Sequencing of VDJ genes in Lepus americanus confirms a correlation between VHn expression and the leporid species continent of origin

The IgA of hares (Lepus sp.) and rabbit confirms that the leporids IgA explosion is old and reveals a new case of trans-species polymorphism

Background

Immunoglobulin A (IgA) is the mammalian mucosal antibody, providing an important line of defense against pathogens. With 15 IgA subclasses, the European rabbit has an extremely complex IgA system, strikingly more complex than most other mammals, which have only one IgA or, in the case of hominoids, two IgA subclasses. Similar to the two hominoid primate IGHA genes, the expansion of the rabbit IGHA genes appears to have begun in an ancestral lagomorph since multiple IgA copies were found by Southern blot analysis for the genera Sylvilagus, Lepus, and Ochotona.

Results

To gain a better insight into the extraordinary lagomorph IgA evolution, we sequenced, for the first time, expressed IgA genes for two Lepus species, L. europaeus and L. granatensis. These were aligned with the 15 rabbit IgA isotypes, and evolutionary analyses were conducted. The obtained phylogenetic tree shows that the Lepus IgA sequences cluster with and among the rabbit IgA isotypes, and the interspecies and intraspecies nucleotide genetic distances are similar. A comparison of the amino acid sequences of the Lepus and rabbit IgA confirms that there are two trans-species polymorphisms and that the rabbit and Lepus sequences share a common genetic pool. In fact, the main differences between the studied leporids IgAs reside in the characteristics of the hinge region.

Conclusion

The Lepus IgA sequences we have obtained strongly suggest that the great expansion of the leporid IGHA genes occurred in a common ancestral species and was then maintained in the descendants. A strong selective pressure caused the extraordinary expansion of the IGHA genes but then subsided, leading to the maintenance of the acquired polymorphisms in the descendants, with little subsequent divergence. This is a unique evolutionary pattern in which an ancient gene expansion has been maintained for approximately 18 million years.

Evolutionary analyses of polymeric immunoglobulin receptor (pIgR) in the mammals reveals an outstanding mutation rate in the lagomorphs

BACKGROUND

The transcytosis of polymeric immunoglobulins, IgA and IgM, across the epithelial barrier to the luminal side of mucosal tissues is mediated by the polymeric immunoglobulin receptor (pIgR). At the luminal side the extracellular ligand binding region of pIgR, the secretory component (SC), is cleaved and released bound to dimeric IgA (dIgA), protecting it from proteolytic degradation, or in free form, protecting the mucosa form pathogens attacks. The pIgR was first cloned for rabbit in early 1980's and since then has been described for all vertebrates, from fish to mammals. The existence of more than one functional pIgR alternative-spliced variant in the European rabbit, the complete pIgR as other mammals and a shorter pIgR lacking two SC exons, raised the question whether other lagomorphs share the same characteristics and how has the PIGR gene evolved in these mammals.

RESULTS

To investigate these questions, we sequenced expressed pIgR genes for other leporid genus, Lepus spp., and obtained and aligned pIgR sequences from representative species of all mammalian orders. The obtained mammalian phylogeny, as well as the Bayesian inference of evolutionary rates and genetic distances, show that Lagomorpha pIgR is evolving at a higher substitution rate. Codon-based analyses of positive selection show that mammalian pIgR is evolving under strong positive selection, with strong incidence in the domains excised from the rabbit short pIgR isoform. We further confirmed that the hares also express the two rabbit pIgR isoforms.

CONCLUSIONS

The Lagomorpha pIgR unique evolutionary pattern may reflect a group specific adaptation. The pIgR evolution may be linked to the unusual expansion of IgA genes observed in lagomorphs, or to neofunctionalization in this group. Further studies are necessary to clarify the driving forces behind the unique lagomorph pIgR evolution.

Lagomorpha as a Model Morphological System

Due to their global distribution, invasive history, and unique characteristics, European rabbits are recognizable almost anywhere on our planet. Although they are members of a much larger group of living and extinct mammals [Mammalia, Lagomorpha (rabbits, hares, and pikas)], the group is often characterized by several well-known genera (e.g., Oryctolagus, Sylvilagus, Lepus, and Ochotona). This representation does not capture the extraordinary diversity of behavior and form found throughout the order. Model organisms are commonly used as exemplars for biological research, but there are a limited number of model clades or lineages that have been used to study evolutionary morphology in a more explicitly comparative way. We present this review paper to show that lagomorphs are a strong system in which to study macro- and micro-scale patterns of morphological change within a clade that offers underappreciated levels of diversity. To this end, we offer a summary of the status of relevant aspects of lagomorph biology.