Positional cloning of the sex‐linked giant egg (Ge) locus in the silkworm,Bombyx mori

The giant egg (Ge) locus is a Z‐linked mutation that leads to the production of large eggs. Cytological observations suggest that an unusual translocation of a large fragment of the W chromosome bearing a putative egg size‐determining gene, Esd, gave rise to giant egg mutants. However, there is currently no molecular evidence confirming either a W–Z translocation or the presence of Esd on the W chromosome. To elucidate the origin of giant egg mutants, we performed positional cloning. We observed that the Bombyx mori. orthologue of the human Phytanoyl‐CoA dioxygenase domain containing 1 gene (PHYHD1) is disrupted in giant egg mutants. PHYHD1 is highly conserved in eukaryotes and is predicted to be a Fe(II) and 2‐oxoglutarate‐dependent oxygenase. Exon skipping in one of the two available Ge mutants is probably caused by the insertion of a non‐long terminal repeat transposon into intron 4 in the vicinity of the 5′ splice site. Segmental duplication in Ge², an independent allele, was caused by unequal recombination between short interspersed elements inserted into introns 3 and 5. Our results indicate that (1) Bombyx PHYHD1 is responsible for the Ge mutants and that (2) the Ge locus is unrelated to the W‐linked putative Esd. To our knowledge, this is the first report describing the phenotypic defects caused by mutations in PHYHD1 orthologues.