Gene Amplification in the Oocyte Nucleus of Mutant and Wild-type Xenopus laevis

Abstract

DURING the development of the amphibian oocyte, the germinal vesicle becomes enlarged and can be seen to contain very many nucleoli^1,2 instead of the four which would be expected from the tetraploidy of the egg nucleus. The RNA of these nucleoli is similar in base composition to ribosomal RNA³ and each nucleolus contains a small amount of DNA^4–6. Amphibian oocyte nucleoli can be hypotonically disrupted to reveal DNP rings in the shape of beaded circles or necklaces^2,6. There is compelling evidence in amphibia^2,6,7 as well as in the insect Tipula⁸ that this DNA is derived from the chromosomal nucleolar organizer region which contains the ribosomal cistrons^9,10. This conclusion is supported by the recent finding that in ovarian tissue of Xenopus there is a disproportionately greater synthesis of ribosomal cistrons¹¹ which leads eventually to a higher proportion of them than that found in DNA of somatic tissues^11,12. In Bufo, the nucleolar DNA migrates as granules from the chromosomal bouquet to the periphery of the nucleus where, at later stages, the granules give rise to nucleoli⁵. A similar process occurs in Xenopus where Feulgen-positive material appears in the oocyte nucleus as a “nuclear cap” at leptotene¹³. After zygotene this cap disperses and numerous nucleoli are formed which contain one or several DNA cores or rings¹⁴.