Influence of Pyriproxyfen on the Expression of Haemolymph Protein Genes in the Colorado Potato Beetle, Leptinotarsa decemlineata

Is a change in juvenile hormone sensitivity involved in range expansion in an invasive beetle?

Introduction

It has been suggested that rapid range expansion could proceed through evolution in the endocrinological machinery controlling life-history switches. Based on this we tested whether the Colorado potato beetle, Leptinotarsa decemlineata, which has rapidly expanded its range across latitudinal regions in Europe, and shows photoperiodic adaptation in overwintering initiation, has different sensitivities to juvenile hormone (JH) manipulation along a latitudinal gradient.

Results

A factorial experiment where beetles were reared either under a long or short day photoperiod was performed. Hormone levels were manipulated by topical applications. An allatostatin mimic, H17, was used to decrease and a juvenile hormone III analogue, pyriproxyfen, was used to increase the hormone levels. The effects of photoperiod and hormone manipulations on fecundity and overwintering related burrowing were monitored. Application of H17 decreased fecundity but did not induce overwintering related burrowing. Manipulation with pyriproxyfen increased fecundity and delayed burrowing. While small population-dependent differences in responsiveness to the topical application treatments were observed in fecundity, none were seen in overwintering related burrowing.

Conclusions

The results indicate that the rapid photoperiodic adaptation manifested in several life-history and physiological traits in L. decemlineata in Europe is unlikely a result of population dependent differences in JH III sensitivity. While other endocrine factors cannot be ruled out, more likely mechanisms could be genetic changes in upstream elements, such as the photoperiodic clock or the insulin signaling pathway.

RNA interference of a putative S-adenosyl-L-homocysteine hydrolase gene affects larval performance in Leptinotarsa decemlineata (Say)

In Leptinotarsa decemlineata, juvenile hormones (JHs) play primary roles in the regulation of metamorphosis, reproduction and diapause. In JH biosynthetic pathway in insect corpora allata, methylation of farnesoic acid or JH acid using S-adenosyl-L-methionine generates a potent feedback inhibitor S-adenosyl-L-homocysteine (AdoHcy). Rapid removal of AdoHcy is hypothesized to be essential for JH synthesis. AdoHcy hydrolase (SAHase) is the only eukaryotic enzyme catalyzing the removal. In the present paper, we firstly cloned a putative LdSAHase gene from L. decemlineata. The cDNA consists of 1806 bp and encodes a 525 amino acid protein. LdSAHase was expressed in all developmental stages. The gene had the highest and the lowest level of transcription respectively in the 3rd- and 4th-instars' heads that contain corpora allata, which was positively correlated with JH titer in the haemolymph and the mRNA level of a JH early-inducible gene, the Krüppel homolog 1 gene (Kr-h1). Secondly, dietary ingestion of bacterially-expressed LdSAHase-dsRNA significantly decreased LdSAHase and LdKr-h1 mRNA levels, reduced JH titer, and caused the death of the larvae, and the failure of pupation and adult emergence. After continuous exposure for 12 days, 42% of the larvae died, 65% of the prepupae failed to pupate and 100% of the pupae failed to emerge. Moreover, RNAi-mediated LdSAHase knockdown also reduced larval developing time, and decreased larval weight. Lastly, application of JH analogue pyriproxyfen to LdSAHase-dsRNA-exposed larvae did not greatly increase LdSAHase expression level and JH content, but up-regulated LdKr-h1 mRNA level. Expectedly, pyriproxyfen application could partially rescue the negative effects on the survival and the development. Thus, our results support the hypothesis that SAHase plays a critical role in JH biosynthesis in insects.

Monitoring diapause development in the Colorado potato beetle, Leptinotarsa decemlineata, under field conditions using molecular biomarkers

A multiplex PCR protocol was developed using five diapause-regulated genes to monitor diapause development of the Colorado potato beetle under field conditions. A total of 870 beetles from the Red River valley of North Dakota and Minnesota, USA, were screened for three consecutive years. Out of the 32 possible expression profiles, eight could be arranged in chronological order of diapause development. These eight profiles account for over 92% of the beetles surveyed. Intra-population variation in diapause phenotypes was observed in the field. Some beetles were already in the diapause initiation phase in June when the day length was greater than 17 h. Inter-seasonal variation in the timing of diapause development was also noted. The greatest differences were before the day length decreased to less than 15 h. Anomalies in the results, e.g., the presence of the diapause maintenance phase profiles in beetles collected on the potato plants, argue that laboratory results are not always equivalent with what is observed under field conditions.

Isolation and characterization of three diapause-associated transcripts from the Colorado potato beetle, Leptinotarsa decemlineata

Using suppressive subtractive hybridization, fragments of three diapause-associated transcripts (DAT-1, 2 and 3) were isolated from the Colorado potato beetle, Leptinotarsa decemlineata. Full-length clones were developed for the transcripts. DAT-1 encodes a deduced protein 286 amino acids in length with limited identity to several proteins with leucine-rich domains. DAT-2 encodes a deduced protein 229 amino acids in length with 27% identity, 40% similarity to the desiccation stress protein from Tenebrio molitor. DAT-3 encodes a deduced protein 97 amino acids in length with identity to no known protein. DAT-1 and 2 have similar expression patterns as determined by northern blot analysis. Trace levels of these two transcripts are first detected in 3-day-old diapause-programmed adults with a significant increase in expression on day 6. Expression of DAT-3 begins on day 12 in diapause-programmed adults and expression levels increase until the beetles enter diapause. Expression of DAT-1, 2 and 3 continues at least 60 days into diapause.

Age- and diapause-related acid and alkaline phosphatase activities in the intestine and malpighian tubules of the Colorado potato beetle, Leptinotarsa decemlineata (Say)

Specific activities for soluble (s) and membrane (m)-bound acid (ACP) and alkaline phosphatases (ALP) were determined in the midgut, hindgut, and Malpighian tubules for developing, prediapausing, and diapausing adult Colorado potato beetles, Leptinotarsa decemlineata (Say). High ACP activities were found in the hindgut and Malpighian tubules while high ALP activities were found in the Malpighian tubules. Variation in both ACP and ALP activities in each tissue reflects fluctuation in protein synthesis and secretion involved with digestion, excretion, and other unknown functions. Phosphatase activities in the tissues examined show the dynamic nature of diapause in this insect. Diapausing beetles showed increases in phosphatase activity after hormone treatments. JHA treatments increased s-ACP and m-ACP activities in all tissues but 20-HE did not increase activity in any tissue. Allatotropin tended to mimic the effects of JHA treatment. The s-ALP activity was also increased in all tissues whereas m-ALP was increased in the midgut and hindgut by JHA treatment. Malpighian tubule m-ALP activity was only increased by 20-HE treatments. Allatotropin was not as effective in increasing ALP activities as it was with ACP activities.

Effect of pyriproxyfen and photoperiod on free amino acid concentrations and proteins in the hemolymph of the Colorado potato beetle, Leptinotarsa decemlineata (Say)

A North Dakota strain of the Colorado potato beetle, Leptinotarsa decemlineata (Say), was reared under both short- (8L:16D) and long-day (17L:7D) conditions. Age-related and pyriproxyfen- (JHA-) induced changes in hemolymph free amino acids and proteins were examined. Under a short-day photoperiod, the total free amino acid concentration in the hemolymph increased gradually up to 20 days of adult life, but the long-day beetles showed marked increases during the first 10 days and then decreased afterwards. Proline, glutamine and valine were the most abundant free amino acids in both sexes of beetles held under either short- or long-day photoregims. JHA treatment of diapausing adults, held under either short- or long-day conditions after treatment, terminated diapause as indicated by re-emergence from the vermiculite, feeding, mating, changes in free amino acid levels, the disappearance of diapause protein 1 and appearance of vitellogenin in the hemolymph. Furthermore, most of the JHA-treated females held under long-day conditions also matured oocytes and oviposited, but those held under short-day conditions did not.

Expression of the juvenile hormone esterase gene in the Colorado potato beetle, Leptinotarsa decemlineata: photoperiodic and juvenile hormone analog response

Metamorphosis and reproduction in insects are controlled by juvenile hormone (JH). One of the factors, which regulate the JH titer in the hemolymph, is the activity of juvenile hormone esterase (JHE). JHE from the Colorado potato beetle, Leptinotarsa decemlineata, consists of two 57kDa subunits. In this study, the JHE-cDNA was used as a probe to examine where and when the gene is transcribed as well as how gene expression responds to photoperiodic treatment and to topical application with a JH analog, pyriproxyfen. JHE transcripts were almost exclusively found in RNA extracts from fat body tissue in both larvae and adults. JHE-mRNA levels in the fat body correlated positively with levels of JHE activity in the hemolymph. In the last larval instar, high levels of JHE-mRNA were found in the feeding stage. In adults, reared under short-day conditions, JHE-mRNA levels were high between day 2 and day 9, which correlated with high JHE activity in the hemolymph. During these conditions, the JH titer decreases in preparation for pupation and diapause, respectively. The JHE-mRNA levels and JHE activity in the hemolymph were higher in short-day than in reproductive long-day adults. If the JH analog pyriproxyfen was applied to animals of the last larval instar on day 0 or day 3, JHE gene expression was enhanced. In contrast, if pyriproxyfen was applied to short-day adults on day 1 or day 4, the mRNA levels and the JHE activity in the hemolymph were suppressed to levels similar to those found in long-day adults. Thus, transcription of JHE is dependent on developmental stage, tissue, photoperiod and the level of its substrate JH.