Hill, Victoria
(2024)
Manipulation of the growth and body composition of mealworms, Tenebrio molitor.
PhD thesis, University of Nottingham.
Abstract
Insects have received increased attention as a potential sustainable source of dietary or feed protein to partially replace feed for livestock. The aim of this thesis was to investigate the potential for manipulation of Tenebrio molitor larvae (mealworm) body composition, ideally increasing the proportion of protein and decreasing fat body composition.
Supplementation of exogenous endocrine compounds into mealworms’ wheat bran feed was one of the avenues pursued in this thesis. Octopamine (low: 5 mg octopamine/g wheat bran, and high: 100 mg octopamine/g wheat bran), which acts as a ß-adrenergic agonist, was trialled in mealworms for a treatment period of 35 days. Whilst no changes to overall mealworm size was observed, significant (p>0.05) changes to pupation were recorded in octopamine treated mealworms, with a low dose of octopamine reducing total pupation. Whole body composition analysis showed a significant (p<0.05) reduction in total fat content in low dose octopamine treated mealworms, but with no effect induced at a high dose, and high dose octopamine treated mealworms fatty acid profile towards a more saturated profile (p<0.05).
The response of mealworms to the juvenile hormone analogue, pyriproxifen, was tested at two doses (low: 2 mg pyriproxifen/ kg wheat bran, and high:15 mg pyriproxifen/kg wheat bran) over treatment period of 35 days. There was no effect on individual mealworm growth (p>0.05) but at both doses pupation significantly inhibited (p<0.001). The fat content of pyriproxifen treated mealworms was significantly reduced with a near 70% reduction (p<0.001), with a concomitant near 50% increase in protein content (p<0.001) between the controls and high dose of pyriproxifen treatment groups. The fatty acid profile, which was significantly altered towards a more saturated profile (p<0.001). The amino acid profile was also shown to be significantly affected by treatment with pyriproxifen (p<0.05) with an increase in lysine proportion (p<0.001). Transcriptomic analysis was performed to investigate the underlying mechanisms of pyriproxifen induced shifts in body compositional profiles by analysing the effect of the agent over a 35 day treatment time course, the transcriptome being assessed at day 7, 14 and 35. Gene enrichment analysis utilising Gene Ontology and Kyoto Encyclopaedia of Genes and Genomes analyses identified lipid-metabolism associated processes being significantly (p<0.05) affected by pyriproxyfen treatment with gene transcripts associated with lipid synthesis and lipid mobilisation and degradation being significantly differentially expressed (p<0.05, ≥log2 fold change). Interestingly, targeted analysis of genes involved in lipid synthesis, acetyl-CoA-carboxylase, and fatty acid synthase, originally hypothesised to be downregulated due to the phenotype of reduced fat, were conversely identified to be upregulated at certain time points. There was large variation in gene expression across replicates, suggesting potential non-responders, leading to a lack of statistical significance when analysing the expression of genes across the time course (p>0.05).
In mammals knock down of diacylglycerol O-acyltransferase 1 (DGAT) responsible triacylglycerol results in reduced fat deposition. RNA interference was used to target mealworm DGAT expression. Microinjection of double stranded RNA (dsRNA) targeting two regions, the 5’ end and the middle of the DGAT mRNA gene transcript did not change the individual mealworm weight (p>0.05), nor induced body compositional changes. The dsRNA targeting the middle of the gene did induce a significant reduction in pupation (p<0.05). Assessment of dsRNA treated mealworms DGAT gene expression gave an indication that this was decreased.
In conclusion, growth, and life traits along with body composition in mealworms can be significantly altered towards a leaner profile by exposing them to a juvenile hormone analogue. This change in body composition significantly alters gene expression associated processes regulating lipid metabolism, but it was not possible to clearly identify what specific aspect of this metabolism was the responsible mechanism for the changes in mealworm fat deposition. However, there is potential to optimise the use of gene editing approaches to attempt to achieve the same nutritional profiles.
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