The disruptive effects of microtubule-specific agents on pronuclear movement illustrate the requirement of an intact cytoskeletal system for movement. In this study, we investigated the effects of high hydrostatic pressure and deuterium oxide (D2O) on fertilized Rana pipiens eggs during the time of pronuclear migration. The eggs were either pulsed for six min with 3000, 5000, or 7000 psi or placed for ten min in 80% D2O between the time of second polar body emission and first cleavage. Both treatments disrupted male pronuclear migration as shown by eccentric first cleavage furrows. Treatment of eggs prior to pronuclear association resulted in haploid production. The androgenetic origin of the haploid embryos was demonstrated using morphological and isozymal markers produced by the cross Rana pipiens female x Rana utricularia male. Eggs treated with D2O also yielded embryos with neural defects identical to those following ultraviolet irradiation. This study complements the recent reports on pressure-suppression of the second polar body and of first cleavage by showing that the selective suppression of microtubular function between these two events produces an entirely different set of genetic and developmental consequences.
Hybrids between species frequently arrest early in development. In the frog hybrid Rana catesbeiana female x Rana clamitans male, the embryo shows a characteristic development to an exogastrula which dies. This hybrid can be rescued by pressure suppression of the second polar body, which results in the addition of another haploid set of R catesbeiana chromosomes to the embryo. The triploid hybrid expresses genes from both species and can develop normally through metamorphosis. The results show that an R catesbeiana egg containing a full haploid set of R clamitans chromosomes is capable of development and that the usual developmental arrest caused by the R clamitans genome responds to chromosomal dosage. Key words: amphibian hybrids, exogastrulation, hybrid lethality, nucleocytoplasmic interactions, trip I o i d y I NTRO DU CTlO NFertilization produces new combinations of nuclei and cytoplasm, which by their interactions, direct early development. It is necessary that these interactions occur properly since alterations of early developmental events often lead to death of the embryo. This is a frequent result when hybrid embryos are produced. The nucleus from the sperm of one species is unable to interact correctly with the nucleus and cytoplasm of the egg of the second species, and the hybrid arrests at an early embryonic stage.On the one hand, we can consider lethal hybrids as developmental deadends, but on the other hand, we can view the hybrid embryo as one following an alternative developmental pathway. This pathway can be analyzed in comparison with the normal one in an attempt to dissect early nucleocytoplasmic interactions. With this aim in mind, hybrids between the green frog Rana clamitans and the bullfrog Rana catesbeiana have been studied. Offspring of the R catesbeiana female x R clamitans male cross arrest at gastrulation, while offspring of the R clamitans female x R catesbeiana male cross are viable and fertile [l-31. The normal development when the two nuclei are in R clamitans egg cytoplasm shows that the genomes from the
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