Molecular Evolution of Duplicated Ray Finned Fish HoxA Clusters: Increased Synonymous Substitution Rate and Asymmetrical Co-divergence of Coding and Non-coding Sequences

In this study the molecular evolution of duplicated HoxA genes in zebrafish and fugu has been investigated. All 18 duplicated HoxA genes studied have a higher non-synonymous substitution rate than the corresponding genes in either bichir or paddlefish, where these genes are not duplicated. The higher rate of evolution is not due solely to a higher non-synonymous-to-synonymous rate ratio but to an increase in both the non-synonymous as well as the synonymous substitution rate. The synonymous rate increase can be explained by a change in base composition, codon usage, or mutation rate. We found no changes in nucleotide composition or codon bias. Thus, we suggest that the HoxA genes may experience an increased mutation rate following cluster duplication. In the non-Hox nuclear gene RAG1 only an increase in non-synonymous substitutions could be detected, suggesting that the increased mutation rate is specific to duplicated Hox clusters and might be related to the structural instability of Hox clusters following duplication. The divergence among paralog genes tends to be asymmetric, with one paralog diverging faster than the other. In fugu, all b-paralogs diverge faster than the a-paralogs, while in zebrafish Hoxa-13a diverges faster. This asymmetry corresponds to the asymmetry in the divergence rate of conserved non-coding sequences, i.e., putative cis-regulatory elements. These results suggest that the 5′ HoxA genes in the same cluster belong to a co-evolutionary unit in which genes have a tendency to diverge together. Reviewing Editor: Dr. Axel Meyer     

Host genotype changes bidirectional to unidirectional cytoplasmic incompatibility in Nasonia longicornis

Wolbachia are the most abundant maternally inherited endosymbionts of insects and cause various reproductive alterations in their hosts. One such manipulation is cytoplasmic incompatibility (CI), which is a sperm-egg incompatibility typically resulting in zygotic death. Nasonia longicornis (Hymenoptera: Pteromalidae) has an A supergroup and two closely related B supergroup Wolbachia infections. The B supergroup bacteria co-diverged in this host genus. Both triple (wNlonAwNlonB1wNlonB2) and double infections (wNlonAwNlonB1, wNlonAwNlonB2) have been obtained from the field. In the present study, CI was determined among the three Wolbachia types in different host genetic backgrounds. Results show that host genetic background determines whether bidirectional CI or unidirectional CI occurs between the two closely related B group Wolbachia. Results show that the wNlonB1-infected males are bidirectionally incompatible with wNlonB2 in their 'native' nuclear genetic background, whereas wNlonB1 males are compatible with wNlonB2 in two other N. longicornis genetic backgrounds, resulting in unidirectional CI. In contrast, wNlonB2-infected males are incompatible with wNlonB1 females in all three host genetic backgrounds. These changes in incompatibility are not due to the loss of the bacteria. We hypothesize that a repressor gene for sperm modification by wNlonB1 is segregating in N. longicornis populations. The relevance of these findings to the potential role of Wolbachia in host-reproductive divergence and speciation is discussed.     

The evolutionary history of parasitic sucking lice and their rodent hosts: A case of evolutionary co-divergences

Parasites are often dependent on their hosts for survival and dispersal and this led to a hypothesis that the evolution of obligate permanent host-specific parasites specifically will show phylogenetic congruence with their hosts. To investigate the factors influencing parasite evolution, mitochondrial- and nuclear DNA sequence data were used to test for genetic co-divergences between Hoplopleura and Polyplax lice occurring on four rodent taxa associated with the Aethomys/Micaelamys rodent complex. Mitochondrial DNA haplotype networks drawn from 24 rodents and 74 obligate permanent lice supported the existence of at least eight genetically distinct parasite lineages. Bayesian and maximum likelihood analyses showed considerable congruence between the phylogenies of the parasites and their hosts, and these finding were also partly supported in a fair amount of overlap in the timing of divergences. Jane co-phylogenetic reconstructions illustrated that co-divergences are the most parsimonious solution to explain the evolution within Polyplax, and also within Hoplopleura. Based on mitochondrial DNA cytochrome oxidase subunit I sequence distances of >13% between strongly supported monophyletic parasite lineages, coupled to some unique morphological features detected for both lice taxa occurring on Micaelamys granti, we propose that the taxonomy of Polyplax and Hoplopleura studied herein is in need of revision. In addition, our findings provide examples of perfect topological co-divergence between parasites and their hosts, and in doing so also provide circumstantial evidence for co-evolution between these permanent host-specific parasites and their hosts. We argue that the signal of co-divergence is partly enforced by limited opportunities for host switching events across the landscape.     

Birds are islands for parasites

Understanding the mechanisms driving the extraordinary diversification of parasites is a major challenge in evolutionary biology. Co-speciation, one proposed mechanism that could contribute to this diversity is hypothesized to result from allopatric co-divergence of host–parasite populations. We found that island populations of the Galápagos hawk (Buteo galapagoensis) and a parasitic feather louse species (Degeeriella regalis) exhibit patterns of co-divergence across variable temporal and spatial scales. Hawks and lice showed nearly identical population genetic structure across the Galápagos Islands. Hawk population genetic structure is explained by isolation by distance among islands. Louse population structure is best explained by hawk population structure, rather than isolation by distance per se, suggesting that lice tightly track the recent population histories of their hosts. Among hawk individuals, louse populations were also highly structured, suggesting that hosts serve as islands for parasites from an evolutionary perspective. Altogether, we found that host and parasite populations may have responded in the same manner to geographical isolation across spatial scales. Allopatric co-divergence is likely one important mechanism driving the diversification of parasites.