Chromosomal Abnormalities and Sex Ratio in Rabbit Blastocysts

Summary. Chromosomal analyses were made of 463 rabbit blastocysts drawn from thirty-nine superovulated does of four strains. Semen containing 1·5% diploid spermatozoa was taken from one strain only. Artificial insemination was conducted with untreated semen or with centrifuged fractions containing either 0·4% or 2·9% diploid spermatozoa. The results indicate that diploid spermatozoa are not a major cause of triploidy among embryos. The sex ratio scored chromosomally among 434 diploid blastocysts was 48·62 (±2·40)% males. With data in the literature incorporated, the average sex ratio among 1077 blastocysts (consistent over three species of mammal) became 50·42 (±1·52)% males. These figures may be taken to estimate the primary sex ratio. Sex ratio among blastocysts is extremely stable over various biological and experimental conditions. Twenty-three (5%) of the 463 analysable blastocysts were heteroploid. They comprised eight triploids (5 XXX, 3 XXY), four trisomics, four diploid/trisomic mosaics, one diploid/monosomic mosaic, four diploid/ tetraploid mosaics, one diploid/triploid mosaic and one triploid/ hexaploid mosaic. `Clustering' of heteroploids occurred in particular dams. Superovulation had no demonstrable effect on the incidence of heteroploidy. There is some evidence of genetic (dam strain) effects on the incidence.