Hemizygous subtelomeres of an African trypanosome chromosome may account for over 75% of chromosome length

Abstract

African trypanosomes are parasitic protozoa that infect a wide range of mammals, including humans. These parasites remain extracellular in the mammalian bloodstream, where antigenic variation allows them to survive the immune response. The Trypanosoma brucei nuclear genome sequence has been published recently. However, the significant chromosome size polymorphism observed among strains and subspecies of T. brucei, where total DNA content may vary up to 30%, necessitates a comparative study to determine the underlying basis and significance of such variation between parasites. In addition, the sequenced strain (Tb927) presents one of the smallest genomes analyzed among T. brucei isolates; therefore, establishing polymorphic regions will provide essential complementary information to the sequencing project. We have developed a Tb927 high-resolution DNA microarray to study DNA content variation along chromosome I, one of the most size-variable chromosomes, in different strains and subspecies of T. brucei. Results show considerable copy number polymorphism, especially at subtelomeres, but are insufficient to explain the observed size difference. Additional sequencing reveals that >50% of a larger chromosome I consists of arrays of variant surface glycoprotein genes (VSGs), involved in avoidance of acquired immunity. In total, the subtelomeres appear to be three times larger than the diploid core. These results reveal that trypanosomes can utilize subtelomeres for amplification and divergence of gene families to such a remarkable extent that they may constitute most of a chromosome, and that the VSG repertoire may be even larger than reported to date. Further experimentation is required to determine if these results are applicable to all size-variable chromosomes.