This study, published December 2012 in the journal PLoS ONE, investigated whether strains of Canine distemper virus (CDV) which co-evolved in homogenous domestic dog populations exhibited specialist traits and those of non-dog species exhibited generalist traits. It has been suggested that organisms evolving in homogenous and heterogenous environments show more specialist and generalist traits respectively. Antagonist pleiotropy describes the effects of a gene which under different environmental circumstances would have deleterious or neutral effects and those effects that are neutral in one environment are deleterious in another. This is due to the gene producing multiple competing effects which may be either detrimental or beneficial to the organism.
CDV is a pantropic virus that is a member of the genus Morbillivirus in the family Paramyxoviridae. Variations in the signalling lymphocyte activation molecule (SLAM) receptor of CDV is thought to permit its wide host range.
CDV enters cells via the SLAM receptor on the host cell. The only receptor for CDV hemagglutinin
protein (CDV-H) which has been identified on host lymphatic cells is the SLAM receptor. Virus
attachment to the host cell occurs via CDV-H and a fusion protein (CDV-F). This study illustrated
an increased similarity in amino acid sequence variations of the SLAM receptor in Canidae compared
to those of other families. Protein sequences for the SLAM binding region of the CDV-H protein are
shown below; the amino acid at position 549 for each strain is indicated. The substitution at
position 549 of tyrosine (Y) by histidine (H) is thought to have allowed the spread of CDV to
non-dog species. In this study the amino acid at position 549 of a CDV-H dog strain was changed.
An increase and decrease in the expression of generalist and specialist traits respectively was evident providing confirmation of antagonistic pleiotropy.
If a virus evolves in one species it is anticipated that it will acquire more specialist traits. These viruses are expected to evolve more quickly as homogenous environments are likely to result in an increased selection against detrimental and for favourable traits. Large populations of domestic dogs has permitted virus co-evolution by creating a large homogenous environment for the virus and as a consequence the selection of specialist traits.
In this study cell cultures were used to test the CDV-H of a dog strain and non-dog strains ability to enter host cells. CDV-H of domestic dogs illustrated specialised traits as it had decreased performance with SLAM receptors from other species. In contrast, CDV-H from other species performed equally well with domestic dog SLAM receptors and other species, illustrating generalist traits, but they performed lower than domestic dogs strains with domestic dogs SLAM receptors as expected for specialist contrasted with generalist traits.
The syncytia formation assay showed the mean number of nuclei (MNN) per syncytium formed by CDV-H proteins from domestic dog and non-dog CDV strains in cells expressing SLAM from the domestic dog. The single substitution at position 549 in the SLAM binding region of the CDV-H protein reduced the MNN per syncytium in cell lines expressing domestic dog SLAM receptors and increased the MNN per syncytia in cells expressing the lion SLAM receptors. Similar MNN per syncytium were seen in lion and domestic dog SLAM from non-dog strains. Higher MNN per syncytium were seen in lion and domestic cat compared to those of domestic dogs with CDV-H of non-dog strains. A virus titer assay for the domestic dog strains produced higher virus titers in cell lines expressing domestic dog SLAM receptors than in cell lines expressing non-dog SLAM. Dog strains did not produce higher titers than non-dog strains in cell lines expressing domestic dog SLAM receptors which conflicted with the predicted specialist traits for this strain. Both non-dog strains replicated to similar titer levels in cell lines expressing dog and lion. Both non-dog strains replicated to higher titer levels than domestic dog strains in cell lines expressing lion SLAM and cat SLAM. These two assays provide more evidence for the generalised trait selection of non-dog strains and specialist trait selection for domestic dogs strains.
The results of this study are consistent with the opinion that CDV underwent marked co-evolution in domestic dog populations. Co-evolution in domestic dogs has been facilitated by creating a large homogenous environment for the virus and as a consequence the selection of specialist traits. Anthropogenic activities have considerably affected the epidemiology of CDV due to changes in density of the host species.
Nikolin VM, Osterrieder K, von Messling V, Hofer H, Anderson D, et al. (2012) Antagonistic Pleiotropy and Fitness Trade-Offs Reveal Specialist and Generalist Traits in Strains of Canine Distemper Virus. PLoS ONE 7(12): e50955. doi:10.1371/journal.pone.0050955