Giant diploid oocytes as a cause of digynic triploidy in mammals

Giant oocytes and zygotes twice the normal size from the Chinese hamster were studied embryologically and cytogenetically in order to clarify their significance as a cause of digynic triploidy. There were 6 (0.62%) giant eggs among 973 primary oocytes, 5 (1.37%) among 365 secondary oocytes from mature follicles missing ovulation, 3 (0.44%) among 677 secondary oocytes from oviducts, 4 (0.45%) among 886 one-cell zygotes, and 3 (0.49%) among 608 two-cell zygotes. The results indicated that follicles containing a giant oocyte might have a tendency to miss ovulation. But once ovulated, the giant eggs were fertilized normally and developed normally. Nuclear and chromosomal aspects were studied in 6 primary and 18 secondary oocytes, and 6 one-cell and 5 two-cells zygotes. There were mononuclear and binuclear giant oocytes, but the latter state was rarely maintained to metaphase II. The primary oocytes showed a diploid number of bivalents (22 tetrads) with normal chiasma configuration, and the secondary oocytes had 22 dyads with normal features, indicating normal segregation. The one-cell zygotes exhibited one set of diploid female pronuclear chromosomes and one set of haploid male pronuclear chromosomes in the late prophase. The two-cell zygotes were all digynic triploids; the tail of a fertilizing spermatozoon was attached. There were five XXX and six XXY giant triploids, giving a ratio of 1:1, as expected. It was suggested that the nuclear-cytoplasmic ratio of giant triploid eggs may be more favorable at least for preimplantation development than that of any other type of triploidy. Giant diploid oocytes may be an important source of human digynic triploids, in spite of the prevailing view from studies of marker chromosomes that suppression of first polar body extrusion is the predominant cause.