Pre-implantation embryos of Chinese hamster. I. Incidence of karyotype anomalies in 226 control embryos.

Karyotyes were determined in 226 pre-implantation embryos (4--8-cell stages) of Chinese hamster. The study was carried out under controlled natural breeding conditions, without superovulation and with the embryos developing in their mothers. A total of 5.3% karyotypically abnormal embryos were found. Over half, 3.1%, were due to ploidy mutations, 5 cases of triploidy and 2 cases of haploidy. Only 0.9% genome mutations were present, consisting of one autosomal trisomy and one autosomal monosomy. Structural aberrations were found in 1.8%, half of these probably due to a balanced maternal aberration and the rest appearing the mosaic condition only. These results are compared with the scarce body of mammalian data from the literature. Compared with the situation in man, the spontaneous aberration rates in the Chinese hamster and other experimental mammals are extremely low. This may be due, in part, to optimal timing of copulation in respect to estrus and ovulation prevailing in these animals but not in man. The low spontaneous aberration rate in the reported system is a valuable asset for purposes of mutagen testing.