The albino–deletion complex in the mouse defines genes necessary for development of embryonic and extraembryonic ectoderm*

Abstract

A detailed embryological analysis has been undertaken on embryos carrying the c^4FR60Hd-, c^5FR60Hg- or c^2YPSj- albino deletions of mouse chromosome 7. Embryos homozygous for the c^4FR60Hd deletion are abnormal at day 7 · 5 of gestation. The extraembryonic ectoderm does not develop, and primitive-streak formation and mesoderm production do not occur. In contrast, extensive development of the extraembryonic ectoderm, as well as mesoderm production, are observed in the c^5FR60Hg-and c^2YPSj- homozygous embryos. The mesoderm does not, however, organize into somites and the neural axis does not form. The embryos are grossly abnormal by day 8-5 of development. There are two other albino deletions (c^6H and c^11DSD) that are known to affect the embryo around the time of gastrulation (Niswander et al. 1988), and the lethal phenotype observed for the c^4FR60Hd - homozygous embryos is similar to that described for c^6H- homozygous embryos, whereas the c^5FR60Hg- and c^2YPSj- homozygous embryos display a phenotype that is similar to c^11DSD -homozygous embryos. A detailed complementation analysis using these five deletions revealed that the c^5FR60Hg, c^2YPSj- and C^11DSD deletions could partially complement the phenotype produced by the c^4FR60Hd and c^6H deletions in any combination. Extensive development of the extraembryonic structures and production of mesoderm occurs in the compound heterozygotes. These results suggest that the distal breakpoints of the c^5FR60Hg, c^2YPSj and c^11DSD deletions lie more proximal than the distal breakpoints of the c^4FR60Hd and c^6H deletions. This arrangement defines new functional units of chromosome 7 such that a gene(s) important for normal development of the extraembryonic ectoderm would be located in the distal region of non-overlap between the two groups of deletions, and a gene(s) important for the development of the embryonic ectoderm would be located in the region deleted by both groups of chromosomes. The c^4FR60Hd and c^6H deletions appear to be missing both genes and belong to one complementation group; whereas the c^5FR60Hg, c^2YPSj and c^11DSD deletions are missing only the gene(s) affecting the development of the embryonic ectoderm and, therefore, belong to another complementation group.