a, C16:0 to C18:0 elongase activity is significantly blunted in Elovl6 mutants, whereas elongase activities on other fatty acids measured are not affected. Microsomal preparations from control or Elovl6 mutant animals were incubated with radioactive malonyl-CoA and the indicated fatty-acyl-CoA. Elongation was quantified by incorporation of the aqueous metabolite malonyl-CoA into lipid-soluble fatty acids, as described previously30. Values represent biological triplicates. b, Gas chromatography flame ionization detector (GC-FID) analysis reveals that Elovl6 mutant larvae have reduced levels of C18:0, the product of Elovl6 elongase activity. Values are the averages of technical duplicates on biological duplicates. Error bars represent standard error of the mean (s.e.m.). c, Lethality of Elovl6 mutants is fully rescued to expected Mendelian ratios by ubiquitous expression (with actin-GAL4) of Elovl6 from a UAS transgene (χ2 test, 1.149 < 3.841 = χ2, where P = 0.05, n = 141; ****P < 0.0001). d, Human ELOVL6 and Drosophila Elovl6 are functionally equivalent, since the lethality of Elovl6 mutant flies is fully rescued to expected Mendelian ratios by ubiquitous expression (with actin-GAL4) of human ELOVL6 from a UAS transgene (χ2 test, 2.38 < 3.841 = χ2, where P = 0.05, n = 76; **P < 0.01). e, The lethality of Elovl6 mutant flies is most strongly rescued by C18:0, the product of Elovl6. Synchronized 1st instar larvae of indicated genotypes were grown on standard food supplemented with indicated fatty acids (5%). The percentage of total animals surviving to pupation was calculated. Values represent average of biological triplicates. f, Elovl6 mutants are not hypersensitivite to drugs such as G418 (protein biosynthesis inhibitor) or etoposide (topoisomerase inhibitor). Thirty synchronized L1 larvae were grown in vials with food supplemented with either G418 (50 µg ml−1) or etoposite (25 µM). Percentage of animals that reach pupation was quantified. Values represent average of four biological replicates. g, Complex IV activity of Elovl6− larvae is not impaired. Complex IV activity of female pre-wandering larvae was measured with Oroboros high-resolution respirometry. Oxygen consumption was measured in the presence of only N,N,N′,N′-tetramethyl-p-phenylenediamine dihydrochloride (TMPD) as substrate, which can be directly oxidized by complex IV. The values were corrected for non-mitochondrial oxygen consumption (oxygen consumption in the presence of complex IV inhibitor potassium cyanide (KCN)) and normalized to tissue weight. n = 3. h, i, Overexpression of Spargel in Elovl6 mutant female pre-wandering larvae leads to increased mitochondrial abundance, assessed by porin levels (h; representative of six biological replicates) and citrate synthase activity (i; n = 4) in pre-wandering larvae. See Supplementary Fig. 12 for image of the uncropped full western blot. j, Drosophila Elovl6 (either N- or C-terminally tagged) localizes to the mitochondrial outer membrane. S2 cell lysates (‘total’) were successively fractionated to yield crude mitochondria (which include mitochondrial-associated membranes (MAMs)), pure mitochondria (lacking MAMs), and mitochondrial outer membranes (OM), inner membranes (IM) and inter-membrane space (IMS). Endogenous porin and ATPsyn-α were used as positive controls for outer membranes and inner membranes, respectively. 7.5 μg of protein from each fraction was loaded per lane. See Supplementary Fig. 13 for image of the uncropped full western blot. Representative of two biological replicates. k, Lipidomic analysis of standard fly food reveals low levels of C18:0 in the food. a, c, d, e, f, g, i, Error bars represent s.d. a, b, e–g, i, *P < 0.05, **P < 0.01, not significant (NS) P ≥ 0.05, two-tailed t-test.