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Background  African trypanosomes (including  Trypanosoma brucei ) are unicellular parasites which multiply in the mammalian bloodstream.  T. brucei  has about twenty telomeric bloodstream form Variant Surface Glycoprotein ( VSG ) expression sites (BESs), of which one is expressed at a time in a mutually exclusive fashion. BESs are polycistronic transcription units, containing a variety of families of expression site associated genes ( ESAG s) in addition to the telomeric  VSG . These polymorphic  ESAG  families are thought to play a role in parasite-host adaptation, and it has been proposed that ESAG diversity might be related to host range. Analysis of the genetic diversity of these telomeric gene families has been confounded by the underrepresentation of telomeric sequences in standard libraries. We have previously developed a method to selectively isolate sets of trypanosome BES containing telomeres using Transformation associated recombination (TAR) cloning in yeast.    Results  Here we describe the isolation of repertoires of BES containing telomeres from three trypanosome subspecies:  Trypanosoma brucei gambiense  DAL 972 (causative agent of West-African trypanosomiasis),  T. b. brucei  EATRO 2340 (a nonhuman infective strain) and  T. equiperdum  STIB 818 (which causes a sexually transmitted disease in equines). We have sequenced and analysed the genetic diversity at four BES loci (BES promoter region,  ESAG6 ,  ESAG5  and  ESAG2 ) from these three trypanosome BES repertoires.    Conclusion  With the exception of ESAG2, the BES sequence repertoires derived from  T. b. gambiense  are both less diverse than and nearly reciprocally monophyletic relative to those from  T. b. brucei  and  T. equiperdum . Furthermore, although we find evidence for adaptive evolution in all three ESAG repertoires in  T. b. brucei  and  T. equiperdum , only ESAG2 appears to be under diversifying selection in  T. b. gambiense . This low level of variation in the  T. b. gambiense  BES sequence repertoires is consistent both with the relatively narrow host range of this subspecies and its apparent long-term clonality. However, our data does not show a clear correlation between size of trypanosome host range and either number of BESs or extent of  ESAG  genetic diversity.

bmc genomics

Isolation and analysis of the genetic diversity of repertoires of VSG expression site containing telomeres from Trypanosoma brucei gambiense, T. b. brucei and T. equiperdum