Expressional divergence of the fatty acid-amino acid conjugate-hydrolyzing aminoacylase 1 (L-ACY-1) in Helicoverpa armigera and Helicoverpa assulta

Consumption of gossypol increases fatty acid-amino acid conjugates in the cotton pests Helicoverpa armigera and Heliothis virescens

Gossypol is a toxic sesquiterpene dimer produced by cotton plants which deters herbivory by insects and vertebrates. Two highly reactive aldehyde groups contribute to gossypol toxicity by cross-linking herbivore proteins. We identified another consequence of consuming gossypol in two insect pests of cotton: increased amounts of fatty acid-amino acid conjugates (FACs). Eight different FACs in the feces of larval Helicoverpa armigera and Heliothis virescens increased when larvae consumed artificial diet containing gossypol, but not a gossypol derivative lacking free aldehyde groups (SB-gossypol). FACs are produced by joining plant-derived fatty acids with amino acids of insect origin in the larval midgut tissue by an unknown conjugase, and translocated into the gut lumen by an unknown transporter. FACs are hydrolyzed back into fatty acids and amino acids by an aminoacylase (L-ACY-1) in the gut lumen. The equilibrium level of FACs in the lumen is determined by a balance between conjugation and hydrolysis, which may differ among species. When heterologously expressed, L-ACY-1 of H. armigera but not H. virescens was inhibited by gossypol; consistent with the excretion of more FACs in the feces by H. armigera. FACs are known to benefit the plant host by inducing anti-herbivore defensive responses, and have been hypothesized to benefit the herbivore by acting as a surfactant and increasing nitrogen uptake efficiency. Thus in addition to its direct toxic effects, gossypol may negatively impact insect nitrogen uptake efficiency and amplify the signal used by the plant to elicit release of volatile compounds that attract parasitoids.

Induction of Heat Shock Protein Genes is the Hallmark of Egg Heat Tolerance in Agasicles hygrophila (Coleoptera: Chrysomelidae)

Insects are ecotothermic organisms. Their development, survival, reproduction as well as distribution and abundance are affected by temperature. Heat shock protein (HSP) gene expression is closely associated with temperature variation and influences the adaptation of organisms to adverse environments. The beetle Agasicles hygrophila has successfully been used for biological control of the invasive plant alligator weed (Alternanthera philoxeroides). As A. hygrophila populations are substantially inhibited by high temperatures in the summer, increasing global temperatures may limit the efficacy of this control agent. We previously established that A. hygrophila eggs have low tolerance to heat and this factored into the decreased numbers of A. hygrophila beetles at temperatures of 37.5°C and above. Here, we identified 26 HSP genes in A. hygrophila and examined the relationship between the transcript levels of these genes and heat tolerance. The temperature at which the expression of these 21 HSP genes peaked (Tpeak) was 37.5°C, which is in line with the limit of the high temperatures that A. hygrophila eggs tolerate. Therefore, we speculate that the Tpeak of HSP gene expression in eggs indicates the upper limit of temperatures that A. hygrophila eggs tolerate. This study identifies HSP genes as potential robust biomarkers and emphasizes that determining species' heat tolerance in their natural habitats remains an important consideration for biocontrol. HSP gene expression data provide information about a species' heat tolerance and may be used to predict its geographical distribution.

Molecular Cloning and Expression Profiles of Thermosensitive TRP Genes in Agasicles hygrophila

Simple Summary

The increase of hot days with temperatures over 37 °C in southern China due to global warming has led to summer collapse of the alligator weed flea beetle, an introduced biological agent for the invasive alligator weed. To promote understanding of the beetle’s adaption/tolerance to hot temperatures, we obtained TRPA1, Painless, and Pyrexia, three thermosensitive transient receptor potential channel genes from the beetle, and analyzed their expression patterns across different developmental stages and hot temperatures. Their constitutive expressions were dramatically different from each other and stage-specific. As temperature increased, their expressions in eggs elevated to their peak levels at 30 or 37.5 °C, and then fell back to their preferred temperature levels at temperatures > their peak temperatures. These results imply that (1) they may have different and stage-specific roles in perceiving high temperatures/chemicals and mediating the corresponding responses; and (2) their expressions may be decoupled from their activation. These findings lay a foundation for further understanding of the summer collapse of the beetle.

Abstract

Global warming has gradually reduced the control efficacy of Agasicles hygrophila against the invasive weed Alternanthera philoxeroides. To better understand the summer collapse of A. hygrophila populations, we cloned the cDNA sequences of the high temperature-sensing TRPA1, Painless, and Pyrexia from A. hygrophila, and analyzed their temporal expressions and the impacts of high temperatures on their expression in eggs, the most vulnerable stage of A. hygrophila to hot temperatures. All the three genes obtained had the signature domains of TRPA channels and were constitutively expressed in eggs, larvae (L1, L2, L3), pupae, and adults, but AhPainless had the highest expression, followed by AhPyrexia, and AhTRPA1. The lowest and highest expression stages were adult and pupae for AhTRPA1, egg and L3 for AhPainless, and pupae/adult and L2 for AhPyrexia. The expressions of AhTRPA1, AhPainless, and AhPyrexia remained low at the preferred temperature range of 25–28 °C, elevated to their peak levels at 37.5, 30, and 30 °C, respectively, then fell to their 25–28 °C levels (AhTRPA1, AhPainless) or a lower level (AhPyrexia) at one or more temperatures >30 or 37.5 °C. These results suggest that their temperature-sensing roles and importance may be different, stage-specific, and their expression may be decoupled from their activation.

Genome Size Reversely Correlates With Host Plant Range in Helicoverpa Species

In organisms with very low percentages of transposable elements (TEs), genome size may positively or negatively correlate with host range, depending on whether host adaptation or host modification is the main route to host generalism. To test if this holds true for insect herbivores with greater percentages of TEs, we conducted flow cytometry to measure the endopolyploidy levels and C-values of the host modification (salivary gland and mandibular gland in head), host adaptation (midgut), and host use-independent tissues (male gonad, hemolymph, and Malpighian tubules) of the generalist Helicoverpa armigera and the head of its older specialist sister H. assulta. Larval salivary gland displayed a consecutive chain of endopolyploidy particles from 8Cx to higher than 32Cx and larval head and midgut had endopolyploidy nuclei clusters of 16Cx and 32Cx, whereas larval male gonad, hemolymph, and Malpighian tubules possessed no endopolyploidy nuclei of higher than 8Cx. The estimated genome size of the Solanaceae plant specialist H. assulta is 430 Mb, significantly larger than that of its older generalist sister Heliothis virescens (408 Mb) and those of its two generalist descendants H. armigera (394 Mb) and H. zea (363 Mb). These data not only reveal a negative correlation between host plant range and genome size in this terminal lineage, but also imply that Helicoverpa species appear to depend more on host modification than on host adaptation to achieve polyphagy.