FIXED PATTERNS OF BRADYCARDIA DURING LATE EMBRYONIC-DEVELOPMENT IN DOMESTIC-FOWL WITH C-LOCUS MUTATIONS

A predictable late embryonic bradycardia (relative to normal White Leg horn chickens) has been documented in chicken strains with C locus mut ations. The basis of the bradycardia remains unknown but clearly is re lated to a mutation at the C locus, which contains the structural gene for tyrosinase. When compared with the heart rate of normal White Leg horns (similar to 295-305 beats/min from day 8 to day 20 of incubation ), c(a)/c(a) and other C locus mutants showed a 10-12% reduction in he art rate during the last 4 days of incubation. Embryonic mortality occ urred in both mutant and normal strains at an equivalent rate (similar to 23%); a significant bradycardia (when compared with surviving embr yos of the same strain) developed on the day before death in White Leg horn but not mutant strains. The bradycardia did not affect embryonic oxygen consumption(similar to 0.2 ml O-2.egg(-1).min(-1) at day 14 and 0.4 ml O-2.egg(-1).min(-1) at day 20), which showed only minor differ ences between strains that can be attributed to differences in embryon ic mass on days 16-20.