Aging is a complex biological process which is accompanied by changes in gene expression and mutational load. In many species including humans, old fathers pass on more paternally-derived de novo mutations, however, the cellular basis and cell types driving this pattern are still unclear. To understand the root causes of this phenomenon, we performed single-cell RNA-sequencing (scRNA-seq) on testes from young and old male Drosophila, as well as genomic sequencing (DNA-seq) on somatic tissue from the same flies. We found that early germ cells from old and young flies have similar mutational loads, but older flies are less able to remove mutations during spermatogenesis. This indicates that germline mutations arise from primarily non-replicative factors, and that the increased mutational load of older males is due to differences in genome maintenance activities such as repairs to DNA damage. We also found that T>A mutations are enriched in older flies, and transcription-related enrichment terms are depleted in older males. Early spermatogenesis-enriched genes have lower dN/dS than late spermatogenesis-enriched genes, supporting the hypothesis that late spermatogenesis is the source of evolutionary innovation. This transcriptional disruption is reflected in the decreased expression of genome maintenance genes in early germ cells of older flies, as well as potentially aberrant transcription of transposable elements in the aging germline. Our results provide novel insights into the transcriptional and mutational signatures of the male germline.
Male-driven de novo mutations in haploid germ cells
