Quantitative and molecular genetic analyses of mutations increasing Drosophila life span

Abstract

Understanding the genetic and environmental factors that affect variation in life span and senescence is of major interest for human health and evolutionary biology. Multiple mechanisms affect longevity, many of which are conserved across species, but the genetic networks underlying each mechanism and cross-talk between networks are unknown. We report the results of a screen for mutations affecting Drosophila life span. One third of the 1,332 homozygous P-element insertion lines assessed had quantitative effects on life span; mutations reducing life span were twice as common as mutations increasing life span. We confirmed 58 mutations with increased longevity, only one of which is in a gene previously associated with life span. The effects of the mutations increasing life span were highly sex-specific, with a trend towards opposite effects in males and females. Mutations in the same gene were associated with both increased and decreased life span, depending on the location and orientation of the P-element insertion, and genetic background. We observed substantial--and sex-specific--epistasis among a sample of ten mutations with increased life span. All mutations increasing life span had at least one deleterious pleiotropic effect on stress resistance or general health, with different patterns of pleiotropy for males and females. Whole-genome transcript profiles of seven of the mutant lines and the wild type revealed 4,488 differentially expressed transcripts, 553 of which were common to four or more of the mutant lines, which include genes previously associated with life span and novel genes implicated by this study. Therefore longevity has a large mutational target size; genes affecting life span have variable allelic effects; alleles affecting life span exhibit antagonistic pleiotropy and form epistatic networks; and sex-specific mutational effects are ubiquitous. Comparison of transcript profiles of long-lived mutations and the control line reveals a transcriptional signature of increased life span.

Figure 1. Cumulative frequency distribution of effects of mutations on life span.

Life spans are averaged across sexes, and expressed as deviations from the mean of co–isogenic controls. Lines with mean life spans exceeding the 95%, 99% and 99.9% confidence intervals are depicted in cyan, dark blue and dark pink, respectively.

Figure 2. Analysis of revertant alleles.

M denotes males and F denotes females. Canton S F control genotypes are depicted in cyan, and mub ^BG00043, crol ^BG00346 and esg ^BG01042 genotypes in dark pink. Grey bars denote the revertant genotypes mub ^rev1 (precise), mub ^rev3 (imprecise), crol ^rev4 (precise) and esg ^rev3 (precise). The letters indicate the results of Tukey tests for significant differences between control, mutant and revertant lines. Genotypes with the same letter are not significantly different from each other.

Figure 3. Epistatic interactions between P–element insert lines associated with increased life span.

Significant SCA effects that enhance the mutant phenotype (i.e., are longer-lived than expected) are indicated by dark pink lines, and significant SCA effects that suppress the mutant phenotype (i.e., are shorter-lived than expected) are indicated by cyan lines. (A) Sexes pooled. (B) Males. (C) Females.

Figure 4. Pleiotropic effects of P–element insert lines associated with increased life span on starvation resistance, chill coma recovery, and climbing activity in males (M) and females (F) at one (1) and six (6) weeks of age.

Dark pink indicates increased fitness (greater resistance to starvation stress and climbing ability, reduced time to recover from chill coma), and cyan indicates decreased fitness (less resistance to starvation stress and climbing ability, increased time to recover from chill coma), relative to the co–isogenic control; grey indicates no significant difference from the control; and black indicates that the measurement was not done.

Figure 5. Survival curves of P–element insertion lines associated with increased life span (diamonds and solid lines) and the co–isogenic control line (Canton S F, squares and dashed lines) used for whole genome microarray profiling.

Cyan lines denote males; dark pink lines denote females. (A) pyd ^BG00028; (B) mub ^BG00043; (C) crol ^BG00346; (D) CG10990 ^BG00495; (E) CG9238 ^BG00761; (F) BG00817; (G) esg ^BG01042.

Figure 6. Pleiotropic effects on of P–element insertion lines associated with increased life span used for whole-genome microarray profiling, expressed as deviations from the co-isogenic Canton S F control line.

SR: Starvation resistance; CC: chill coma recovery; CA: climbing activity. Numbers in parentheses refer to assays at week 1 or week 6. Cyan denotes males; dark pink denotes females. (A) pyd ^BG00028; (B) mub ^BG00043; (C) crol ^BG00346; (D) CG10990 ^BG00495; (E) CG9238 ^BG00761; (F) BG00817; (G) esg ^BG01042. Asterisks denote significant deviation from the control.