Studies on the maternal effect of the semilethal gene, v, in the Mexican axolotl. I. Influence of temperature on the expression of the effect. II. Cytological changes in the affected embryos

Abstract

The gene, v, in the Mexican axolotl has recently been shown to affect the egg cytoplasm during oogenesis in such a way as to lead to abnormalities in development following fertilization. Affected embryos frequently are arrested in blastula stages and display a remarkable appearance resulting from a stratification of the contents of the blastomeres. Or they may continue to later stages before the maternal effect becomes apparent ( Humphrey, 1962). The present paper is concerned with the influence of temperature on the expression of this maternal effect and with the cytological changes occurring in the affected embryos. The influence of temperature on the maternal effect was investigated by rearing the embryos at 25° C and at 14–20° C. (Both temperatures are well within the range permitting normal development of control eggs.) At 25° a large majority (88%) of the embryos were arrested in mid-blastula stages and exhibited a striking appearance due to a stratification of the contents of the blastomeres. At lower temperatures (14–20°) the expression of the maternal effect was delayed. Less than 1% of the embryos were arrested in mid-blastula stages. Approximately 70% developed to gastrula and neurula stages and 30% to more advanced embryonic stages. Stratification of cell contents was observed in the gastrulae and some neurulae but was less pronounced than that seen in arrested blastulae. The maternal effect results in a striking rearrangement of cell components in embryos arrested in blastula and gastrula stages. The first cytologically detectable change consists in a concentration of the RNA-containing cytoplasm into the upper (animal) parts of the cells. Nuclei are similarly displaced from their normal central location and are found in the upper parts of the cells, wholly or partly within the RNA-containing cytoplasm. These changes in the distribution of nuclei and of cytoplasmic RNA are followed by a sedimentation of yolk platelets and then of pigment granules. Eventually the cells take on a highly stratified appearance. For a time after the beginning of the cytoplasmic stratification the nuclei continue mitotic activity which, however, is usually of an abnormal type. The most common abnormality (at 25°) is an extensive polyploidization, indicating that chromosomal replication continues for a time after nuclear and cell division have been suppressed.