Reproductive performance in C57BL and I strain mice.

Mice of the I strain are regarded as difficult to breed. To characterize and elucidate the cause of poor reproductive performance in I strain mice, records of reproductive performance were analyzed from Cancer Research Laboratory, Kirschbaum Memorial Laboratory and Strong Research Laboratories which had bred the I strain mice and a control strain, C57BL, and compared with data from Jackson Laboratory and from three dietary studies conducted in this laboratory. Reproductive performance in the I strain mice is characterized by fewer productive matings, fewer litters and smaller litters than C57BL mice. Also, fewer mice of the I strain survive to weaning. These factors result in the production of only half as many mice per dam in the I strain as the C57BL mice. The fewer number of litters is not due to a greater proportion of resorptions in the I mice, nor are there differences in the survival of either sex. Diets which contain slightly higher (8-12 mg per kg of diet) amounts of pyridoxine (PN) than recommended (1-6 mg per kg of diet) improve performance in the I strain. However, 410 or 1230 mg PN does not increase productivity further. That survival rate of the C57BL mice improves with higher amounts of PN (410 and 1230 mg diets), suggests another dietary factor may be limiting for I strain mice. The reproductive performance in I strain mice is better with diets which result in a higher body weight.(ABSTRACT TRUNCATED AT 250 WORDS)