Sex pheromones and reproductive isolation in five mirid species

Pheromone Response and Mating Behavior in Fission Yeast

Most ascomycete fungi, including the fission yeast Schizosaccharomyces pombe, secrete two peptidyl mating pheromones: C-terminally modified and unmodified peptides. S. pombe has two mating types, plus and minus, which secrete two different pheromones, P-factor (unmodified) and M-factor (modified), respectively. These pheromones are specifically recognized by receptors on the cell surface of cells of opposite mating types, which trigger a pheromone response. Recognition between pheromones and their corresponding receptors is important for mate discrimination; therefore, genetic changes in pheromone or receptor genes affect mate recognition and cause reproductive isolation that limits gene flow between populations. Such genetic variation in recognition via the pheromone/receptor system may drive speciation. Our recent studies reported that two pheromone receptors in S. pombe might have different stringencies in pheromone recognition. In this review, we focus on the molecular mechanism of pheromone response and mating behavior, emphasizing pheromone diversification and its impact on reproductive isolation in S. pombe and closely related fission yeast species. We speculate that the "asymmetric" system might allow flexible adaptation to pheromone mutational changes while maintaining stringent recognition of mating partners. The loss of pheromone activity results in the extinction of an organism's lineage. Therefore, genetic changes in pheromones and their receptors may occur gradually and/or coincidently before speciation. Our findings suggest that the M-factor plays an important role in partner discrimination, whereas P-factor communication allows flexible adaptation to create variations in S. pombe. Our inferences provide new insights into the evolutionary mechanisms underlying pheromone diversification.

Biology, Ecology, and Pest Management of the Tarnished Plant Bug, Lygus lineolaris (Palisot de Beauvois) in Southern Row Crops

Simple Summary

The tarnished plant bug, Lygus lineolaris, is a polyphagous, sap-feeder that causes significant economic damage in several field crops, especially cotton (Gossypium hirsutum L.) in the mid-southern United States. In 2020, it was reported that 4.8 million acres of cotton were infested by Lygus spp. in the United States. A broad host range, polyphagous feeding behavior and high mobility of this pest along with resistance development to conventional pesticides helped them establish as a significant pest of concern for cotton growers in the mid-south. Since the publication of a review by Layton (2000) on damage caused by Lygus lineolaris, many new research studies have been published on the Lygus biology, ecology, and integrated pest management strategies. A comprehensive review paper that summarizes these latest research developments and Lygus management strategies will be useful for researchers and cotton growers. In this review, we report and discuss the latest developments in Lygus research and the new control strategies that have been developed in the last two decades.

Abstract

The tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), (Hemiptera: Miridae) is considered the most damaging pest of cotton (Gossypium hirsutum L.) in the mid-southern United States, although it is established throughout the United States, southern Canada, and northern Mexico. The introduction of transgenic crops for the control of moths in the Heliothine complex and eradication of the boll weevil, Anthonomus grandis, from much of the United States led to greatly reduced pesticide use in cotton fields, which allowed L. lineolaris to emerge as a new primary pest of cotton in the mid-southern United States. Since the publication of a review by Layton (2000) on damage caused by Lygus lineolaris, many new studies have been published on the changes in host range, population dynamics, sampling methods and thresholds, cultural practices, sex pheromones and attractant blends, novel pesticides and insecticide resistance mechanisms, olfactory and feeding behaviors, introduction of biological control agents, host-plant resistance mechanisms, and new molecular and genetic tools for integrated pest management of Lygus species in cotton and other important crops. Here, we review and discuss the latest developments in L. lineolaris research in the last two decades.

Optimization and field demonstration of the Lygus pratensis (Hemiptera: Miridae) sex pheromone

BACKGROUND

The plant bug Lygus pratensis Linnaeus is a widely distributed polyphagous herbivore that increasingly attains outbreak population levels on cotton in northwestern China. Although the sex pheromone of L. pratensis from the United Kingdom has been identified as hexyl butyrate, (E)-2-hexenyl butyrate and (E)-4-oxo-2-hexenal, at a ratio of 100:25:24, this volatile blend does not prove attractive to Chinese field populations.

RESULTS

In this study, we identified and optimized the sex pheromone of L. pratensis strains from northwestern China. In coupled gas chromatography and electro-antennogram detection (GC-EAD) assays, three compounds within whole-body extracts of virgin L. pratensis females elicited antennal responses: hexyl butyrate, (E)-2-hexenyl butyrate and (E)-4-oxo-2-hexenal. In field trials, a 20:1:30 ratio blend was the most attractive to L. pratensis males.

CONCLUSION

Traps baited with this synthetic pheromone blend present considerable advantages over traditional sweep-net sampling for L. pratensis population monitoring. It can readily be incorporated into monitoring schemes and integrated pest management packages.

Transcriptome Analysis of Ostrinia furnacalis Female Pheromone Gland: Esters Biosynthesis and Requirement for Mating Success

Female moths use sex pheromones to attract males, and corresponding regulatory mechanism underlying sex pheromone biosynthesis is species-dependent. However, the detailed mechanism involved in sex pheromone biosynthesis in Ostrinia furnacalis has not yet been fully addressed. In the present study, transcriptome sequencing of O. furnacalis pheromone glands screened a serials of candidate genes involved in sex pheromone biosynthesis. Our analysis showed that sex pheromone release in O. furnacalis females arrives its peak at the 2^nd scotophase, consistent with its mating behavior. Pheromone biosynthesis-activating neuropeptide (PBAN) was confirmed to regulate sex pheromone biosynthesis, and Ca²⁺ is the secondary messenger of PBAN signaling in O. furnacalis. The functional analysis of candidate genes demonstrated that the decreased mRNA levels or activities of calcineurin (CaN) and acetyl-CoA carboxylase (ACC) led to significant decrease in sex pheromone production and female capability to attract males, as demonstrated by RNAi-mediated knockdown and pharmacological inhibitor assay. Most importantly, the activities of CaN and ACC depend on the activation of PBAN/PBANR/Ca²⁺. Furthermore, fatty-acyl reductase 14 was involved in PBAN-mediated sex pheromone biosynthesis. Altogether, our results demonstrated that PBAN regulates sex pheromone biosynthesis through PBANR/Ca²⁺/CaN/ACC pathway to promote sex pheromone biosynthesis in O. furnacalis and provided a reference for non-model organism to study neuropeptide signal transduction.

Identification and field evaluation of the sex pheromone of Apolygus lucorum (Hemiptera: Miridae) in China

BACKGROUND

The plant bug, Apolygus lucorum Meyer-Dür, has begun a resurgence and has become a key pest in cotton in northern China, with the wide-scale adoption of transgenic Bt cotton. We attempted to develop a new approach to the control of this plant bug by identifying and utilizing its sex pheromone.

RESULTS

Extracts from A. lucorum adults and nymphs were analyzed using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-electroantennogram detection (GC-EAD) to identify the sex pheromone components. (E)-4-Oxo-2-hexenal and (E)-2-hexenyl butyrate were the major sex pheromone components from females and were also electrophysiology-active towards male antennae. For males, (E)-4-oxo-2-hexenal and hexyl butyrate were the major sex pheromone components. These three components, however, were not detected in nymphs. Field tests showed that binary blends of (E)-4-oxo-2-hexenal and (E)-2-hexenyl butyrate at a ratio of 3:2 resulted in trapping the greatest numbers of A. lucorum males. A long-term dispenser was developed by loading the pheromone components dissolved in sunflower oil into polyethylene vials. Further field evaluation showed that the polyethylene dispensers attracted significantly more bugs even when deployed in the field for 5 weeks.

CONCLUSION

We identified the sex pheromone of A. lucorum and developed a type of high-efficiency and long-term lure. Our results could expand knowledge of the pheromones of plant bugs, and provide novel technologies to monitor and control this pest. © 2019 Society of Chemical Industry.

Speciation and hybridization in invasive fire ants

BACKGROUND

A major focus of evolutionary biology is the formation of reproductive barriers leading to divergence and ultimately, speciation. Often, it is not clear whether the separation of populations is complete or if there still is ongoing gene flow in the form of rare cases of admixture, known as isolation with migration. Here, we studied the speciation of two fire ant species, Solenopsis invicta and Solenopsis richteri, both native to South America, both inadvertently introduced to North America in the early twentieth century. While the two species are known to admix in the introduced range, in the native range no hybrids were found.

RESULTS

We conducted a population genomic survey of native and introduced populations of the two species using reduced representation genomic sequencing of 337 samples. Using maximum likelihood analysis over native range samples, we found no evidence of any gene flow between the species since they diverged. We estimated their time of divergence to 190,000 (100,000-350,000) generations ago. Modelling the demographic history of native and introduced S. invicta populations, we evaluated their divergence times and historic and contemporary population sizes, including the original founder population in North America, which was estimated at 26 (10-93) unrelated singly-mated queens.

CONCLUSIONS

We provide evidence for complete genetic isolation maintained between two invasive species in their natïve range, based, for the first time, on large scale genomic data analysis. The results lay the foundations for further studies into different stages in the formation of genetic barriers in dynamic, invasive populations.

Asymmetric diversification of mating pheromones in fission yeast

In fungi, mating between partners depends on the molecular recognition of two peptidyl mating pheromones by their respective receptors. The fission yeast Schizosaccharomyces pombe (Sp) has two mating types, Plus (P) and Minus (M). The mating pheromones P-factor and M-factor, secreted by P and M cells, are recognized by the receptors mating type auxiliary minus 2 (Mam2) and mating type auxiliary plus 3 (Map3), respectively. Our recent study demonstrated that a few mutations in both M-factor and Map3 can trigger reproductive isolation in S. pombe. Here, we explored the mechanism underlying reproductive isolation through genetic changes of pheromones/receptors in nature. We investigated the diversity of genes encoding the pheromones and their receptor in 150 wild S. pombe strains. Whereas the amino acid sequences of M-factor and Map3 were completely conserved, those of P-factor and Mam2 were very diverse. In addition, the P-factor gene contained varying numbers of tandem repeats of P-factor (4–8 repeats). By exploring the recognition specificity of pheromones between S. pombe and its close relative Schizosaccharomyces octosporus (So), we found that So-M-factor did not have an effect on S. pombe P cells, but So-P-factor had a partial effect on S. pombe M cells. Thus, recognition of M-factor seems to be stringent, whereas that of P-factor is relatively relaxed. We speculate that asymmetric diversification of the two pheromones might be facilitated by the distinctly different specificities of the two receptors. Our findings suggest that M-factor communication plays an important role in defining the species, whereas P-factor communication is able to undergo a certain degree of flexible adaptation–perhaps as a first step toward prezygotic isolation in S. pombe.

An asymmetric pheromone/receptor system in the fission yeast Schizosaccharomyces pombe might allow flexible adaptation of pheromones to mutational changes while maintaining stringent recognition for mating partners, perhaps as a first step toward prezygotic mating isolation.

The emergence of a new species might occur when two groups can no longer mate. Although such reproductive isolation is considered a key evolutionary process, the mechanisms by which it actually occurs have been confined to conjecture. The two sexes (Plus [P] and Minus [M]) of S. pombe each secrete a pheromone (P-factor and M-factor), which binds to a corresponding receptor (mating type auxiliary minus 2 [Mam2] and mating type auxiliary plus 3 [Map3]) on cells of the opposite sex. The interaction between a pheromone and its receptor is essential for successful mating. Here, we explored conservation of the mating pheromone communication system among 150 wild S. pombe strains of different geographical origins and the closely related species S. octosporus. We found that 1) the M-factor/Map3 interaction was completely conserved, whereas the P-factor/Mam2 interaction was very diverse in the strains investigated, and 2) most of the P-factor variants were functional across species. Thus, we have revealed an asymmetric pheromone/receptor system in fungal mating: namely, whereas M-factor communication operates extremely stringently, P-factor communication has the flexibility to create variations, perhaps facilitating prezygotic isolation in S. pombe.

COI barcoding of plant bugs (Insecta: Hemiptera: Miridae)

The family Miridae is the most diverse and one of the most economically important groups in Heteroptera. However, identification of mirid species on the basis of morphology is difficult and time-consuming. In the present study, we evaluated the effectiveness of COI barcoding for 123 species of plant bugs in seven subfamilies. With the exception of three Apolygus species-A. lucorum, A. spinolae, and A. watajii (subfamily Mirinae)-each of the investigated species possessed a unique COI sequence. The average minimum interspecific genetic distance of congeners was approximately 37 times higher than the average maximum intraspecific genetic distance, indicating a significant barcoding gap. Despite having distinct morphological characters, A. lucorum, A. spinolae, and A. watajii mixed and clustered together, suggesting taxonomic revision. Our findings indicate that COI barcoding represents a valuable identification tool for Miridae and can be economically viable in a variety of scientific research fields.

Identification of a sex pheromone of the chrysanthemum lace bug Corythucha marmorata (Hemiptera: Tingidae)

Although the nymphs of Corythucha marmorata form clusters on the undersides of host plant leaves, as frequently observed for Hemiptera, the adults are scattered in the vicinity of the nymph population. By investigating the biological activities of volatile secretions from the adult, we found that the secretions activated male mounting behaviour. A chemical analysis revealed that borneol was a common component of the secretions from both sexes. The absolute configuration of the natural product was the (+)-enantiomer of borneol and the optical isomer was undetectable. Although (+)-borneol showed significant sex pheromone activity against males, the antipode (−)-borneol also induced sex pheromone activity, albeit only slightly. Males may not have a strict identification mechanism based on stereochemistry. To verify the origin of this sex pheromone, we analysed the components of the essential oil of the leaves of Solidago canadensis L. (Compositae: Asteraceae), a host plant; bornyl acetate was detected to be a major component. The plant-produced bornyl acetate had different stereochemistry from the sex pheromone. The results suggested that the adults do not utilise the secondary metabolites of plants but biosynthesise this sex pheromone themselves. This is the first report on sex pheromone identification in Tingidae.

Field Evaluation of Potential Pheromone Lures for Lygus lineolaris (Hemiptera: Miridae) in the Mid-South

Plant bugs (Hemiptera: Miridae) are phytophagous pests of cultivated plants around the world. In the mid-South region of the United States, Lygus lineolaris (Palisot de Beauvois) is a primary pest of cotton, and causes economic damage. Previously published research about the volatiles produced by members of the genus Lygus, and other closely related groups, indicated that they produce blends of hexyl butyrate, (E)-2-hexenyl butyrate, and (E)-4-oxo-2-hexenal. Varying ratios of the three compounds were loaded into pipette tips, and screened in combination with non-UV white sticky cards for attractiveness to field populations of L. lineolaris in Mississippi. Field screening indicated that a lure expressing a ratio of 4:10:7 was the most effective at collecting L. lineolaris, and collected similar numbers of individuals to those reported in other studies using traps baited with live virgin insects over a similar period of time. Availability of a synthetic pheromone usable in the climate of the mid-South will enable broader scale landscape level monitoring for populations of L. lineolaris before movement into cotton fields and resulting economic damage.

Central Projection of Antennal Sensory Neurons in the Central Nervous System of the Mirid Bug Apolygus lucorum (Meyer-Dür)

The mirid bug Apolygus lucorum (Meyer-Dür), a polyphagous pest, is dependent on olfactory cues to locate various host plant species and mates. In this study, we traced the projection pathway of the antennal sensory neurons and visualized their projection patterns in the central nervous system of A. lucorum through confocal microscopy and digital reconstructions. We also examined the glomerular organization of the primary olfactory center of the brain, the antennal lobe, and created a three-dimensional model of the glomeruli. We found that the axons of the sensory neurons project into the brain via the ipsilateral antennal nerve, and descend further into the gnathal ganglion, prothoracic ganglion, mesothoracic ganglion, and metathoracic ganglion, and reach as far as to the abdominal ganglion. Such a projection pattern indicates that antennal sensory neurons of A. lucorum may be potentially directly connected to motor neurons. The antennal lobe, however, is the major target area of antennal sensory neurons. The antennal lobe is composed of a large number of glomeruli, i.e. 70–80 glomeruli in one AL of A. lucorum. The results of this study which provide information about the basic anatomical arrangement of the brain olfactory center of A. lucorum, are important for further investigations of chemosensory encoding mechanisms of the mirid bug.