Mosaic/chimeras and twinning in the current reproductive genetics perspective.

Sir, My thanks to Dr Golubovsky for his incisive questions and comments. It seems he pleads the case for the common knowledge that (i) there are two kinds of twins, (ii) we understand clear differences in the cellular and genetic origins thereof and (iii) we can use those differences with confidence as bases for productive genetic analyses. Thirty-five years ago I believed that too, and it occurred to me that we might learn about embryogenesis by studying developmental differences between monozygotic (MZ) twins (who must have experienced unusual embryogenesis and were supposed to be plagued by great excesses of embryogenic errors) and dizygotic (DZ) twins (who were thought to be doing embryogenesis quite normally and independently, just like singletons). Every result from those studies has shown that picture of DZ development to be wrong. The differences between MZ and DZ twins are of little interest compared with those between all twins and singletons. Measurable traces of embryogenic a/symmetries (and errors in establishing them) in twins and their first-degree relatives are clearly and consistently incompatible with the independent double-ovulation hypothesis of DZ twin origin. The common knowledge, however, remains common. Some credit may be given for a particular difficulty: it can be hard to accept that DZ twins do not normally arise from double ovulation without a clear picture of an alternate cellular mechanism for their origin. A fuller understanding of chimerism brings new light. By every measure appropriately tested, twins, both MZ and DZ grow heads, central nervous systems, faces and hearts—midline-asymmetrical structures in general—to patterns different from those defining those same structures for the singleton majority. The excesses of all of those anomalies among twins have long been attributed to MZs because ‘everyone knows’ MZs have odd embryogeneses and ‘everyone knows’ DZs develop independently, just like singletons. In fact, all of those oddities of developmental or behavioural asymmetries are associated with DZ twinning no less than with MZs, and in some cases more so. DZ twins develop at least as differently from singletons as the MZs do and differ in much the same ways—mostly having to do with asymmetries of embryogenesis, in stages that begin inside the zona. The firstdegree relatives of twins—of both zygosities equally except where the association is stronger with DZ twinning—share the excesses of those anomalies among twins. The embryogenesis of DZ twins is at least as odd as that of the MZs. It is not compatible with double-independent progress from gametogenesis to embryogenesis. There has never been any direct evidence for a doubleovulation origin for naturally conceived human DZ twins. There are twin pairs whose members seem to have arisen from a single genetic source and others who appear to have arisen from two sources. That much Aristotle noted, over 2000 years before anyone knew that life is cellular or any other part of a definition of zygote. He declared that monsters (the root, and the probable meaning then, was ‘manifestations’: oddities bringing warnings or omens) such as human twins arose from untoward variations of volume or temperature or pressure in the conceptive interactions between seminal and catamenial fluids (Aristotle, ∼350 B.C.E.-a,b). Aristotle’s channel to medieval Europe, Albertus Magnus, suggested as cause an excess of coital pleasure on the part of the mother (Thijssen, 1987). The common knowledge double-ovulation hypothesis is of related provenance and similar scientific utility. The only thing that we have firmly established about how MZ and DZ twins differ in coming to be as they are is our ignorance. Every scrap of literature that supports any version of a double-ovulation origin for DZ twinning either states that it is just so with no offer of evidence or offers as authority a citation to an earlier article, which in its turn states with no offer of evidence that DZ twins just do arise from double ovulation. The articles from Leroy and Hall cited as authority in Dr Golubovsky’s letter are excellent examples of the conduits of ‘common knowledge’. ‘Everyone knows’ this is so. It is what Aristotle might call a first principle. There is no need of evidence in its favour and no attention for evidence against it. Today, we also know there are people whose single individual bodies arose from two zygotes, with tissues made of cells from both members of an embryonic DZ pair. Some are born as twins, most as sole survivors with no inkling of their twinship. Common knowledge says chimerism arises from anastomoses between placentae of dichorionic twins, but the two phenomena bear a quantitative discrepancy of three or more orders of magnitude in frequency. Chimerism may help us to understand what cells actually do in bringing about zygosis, embryogenesis and the establishment of body plans. With considerable and yet underestimated frequency, chimerism gives rise to monochorionic (MC) DZs. The existence of MC DZs might explain frequent discordance of supposed MZ twins (if they are same-sexed and monochorionic, then they are presumed to be monozygotic, with no reason to doubt or test it.) for otherwise highly heritable anomalies. Chimerism is strongly associated with disrupted timing of oöcyte maturation in assisted reproduction techniques (ART), an association shared with both MZ and DZ twinning and with anomalies of genomic imprinting. Clearly, DZ twins can arise from single embryonic cell masses, because they do so frequently. If the embryonic axes of DZ twins are determined in the same cellular circumstances and by the same molecular mechanisms as are those of MZ twins, then it follows that the developmental patterns of MZ and DZ twins as groups should differ from those of singletons in similar directions and distances, as they do. When evidence is involved, no ‘heat of the argument’ is required to deny independent double ovulation. Nor is any ‘heat of the argument’ required to reject every result from Weinberg method estimates (the theory of which depends absolutely on independence of the events under consideration; Boklage, 2005). Every question ever ‘answered’ with Weinberg results, if it ever needed to be asked, needs to be asked again, now that proper genotyping is simple and inexpensive. The ‘roughly’ (i.e. Weinberg) estimated ratio of DZ/MZ births, which Golubovsky cites from Tong and Short, is no measure of human fertility at all, let alone a ‘reasonable’ one;