Increasing Precision in Development-Based Postmortem Interval Estimates: What's Sex Got to Do With It?

Abstract

Forensic entomologists typically use either succession models for postmortem interval (PMI) estimates or development-based models for minimum PMI (PMI(MIN) estimates. Development-based age estimates are calculated with durations of immature stadia and can also include morphological data such as larval size. For developmental data, the first and second instar stages are typically brief with little variation in larval length. The third instar, a much longer stage by comparison, is prone to considerable variation. This variation is, in part, because of the nonlinear growth during the third instar. There is evidence that genetic and environmental factors influence growth curve divergence during this stage. We chose to investigate one genetic factor, sex, as numerous insect species exhibit sex-specific immature growth patterns. The development rate of Lucilia sericata (Meigen) (Diptera: Calliphoridae) males and females is considered here. We previously determined the genome sizes of L. sericata and found significant sexually dimorphic genome sizes. This difference can be exploited to identify larval sex to evaluate male and female immature growth curves. A preliminary development study encompassing the third larval instar was conducted to compare larval lengths for each sex. Results showed length (P < 0.0001) and sex (P < 0.01) were statistically significant predictors of age at two temperatures (30 and 33.5 degrees C), and that total male development was significantly shorter (P < 0.001). These results introduce a new tool, assessment of sex-specific growth, that has the potential to reduce noise in PMI(MIN) estimates when using third instar larvae.