David M. Rand, Michele Dorfsman and Lisa M. Kann
Department of Ecology and Evolutionary Biology
Brown University
Providence, RI  02912
 

Address for correspondence:

Dr. David M. Rand
Department of Ecology and Evolutionary Biology
Brown University
Box G-W
Providence, RI  02912

Phone  (401) 863-2890
Fax  (401) 863-2166
email  David_Rand@brown.edu

Key words: mtDNA / neutral theory / Drosophila / selection / nucleotide polymorphism / molecular evolution
 

Running Head: Neutrality tests of mtDNA
 
 

 Abstract
 To test hypotheses of neutral evolution of mitochondrial DNA (mtDNA), nucleotide sequences were determined for 1515 base pairs of the NADH dehydrogenase subunit 5 (ND5) gene in the mitochondrial DNA of 29 lines of Drosophila melanogaster and nine lines of its sibling species D. simulans.  In contrast to the patterns for nuclear genes, where D. melanogaster generally exhibits much less nucleotide polymorphism, the number of segregating sites was slightly higher in a global sample of nine ND5 sequences in D. melanogaster (s = 8) than in the nine lines of D. simulans (s = 6).  When compared to variation at nuclear loci, the mtDNA variation in D. melanogaster does not depart from neutral expectations.  The ND5 sequences in D. simulans, however, show fewer than half the number of variable sites expected under neutrality when compared to sequences from the period locus.  While this reduction in variation is not significant at the 5% level,  HKA tests with published restriction data for mtDNA in D. simulans do show a significant reduction of variation suggesting a selective sweep of variation in the mtDNA in this species.  Tests of neutral evolution based on the ratios of synonymous and replacement polymorphism and divergence are generally consistent with neutral expectations, although a significant excess of amino acid polymorphism within both species is localized in one region of the protein.  The rate of mtDNA evolution has been faster in D. melanogaster than in D. simulans and the population structure of mtDNA is distinct in these species.  The data reveal how different rates of mtDNA evolution between species and different histories of neutral and adaptive evolution within species can compromise historical inferences in population and evolutionary biology.