Cis-regulatory polymorphism at fiz ecdysone oxidase contributes to polygenic evolutionary response to malnutrition in Drosophila

Shared genetic architecture links energy metabolism, behavior and starvation resistance along a power-endurance axis

Shared developmental, physiological, and molecular mechanisms can generate strong genetic covariances across suites of traits, constraining genetic variability, and evolvability to certain axes in multivariate trait space ("variational modules" or "syndromes"). Such trait suites will not only respond jointly to selection; they will also covary across populations that diverged from one another by genetic drift. We report evidence for such a genetically correlated trait suite that links traits related to energy metabolism along a "power-endurance" axis in Drosophila melanogaster. The "power" pole of the axis is characterized by high potential for energy generation and expenditure-high expression of glycolysis and TCA cycle genes, high abundance of mitochondria, and high spontaneous locomotor activity. The opposite "endurance" pole is characterized by high triglyceride (fat) reserves, locomotor endurance, and starvation resistance (and low values of traits associated with the "power" pole). This trait suite also aligns with the first principal component of metabolome; the "power" direction is characterized by low levels of trehalose (blood sugar) and high levels of some amino acids and their derivatives, including creatine, a compound known to facilitate energy production in muscles. Our evidence comes from six replicate "Selected" populations adapted to a nutrient-poor larval diet regime during 250 generations of experimental evolution and six "Control" populations evolved in parallel on a standard diet regime. We found that, within each of these experimental evolutionary regimes, the above traits strongly covaried along this "power-endurance" axis across replicate populations which diversified by drift, indicating a shared genetic architecture. The two evolutionary regimes also drove divergence along this axis, with Selected populations on average displaced towards the "power" direction compared to Controls. Aspects of this "power-endurance" axis resemble the "pace of life" syndrome and the "thrifty phenotype"; it may have evolved as part of a coordinated organismal response to nutritional conditions.

Genetic characterization of candidate ecdysteroid kinases in Drosophila melanogaster

Ecdysteroids are major hormones in insects and control molting, growth, reproduction, physiology, and behavior. The biosynthesis of ecdysteroids such as 20-hydroxyecdysone (20E) from dietary sterols is well characterized, but ecdysteroid catabolism is poorly understood. Ecdysteroid kinases (EcKs) mediate the reversible phosphorylation of ecdysteroids, which has been implicated in ecdysteroid recycling during embryogenesis and reproduction in various insects. However, to date, only 2 EcK-encoding genes have been identified, in the silkworm Bombyx mori and the mosquito Anopheles gambiae. Previously, we identified 2 ecdysteroid kinase-like (EcKL) genes-Wallflower (Wall) and Pinkman (pkm)-in the model fruit fly Drosophila melanogaster that are orthologs of the ecdysteroid 22-kinase gene BmEc22K. Here, using gene knockdown, knockout, and misexpression, we explore Wall and pkm's possible functions and genetically test the hypothesis that they encode EcKs. Wall and pkm null mutants are viable and fertile, suggesting that they are not essential for development or reproduction, whereas phenotypes arising from RNAi and somatic CRISPR appear to derive from off-target effects or other artifacts. However, misexpression of Wall results in dramatic phenotypes, including developmental arrest, and defects in trachea, cuticle, and pigmentation. Wall misexpression fails to phenocopy irreversible ecdysteroid catabolism through misexpression of Cyp18a1, suggesting that Wall does not directly inactivate 20E. Additionally, Wall misexpression phenotypes are not attenuated in Cyp18a1 mutants, strongly suggesting that Wall is not an ecdysteroid 26-kinase. We hypothesize that the substrate of Wall in this misexpression experiment and possibly generally is an unknown, atypical ecdysteroid that plays essential roles in Drosophila development, and may highlight aspects of insect endocrinology that are as-yet uncharacterized. We also provide preliminary evidence that CG5644 encodes an ecdysteroid 22-kinase conserved across Diptera.