Mapping the genome of a model protochordate. I. A low resolution genetic map encompassing the fusion/histocompatibility (Fu/HC) locus of Botryllus schlosseri.

The colonial protochordate, Botryllus schlosseri, undergoes a genetically defined, natural transplantation reaction when the edges of two growing colonies interact. Peripheral blood vessels of each colony touch and will either fuse together to form a common vasculature between the colonies, or reject each other in an active blood-based inflammatory process in which the interacting vessels are cut off and the two colonies no longer interact. Previous studies have demonstrated that allorecognition in Botryllus is principally controlled by a single Mendelian locus named the fusion/histocompatibility (Fu/HC) locus, with multiple codominantly expressed alleles. However, identification and cloning of this locus has been difficult. We are taking a genomic approach in isolating this locus by creating a detailed genetic linkage map of the 725 Mbp Botryllus genome using DNA polymorphisms (primarily identified as AFLPs) as molecular genetic markers. DNA polymorphisms are identified in inbred laboratory strains of Fu/HC defined Botryllus, and their segregation and linkage is analyzed in a series of defined crosses. Using bulk segregant analysis, we have focused our mapping efforts on the Fu/HC region of the genome, and have generated an initial map which delineates the Fu/HC locus to a 5.5 cM region.