We conclude that the close-range mechanism ensuring assortative mating between the E and Z ECB pheromone races is unrelated to the difference in female sex pheromone. Although the nature of this mechanism remains elusive, our results show that it is expressed in females, acts at close range, segregates independently of the autosome carrying Pher and of both sex chromosomes, and is widely distributed since it occurs both in France and in the USA.

Pheromone component patterns of moth evolution revealed by computer analysis of the Pherolist

1. The Pherolist internet site listing moth sex pheromone components reported in the literature was downloaded and processed by a basic program into a database with 2931 combinations of 377 unique chemical names of sex pheromone attractants used by 1572 moth species in 619 genera and 49 families. Names of pheromone compounds were analysed for aliphatic chain length, unsaturation position, geometric configuration, functional group (aldehyde, alcohol, acetate, epoxide, methyl-branched and hydrocarbon) and number of instances such combinations are used by species and families. 2. The analyses revealed pheromone blends of species ranged from one to eight components (45% species with one component, 36% two, 12% three, 5% four, 1% five, < or = 0.5% for > or = six). The numbers of different components of various chain lengths and functional groups, the numbers of instances such compounds are used by species and the numbers of species using such compounds are presented. 3. The average number of pheromone components per species increased as the number of species in a family increased based on linear regression of components in the 10 largest families, with species numbers ranging from 19 to 461. Pooling the four largest families gave a mean of 1.96 components per species that was significantly greater than the mean of the next 14 smaller families (1.63). Because related species in a large family would need more communication channels, this suggests that these species on average evolved to produce and detect more components in their pheromone blends to achieve a unique communication channel than was needed by species in smaller families. 4. Speciation in moths would entail evolutionary changes in both pheromone biosynthetic and sensory systems that avoided competition for communication channels of existing species. Regression analysis indicated that the more species in a family the more unique pheromone components, but the increase diminishes progressively. This suggests that, as the number of components increases with species number in a family, new species are more likely to evolve a unique blend comprising a communication channel from among existing components of the family.