A selection experiment for distinct types of 6‐aminonicotinamide‐induced cleft lip in mice

The face shape hypothesis (Trasler, '68) suggests that the face shape of the embryo at the time of lip formation plays a role in the genetic predisposition to lateral cleft lip (LCL). Predisposed strains like A/J and CL/Fr have narrower inter‐nasal‐pit distances than those not predisposed such as C57BL/6 and C3H. The present paper reports a test of the hypothesis by deriving two new lines of mice from the same heterogeneous stock, selecting for differences in type of response on day 9.5 of gestation to the teratogen 6‐aminonicotinamide (6AN). The M line responds with a high frequency of induced median cleft lip (MCL) and the L line with a low frequency of MCL. The M line has a wider inter‐pit‐distance than the L line. The L line has a 3 to 9% spontaneous LCL frequency and has the same narrow inter‐pit‐distance component of embryonic face shape found in A/J and CL/Fr. The face measurements have already been reported (Juriloff and Trasler, '76). The present paper outlines the selection experiment and discusses the differences obtained between lines for induced CL and for spontaneous LCL. These findings have provided independent support for the face shape hypothesis.