Transposable elements (TEs) are mobile, selfish genetic factors that are present in large numbers in the genomes of many organisms. TE insertions can lead to deleterious consequences for organism function, and indeed TE proliferation has been linked to a decline in function of the central nervous system and the onset of neurological disease. In addition, age-dependent changes in the expression of certain TE families has been seen in Drosophila brain tissue. In order to determine the effects of TE expression and mobilization in the aging brain it is critical to first detail the pattern of TE expression change during the aging process.
Here, we use mRNAseq to evaluate the regulation of TE expression throughout the lifespan of multiple, genetically-distinct, inbred Drosophila genotypes, focusing on head-specific expression. In our first experiment we made use of samples of young and old mated females collected from multiple DSPR (Drosophila Synthetic Population Resource) strains aged under standard laboratory conditions. Following RNA extraction, library generation and sequencing, we tested for gene expression differences between young and old flies, and for age-related expression changes at 126 TEs. We confirmed previous reports that Hsp22 expression increases markedly in the heads of old flies, and validated work showing that expression of antimicrobial genes also increases as flies age. We found that 8 TEs show a significant change in expression during aging (p < 0.05), all but one of which are Class I retrotransposons. However only 2/8 show the predicted increase in expression with age. For example, while expression of the Tabor element (an LTR retrotransposon) increases with age, expression of the GATE element (another LTR element) is reduced with age. We found no evidence for the previously reported increased expression of R1 and R2 LINE-like retrotransposons or gypsy LTR retrotransposons in aged heads of any strain examined.
Concerns with this experiment are the lack of replication within strains, and the use of just two samples of head tissue - young and old - per strain. To both increase our power and more precisely describe the change in TE expression during lifespan, we generated large numbers of mated females from two strains with marked differences in overall lifespan, and sampled flies for mRNAseq at several points throughout the aging process. We anticipate this new experiment will provide a more accurate picture of age-related changes in TE expression in the Drosophila head.