The fate of parthenogenetic cells was investigated during development of fetal and early postnatal chimeras. On day 13 of embryonic development, considerable contribution of parthenogenetic cells was observed in all tissues of chimeric embryos, although selection against parthenogenetic cells seemed to start before day 13. Between days 13 and 15 of development, parthenogenetic cells came under severe selective pressure, which was most striking in tongue. The disappearance of parthenogenetic cells from tongue coincided with the beginning of myoblast fusion in this tissue. Severe selection against parthenogenetic cells was also observed in pancreas and liver, although in the latter, parthenogenetic cells were eliminated later than in skeletal muscle or pancreas. In other tissues, parthenogenetic cells may persist and participate to a considerable extent throughout the gestation period and beyond, although a significant decrease was observed in all tissues. Parthenogen-etic↔fertilized chimeras were significantly smaller than their non-chimeric littermates at all developmental stages. These results suggest that the absence of paternal chromosomes is largely incompatible with the maintenance of specific differentiated cell types. Furthermore, paternally derived genes seem to be involved in the regulation of proliferation of all cell types, as indicated by the drastic growth deceleration of parthenogen-etic↔fertilized chimeras and the overall decrease of parthenogenetic cells during fetal development. Chromosomal imprinting may have a role in maintaining a balance between cell growth and differentiation during embryonic development.

The major exception to the selective elimination of parthenogenetic cells appear to be the germ cells; viable offspring derived from parthenogenetic oocytes were detected, sometimes at a high frequency in litters of female parthenogenetic↔fertilized chimeras.