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

Abstract

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.

Improving accuracy of age estimates for insect evidence—calibration of physiological age at emergence (k) using insect size but without “k versussize” model

“…As a rule, forensic entomologists use developmental data for species or their geographic subpopulations [ 13 , 20 , 21 ]. More detailed data and tools to collect them were developed only recently, with the focus on sex and size of insects [ 8–10 , 14 , 15 , 22–25 ]. Further studies in this field are necessary and in particular we should look for the new traits and tools that could be useful to calibrate developmental data for narrow subpopulations of insect species.…”

“…Although thermal summation constants are sometimes regarded as truly constant for an insect species or population, plenty of data have been accumulated that demonstrate their substantial variation. For example, k was found to differ between the sexes within a species [ 8–10 ], between different geographic populations of a species [ 11–13 ] and between different sized insects within the local population of a species [ 14 , 15 ]. Essentially, k is a central point (an average) of an insect group (i.e.…”

Evaluating the impact of hot water killing larvae on gene expression using the transformer gene in Cochliomyia macellaria (Diptera: Calliphoridae)

“…The primary use of the transformer gene in forensic entomology is for the identification of sex in the immature stages. Studies have evaluated whether a difference in development rate is present because of sex [9,17,21,22]. The work focused on assessing sex-based differences in development rate in the larval stages [9,17,21] suffers from comparing larvae in the absence of hot water killing, meaning the size results reported likely do not closely replicate what the sizes would be if stored under casework storage conditions.…”