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Review: Disparate Rates of Molecular Evolution in Cospeciating Hosts and Parasites

Citation

  • Hafner, M. S., Sudman, P. D., Villablanca, F. X., Spradling, T. A., Demastes, J. W. & Nadler, S. A. (1994). Disparate rates of molecular evolution in cospeciating hosts and parasites. Science, 265(5175), 1087–1090.
  • DOI

Abstract

Mitochondrial cytochrome oxidase I sequences from pocket gophers and their chewing-louse ectoparasites confirm that the two groups have cospeciated: the host and parasite phylogenies are congruent beyond chance. The rate of molecular evolution, however, differs markedly between them. Overall nucleotide substitution is about three times faster in lice, and synonymous substitution at fourfold-degenerate sites is roughly an order of magnitude faster, a difference that correlates with the difference in generation time between the two groups.

Where the 1988 study used allozymes, this one uses DNA sequence (mitochondrial COI) for the same pocket-gopher and chewing-louse system, and it separates two questions. The first is topological: multiple methods (parsimony, maximum likelihood, Fitch-Margoliash, and neighbor-joining) recover congruent host and parasite trees, and Mantel tests confirm the genetic distances are correlated, so the cospeciation signal holds under sequence data and under several inference models.

The second question is about rate. Once the trees are aligned, the branch lengths are not: louse branches are consistently longer. The overall substitution rate is about threefold higher in lice, and at fourfold-degenerate (silent) sites the rate is about elevenfold higher. The authors trace this to generation time, roughly one year in gophers against about forty days in lice, consistent with a generation-time effect under the neutral theory once the rate is expressed per generation. Congruent topology and disparate branch length coexist.

Relevance to Hifuku

The finding that motivates Hifuku is that two lineages can share a branching history yet evolve at very different rates, so their trees agree in topology while disagreeing in branch length. Hifuku's tree metric is built to see exactly this split: the Robinson-Foulds term measures topological agreement and the clade branch score measures branch-length agreement, and the weight \( w \) tunes between them (Section 5). Surveying several genes on one shared chart lets a reader see where gene maps coincide in topology but separate in branch length, the sequence-level analogue of the host-parasite comparison here. Rate variation of this kind is also why the likelihood model carries among-site rate heterogeneity (see Yang 1994). See Hafner & Nadler 1988 and Demastes et al. 2012 for the same assemblage.