A brief look at the origin of tetraploidy

ABC Fax + 41 61 306 12 34 E-mail karger@karger.ch www.karger.com © 2004 S. Karger AG, Basel 0301–0171/04/1072–0128$21.00/0 Accessible online at: www.karger.com/cgr A tetraploid embryo with the rare karyotype 92,XXXY that most probably received an additional maternal set of chromosomes due to incorporation of the first polar body and an additional paternal set due to dispermy has recently been described by Baumer et al. (2003). Their report contains two remarkable comments: (a) the mode of formation of only six tetraploidies has been elucidated as yet and (b) there is little direct evidence for the assumption that the majority of tetraploidies arise through mitotic failures during the first (in non-mosaic cases) or later divisions (in mosaics). This predominance of errors at early cleavage division had been deduced from previous large surveys of spontaneous abortions that exclusively demonstrated the presence of XXXX or XXYY chromosome constitutions in tetraploid conceptuses (Table 1). In this context, particular observations such as a 94,XXXX,+16,+16 karyotype could be explained by the duplication of a zygote with trisomy 16 (Boué and Boué, 1976). With regard to the origin of the supernumerary chromosome complements we should like to draw attention to information gathered so far in programs of assisted reproduction. Here, tetraploid preimplantation embryos have been sporadically observed e.g., an embryo that was considered a 2n/4n mosaic with the sex chromosomes XXYY in the tetraploid line (Munné et al., 1993). Hardarson and colleagues (2001) analysed unevenly cleaved embryos and found two 2n/4n mosaics that revealed multinuclear but, in total, tetraploid blastomeres (sex chromosomes XXYY and XXXX, respectively). Whereas these few selected examples again point to a mitotic failure, the mode of formation must have been different in another case. Within a total of 118 poor quality embryos with irregular blastomeres and excessive fragmentation, four were found to be tetraploid (Pellestor et al., 1994). One embryo provided two metaphases that could be karyotyped (89,XXXY,–2A,–E and 85,XXXY,–2A,–B,–3C,–E, respectively). Chromosome duplication or multiple sperm penetration are mechanisms that might have led to the observed sex chromosome constitution in this embryo (Pellestor et al., 1994). Since at least the latter event should be recognized by the number of pronuclei in the ooplasm, it seems worthwhile going back one more step and looking at the level of the so-called pre-zygote (Veeck, 1999).