Pattern and timing of diversification in the African freshwater fish genus Distichodus (Characiformes: Distichodontidae)

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Abstract

Background Distichodus is a clade of tropical freshwater fishes currently comprising 25 named species distributed continent-wide throughout the Nilo-Sudan and most Sub-Saharan drainages. This study investigates the phylogenetic relationships, timing of diversification, and biogeographic history of the genus from a taxonomically comprehensive mutilocus dataset analyzed using Maximum Likelihood and Bayesian methods of phylogenetic inference, coalescence-based species-tree estimation, divergence time estimation, and inference of geographic range evolution. Results Analyses of comparative DNA sequence data in a phylogenetic context reveal the existence of two major clades of similar species-level diversity and provide support for the monophyletic status of most sampled species. Biogeographic reconstruction on a time-scaled phylogeny suggest that the origins of the genus date back to the late Oligocene and that current geographic distributions are the result of a Congo Basin origin followed by dispersal and range expansion into adjacent ichthyofaunal provinces at different times during the evolutionary history of the group. Conclusions We present the most comprehensive phylogenetic, chronological, and biogeographic treatment yet conducted for the genus. The few instances of species paraphyly (D. teugelsi, D. fasciolatus) revealed by the resulting phylogenies are likely a consequence of post-divergence introgressive hybridization and/or incomplete lineage sorting due to recent speciation. Historical biogeographic findings are both in agreement and conflict with previous studies of other continent-wide African freshwater fish genera, suggesting a complex scenario for the assemblage of Africa’s continental ichthyofaunal communities.

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Codon-substitution models for heterogeneous selection pressure at amino acid sites.

Shira N Goldman, Q. Z. Yang, Gloria A. Pedersen … (2000)

Comparison of relative fixation rates of synonymous (silent) and nonsynonymous (amino acid-altering) mutations provides a means for understanding the mechanisms of molecular sequence evolution. The nonsynonymous/synonymous rate ratio (omega = d(N)d(S)) is an important indicator of selective pressure at the protein level, with omega = 1 meaning neutral mutations, omega 1 diversifying positive selection. Amino acid sites in a protein are expected to be under different selective pressures and have different underlying omega ratios. We develop models that account for heterogeneous omega ratios among amino acid sites and apply them to phylogenetic analyses of protein-coding DNA sequences. These models are useful for testing for adaptive molecular evolution and identifying amino acid sites under diversifying selection. Ten data sets of genes from nuclear, mitochondrial, and viral genomes are analyzed to estimate the distributions of omega among sites. In all data sets analyzed, the selective pressure indicated by the omega ratio is found to be highly heterogeneous among sites. Previously unsuspected Darwinian selection is detected in several genes in which the average omega ratio across sites is 1. Genes undergoing positive selection include the beta-globin gene from vertebrates, mitochondrial protein-coding genes from hominoids, the hemagglutinin (HA) gene from human influenza virus A, and HIV-1 env, vif, and pol genes. Tests for the presence of positively selected sites and their subsequent identification appear quite robust to the specific distributional form assumed for omega and can be achieved using any of several models we implement. However, we encountered difficulties in estimating the precise distribution of omega among sites from real data sets.

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A likelihood framework for inferring the evolution of geographic range on phylogenetic trees.

Richard Ree, Brian R. Moore, Campbell Webb … (2005)

At a time when historical biogeography appears to be again expanding its scope after a period of focusing primarily on discerning area relationships using cladograms, new inference methods are needed to bring more kinds of data to bear on questions about the geographic history of lineages. Here we describe a likelihood framework for inferring the evolution of geographic range on phylogenies that models lineage dispersal and local extinction in a set of discrete areas as stochastic events in continuous time. Unlike existing methods for estimating ancestral areas, such as dispersal-vicariance analysis, this approach incorporates information on the timing of both lineage divergences and the availability of connections between areas (dispersal routes). Monte Carlo methods are used to estimate branch-specific transition probabilities for geographic ranges, enabling the likelihood of the data (observed species distributions) to be evaluated for a given phylogeny and parameterized paleogeographic model. We demonstrate how the method can be used to address two biogeographic questions: What were the ancestral geographic ranges on a phylogenetic tree? How were those ancestral ranges affected by speciation and inherited by the daughter lineages at cladogenesis events? For illustration we use hypothetical examples and an analysis of a Northern Hemisphere plant clade (Cercis), comparing and contrasting inferences to those obtained from dispersal-vicariance analysis. Although the particular model we implement is somewhat simplistic, the framework itself is flexible and could readily be modified to incorporate additional sources of information and also be extended to address other aspects of historical biogeography.

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A likelihood approach for comparing synonymous and nonsynonymous nucleotide substitution rates, with application to the chloroplast genome.

S Muse, B. S. Gaut (1994)

A model of DNA sequence evolution applicable to coding regions is presented. This represents the first evolutionary model that accounts for dependencies among nucleotides within a codon. The model uses the codon, as opposed to the nucleotide, as the unit of evolution, and is parameterized in terms of synonymous and nonsynonymous nucleotide substitution rates. One of the model's advantages over those used in methods for estimating synonymous and nonsynonymous substitution rates is that it completely corrects for multiple hits at a codon, rather than taking a parsimony approach and considering only pathways of minimum change between homologous codons. Likelihood-ratio versions of the relative-rate test are constructed and applied to data from the complete chloroplast DNA sequences of Oryza sativa, Nicotiana tabacum, and Marchantia polymorpha. Results of these tests confirm previous findings that substitution rates in the chloroplast genome are subject to both lineage-specific and locus-specific effects. Additionally, the new tests suggest tha the rate heterogeneity is due primarily to differences in nonsynonymous substitution rates. Simulations help confirm previous suggestions that silent sites are saturated, leaving no evidence of heterogeneity in synonymous substitution rates.