Behavioral Ecology Blog

<-- H. glaber is one UGLY rodent!!

In The current issue of The Biological Journal of the Linnean Society, SAMIT KUNDU, CHRISTOPHER G. FAULKES present some results in a paper, “A tangled history: patterns of major histocompatibility complex evolution in the African mole-rats (Family: Bathyergidae)“

Abstract:Phylogenetic trees based upon major histocompatibility complex (MHC) gene sequences, particularly those encompassing sites encoding the antigen recognition site, are often discordant with the species tree. It has been argued that the principal cause of such discordance is the presence of ancestrally derived polymorphisms persisting through speciation events as a consequence of selection. In the present study, we examine the evolution of the MHC class II DQα1 gene in an unusual family of hystricomorph rodents, the African mole-rats (Family: Bathyergidae). We show that there is a high level of trans-species polymorphism and that this is a result of positive selection. Furthermore, the major lineages of the gene tree are characterized by allelic motifs occurring in regions that coincide with the pocket domains of the putative antigen recognition site, a region that has been shown to be under positive selection in a number of MHC genes from a range of species. Finally, these alleles may have been retained for at least 48 million years. This is significantly older than the estimate for the equivalent primate locus and appears to be one of the oldest documented sets of MHC alleles. We suggest that these allelic motifs possess polymorphisms that have been immunologically important to African mole-rats over long periods of evolutionary history.

So what is trans species polymorphism???

• usually assumed to be likely when allelic lineages from seemingly divergent species appear to intermingle. See tanglegram to the right…
• where selection is postulated to have retained ancestral polymorphisms through speciation events known as deep coalescences

So what’s so cool about this?
Trams species polymorphism is a commonly invoked hypothesis for gene tree/species tree disconcordance- particularly with MHC genes that are thought to be under strong selection, but the Bathyergidae is a super-old group. Divergence is thought to have occured some 48 million years ago- alleles from the naked mole-rat (H. glaber), representing the basal lineage, occur in all four of the major BLA-DQα1 lineages, and are significantly older than the estimate for the equivalent primate locus and appears to be one of the oldest documented sets of MHC alleles.

They go on to say:

The African mole-rats in the present study occupy a wide range of habitat types and encompass a variety of social and mating systems. Interestingly, the persistence over a long period of evolution of polymorphisms at the antigen recognition site shared by these mole-rat species suggests that, despite their distinct life histories, there is a degree of homogeneity in the nature of parasitism occurring in these species (at least for the parasites affected by BLA-DQα1).

For those of you that are paying attention- they attempt to rule out convergence, by looking at intron 2- a linked “neutral” chunk of DNA. They find a similar pattern indicating trans species polymorphism- NOT convergence…