Quantitative RT-PCR analysis of pacifastin-related precursor transcripts during the reproductive cycle of solitarious and gregarious desert locusts

Comparative analysis of phenotypic plasticity sheds light on the evolution and molecular underpinnings of locust phase polyphenism

Locusts exhibit one of nature's most spectacular examples of complex phenotypic plasticity, in which changes in density cause solitary and cryptic individuals to transform into gregarious and conspicuous locusts forming large migrating swarms. We investigated how these coordinated alternative phenotypes might have evolved by studying the Central American locust and three closely related non-swarming grasshoppers in a comparative framework. By experimentally isolating and crowding during nymphal development, we induced density-dependent phenotypic plasticity and quantified the resulting behavioural, morphological, and molecular reaction norms. All four species exhibited clear plasticity, but the individual reaction norms varied among species and showed different magnitudes. Transcriptomic responses were species-specific, but density-responsive genes were functionally similar across species. There were modules of co-expressed genes that were highly correlated with plastic reaction norms, revealing a potential molecular basis of density-dependent phenotypic plasticity. These findings collectively highlight the importance of studying multiple reaction norms from a comparative perspective.

There is no magic bullet: the importance of testing reference gene stability in RT-qPCR experiments across multiple closely related species

Reverse Transcriptase quantitative Polymerase Chain Reaction (RT-qPCR) is the current gold standard tool for the study of gene expression. This technique is highly dependent on the validation of reference genes, which exhibit stable expression levels among experimental conditions. Often, reference genes are assumed to be stable a priori without a rigorous test of gene stability. However, such an oversight can easily lead to misinterpreting expression levels of target genes if the references genes are in fact not stable across experimental conditions. Even though most gene expression studies focus on just one species, comparative studies of gene expression among closely related species can be very informative from an evolutionary perspective. In our study, we have attempted to find stable reference genes for four closely related species of grasshoppers (Orthoptera: Acrididae) that together exhibit a spectrum of density-dependent phenotypic plasticity. Gene stability was assessed for eight reference genes in two tissues, two experimental conditions and all four species. We observed clear differences in the stability ranking of these reference genes, both between tissues and between species. Additionally, the choice of reference genes clearly influenced the results of a gene expression experiment. We offer suggestions for the use of reference genes in further studies using these four species, which should be taken as a cautionary tale for future studies involving RT-qPCR in a comparative framework.

A shrimp pacifastin light chain-like inhibitor: molecular identification and role in the control of the prophenoloxidase system

Pacifastin is a recently classified family of serine proteinase inhibitors that play essential roles in various biological processes, including in the regulation of the melanization cascade. Here, a novel pacifastin-related gene, termed PmPacifastin-like, was identified from a reverse suppression subtractive hybridization (SSH) cDNA library created from hemocytes of the prophenoloxidase PmproPO1/2 co-silenced black tiger shrimp Penaeus monodon. The full-length sequences of PmPacifastin-like and its homologue LvPacifastin-like from the Pacific white shrimp Litopenaeus vannamei were determined. Sequence analysis revealed that both sequences contained thirteen conserved pacifastin light chain domains (PLDs), followed by two putative kunitz domains. Expression analysis demonstrated that the PmPacifastin-like transcript was expressed in all tested shrimp tissues and larval developmental stages, and its expression responded to Vibrio harveyi challenge. To gain insight into the functional roles of PmPacifastin-like protein, the in vivo RNA interference experiment was employed; the results showed that PmPacifastin-like depletion strongly increased PO activity. Interestingly, suppression of PmPacifastin-like also down-regulated the expression of the proPO-activating enzyme PmPPAE2 transcript; the PmPacifastin-like transcript was down-regulated after the PmproPO1/2 transcripts were silenced. Taken together, these results suggest that PmPacifastin-like is important in the shrimp proPO system and may play an essential role in shrimp immune defense against bacterial infection. These results also expand the knowledge of how pacifastin-related protein participates in the negative regulation of the proPO system in shrimp.

The first homolog of pacifastin-related precursor in the swimming crab (Portunus trituberculatus): characterization and potential role in immune response to bacteria and fungi

Among all the serine proteinase inhibitor families (SPIs), the pacifastin-related inhibitor is seldom isolated. A pacifastin-related SPI named as PtPLC was identified from the swimming crab Portunus trituberculatus in this study. The full-length of PtPLC was cloned from haemocytes cDNA library by the combination of homology cloning and rapid amplification of cDNA ends (RACE). The PtPLC contained an open reading frame (ORF) of 1098 bp encoding a putative pacifastin-related precursor of 365 amino acids, a 5'-untranslated region (UTR) of 33 bp, and a 3'-UTR of 524 bp. The estimated molecular weight of mature PtPLC was 40.51 kDa and its isoelectric point was 5.04. Eight PLD domains in PtPLC shared a common characteristic of conserved array of six cysteine residues (Cys-Xaa(9-12)-Cys-Asn-Xaa-Cys-Xaa- Cys-Xaa(2-3)-Gly-Xaa(3-4)-Cys-Thr-Xaa(3)-Cys). The mRNA expression of PtPLC transcripts was highly detected in haemocytes, gill, hepatopancreas and stomach. The temporal expression levels of PtPLC transcripts in haemocytes showed different expression patterns after challenged by Gram-negative bacteria Vibrio alginolyticus, Gram-positive bacteria Micrococcus luteus and fungus Pichia pastoris. There were two peaks in the mRNA expression profile after M. luteus stimulation. And after V. alginolyticus and P. pastoris stimulation, there were three peaks in the mRNA expression profiles. These findings suggest that PtPLC is involved in the antibacterial defense mechanism of P. tritubercualtus.

Microarray-based transcriptomic analysis of differences between long-term gregarious and solitarious desert locusts

Desert locusts (Schistocerca gregaria) show an extreme form of phenotypic plasticity and can transform between a cryptic solitarious phase and a swarming gregarious phase. The two phases differ extensively in behavior, morphology and physiology but very little is known about the molecular basis of these differences. We used our recently generated Expressed Sequence Tag (EST) database derived from S. gregaria central nervous system (CNS) to design oligonucleotide microarrays and compare the expression of thousands of genes in the CNS of long-term gregarious and solitarious adult desert locusts. This identified 214 differentially expressed genes, of which 40% have been annotated to date. These include genes encoding proteins that are associated with CNS development and modeling, sensory perception, stress response and resistance, and fundamental cellular processes. Our microarray analysis has identified genes whose altered expression may enable locusts of either phase to deal with the different challenges they face. Genes for heat shock proteins and proteins which confer protection from infection were upregulated in gregarious locusts, which may allow them to respond to acute physiological challenges. By contrast the longer-lived solitarious locusts appear to be more strongly protected from the slowly accumulating effects of ageing by an upregulation of genes related to anti-oxidant systems, detoxification and anabolic renewal. Gregarious locusts also had a greater abundance of transcripts for proteins involved in sensory processing and in nervous system development and plasticity. Gregarious locusts live in a more complex sensory environment than solitarious locusts and may require a greater turnover of proteins involved in sensory transduction, and possibly greater neuronal plasticity.

The first pacifastin elastase inhibitor characterized from a blood sucking animal

Pacifastin-like protease inhibitors belong to a recent classified protease inhibitor family and they are the smallest protease inhibitors described in animals. In this work, we purified and characterized, for the first time, two neutrophil elastase inhibitors belonging to the pacifastin family from the blood sucking insect Triatoma infestans eggs. The inhibitors showed the same N-terminal sequences, molecular masses of 4257 and 4024Da by MALDI-TOF mass spectrometry and dissociation constants (Ki) for neutrophil elastase of 0.52 and 0.29nM, respectively. Using a fat body cDNA library, we cloned a pacifastin precursor containing two protease inhibitor domains similar to locust pacifastins. The first pacifastin domain translated to T. infestans purified protein, named TIPI1. Recombinant TIPI1 expressed in Pichia pastoris system showed similar inhibitory activities compared to the native inhibitor. Its precursor, called TiPP1, is mainly expressed in fat body, and it is up-regulated after blood feeding. The immune challenges of 1(a) instar T. infestans nymph with bacteria or dsRNA strongly stimulated TiPP1 expression in fat body, suggesting a possible role of TiPP1 in T. infestans immunity. This work is the first to characterize a blood feeding insect pacifastin inhibitor.

A lepidopteran pacifastin member: cloning, gene structure, recombinant production, transcript profiling and in vitro activity

Members of the pacifastin family have been characterized as serine peptidase inhibitors (PI), but their target enzyme(s) are unknown in insects. So far, the structural and biochemical characteristics of pacifastin-like PI have only been studied in locusts. Here we report the molecular identification and functional characterization of a pacifastin-like precursor in a lepidopteran insect, i.e. the silkworm Bombyx mori. The bmpp-1 gene contains 17 exons and codes for two pacifastin-related precursors of different length. The longest splice variant encodes 13 inhibitor domains, more than any other pacifastin-like precursor in arthropods. The second transcript lacks two exons and codes for 11 inhibitor domains. By studying the expression profile of the Bombyx pacifastin-like gene a different expression pattern for the two variants was observed suggesting functional diversification. Next, several PI domains of BMPP-1 were produced and, contrary to locust pacifastin peptides, they were found to be potent inhibitors of both bovine trypsin and chymotrypsin. Surprisingly, the same Bombyx PI are only weak inhibitors of endogenous digestive peptidases, indicating that other peptidases are the in vivo targets. Interestingly, the Bombyx PI inhibit a fungal trypsin-like cuticle degrading enzyme, suggesting a protective function for BMPP-1 against entomopathogenic fungi.

Identification and validation of housekeeping genes in brains of the desert locust Schistocerca gregaria under different developmental conditions

BACKGROUND

To obtain reliable quantitative RT-PCR data, normalization relative to stable housekeeping genes is required. However, in practice, expression levels of 'typical' housekeeping genes have been found to vary between tissues and under different experimental conditions. To date, validation studies of reference genes in insects are extremely rare and have never been performed in locusts. In this study, putative housekeeping genes were identified in the desert locust, Schistocerca gregaria and two different software programs (geNorm and Normfinder) were applied to assess the stability of these genes.

RESULTS

We have identified seven orthologs of commonly used housekeeping genes in the desert locust. The selected genes were the orthologs of actin, EF1a, GAPDH, RP49, TubA1, Ubi, and CG13220. By employing real time RT-PCR we have analysed the expression of these housekeeping genes in brain tissue of fifth instar nymphs and adults. In the brain of fifth instar nymphs geNorm indicated Sg-EF1a, Sg-GAPDH and Sg-RP49 as most stable genes, while Normfinder ranked Sg-RP49, Sg-EF1a and Sg-ACT as most suitable candidates for normalization. The best normalization candidates for gene expression studies in the brains of adult locusts were Sg-EF1a, Sg-GAPDH and Sg-Ubi according to geNorm, while Normfinder determined Sg-GAPDH, Sg-Ubi and Sg-ACT as the most stable housekeeping genes.

CONCLUSION

To perform transcript profiling studies on brains of the desert locust, the use of Sg-RP49, Sg-EF1a and Sg-ACT as reference genes is proposed for studies of fifth instar nymphs. In experiments with adult brains, however, the most preferred reference genes were Sg-GAPDH, Sg-Ubi and Sg-EF1a. These data will facilitate transcript profiling studies in desert locusts and provide a good starting point for the initial selection of genes for validation studies in other insects.

Endocrinology of reproduction and phase transition in locusts

In the last decade, important progress has been made in the experimental analysis of the endocrine mechanisms controlling reproduction and phase transition in locusts. Phase transition is a very fascinating, but complex, phenomenon of phenotypic plasticity that is triggered by changes in population density and can lead to the formation of extremely devastating hopper bands and adult gregarious locust swarms. While some phase characteristics change within hours, others appear more gradually in the next stage(s), or even in the next generation(s). In adults, the phase status also has a major influence on the process of reproduction. A better understanding of how solitarious locusts become gregarious and how this switch affects reproductive physiology may result in novel strategies to fight locust plagues. In this paper, we will review the current knowledge concerning this close interaction between locust phase polyphenism and reproduction.

Identification, distribution and molecular evolution of the pacifastin gene family in Metazoa

BACKGROUND

Members of the pacifastin family are serine peptidase inhibitors, most of which are produced as multi domain precursor proteins. Structural and biochemical characteristics of insect pacifastin-like peptides have been studied intensively, but only one inhibitor has been functionally characterised. Recent sequencing projects of metazoan genomes have created an unprecedented opportunity to explore the distribution, evolution and functional diversification of pacifastin genes in the animal kingdom.

RESULTS

A large scale in silico data mining search led to the identification of 83 pacifastin members with 284 inhibitor domains, distributed over 55 species from three metazoan phyla. In contrast to previous assumptions, members of this family were also found in other phyla than Arthropoda, including the sister phylum Onychophora and the 'primitive', non-bilaterian Placozoa. In Arthropoda, pacifastin members were found to be distributed among insect families of nearly all insect orders and for the first time also among crustacean species other than crayfish and the Chinese mitten crab. Contrary to precursors from Crustacea, the majority of insect pacifastin members contain dibasic cleavage sites, indicative for posttranslational processing into numerous inhibitor peptides. Whereas some insect species have lost the pacifastin gene, others were found to have several (often clustered) paralogous genes. Amino acids corresponding to the reactive site or involved in the folding of the inhibitor domain were analysed as a basis for the biochemical properties.

CONCLUSION

The absence of the pacifastin gene in some insect genomes and the extensive gene expansion in other insects are indicative for the rapid (adaptive) evolution of this gene family. In addition, differential processing mechanisms and a high variability in the reactive site residues and the inner core interactions contribute to a broad functional diversification of inhibitor peptides, indicating wide ranging roles in different physiological processes. Based on the observation of a pacifastin gene in Placozoa, it can be hypothesized that the ancestral pacifastin gene has occurred before the divergence of bilaterian animals. However, considering differences in gene structure between the placozoan and other pacifastin genes and the existence of a 'pacifastin gene gap' between Placozoa and Onychophora/Arthropoda, it cannot be excluded that the pacifastin signature originated twice by convergent evolution.

Pacifastin-related peptides: structural and functional characteristics of a family of serine peptidase inhibitors

Members of the pacifastin family are serine peptidase inhibitors, found in arthropods and have many members within different insect orders. Based on their structural characteristics, inhibitors of this peptide family are divided into two groups (I and II). Members of both groups exhibit specificity towards different types of serine peptidases. In addition, group I inhibitors display species selectivity. The specificity and selectivity of these inhibitors depends on the nature of their P1 residue and on additional interaction sites at the inhibitor's surface. Functional analysis studies have shown that crustacean pacifastin plays a key role in the immune response, whereas insect pacifastin-like peptides have multiple regulatory functions in processes involved in immunity, reproduction, phase transition, etc.