Postzygotic diploidization of triploids as a source of unusual cases of mosaicism, chimerism and twinning.

Triploidy is one of the most frequent chromosomal errors responsible for reproduction failure. This paper encompasses, in one conceptual frame, four recent findings in reproduction biology: predominant dispermic origin of triploids, paternal centrosome inheritance, eccentric cleavage divisions of dispermic triploid zygotes and certain intricate cases of mosaicism/chimerism. It is argued that dispermic zygotes, in contrast to digynic ones, are characterized by cytogenetic phenomenon described here as postzygotic diploidization of triploids (PDT). PDT embraces three main developmental scenarios: (i) the maintenance of the triploid state accompanied by regular segregation of 2n cells and the 2n/3n mixoploid populations; (ii) immediate diploidization with elimination of an odd haploid set of chromosomes and regular appearance of 1n/2n, 2n/3n and other mixoploids and (iii) tripolar spindle formation leading to gross aneuploidy, cell death with occasional survival of 2n+1 or 2n+1+1 trisomics and uniparental disomics. According to the PDT concept, a trisomy and disomy might occur due to generalized karyotype instability of dispermic triploids. PDT may provide a natural explanation for the regular appearance of 2n homozygous androgenic moles, various 2n/3n, 2n/2n molar/twin complexes without necessitating the concept of the 'empty' oocyte fertilization. Convincing evidence for a reservoir of anuclear oocytes does not exist. Peculiar implications are expected in the case of two rounds of diploidizations or involvement of triploid cell derivatives in the twinning process. Cryptic mosaic/chimeras and unusual twins intermediate between monozygotic (MZ) and dizygotic (DZ) are expected. Thus, PDT could have an explanation for the broad spectrum of odd reproductive cytogenetic events and might provide additional alternatives and definite predictions.