Gene expression during blow fly development: improving the precision of age estimates in forensic entomology

Establishment of a multiplex PCR-CE assay for the simultaneous and rapid analysis of age markers for Calliphora vicina pupae

Insects, especially blow flies, are forensically relevant to determine the minimal postmortem interval (PMImin), based on the fact that they are usually the first colonisers of a body. By estimating the age of immature blow flies, interferences can be made about the time since death. Whilst morphological parameters are valuable for age estimation of blow fly larvae, gene expression profiling is more applicable for blow fly pupae. Here, the age-dependent changes in the gene expression levels during the development are analysed. 28 temperature-independent markers have already been described for the age estimation of pupae of the forensically important blow fly Calliphora vicina and are analysed by RT-qPCR. To allow simultaneous analysis of these age markers, a multiplex assay was developed in the present study. After reverse transcription, the markers are analysed simultaneously in an endpoint PCR and subsequently separated by capillary electrophoresis (CE). This method is highly attractive due to its quick and easy procedure and interpretation. The present age prediction tool was adapted and validated. The multiplex PCR assay reproduced the same expression profiles as the RT-qPCR assay based on the same markers. The statistical evaluation shows that the new assay has a lower precision but a better trueness for age determination compared to the RT-qPCR assay. Since the new assay is also qualified to estimate the age of C. vicina pupae and is practical, cost-effective and, even more importantly, time-saving, it is attractive for use in forensic casework.

Estimating the intra-puparial period of Hydrotaea spinigera (Stein,1910) (Diptera: Muscidae) with morphological and gene expression changes

Hydrotaea spinigera (Stein, 1910) (Diptera: Muscidae) is a forensically important sarcosaprophagous species widely distributed throughout the Oriental and Australasian regions. At the advanced decomposition stage or the skeletonize stage, the immature stages of H. spinigera, especially the pupae, can still be found in large quantities and could be used as important indicators to estimate the minimum postmortem interval (PMI_min). However, there have been no studies on the intra-puparial period of this species. Herein, we studied morphological and differential gene expression changes during the intra-puparial development of H. spinigera, aiming to accurately estimate the intra-puparial age of H. spinigera. The intra-puparial morphological changes of H. spinigera were observed at seven constant temperatures ranging from 16°C to 34°C and divided into 12 sub-stages. Structures that could be used to estimate the intra-puparial age, such as compound eyes, mouthparts, antennae, thorax, legs, wings, and abdomen, were observed in detail, and the developmental process of each structure was divided into 5 to 10 stages. The time range of each sub-stage, or when a structure appeared, was recorded. For the gene expression section, the most suitable reference genes were screened by geNorm, NormFinder, BestKeeper, and ΔCt methods. Based on the selected reference genes, real-time quantitative PCR (RT qPCR) was used to detect the expression changes of the white and hsp90 genes with developmental time at 19°C, 25°C, and 31°C. Results showed that the trend of Hsp90 gene expression under different temperatures was not consistent, while white genes exhibited regular changes during development, and could thus be used for age estimation of H. spinigera. This study provides an important basis for forensic entomology to use morphological and differential gene expression for estimating the age of H. spinigera during the intra-puparial period. Moreover, the combination of the two methods can produce a more accurate minimum postmortem interval (PMI_min) estimate compared to when each method is used separately.

Multimethod combination for age estimation of Sarcophaga peregrina (Diptera: Sarcophagidae) with implications for estimation of the postmortem interval

Sarcophaga peregrina (Robineau-Desvoidy, 1830) (Diptera: Sarcophagidae) is a forensically important flesh fly with potential value for estimating the minimum postmortem interval (PMI_min). Here, the developmental patterns of S. peregrina were investigated at 5 constant temperatures (15-35 °C). Morphological changes at different developmental stages of the pupa were observed at 4 constant temperatures (15-30 °C) by removing the puparium and staining the pupa with hematoxylin and eosin. Furthermore, differentially expressed genes (DEGs) were analyzed at 25 °C in the intrapuparial period to estimate the age of S. peregrina during the intrapuparial stage. S. peregrina completed development from larviposition to adult eclosion at 15 °C, 20 °C, 25 °C, and 30 °C; the developmental durations were 1090.3 ± 30.6 h, 566.6 ± 21.9 h, 404.6 ± 13.01 h, and 280.3 ± 4.5 h, respectively, while the development could not be completed at 35 °C. The intrapuparial period of S. peregrina was divided into 12 sub-stages on the basis of the overall external morphological changes; 6 sub-stages on the basis of individual morphological structures such as the compound eyes, antennae, thorax, legs, wings, and abdomen; and 10 sub-stages on the basis of internal morphological changes detected using histological analysis. The period of each sub-stage or structure that appeared was determined. Moreover, we found that 6 genes (NDUFS2, CPAMD8, NDUFV2, Hsp27, Hsp23, and TPP) with differential expression can be used for the precise age estimation of S. peregrina during the intrapuparial period. This study provided basic developmental data for the use of S. peregrina in PMI_min estimation, and we successfully estimated PMI_min in a real forensic case by using a multimethod combination.

Identification and Characterization of Small RNA Markers of Age in the Blow Fly Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae)

Blow fly development is important in decomposition ecology, agriculture, and forensics. Much of the impact of these species is from immature samples, thus knowledge of their development is important to enhance or ameliorate their effects. One application of this information is the estimation of immature insect age to provide temporal information for death investigations. While traditional markers of age such as stage and size are generally accurate, they lack precision in later developmental stages. We used miRNA sequencing to measure miRNA expression, throughout development, of the secondary screwworm, Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae) and identified 217 miRNAs present across the samples. Ten were identified to be significantly differentially expressed in larval samples and seventeen were found to be significantly differentially expressed in intrapuparial samples. Twenty-eight miRNAs were identified to be differentially expressed between sexes. Expression patterns of two miRNAs, miR-92b and bantam, were qPCR-validated in intrapuparial samples; these and likely food-derived miRNAs appear to be stable markers of age in C. macellaria. Our results support the use of miRNAs for developmental markers of age and suggest further investigations across species and under a range of abiotic and biotic conditions.

Temporal Expression Profiles Reveal Potential Targets during Postembryonic Development of Forensically Important Sarcophaga peregrina (Diptera: Sarcophagidae)

Sarcophaga peregrina (Robineau-Desvoidy, 1830) is a species of medical and forensic importance. In order to investigate the molecular mechanism during postembryonic development and identify specific genes that may serve as potential targets, transcriptome analysis was used to investigate its gene expression dynamics from the larval to pupal stages, based on our previous de novo-assembled genome of S. peregrina. Totals of 2457, 3656, 3764, and 2554 differentially expressed genes were identified. The specific genes encoding the structural constituent of cuticle were significantly differentially expressed, suggesting that degradation and synthesis of cuticle-related proteins might actively occur during metamorphosis. Molting (20-hydroxyecdysone, 20E) and juvenile (JH) hormone pathways were significantly enriched, and gene expression levels changed in a dynamic pattern during the developmental stages. In addition, the genes in the oxidative phosphorylation pathway were significantly expressed at a high level during the larval stage, and down-regulated from the wandering to pupal stages. Weighted gene co-expression correlation network analysis (WGCNA) further demonstrated the potential regulation mechanism of tyrosine metabolism in the process of puparium tanning. Moreover, 10 consistently up-regulated genes were further validated by qRT-PCR. The utility of the models was then examined in a blind study, indicating the ability to predict larval development. The developmental, stage-specific gene profiles suggest novel molecular markers for age prediction of forensically important flies.

Characterized Gene Repertoires and Functional Gene Reference for Forensic Entomology: Genomic and Developmental Transcriptomic Analysis of Aldrichina grahami (Diptera: Calliphoridae)

Many flies of Diptera are common entomological evidence employed in forensic investigation. Exploring the existence of inter- and intra-species genomic differences of forensically relevant insects is of great importance. Aldrichina grahami is a common blow fly species of forensic importance. The present study characterized the gene repertoires of A. grahami, and provides insights into issues related to forensic entomology, such as necrophagous behavior, gene family features, and developmental patterns. Gene families were clustered and classified according to their function in different aspects of the necrophagous lifestyle, generating several gene repertoires. The genes under positive selection pressure and evolutionary changes were screen and identified. Moreover, genes that exhibited potential prediction value in the post mortem interval (PMI) estimation and development of immature stages were subjected to analysis based on the developmental transcriptome. Related insect species were compared at the genomic level to reveal the genes associated with necrophagous behaviors. The expression of selected genes in separated repositories was verified using qPCR. This work was conducted using a high-quality chromosome-level genome assembly of A. grahami and its developmental transcriptome. The findings will facilitate future research on A. grahami and the other forensically important species.

Intrapuparial Development and Age Estimation of Calliphora grahami (Diptera: Calliphoridae) for Postmortem Interval Estimation

Calliphora grahami (Aldrich, 1930) (Diptera: calliphoridae) is a forensically important blow fly that is widely distributed across Asia, North America, Russia, and Mexico. Calliphora grahami is frequently found on corpses during the spring, autumn, and winter seasons. It is among the early colonizers of cadavers during the cold season, and sometimes, the only necrophagous blow fly on cadavers. Therefore, this species is of forensic significance, although very few studies have explored the application of its intrapuparial age for PMI estimation. This study aimed to examine the intrapuparial development of C. grahami and establish a method for estimating its intrapuparial age. Herein, the C. grahami puparia were studied under six different temperatures (13, 16, 19, 22, 25, and 28°C) for the intrapuparial age estimation, and a total of 5776 puparia were sampled. The morphological changes were divided into 11 stages based on the 1) development of legs and wings, 2) differentiation of the head, thorax, and abdomen, 3) growth and color of the bristles, and 4) color changes of the compound eyes. The corresponding time of each stage was determined. Moreover, the observation and classification of individual morphological features, including compound eyes, antennae, mouthparts, thorax, abdomen, legs, and wings was used to improve the precision of intrapuparial age estimation. The findings of this study provide important information on the use of C. grahami intrapuparia to estimate the minimum postmortem interval (PMImin).

Research Progress on Developmental Biology of Sarcosaprophagous Insects

Forensic entomology provides a feasible way to estimate postmortem interval (PMI), of which the growth and development of sarcosaprophagous insects is the most widely used indicator in forensic practice. Over the years, forensic entomologists have carried out a large number of studies on the development biology of sarcosaprophagous insects. This paper illustrates the main factors that affect the development of sarcosaprophagous insects, including temperature, humidity, light, food types and poisons. The development indicators of sarcosaprophagous insects were reviewed from the perspectives of morphology, differential gene expression and biochemical characteristics. It is emphasized that future research of development biology on sarcosaprophagous insects should fully absorb and integrate the methods of artificial intelligence and omics, and the research object also needs further expansion in order to establish a more objective and more accurate PMI estimation method.

Genome and transcriptome sequencing of the green bottle fly, Lucilia sericata, reveals underlying factors of sheep flystrike and maggot debridement therapy

The common green bottle blow fly Lucilia sericata (family, Calliphoridae) is widely used for maggot debridement therapy, which involves the application of sterile maggots to wounds. The larval excretions and secretions are important for consuming necrotic tissue and inhibiting bacterial growth in wounds of patients. Lucilia sericata is also of importance as a pest of sheep and in forensic studies to estimate a postmortem interval. Here we report the assembly of a 565.3 Mb genome from long read PacBio DNA sequencing of genomic DNA. The genome contains 14,704 predicted protein coding genes and 1709 non-coding genes. Targeted annotation and transcriptional analyses identified genes that are highly expressed in the larval salivary glands (secretions) and Malpighian tubules (excretions) under normal growth conditions and following heat stress. The genomic resources will underpin future genetic studies and in development of engineered strains for genetic control of L. sericata and for biotechnology-enhanced maggot therapy.

Age-dependent gene expression of Calliphora vicina pupae (Diptera: Calliphoridae) at constant and fluctuating temperatures

Estimating the age of the developmental stages of the blow fly Calliphora vicina (Diptera: Calliphoridae) is of forensic relevance for the determination of the minimum post-mortem interval (PMI_min). Fly eggs and larvae can be aged using anatomical and morphological characters and their modification during development. However, such methods can only hardly be applied for aging fly pupae. Previous study described age estimation of C. vicina pupae using gene expression, but just when reared at constant temperatures, but fluctuating temperatures represent a more realistic scenario at a crime scene. Therefore, age-dependent gene expression of C. vicina pupae were compared at 3 fluctuating and 3 constant temperatures, the latter representing the mean values of the fluctuating profiles. The chosen marker genes showed uniform expression patterns during metamorphosis of C. vicina pupae bred at different temperature conditions (constant or fluctuating) but the same mean temperature (e.g. constant 10 °C vs. fluctuating 5–15 °C). We present an R-based statistical tool, which enables estimation of the age of the examined pupa based on the analysed gene expression data.

Sexual Dimorphism in Growth Rate and Gene Expression Throughout Immature Development in Wild Type Chrysomya rufifacies (Diptera: Calliphoridae) Macquart

Reliability of forensic entomology analyses to produce relevant information to a given case requires an understanding of the underlying arthropod population(s) of interest and the factors contributing to variability. Common traits for analyses are affected by a variety of genetic and environmental factors. One trait of interest in forensic investigations has been species-specific temperature-dependent growth rates. Recent work indicates sexual dimorphism may be important in the analysis of such traits and related genetic markers of age. However, studying sexual dimorphic patterns of gene expression throughout immature development in wild-type insects can be difficult due to a lack of genetic tools, and the limits of most sex-determination mechanisms. Chrysomya rufifacies, however, is a particularly tractable system to address these issues as it has a monogenic sex determination system, meaning females have only a single-sex of offspring throughout their life. Using modified breeding procedures (to ensure single-female egg clutches) and transcriptomics, we investigated sexual dimorphism in development rate and gene expression. Females develop slower than males (9 h difference from egg to eclosion respectively) even at 30°C, with an average egg-to-eclosion time of 225 h for males and 234 h for females. Given that many key genes rely on sex-specific splicing for the development and maintenance of sexually dimorphic traits, we used a transcriptomic approach to identify different expression of gene splice variants. We find that 98.4% of assembled nodes exhibited sex-specific, stage-specific, to sex-by-stage specific patterns of expression. However, the greatest signal in the expression data is differentiation by developmental stage, indicating that sexual dimorphism in gene expression during development may not be investigatively important and that markers of age may be relatively independent of sex. Subtle differences in these gene expression patterns can be detected as early as 4 h post-oviposition, and 12 of these nodes demonstrate homology with key Drosophila sex determination genes, providing clues regarding the distinct sex determination mechanism of C. rufifacies. Finally, we validated the transcriptome analyses through qPCR and have identified five genes that are developmentally informative within and between sexes.

Evaluation of Development Datasets for Hermetia illucens (L.) (Diptera: Stratiomyidae) for Estimating the Time of Placement of Human and Swine Remains in Texas, USA

A basic tenet of forensic entomology is development data of an insect can be used to predict the time of colonization (TOC) by insect specimens collected from remains, and this prediction is related to the time of death and/or time of placement (TOP). However, few datasets have been evaluated to determine their accuracy or precision. The black soldier fly, Hermetia illucens (L.) (Diptera: Stratiomyidae) is recognized as an insect of forensic importance. This study examined the accuracy and precision of several development datasets for the black soldier fly by estimating the TOP of five sets of human and three sets of swine remains in San Marcos and College Station, TX, respectively. Data generated from this study indicate only one of these datasets consistently (time-to-prepupae 52%; time-to-eclosion 75%) produced TOP estimations that occurred within a day of the actual TOP of the remains. It is unknown if the precolonization interval (PreCI) of this species is long, but it has been observed that the species can colonize within 6 d after death. This assumption remains untested by validation studies. Accounting for this PreCI improved accuracy for the time-to-prepupae group, but reduced accuracy in the time-to-eclosion group. The findings presented here highlight a need for detailed, forensic-based development data for the black soldier fly that can reliably and accurately be used in casework. Finally, this study outlines the need for a basic understanding of the timing of resource utilization (i.e., duration of the PreCI) for forensically relevant taxa so that reasonable corrections may be made to TOC as related to minimum postmortem interval (mPMI) estimates.

The Growth Model of Forensically Important Lucilia sericata (Meigen) (Diptera: Calliphoridae) in South Korea

Simple Summary

This study provides a detailed growth data for Lucilia sericata (Meigen) collected in South Korea. With the growth data, authors compared different minimum ADH models and found little differences. However, the logalithmic model was the best fit among differenct models.

Abstract

Development of forensically important Lucilia sericata (Meigen) was analyzed in South Korea. Rearing was replicated five times at seven constant temperatures between 20–35 °C to elucidate changes in accumulated degree hours, based on developmental stage and body length, and 2673 individuals were statistically analyzed. The results indicated that the optimum temperature, the base temperature, and the overall thermal constant were 22.31 °C (±1.21 °C, 95% CI), 9.07 °C, and 232.81 ± 23 (mean ± SD) accumulated degree days, respectively. In the minimum ADH models of each development stage, nonlinear regression graphs were parallel at the immature stages. Based on the scatter plot (n = 973) of immature stages using ADH values and body length, the logarithmic model using Log₁₀ADH as the dependent variable was identified as the best fitting regression model. Additionally, the adjusted R² value and mean square of error were 0.911 and 0.007, respectively. This is the first forensically focused study on the development of L.sericata for the estimation of minimum postmortem interval in South Korea. In future studies, we intend to study the development of other necrophagous fly species and to identify parameters for the determination of age at post-feeding and pupal stages.

Intrapuparial Age Estimation of Forensically Important Dohrniphora cornuta (Diptera: Phoridae)

Dohrniphora cornuta (Bigot) is a forensically important phorid fly indoors and in burial environments. The determination of a minimum postmortem interval (PMImin) often relies on the determination of the age of the immatures. Although the larval development data of D. cornuta under different temperatures has been established, the intrapuparial stage which lasts for about half of the total immature development is scarce. In this study, we investigated the key morphological changes during intrapuparial development at constant temperatures (15, 18, 21, 24, 27, 30, 33, and 36°C), with an aim to estimate the intrapuparial age of D. cornuta. Puparia were sampled at 12-h (24, 27, 30, and 33°C), 24-h (18 and 21°C), and 48-h (15°C) intervals. The morphological developments within the puparium were analyzed using a stereomicroscope after the puparium was removed. The average minimum duration of intrapuparial stage was inversely related to temperature, ranging from 184.79 ± 3.00 h at 30°C to 1102.86 ± 25.55 h at 15°C for female, and 197.40 ± 4.12 h at 30°C to 1175.33 ± 18.55 h at 15°C for male. It did not develop at 36°C. Some morphological traits that changed during development within the puparium could be used as age markers. According to these changes, the intrapuparial stage of D. cornuta was divided into nine stages which could be used for both sexes. This study provides relatively systematic development data of D. cornuta intrapuparial for the estimation of PMImin in forensic entomology.

Forensic Entomology in China and Its Challenges

Simple Summary

Forensic entomologists utilize sarcosaprophagous insect species to estimate the postmortem interval to aid death investigations. In this paper, we present the recent chronology of forensic entomology in China and illustrate how identification, development, and succession data are obtained and applied at the scale of such a large country. To overcome the difficulties and challenges forensic entomology faces in China, a number of countermeasures are provided.

Abstract

While the earliest record of forensic entomology originated in China, related research did not start in China until the 1990s. In this paper, we review the recent research progress on the species identification, temperature-dependent development, faunal succession, and entomological toxicology of sarcosaprophagous insects as well as common applications of forensic entomology in China. Furthermore, the difficulties and challenges forensic entomologists face in China are analyzed and possible countermeasures are presented.

Time Flies-Age Grading of Adult Flies for the Estimation of the Post-Mortem Interval

The estimation of the minimum time since death is one of the main applications of forensic entomology. This can be done by calculating the age of the immature stage of necrophagous flies developing on the corpse, which is confined to approximately 2–4 weeks, depending on temperature and species of the first colonizing wave of flies. Adding the age of the adult flies developed on the dead body could extend this time frame up to several weeks when the body is in a building or closed premise. However, the techniques for accurately estimating the age of adult flies are still in their beginning stages or not sufficiently validated. Here we review the current state of the art of analysing the aging of flies by evaluating the ovarian development, the amount of pteridine in the eyes, the degree of wing damage, the modification of their cuticular hydrocarbon patterns, and the increasing number of growth layers in the cuticula. New approaches, including the use of age specific molecular profiles based on the levels of gene and protein expression and the application of near infrared spectroscopy, are introduced, and the forensic relevance of these methods is discussed.

Evaluation of Reference Genes and Age Estimation of Forensically Useful Aldrichina grahami (Diptera: Calliphoridae) During Intrapuparial Period

The minimum postmortem interval (PMImin) could be evaluated from the developmental stage of forensically important insects colonize a corpse, such as blow flies (Diptera: Calliphoridae). Unlike larvae, the developmental stage of which is well established according to their morphology, estimating the age of pupae is proven to be challenging. Recently, several studies reported the regulation of special genes during the development of blow fly pupae. However, gene regulation in Aldrichina grahami during the intrapuparial period remains to be studied. Therefore, we set out to investigate the mRNA levels of heat shock protein 23 (Hsp23), heat shock protein 24 (Hsp24), and 1_16 during the metamorphosis of A. grahami pupae. First, we examined seven candidate reference genes (ribosomal protein 49 (RP49), 18S ribosomal RNA (18S rRNA), 28S ribosomal RNA (28S rRNA), beta-tubulin at 56D (β-tubulin), Ribosomal protein L23 (RPL23), glutathione S-transferase (GST1), and Actin. Three widely used algorithms (NormFinder, BestKeeper, and geNorm) were applied to evaluate the mRNA levels of reference gene candidates in puparium at three stable temperatures (15, 22, and 27°C). Next, mRNA expression of Hsp23, Hsp24, and 1_16 during A. grahami metamorphosis was examined. We demonstrated that mRNA expression levels of Hsp23, Hsp24, and 1_16 showed time-specific regulation. In summary, our study identified three gene markers for the intrapuparial period of A. grahami and might provide a potential application in PMImin estimation.

Gene expression as age estimation marker in the larval stages of the forensic blowfly, Chrysomya albiceps, at different temperatures

The blowfly Chrysomya albiceps (Diptera: Calliphoridae) has been known to breed in both animal and human carcasses in different geographical regions and is considered an important post-mortem indicator in forensic entomology. Determining the age of its larvae is an essential tool for the accurate determination of post-mortem intervals (PMI). This study adopted a molecular-based approach for age determination of the immature stages of the blowfly, C. albiceps, by evaluating the genes 15_2, 2014192, EcdR, AR, hsp90, and Actin, during larval development at different temperatures. Targeted genes were amplified by real-time PCR (RT-PCR), and the resulting amplicons were normalized against the two housekeeping genes, rp49 and 19_2. Data showed that the expression profile of AR was constant throughout all larval developmental stages at all tested temperatures. 2014192 showed low expression levels throughout the 1st, 2nd, and 3rd larval stages. Hsp90 and EcdR showed different expression profiles. The expression of 15_2 was low at the tested temperatures and was undetectable on most days. Collectively, the results of this study indicate that larvae exhibit temperature-dependent and age-specific up- and down-regulation in the expression profile of the targeted genes. This may indicate a possibility to be utilized as age estimation markers for C. albiceps.

Development of Sarcophaga dux (diptera: Sarcophagidae) at constant temperatures and differential gene expression for age estimation of the pupae

Sarcophaga dux (Diptera: Sarcophagidae) is a necrophagous flesh fly species with potential forensic value for estimating minimum postmortem interval (PMI_min). The basic developmental data and precise age estimates of the pupae are significant for PMI_min estimation in forensic investigations. In the present study, we investigated the development data of that species at seven constant temperatures varying from 16 °C to 34 °C, including body length changes of the larve, developmental duration and accumulated degree hours of the preadults. Several reference genes for relative quantification of the differentially expressed genes (DEGs) were firstly selected and evaluated in the pupae of different ages under different temperatures. The DEGs of the insects during the pupal period at different constant temperatures (34, 25 and 16 °C) were further analyzed for more precise age estimation. The results showed that the developmental durations of the preadults at 16, 19, 22, 25, 28, 31 and 34 °C were 1478.6 ± 18.3 h, 726.1 ± 15.8 h, 538.5 ± 0.9 h, 394.1 ± 9.5 h, 375.6 ± 10.8 h, 284.1 ± 7.3 h, and 252.5 ± 6.1 h, respectively. The developmental threshold temperature the flies was 12.27 ± 0.35 °C, and the thermal summation constant was 5341.71 ± 249.29° hours. The most reliable reference genes during the pupal period at different temperatures were found: GST1 and 18S rRNA for the 34 °C group, GST1 and RPL49 for 25 °C, and 18S rRNA and 28S rRNA for 16 °C. The four differential expression genes (Hsp60, A-alpha, ARP, and RPL8) have the potential to be used for more precise age estimation of pupal S. dux. This work provides important basic developmental data and a more precise age estimation method for pupal S. dux, and improves the value of this species for PMI_min estimation in forensic investigations.

Eye-background contrast as a quantitative marker for pupal age in a forensically important carrion beetle Necrodes littoralis L. (Silphidae)

Insect pupae sampled at a death scene may be used to estimate the post-mortem interval. The pupal age is however difficult to estimate, as there are no good quantitative markers for the age of a pupa. We present a novel method for pupal age estimation based on the quantification of contrast in intensity between the eyes of a pupa and the middle grey photography card as a standard background. The intensity is measured on a standardized scale from 0 (perfect black) to 255 (perfect white) using computer graphical software and pictures of the eye and the background taken with a stereomicroscope. Eye-background contrast is calculated by subtracting the average intensity of the eye from the average intensity of the background. The method was developed and validated using pupae of Necrodes littoralis (Linnaeus, 1758) (Coleoptera: Silphidae), one of the most abundant beetle species on human cadavers in Central Europe. To develop the model, pupae were reared in 17, 20 and 23 °C, with a total of 120 specimens. The method was validated by three raters, using in total 182 pupae reared in 15, 17, 20, 23 and 25 °C. We found a gradual increase in eye-background contrast with pupal age. Changes followed generalized logistic function, with almost perfect fit of the model. Using our method pupal age was estimated with the average error of 8.1 accumulated degree-days (ADD). The largest error was 27.8 ADD and 95% of age estimates had errors smaller than 20 ADD. While using the method, different raters attained similar accuracy. In conclusion, we have demonstrated that eye-background contrast is a good quantitative marker for the age of N. littoralis pupae. Contrast measurements gave accurate estimates for pupal age. Our method is thus proven to be a candidate for a reliable approach to age insect pupae in forensic entomology.

An Evaluation of Differentially Spliced Genes as Markers of Sex for Forensic Entomology,

Blow flies (Calliphoridae) are important medically and economically and are commonly used in forensics as temporal markers in death investigations. While phenotypic traits in adult flies can be sexually dimorphic, sex identification in immatures is difficult. Consequently, little is known about how sex may result in developmental disparities among sexes even though there are indications that they may be important in some instances. Since genetic mechanisms for sex are well studied in model flies and species of agricultural and medical importance, we exploit the sex-specifically spliced genes transformer (tra) and doublesex (dsx) in the sex determination pathway to optimize a sex identification assay for immatures. Using known primer sets for tra and with a novel one for dsx, we develop PCR assays for identifying sex in four forensically relevant Calliphoridae species: Lucilia sericata (Meigen), Lucilia cuprina (Wiedemann), Cochliomyia macellaria (Fabricius), and Chrysomya rufifacies (Macquart) and evaluated their performance. Band detection rates were found to range from 71 to 100%, call rates ranged from 90 to 100%, and no error was found when bands could be called. Such information is informative for purposes of testimony and in preparation for development studies. The developed assays will assist in further differentiating sexually dimorphic differences in development of the Calliphoridae and aid in more accurately estimating insect age when age predictive markers (size, development time, molecular expression) are sexually dimorphic.

Development of Necrobia rufipes (De Geer, 1775) (Coleoptera: Cleridae) under constant temperatures and its implication in forensic entomology

After the death of humans or animals, the odors released at different stages of decay attract various insects, and other arthropods, to the corpses. Therefore, the development of insects, and other arthropods present on corpses, can be assessed to estimate the minimum postmortem interval since death. In general, necrophagous blow flies are the insects that first colonize corpses. With progressing decay, other necrophagous and predatory insects arrive at the corpses, which will develop on or around these either by feeding directly on the corpses or by prey on other immature insects. Beetles (Coleoptera) mainly arrive at the corpses during the later stages of decay, and play important roles in cases with longer postmortem intervals. Necrobia rufipes (De Geer, 1755) (Coleoptera: Cleridae) is an important stored-product species with world-wide distribution. Moreover, it is also a forensically important insect species. At temperatures of 22, 25, 28, 31, 34, and 36°C (±0.5°C), the developmental periods from egg to adult were 113.20±2.96, 66.16±3.22, 50.61±1.95, 38.26±2.48, 37.97±2.40, and 31.20±2.11 days, respectively. In vivo measurements obtained the morphological indexes of larvae. The growth curve and the equation of the relationship between development time, body lengths, and mesonotum widths were simulated. The isomorphen diagram model, the isomegalen diagram model, and the thermal summation model were established. In addition, the widths of head capsules and pronota of larvae at different instars were determined by cluster analysis. Classifiers were created and validated by linear discriminant analysis.

Differential Gene Expression for Age Estimation of Forensically Important Sarcophaga peregrina (Diptera: Sarcophagidae) Intrapuparial

Sarcophaga peregrina is an important flesh fly species for estimating the minimum postmortem interval (PMImin) in forensic entomology. The accurate determination of the developmental age is a crucial task for using necrophagous sarcophagids to estimate PMImin. During larval development, the age determination is straight forward by the morphological changes and variation of length, weight, and width; however, the age estimation of sarcophagid intrapuparial is more difficult due to anatomical and morphological changes not being visible. The analysis of differentially expressed genes (DEGs) during sarcophagid metamorphosis is a potential method for age estimation of intrapuparial. In the present study, real-time quantitative polymerase chain reaction (RT-qPCR) was used to analyze the differential gene expression level of S. peregrina intrapuparial in different constant temperatures (35°C, 25°C, and 15°C). In addition, the appropriate reference genes of S. peregrina were selected in the intrapuparial and at different temperatures to obtain reliable and valid gene expression profiles. The results indicated that two candidate genes (18S rRNA and 28S rRNA) were the most reliable reference genes, and four DEGs (Hsp90, A-alpha, AFP, AFBP) have the potential to be used to more accuracy estimate the age of S. peregrina intrapuparial.

Field Validation of a Development Data Set for Cochliomyia macellaria (Diptera: Calliphoridae): Estimating Insect Age Based on Development Stage

Insect age estimates can be useful for estimating the postmortem interval when certain assumptions are met. Such estimates are based on species-specific development data that are temperature-dependent and variable, and therefore prone to different degrees of error depending on the combination of data sets, calculations, and assumptions applied in a specific instance. Because of this potential error, validating the methods employed is necessary for determining accuracy and precision of a given technique. For forensic entomology, validation of development data sets is one approach for identifying the uncertainty associated with insect age estimates. Cochliomyia macellaria (Fabricius) is a primary colonizer of remains across the United States and is commonly encountered in forensic investigations. A development study for this species was produced for a central Texas, U.S. population; the variation associated with this data set and the pre-appearance interval were previously explored in an ecological model. The objectives of this study were to determine the accuracy of the development data and the validity of the ecological model when applied to immatures of known age developing under field conditions. Results indicate this data set is an accurate predictor of insect age when using development stage, supporting the validity of the ecological model in central Texas. Age predictions made with all stages present in a sample were more accurate than predictions made with the most developed stage in a sample, and estimates of age when using the prepupal stage were overestimated regardless of prediction method, though thermal requirements for total development were similar.

Succession of oral microbiota community as a tool to estimate postmortem interval

The establishment of postmortem interval is one of the most important aspects of forensic expertise. Microbes may provide a novel way to estimate the postmortem intervals in order to avoid many of these limitations. The oral cavity harbors one of the most diverse microbiomes that play a key role in the decomposition of corpses. In this study, the oral bacterial community showed obvious changes in relative abundance during the process of mice decomposition. Meanwhile, at different taxonomic levels, specific bacteria were found to be significantly correlated with the postmortem interval. Linear regression models between relative abundance and the postmortem interval were constructed. Among these species, Gamma-proteobacteria and Proteus were the best ones that can be used to infer the postmortem interval, especially late postmortem interval. Therefore, we suggest that succession of oral microbial community can be developed as a forensic tool for estimating the postmortem interval.

Development of Chrysomya rufifacies (Diptera: Calliphoridae) at Constant Temperatures Within its Colony Range in Yangtze River Delta Region of China

The age of insects found on corpses is often used to estimate the minimum postmortem interval by forensic entomologists. Insect development is affected by biotic and abiotic factors, and temperature is the most important environmental factor that determines the length of insect development. Chrysomya rufifacies (Macquart) (Diptera: Calliphoridae) is a globally distributed fly that is commonly found on corpses, and this study investigated the development of C. rufifacies from China at various constant temperatures. At 16, 19, 22, 25, 28, 31, and 34°C, the developmental time from egg to adult was 870.17 ± 11.50, 566.20 ± 8.79, 406.38 ± 10.98, 291.14 ± 4.71, 232.59 ± 5.96, 192.47 ± 3.45, and 160.48 ± 7.15 h, respectively. We established three developmental models for C. rufifacies to estimate the age of the developing insect: the isomorphen diagram model, the isomegalen diagram model and the thermal summation model. Regression analysis was conducted to obtain the relationship between body length and development time of the larvae from hatching to wandering. The developmental threshold temperatures of the egg, 1st instar, 2nd instar, 3rd instar, and wandering larvae, and intra-puparial stages were 12.28 ± 0.30, 11.74 ± 0.95, 12.70 ± 0.55, 11.68 ± 0.96, 10.53 ± 1.53, and 12.51 ± 0.41°C, respectively. The developmental threshold temperature, and the thermal summation constant during the entire developmental period were 3759.95 ± 170.80 degree hours and 11.96 ± 0.38°C, respectively. This study provides an improved dataset to estimate the postmortem interval of corpses using C. rufifacies.

Technical note: A rapid, non-invasive method for measuring live or preserved insect specimens using digital image analysis

The measurement of insects is an important component of many entomological applications, including forensic evidence, where larvae size is used as a proxy for developmental stage, and hence time since colonization/death. Current methods for measuring insects are confounded by varying preservation techniques, biased and non-standardized measurements, and often a lack of sample size given practical constraints. Towards enhanced accuracy and precision in measuring live insects to help avoid these variables, and that allows for different measurements to be analyzed, we developed a non-invasive, digital method using widely available free analytical software to measure live blow fly larvae. Using crime scene photographic equipment currently standard in investigation protocols, we measured the live length of 282 Phormia regina larvae. Repeated measurements of maggots, for all instars, were performed for several orientations and images. Most accurate measurements were obtained when maggots were oriented in their natural full extension. Killed specimens resulted in greater length measurements (Mean 1.79 ± 1.11 mm) when compared to live length. Herein, we report a technically simple, fast, and accurate measurement technique adapted for field and lab-based measurements, as well as, a simple linear equation for conversion of live length to standard killed length measurements. We propose this method be utilized for the standardization of forensic entomological evidence collection and development model creation.

The Effects of Diets and Long-term Laboratory Rearing on Reproduction, Behavior, and Morphology of Lucilia cuprina (Diptera: Calliphoridae)

The Australian sheep blow fly, Lucilia cuprina (Wiedemann), is commonly reared in the laboratory for many sequential generations on simple, fixed diets, so it can be used in veterinary, medical, and forensic studies. To investigate the effect of diet and long-term laboratory rearing on L. cuprina, flies were fed with two different diets (sugar and milk-sugar) over a year and F1, F6, and F11 generations were used for comparisons based on the number of eggs, attraction to wool and liver, and wing size. The results showed that the number of eggs of gravid flies, and the attractiveness of wool and liver did not differ significantly between diets and generations, but gravid flies were more attracted to wool and liver than non-gravid flies (P < 0.05). Moreover, in the F1 generation, thorax length and wing aspect ratio were significantly longer than in the F6 and F11 generations (P < 0.05), and the wing length was significantly longer than in the F11 generation (P < 0.05). It was concluded that neither diet nor long-term laboratory rearing affect potential fecundity or the behavioral responses of L. cuprina, but the gravidity of flies affects their behavioral response, and long-term laboratory rearing significantly affects fly morphology, apparently explaining a loss in flight performance.

Dating Pupae of the Blow Fly Calliphora vicina Robineau-Desvoidy 1830 (Diptera: Calliphoridae) for Post Mortem Interval-Estimation: Validation of Molecular Age Markers

Determining the age of juvenile blow flies is one of the key tasks of forensic entomology when providing evidence for the minimum post mortem interval. While the age determination of blow fly larvae is well established using morphological parameters, the current study focuses on molecular methods for estimating the age of blow flies during the metamorphosis in the pupal stage, which lasts about half the total juvenile development. It has already been demonstrated in several studies that the intraspecific variance in expression of so far used genes in blow flies is often too high to assign a certain expression level to a distinct age, leading to an inaccurate prediction. To overcome this problem, we previously identified new markers, which show a very sharp age dependent expression course during pupal development of the forensically-important blow fly Calliphora vicina Robineau-Desvoidy 1830 (Diptera: Calliphoridae) by analyzing massive parallel sequencing (MPS) generated transcriptome data. We initially designed and validated two quantitative polymerase chain reaction (qPCR) assays for each of 15 defined pupal ages representing a daily progress during the total pupal development if grown at 17 °C. We also investigated whether the performance of these assays is affected by the ambient temperature, when rearing pupae of C. vicina at three different constant temperatures-namely 17 °C, 20 °C and 25 °C. A temperature dependency of the performance could not be observed, except for one marker. Hence, for each of the defined development landmarks, we can present gene expression profiles of one to two markers defining the mentioned progress in development.

Rapid molecular identification of necrophagous diptera by means of variable-length intron sequences in the wingless gene

The arrival of arthropods at a corpse exhibits specific temporal patterns, and Diptera play a key role in the initial stages of the decomposition process. Thus, the correct species assignment of the insect larvae found on a decomposing body is an important step in forensic investigations. Here, we describe a molecular procedure to define the species at larval age found on a corpse more quickly and easily than current systems. Our method involves a unique PCR amplification of a DNA segment within the evolutionarily conserved wingless gene, involved in embryo development. The amplified DNA segment contains the fourth intron of wingless, which we found to be variable in length, from about 800 to 3000 bp, among species of necrophagous Diptera. The identification of the amplified segment size in species from Lucilia, Calliphora and Sarcophaga genera, allowed us to determine the species at larval age collected in the early stages of a decomposing body, with a simple PCR amplification and subsequent electrophoresis. This procedure may help in forensic investigations to estimate the minimum Post Mortem Interval (PMI-min) of a body colonized by these larvae, avoiding the use of time-consuming and/or more expensive procedures.

Hyperspectral measurements of immature Lucilia sericata (Meigen) (Diptera: Calliphoridae) raised on different food substrates

Immature Lucilia sericata (Meigen) raised on beef liver, beef heart, pork liver and pork heart at a mean temperature of 20.6°C took a minimum of 20 days to complete development. Minimum development time differences within stages were observed between the meat types (pork/beef), but not the organ types (liver/heart). Daily hyperspectral measurements were conducted and a functional regression was completed to examine the main effects of meat and organ type on daily spectral measurements. The model examined post feeding larval spectral measurements of insects raised on beef liver alone, the effect of those raised on pork compared with those raised on beef, the effect of those raised on heart compared with those raised on liver and the interactional effect of those raised on pork heart compared with those raised on beef liver. The analyses indicated that the spectral measurements of post feeding L. sericata raised on pork and beef organs (liver and heart) are affected by the meat and organ type.

Adult fly age estimations using cuticular hydrocarbons and Artificial Neural Networks in forensically important Calliphoridae species

Blowflies (Diptera: Calliphoridae) are forensically important as they are known to be one of the first to colonise human remains. The larval stage is typically used to assist a forensic entomologists with adult flies rarely used as they are difficult to age because they remain morphologically similar once they have gone through the initial transformation upon hatching. However, being able to age them is of interest and importance within the field. This study examined the cuticular hydrocarbons (CHC) of Diptera: Calliphoridae species Lucilia sericata, Calliphora vicina and Calliphora vomitoria. The CHCs were extracted from the cuticles of adult flies and analysed using Gas Chromatography-Mass Spectrometry (GC-MS). The chemical profiles were examined for the two Calliphora species at intervals of day 1, 5, 10, 20 and 30 and up to day 10 for L. sericata. The results show significant chemical changes occurring between the immature and mature adult flies over the extraction period examined in this study. With the aid of a Principal Component Analysis (PCA) and Artificial Neural Networks (ANN), samples were seen to cluster, allowing for the age to be established within the aforementioned time frames. The use of ANNs allowed for the automatic classification of novel samples with very good performance. This was a proof of concept study, which developed a method allowing to age post-emergence adults by using their chemical profiles.

Initial investigations of spectral measurements to estimate the time within stages of Protophormia terraenovae (Robineau-Desvoidy) (Diptera: Calliphoridae)

Current applications of forensic entomology to post-mortem interval estimations involve ageing the insects colonizing the remains based on minimum time to reach the oldest stage of development. Immature species of blow fly develop at a predictable rate to each stage of development in their lifecycle. Unfortunately, the minimum time to reach a stage of development can be a rather unrefined estimate of tenure on the body in the sometimes lengthy time frame of the later stages. In a successful attempt to narrow this time frame, daily spectral measurements of the immature stages of Protophormia terraenovae (Robineau-Desvoidy) raised at a mean temperature of 24.6°C were collected and the functional data analysis was completed. Functional regressions and coefficient functions were examined for model prediction and generalization. P. terraenovae is a Holarctic species as well as an early colonizer of human remains and is therefore, an excellent indicator species in North American death investigations. Spectral measurements can be used successfully to estimate the day of development in the third instar including post feeding stage. In the intra-puparial period, however, only the last day of development could be distinguished from the earlier days of the intra-puparial period. Distinguishing day within second instar is also possible for P. terraenovae raised at a mean temperature of 24.6°C and, although not fully within the pointwise 95% confidence interval, it still accurately predicts the day. The results of this proof of concept research are promising and show a potential method for narrowing the original death estimates and offering a better overall estimate of age of P. terraenovae larvae and, therefore; estimated time since death.

The Battle Against Flystrike - Past Research and New Prospects Through Genomics

Flystrike, or cutaneous myiasis, is caused by blow fly larvae of the genus Lucilia. This disease is a major problem in countries with large sheep populations. In Australia, Lucilia cuprina (Wiedemann, 1830) is the principal fly involved in flystrike. While much research has been conducted on L. cuprina, including physical, chemical, immunological, genetic and biological investigations, the molecular biology of this fly is still poorly understood. The recent sequencing, assembly and annotation of the draft genome and analyses of selected transcriptomes of L. cuprina have given a first global glimpse of its molecular biology and insights into host-fly interactions, insecticide resistance genes and intervention targets. The present article introduces L. cuprina, flystrike and associated issues, details past control efforts and research foci, reviews salient aspects of the L. cuprina genome project and discusses how the new genomic and transcriptomic resources for this fly might accelerate fundamental molecular research of L. cuprina towards developing new methods for the treatment and control of flystrike.

Factors Affecting Species Identifications of Blow Fly Pupae Based upon Chemical Profiles and Multivariate Statistics

Alternative methods for the identification of species of blow fly pupae have been developed over the years that consist of the analyses of chemical profiles. However, the effect of biotic and abiotic factors that could influence the predictive manner for the tests have not been evaluated. The lipids of blowfly pupae (Cochliomyia macellaria, Lucilia cuprina, Lucilia sericata, and Phormia regina) were extracted in pentane, derivatized, and analyzed by total-vaporization solid phase microextraction gas chromatography-mass spectrometry (TV-SPME GC-MS). Peak areas for 26 compounds were analyzed. Here we evaluated one biotic factor (colonization) on four species of blow flies to determine how well a model produced from lipid profiles of colonized flies predicted the species of flies of offspring of wild-caught flies and found very good species identification following 10 generations of inbreeding. When we evaluated four abiotic factors in our fly rearing protocols (temperature, humidity, pupation substrate, and diet), we found that the ability to assign the chemical profile to the correct species was greatly reduced.

Spectral Signatures of Immature Lucilia sericata (Meigen) (Diptera: Calliphoridae)

Hyperspectral remote sensing is an innovative technology with applications in many sciences and is a non-destructive method that may offer more precise aging within development stages. Hyperspectral reflectance measurements from the anterior, midsection, and posterior of Lucilia sericata (Meigen) larvae and pupae were conducted daily from samples of the developing insects beginning at second instar. Only midsection measurements were conducted on second instar larvae due to their size, to ensure that the measurement was not of reflective surroundings. Once measured, all insects were washed with deionized water, blotted with filter paper, and re-measured. Daily age prediction during the post-feeding stage was not impacted by the unwashed insect measurements and was best predicted based on posterior measurements. The second and third instar larvae, which move about their food source, had different contributing coefficients to the functional regression model for the hyperspectral measurements of the washed compared with unwashed specimens. Although washing did not affect the daily prediction within these stages, it is still encouraged in order to decrease the effect of food source on spectral reflectance. Days within the intra-puparial period were best predicted based on anterior measurements and were not well distinguished from one another in the first few days based on midsection and posterior measurements.

Forensic Entomology: Evaluating Uncertainty Associated With Postmortem Interval (PMI) Estimates With Ecological Models

Estimates of insect age can be informative in death investigations and, when certain assumptions are met, can be useful for estimating the postmortem interval (PMI). Currently, the accuracy and precision of PMI estimates is unknown, as error can arise from sources of variation such as measurement error, environmental variation, or genetic variation. Ecological models are an abstract, mathematical representation of an ecological system that can make predictions about the dynamics of the real system. To quantify the variation associated with the pre-appearance interval (PAI), we developed an ecological model that simulates the colonization of vertebrate remains by Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae), a primary colonizer in the southern United States. The model is based on a development data set derived from a local population and represents the uncertainty in local temperature variability to address PMI estimates at local sites. After a PMI estimate is calculated for each individual, the model calculates the maximum, minimum, and mean PMI, as well as the range and standard deviation for stadia collected. The model framework presented here is one manner by which errors in PMI estimates can be addressed in court when no empirical data are available for the parameter of interest. We show that PAI is a potential important source of error and that an ecological model is one way to evaluate its impact. Such models can be re-parameterized with any development data set, PAI function, temperature regime, assumption of interest, etc., to estimate PMI and quantify uncertainty that arises from specific prediction systems.

Thermal summation model and instar determination of all developmental stages of necrophagous beetle, Sciodrepoides watsoni (Spence) (Coleoptera: Leiodidae: Cholevinae)

Necrophagous beetles are underrepresented in forensic entomology studies despite their undeniable utility for the field. In the present article, information is presented regarding the developmental biology and instar determination of Sciodrepoides watsoni (Spence, 1813), a very common species occurring across the Holarctic region. Wild collected beetles were kept in climate chambers at constant temperature (12, 15, 18, 21 and 28 °C) and their development was regularly documented. Parameters of thermal summation models and standard errors were calculated for each developmental stage. These models may be used for an estimation of post-mortem interval in legal investigations after further validation on local populations of S. watsoni. An additional methodology is introduced for future studies of size-based characteristics, addressing instar identification bias. The methodology provided estimations (mean, standard error and standard deviation) of S. watsoni larval head capsule width for preliminary larval instar determination. The methodology may be used with other morphological features to improve instar determination accuracy.