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Summary
The study by Starmer, Heed, and Rockwood- Summary
The study by Starmer, Heed, and Rockwood-Slusser (1977) investigates the extension of longevity in Drosophila mojavensis when exposed to environmental ethanol and explores the genetic and ecological factors underlying this phenomenon. The authors focus on differences between subraces of D. mojavensis, emphasizing the role of alcohol dehydrogenase (ADH) isozyme polymorphisms, environmental heterogeneity of host plants, and related genetic elements.
Core Findings
Longevity Increase by Ethanol Exposure: Adult D. mojavensis flies, which breed and feed on necrotic cacti, show a significant increase in longevity when exposed to atmospheric ethanol. This longevity extension is:
Diet-independent (i.e., does not depend on yeast ingestion).
Accompanied by retention of mature ovarioles and eggs in females, indicating not just longer life but maintained reproductive potential.
Subrace Differences: Longevity increases differ among strains from different geographic regions:
Flies from Arizona and Sonora, Mexico (subrace BI) exhibit the greatest increase in longevity.
Flies from Baja California, Mexico (subrace BII) show the least increase.
Genetic Correlations:
The longevity response correlates with the frequency of alleles at the alcohol dehydrogenase locus (Adh).
Adh-S allele (slow electrophoretic form) is prevalent in Arizona and Sonora populations; its enzyme product is more heat- and pH-tolerant.
Adh-F allele (fast electrophoretic form) predominates in Baja California populations; its enzyme product is heat- and pH-sensitive but shows higher activity with isopropanol as substrate.
Modifier genes, including those associated with chromosomal inversions on the second chromosome (housing the octanol dehydrogenase locus), may also influence longevity response.
Environmental Heterogeneity: Differences in longevity and allele frequencies correspond to the distinct physical and chemical environments of the host cacti:
Arizona-Sonora flies breed on organpipe cactus (Lemaireocereus thurberi), which exhibits extreme temperature and pH variability.
Baja California flies breed on agria cactus (Machaerocereus gummosus), which shows moderate temperature and pH but contains relatively high concentrations of isopropanol.
The interaction between substrate alcohol content, temperature, and pH likely maintains the polymorphism at the ADH locus and influences evolutionary adaptations.
Experimental Design and Key Results
Experimental Setup
Flies were exposed to various concentrations of atmospheric ethanol (0.0% to 8.0% vol/vol) in sealed vials containing cotton soaked with ethanol solutions.
Longevity was measured as the lifespan of adult flies exposed to ethanol vapors, and data were log-transformed (ln[hr]) for statistical analysis.
Different strains from Baja California, Sonora, and Arizona were tested, alongside analysis of ADH allele frequencies and chromosomal inversions.
Axenic (microbe-free) strains were used to test the effect of yeast ingestion on longevity.
Summary of Key Experiments
Experiment Purpose Main Result
1 (Ethanol dose response) Test longevity response of D. mojavensis adults to ethanol vapors at different concentrations Longevity increased significantly at 1.0%, 2.0%, and 4.0% ethanol; highest female longevity observed in 4.0% ethanol group, with retention of mature eggs
2 (Yeast dependence) Assess whether longevity increase depends on live yeast ingestion Longevity increase occurred regardless of yeast treatment; live yeasts (Candida krusei or Kloeckera apiculata) not essential for enhanced longevity
3 (Subrace and sex differences) Compare longevity response among strains from different regions and sexes Females from Arizona-Sonora (subrace BI) showed significantly greater relative longevity increase than Baja California (subrace BII); males showed less pronounced differences
4 (Isozyme stability tests) Measure heat and pH stability of ADH-F and ADH-S isozymes ADH-F enzyme less stable at high temperature (45°C) and acidic pH compared to ADH-S; ADH-F activity reduced after 7-11 minutes heat exposure
Quantitative Data Highlights
Longevity Response to Ethanol Concentrations (Experiment 1)
Ethanol Concentration (%) Effect on Longevity
0.0 (Control) Baseline
0.5 No significant increase
1.0 Significant increase
2.0 Significant increase (highest relative longevity)
4.0 Significant increase
8.0 No increase (toxicity likely)
Analysis of Variance (Table 1 and Table 3)
Source of Variation Significance (p-value) Effect Description
Ethanol treatment p < 0.001 Strong effect on longevity
Yeast treatment Not significant No strong effect on longevity
Interaction (Ethanol x Yeast) p < 0.05 Minor effects, but overall yeast not required
Subrace p < 0.001 Significant effect on relative longevity
Sex Not significant Sex alone not significant, but sex x subrace interaction significant
Subrace x Sex interaction p < 0.001 Males and females respond differently across subraces
Ethanol treatment (dose) p < 0.01 Different doses produce varying longevity effects
Correlation Coefficients (Longevity Response vs. Genetic Factors)
Genetic Factor Correlation with Longevity Response at 2.0% Ethanol Correlation at 4.0% Ethanol
Frequency of Adh-F allele -0.633 (negative correlation) -0.554 (negative correlation)
Frequency of ST chromosomal arrangement (3rd chromosome) -0.131 (non-significant) 0.004 (non-significant)
Frequency of LP chromosomal arrangement (2nd chromosome) -0.694 (negative correlation) -0.713 (negative correlation)
Ecological and Genetic Interpretations
The Adh-S allele product is more heat- and pH-tolerant, which suits the variable, extreme environment of the organpipe cactus in Arizona and Sonora.
The Adh-F allele product is less stable under heat and acidic conditions but metabolizes isopropanol effectively, aligning with the chemical environment of Baja California’s agria cactus.
The distribution of Adh alleles matches the physical and chemical characteristics of the host cactus substrates, suggesting natural selection shapes the genetic polymorphism at the ADH locus.
The presence of isopropanol in agria cactus tissues may favor the Adh-F allele, as its enzyme shows higher activity with isopropanol.
The second chromosome inversion frequency correlates with longevity response, implicating the octanol dehydrogenase locus and potential modifier genes in ethanol tolerance.
Biological Significance and Implications
The study supports the hypothesis that environmental ethanol serves as a selective agent influencing longevity and allele frequencies in desert-adapted Drosophila.
The increased longevity and maintained reproductive capacity in ethanol vapor suggest a fitness advantage and physiological adaptation.
Findings align with broader research on **genetic polymorphisms in Dros
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