Carolyn M. Hutter and David M. Rand*
Department of Ecology and Evolutionary Biology
Brown University
Providence, RI  02912
 
 
 

*Address for correspondence and reprints:
David M. Rand
Dept. of Ecology and Evolutionary Biology
Box G-W, Brown University
Providence, RI  02912
Ph: (401) 863-2890
Fax: (401) 863-2166
email: David_Rand@brown.edu
 

Running Head:  Fitness of mtDNAs in Drosophila

Key Words:  Mitochondrial DNA, coevolution, nuclear cytoplasmic interaction, fitness, selection, Drosophila, population genetics

 Abstract
 A test for coadaptation of nuclear and mitochondrial genomes was performed using the sibling species, Drosophila pseudoobscura and D. persimilis.   Two lines of flies with ìdisruptedî cytonuclear genotypes were constructed by repeated backcrossing of males from one species to females carrying mtDNA from the other species.  Each "disrupted" strain was competed in population cages with the original stock of each species from which the recurrent males were obtained during the backcrossing.  As such, the two species' mitochondrial types were competed reciprocally in the nuclear genetic environments of each species.  The trajectories of mtDNA haplotypes were followed in discrete-generation population cages using a PCR-four-cutter approach.  A significant increase in the frequency of D. pseudoobscura mtDNA was observed in each of four replicate cages with a D. pseudoobscura nuclear background.  In the D. persimilis nuclear background, one cage actually showed an increase in frequency of D. pseudoobscura mtDNA, although together the four replicate cages show little change in frequency.  These results were repeated after frequency perturbations and re-initiation of each cage.  An analysis of fitness components revealed that fertility selection greatly outweighed viability selection in these cytonuclear competition experiments.  The asymmetry of the fitnesses of the mtDNA haplotypes on the two genetic backgrounds is consistent in direction with the previously reported asymmetry of female fertility in backcrosses between these two species.  While our experiments do not allow us to identify mtDNA as the sole source of fitness variation, at a minimum the data indicate a fitness association between nuclear fertility factors and the D. pseudoobscura mtDNA on its own genetic background.