Department of Biology

Program

Evolution, Ecology & Behavior

Research Area

Evolution

Education

Postdoctoral Research, Brown University, 2005-2007

Ph.D. Genetics, Cornell University, 2005

B.S. Biology, University of California Irvine, 1998

Awards

NSF CAREER award: Physiological adaptation to a complex environment, 2012-2017

Research Description

Overview
Tracing the reciprocal paths from nucleotide variation to organismal fitness variation is of key importance to understanding:

1) how physiological traits evolve to fit organisms to their ecologies, and
2) what evolutionary forces shape the genetic and biochemical pathways underlying physiological change.

Energetic pathways provide systems of genes within which we can link genetic variation and divergence in gene expression, enzyme activity and whole-organism physiological performance, such as flight velocity or ethanol tolerance, under controlled experimental manipulation. Our research program integrates experimental, comparative, quantitative genetic, population genetic/genomic, bioinformatic and molecular genetic approaches to link genes to their evolutionarily significant function.

Drosophila have a unique ecology and physiology, acquiring nutrient resources in habitats ranging from desert cactus rots to ethanol-rich vineyards and using these resources to maintain aerobic metabolism during hovering flight. While Drosophilids are particularly amenable to combining comparative, population genetic/genomic and molecular genetic techniques, our approach is not limited to working with “model” organisms. Members of the lab are encouraged to choose study organisms based on the investigation of physiological adaptations that offer insight into the evolutionary forces shaping genetic and phenotypic variation and divergence in natural populations.

Examples of physiological systems within which we study the evolutionary process are:

Evolution of pathways underlying ethanol and temperature tolerance
Environmental ethanol and acetic acid present a toxic challenge to species that inhabit rotting fruit, but efficient catabolism of these compounds yields a valuable pool of acetyl-CoA to fuel metabolic processes. D. melanogaster has evolved a remarkable ability to tolerate and utilize ethanol and acetic acid, presumably allowing for niche expansion. This project characterizes how ethanol metabolism and membrane physiology interact in the evolution of toxin and temperature tolerance.

A. Modeling physiological performance as a function of biochemical flux. A model of biochemical flux through the three-step ethanol metabolic pathway reveals a ridge of high ethanol tolerance in the phenotypic landscape relating ADH and ACS activities to tolerance. Genotypes with high ADH activity can nonetheless have low tolerance when paired with low ACS activity, presumably due to accumulation of toxic intermediates. This suggests an interesting evolutionary dynamic that we are modeling and empirically testing, where the selective effects of genetic variants that enhance ADH activity depend upon the genetic background and activity of Acs.

B. Interactions between membranes, temperature and toxins. Organisms in nature do not experience single, isolated selection pressures. A remaining challenge is to describe phenotypic evolution as the integrated outcome of multiple selection pressures. Ethanol disrupts membrane function by making membranes more fluid. Because flies are ectotherms, their survival depends upon adjusting membrane fluidity in response to both environmental temperature and ethanol. Membrane fluidity can significantly affect ethanol tolerance. Moreover, a gene regulating membranes, the dSREBP transcription factor, also regulates expression of ethanol metabolism genes! We are combining experimental and lab-evolution manipulation of ethanol, acetic acid and temperature to link expression differences within candidate pathways to toxin and temperature tolerance. Of particular interest are gene/enzymes, such as Pld, that respond to cold acclimation but also affect ethanol tolerance. Investigating gene/enzymes with pleiotropic effects on multiple physiological traits is key to understanding,
1) how multiple functions of a single gene constrain or enhance trait evolution, and
2) how enzymes with conserved function evolve new roles in particular species.

Evolutionary genetics of energetics
Energetic phenotypes, such as ATP levels, must be maintained as organisms adapt to differing ecologies. This project relates genomic change to biochemical change in energetic pathways with consequences for whole-organism performance. We are interested in the question: How do the pathways underlying largely homeostatic and conserved physiological systems nevertheless harbor variation within populations and evolve across species?

A. The glucose-6-phosphate (G6P) branchpoint
Traits such as glycogen storage, metabolic rate or flight performance are likely a function of flux through the G6P branchpoint in glycolysis. In D. melanogaster, we identified a genetic locus (QTL) underlying enzyme activities at the G6P branchpoint that also affects metabolic rate and flight velocity. This indicates that genetic variation in metabolic pathways can impact performance and motivates our use of enzymatic flux equations to model naturally-occurring variation in energetic traits as a function of variation in gene expression and enzyme activity at the G6P branchpoint.

B. Linking genomic, biochemical and physiological change on a phylogeny
Drosophilids are uniquely poised for a comparative approach to study the evolution of energetics. The genomes of twelve Drosophila species have been sequenced, and, in collaboration with David Rand (Brown University), we have assembled and analyzed the mitochondrial genomes of these species. We are attempting to model pathways such as the G6P branchpoint and mitochondrial oxidative phosphorylation in a phylogenetic context to test the hypothesis that relationships between genetic, biochemical and physiological variation within populations also hold for genetic and phenotypic divergence between species. This comparative approach measures traits among multiple genetic isolates within Drosophila species, available for the D. melanogaster, D. simulans, D. sechellia; the D. yakuba, D. teissieri, D. santomea; and the D. pseudoobscura, D. persimilis species groups. Using this experimental design we can scale between species phenotypic divergence by within species variation, shedding light on the evolutionary forces acting on physiological traits. For example, do populations harbor more phenotypic variation than expected given the amount of phenotypic divergence observed across the phylogeny?

Select Publications

Meiklejohn CD, Holmbeck MA, Siddiq MA, Abt DN, Rand DM, and KL Montooth (2013) An Incompatibility between a Mitochondrial tRNA and Its Nuclear-Encoded tRNA Synthetase Compromises Development and Fitness in Drosophila. PLoS Genet 9(1): e1003238. doi:10.1371/journal.pgen.1003238  [article]

Kobey, RL, Montooth, KL. Mortality from desiccation contributes to a genotype-by-temperature interaction for cold survival in Drosophila melanogaster. J Exp Biol. Published online Nov 29, 2012.  [article]

Cooper BS, Hammad LA, Fisher NP, Karty JA and KL Montooth. In a variable thermal environment selection favors greater plasticity of cell membranes in Drosophila melanogaster. Evolution 66(6):1976-1984      [article]

Montooth, KL, Meiklejohn, CD, Abt, DN and DM Rand (2010) Mitochondrial-nuclear epistasis affects fitness within species but does not contribute to fixed incompatibilities between species of Drosophila.  Evolution 64(12):3364-3379    [article]

Montooth KL, Abt DN, Hofmann JW, Rand DM (2009) Comparative genomics of Drosophila mtDNA:  novel features of conservation and change across functional domains and lineages. Journal of Molecular Evolution 69(1): 94-114.  [article]

Montooth KL, Rand DM (2008) The Spectrum of Mitochondrial Mutation Differs across Species. PLoS Biol 6(8): e213.  [article]

Meiklejohn CD, Montooth KL, Rand DM (2007) Positive and negative selection on the mitochondrial genome. Trends in Genetics 23(6): 259-263.  [article]

Montooth KL, Siebenthall KT, Clark AG (2006) Membrane lipid physiology and toxin catabolism underlie ethanol and acetic acid tolerance in Drosophila melanogaster. J Exp Biol 209(19): 3837-3850.  [article]

Montooth KL, Marden JH, Clark, AG (2003) Mapping determinants of variation in energy metabolism, respiration and flight in Drosophila. Genetics 165(2): 623-635.  [article]

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