Wednesday, February 22, 2012

Parasite Burden, MHC & the Malagasy Mouse Lemur

In the February 2005 issue of the International Journal of Evolution there is a study published by Schad et al. that analyzes the role of parasite burden and the major histocompatability (MHC) complex in the Malagasy mouse lemur (Microcebus murinus). The study is notable as the first to examine the relationship between parasite burden and MHC variation in a free-ranging primate. Schad et al. hypothesized that pathogen-driven selection acting by frequency-dependent selection maintains MHC polymorphisms in M. murinus. The team examined four littoral forest fragments in southeastern Madagascar and found that "fourteen different MHC class II DRB-exon 2 alleles were found in 228 individuals with high levels of sequence divergence between alleles." Nematode infection status (either infected or not) differed between the four forest areas, with heterozygosity not statistically correlated to the number of nematode morphotypes nor to the fecal egg count values per individual. 

A positive relationship though, was found between specific parallel loads and three specific alleles that had unique amino acide motifs in the antigen binding sites; supporting the team's original hypothesis. Schad et al. conclude that further investigation is needed to support selection hypotheses and that allele frequencies and that parasite burdens need to be followed through time in order to better unsterstand the role of parasite selection and MHC diversity in free-ranging populations.

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