Rhizozygotic twins beat the frozen odds

Scientific American, 14 June 2007

Emmett and Otto Ramstad present a fascinating glimpse into the diversity of biology. They are rhizozygotic twins. Rhizozygotic twins are an extremely rare form of multiple birth, enabled by the wonders of assisted reproductive technology. On September 20 1975, an egg, or blastocyst, was harvested from the Ramstad twins’ mother Josie Winship and cryogenically frozen. Four years later, in January 1979, Ms. Winship decided to have the frozen egg thawed to try for pregnancy. Because doctors were unsure whether this blastocyst would survive the perils of cryopreservation, they harvested a fresh blastocyst. Both eggs were fertilized. At the time, it was standard procedure to keep multiple fertilized embryos in the same test-tube. In a scientific first, the two embryos fused in the test-tube and were injected together into the mother’s womb. The two embryos shared a placenta and grew to term; they were born on September 28 1979 and named Otto and Emmett.

At the time, this procedure was radically experimental. Scientists began developing methods of cryopreservation, or slow programmable freezing (SPF) in the early 1970s. The first frozen human embryo and IVF baby, Louise Brown, was born in 1978. The success rates of cryogenically frozen embryos were very low at the time. Nowadays, however, scientists estimate that around 20% of babies born through IVF were cryogenically frozen embryos.

The amount of genetic material that both twins share is open to debate. Their DNA profiles are like those of monozygotic twins, or twins that come from the same egg. However, they did not come from the same blastocyst, so how they share so identical a DNA profile is quite a mystery. Dr Maynard Howe, of Minnesota State University, has been working on the biology of rhizozygotic twins since the birth of the Ramstad twins. He coined the term “rhizozygotic”. He argues that the polyembryony of rhizozygosis is similar to some species of armadillos. “I believe the Ramstad twins may have been produced through a biological anomaly seen in Euphractus armadilloes called monchorial twins. In these species of armadillo, two separate eggs fuse in the womb and produce a set of twins that look very much like identical twins, but are not,” he explains.

Scientific anomalies like this are rare, but not as rare as you might think. A Texas mother of twins discovered in 2009 that her children carried DNA from two different fathers. She had released two separate eggs and slept with two different men around the same time. Both eggs fertilized with different sperm, and matured in the uterus together. They were born seven minutes apart. The scientific term for this is heteropaternal superfecundation.