Systematic Relationships Among Waterfowl (Anatidae) Inferred from Restriction Endonuclease Analysis of Mitochondrial DNA

Abstract

To evaluate the potential of mitochondrial DNA (mtDNA) analysis for avian systematics, we have assayed mtDNA differences among 13 species of waterfowl in the genera Anas and Aythya (Anseriformes: Anatidae). Purified mtDNA was digested with each of 15 different type II restriction endonucleases which cleave at five- or six-base recognition sequences. Side-by-side comparisons of digestion profiles permitted the estimation of levels of fragment homology and nucleotide sequence divergence (p). Among nine Anas species, mean sequence divergence (p) was 0.062 (range 0.004-0.088); among four Aythya species, 0.034 (0.025-0.043); between selected species belonging to separate genera, 0.109. Phylogenetic trees and dendrograms were constructed from qualitative and quantitative data bases by a variety of procedures including undirected parsimony (Penny and Wagner algorithms), undirected compatibility (Estabrook algorithms), and phenetic clustering. These trees were highly concordant with one another, and with traditional phylogenies derived from independent sources of information. Previously published evidence from mammals has suggested a "saturation effect" on level of mtDNA differentiation: for p less than 0.15-0.20, mtDNA distances are reportedly linearly related to time since common ancestry, but for larger p values the relationship becomes curvilinear as differentiation approaches an observed plateau at a p of approximately 0.30. Our estimates of mtDNA sequence divergence among congeneric waterfowl fall in a broad range well within the expected linear portion of the curve. This observation, coupled with the general concordance of mtDNA-generated trees with those derived from independent information, demonstrates that the restriction fragment approach to mtDNA analysis should provide an important new molecular technique for studying evolutionary relationships among lower taxonomic levels in Aves.