title: NextGen sequencing reveals short double crossovers contribute disproportionately to genetic diversity in Toxoplasma gondii.
abstract: BACKGROUND
  Toxoplasma gondii is a widespread protozoan parasite of animals that causes zoonotic disease in humans. Three clonal variants predominate in North America and Europe, while South American strains are genetically diverse, and undergo more frequent recombination. All three northern clonal variants share a monomorphic version of chromosome Ia (ChrIa), which is also found in unrelated, but successful southern lineages. Although this pattern could reflect a selective advantage, it might also arise from non-Mendelian segregation during meiosis. To understand the inheritance of ChrIa, we performed a genetic cross between the northern clonal type 2 ME49 strain and a divergent southern type 10 strain called VAND, which harbors a divergent ChrIa.
  
  
  RESULTS
  NextGen sequencing of haploid F1 progeny was used to generate a genetic map revealing a low level of conventional recombination, with an unexpectedly high frequency of short, double crossovers. Notably, both the monomorphic and divergent versions of ChrIa were isolated with equal frequency. As well, ChrIa showed no evidence of being a sex chromosome, of harboring an inversion, or distorting patterns of segregation. Although VAND was unable to self fertilize in the cat, it underwent successful out-crossing with ME49 and hybrid survival was strongly associated with inheritance of ChrIII from ME49 and ChrIb from VAND.
  
  
  CONCLUSIONS
  Our findings suggest that the successful spread of the monomorphic ChrIa in the wild has not been driven by meiotic drive or related processes, but rather is due to a fitness advantage. As well, the high frequency of short double crossovers is expected to greatly increase genetic diversity among progeny from genetic crosses, thereby providing an unexpected and likely important source of diversity.
authors.forename: Asis
authors.initials: A
authors.lastname: Khan
authors.forename: Jahangheer S
authors.initials: JS
authors.lastname: Shaik
authors.forename: Michael
authors.initials: M
authors.lastname: Behnke
authors.forename: Qiuling
authors.initials: Q
authors.lastname: Wang
authors.forename: Jitender P
authors.initials: JP
authors.lastname: Dubey
authors.forename: Hernan A
authors.initials: HA
authors.lastname: Lorenzi
authors.forename: James W
authors.initials: JW
authors.lastname: Ajioka
authors.forename: Benjamin M
authors.initials: BM
authors.lastname: Rosenthal
authors.affiliation: Department of Molecular Microbiology, Washington University School of Medicine, Campus Box 8230, 660S, Euclid Ave,, St, Louis, Mo 63110, USA. sibley@wustl.edu.
authors.forename: L David
authors.initials: LD
authors.lastname: Sibley
mesh_terms: D000818:Animals; D002415:Cats; D002874:Chromosome Mapping; D002875:Chromosomes; D003433:Crosses, Genetic; D003434:Crossing Over, Genetic; D019143:Evolution, Molecular; D008040:Genetic Linkage; D014644:Genetic Variation; D018503:Genome, Protozoan; D023281:Genomics; D059014:High-Throughput Nucleotide Sequencing; D051379:Mice; D020641:Polymorphism, Single Nucleotide; D011995:Recombination, Genetic; D014122:Toxoplasma / Q000235:genetics*