Citation:
Balakrishnan CN, Ekblom R, Volker M, Westerdahl H, Godinez R, Kotkiewicz H, Burt DW, Graves T, Griffin DK, Warren WC, et al. Gene duplication and fragmentation in the zebra finch major histocompatibility complex. BMC Biology. 2010;8 (29) :(1 April 2010)-(1 April 2010).
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Abstract:
Background:
Due to its high polymorphism and importance for disease resistance, the major histocompatibility
complex (MHC) has been an important focus of many vertebrate genome projects. Avian MHC organization is of
particular interest because the chicken
Gallus gallus
, the avian species with the best characterized MHC, possesses a
highly streamlined
minimal essential
MHC, which is linked to resistance against specific pathogens. It remains
unclear the extent to which this organization describes the situation in other birds and whether it represents a
derived or ancestral condition. The sequencing of the zebra finch
Taeniopygia guttata
genome, in combination
with targeted bacterial artificial chromosome (BAC) sequencing, has allowed us to characterize an MHC from a
highly divergent and diverse avian lineage, the passerines.
Results:
The zebra finch MHC exhibits a complex structure and history involving gene duplication and
fragmentation. The zebra finch MHC includes multiple Class I and Class II genes, some of which appear to be
pseudogenes, and spans a much more extensive genomic region than the chicken MHC, as evidenced by the
presence of MHC genes on each of seven BACs spanning 739 kb. Cytogenetic (FISH) evidence and the genome
assembly itself place core MHC genes on as many as four chromosomes with TAP and Class I genes mapping to
different chromosomes. MHC Class II regions are further characterized by high endogenous retroviral content.
Lastly, we find strong evidence of selection acting on sites within passerine MHC Class I and Class II genes.
Conclusion:
The zebra finch MHC differs markedly from that of the chicken, the only other bird species with a
complete genome sequence. The apparent lack of synteny between
TAP
and the expressed MHC Class I locus is in
fact reminiscent of a pattern seen in some mammalian lineages and may represent convergent evolution. Our
analyses of the zebra finch MHC suggest a complex history involving chromosomal fission, gene duplication and
translocation in the history of the MHC in birds, and highlight striking differences in MHC structure and
organization among avian lineages