Circletail, a new mouse mutant with severe neural tube defects: Chromosomal localization and interaction with the loop-tail mutation

Circletail (Crc) is a new mouse mutant that exhibits a severe form of neura l tube defect, craniorachischisis, in which almost the entire neural tube f ails to close. This phenotype is seen in very few other mutants, the best c haracterized of which is loop-tail (Ltap(Lp), referred to hereafter as Lp). We tested the possibility of allelism between Lp and Crc by intercrossing Lp/+ and Crc/+ mice. A proportion of double heterozygotes (Lp/+,Crc/+) exhi bit craniorachischisis, revealing failure of complementation. However, gene tic analysis shows that Crc is not linked to the markers that flank the Lp locus and cannot, therefore, be an allele of Lp. A genome-wide scan has loc alized the Crc gene to a region of 8.8 cM on central chromosome 15. Partial penetrance of the craniorachischisis phenotype in Crc/+,Lp/+ double hetero zygotes suggests the existence of a third, unlinked genetic locus that infl uences the interaction between Crc and Lp.