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 genetica lly defined, natural transplantation reaction when the edges of two gr owing 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 colon ies no longer interact. Previous studies have demonstrated that allore cognition in Botryllus is principally controlled by a single Mendelian locus named the fusion/histocompatibility (Fu/HC) locus, with multipl e 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 identifie d as AFLPs) as molecular genetic markers. DNA polymorphisms are identi fied in inbred laboratory strains of Fu/HC defined Botryllus, and thei r 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 whic h delineates the Fu/HC locus to a 5.5 cM region.