Levitus and his department produced a short video about his research exploring “Why Do So Many Offspring Die?”
Dan Levitis and his wife, Iris, were living in Germany when they lost their first pregnancy. An ocean away from their families, they had few people they could talk to about their loss. Then they had a second miscarriage and were devastated.
As they visited doctors for answers, Levitis dove into the scientific literature. “As a scientist I wanted to understand why this had happened, so I started reading the literature about pregnancy loss, from a wide range of angles.” At the time, he was working at Max Planck Institute studying demographics and mortality.
Now a scientist in the botany department at UW-Madison, Levitis and co-authors, Anne Pringle and Kolea Zimmerman, have published a new understanding of offspring loss. They find that while sexual reproduction has the advantage of creating more genetically diverse offspring, it is a delicate process with a high failure rate among all species that reproduce this way.
Levitis learned from his literature reading that miscarriage is common in humans, occurring in roughly half of all pregnancies. While there’s a wide range of causes for these losses, the majority stem from chromosomal abnormalities resulting from the process of cell division called meiosis.
Meiosis (not to be confused with the simpler cell dividing mitosis) is the process that divides cells into gametes or reproductive cells — in the case of humans, sperm and eggs. These each have half the number of chromosomes found in regular cells so that they can combine with another gamete to form a complete set.
Meiosis is a multi-stage process that first involves chromosomes swapping genes for individual traits. This allows each of a woman’s egg cells to contain a distinct combination of the genetic material she carries from both of her parents.Female mammals are born with eggs that have already started meiosis. The chromosomes in these eggs have gone through the genetic cut and paste, but the cells haven’t yet divided down into gametes ready to pair up with a counterpart.
Those final steps don’t happen until each time an egg matures and a woman ovulates. As the woman gets older, this pause in meiosis grows longer and longer, perhaps explaining why more chromosomal defects happen in babies born to older women.
From an evolutionary perspective Levitis wondered how a process with such a high error rate has persisted. He also wondered whether the high percentage of failures was limited to humans, since meiosis occurs in all sexually reproducing organisms. “Biologists are trained to be cautious of any statement that humans are in some way different from other species,” Levitis says.
If meiosis causes human pregnancy loss, maybe it is also to blame for offspring loss in other species, Levitis reasoned. He screened thousands of studies looking at survival rates of offspring in plant and animal species, ranging from lizards to palm trees, damselflies to turkeys.
He compared offspring survival rates among organisms that use meiosis and those that don’t. He also looked at asexually reproducing species that still use meiosis. Most asexual species (who basically reproduce by cloning) don’t need meiosis but some still use it. In these organisms it’s an even more complicated process.
With more complexity comes more chance for error, so Levitis predicted these organisms would have an even higher reproductive failure rate than their sexual counterparts. He also predicted the asexual species that don’t use meiosis would have the lowest rate of failure relative to the other groups.
Of the thousands of studies Levitis screened, 44 met the criteria he was looking for, including that they quantified the actual numbers of surviving offspring. Of those, 42 supported his predictions. In scientific research, that’s a conclusive result. “My response when I saw that was I thought I must have done something wrong,” Levitis says.
He went over the studies again and the results held up. “The fact that there’s that strong a predictive power across many different kinds of animals and plants implies that meiosis is having a major effect on viability everywhere it’s used.” The many steps in the process seem to mean many opportunities for something to go wrong.
Despite the huge disadvantage of high offspring loss, a majority of species still use meiosis, most reproducing sexually. This is likely because of the great advantages it provides, namely increasing genetic diversity, which helps species adapt and survive. “You can’t get the advantages of sex, without the disadvantages of sex,” Levitis says. “In that way, pregnancy loss can be seen as a cost of sexual reproduction.”
The findings of the study are published in the August issue of Proceedings of the Royal Society B. For the non-scientists, Levitis’ department created a video to help explain the study’s meaning.
Levitis is eager to hear feedback on the idea that meiosis isn’t only costly in human reproduction, but in all species that use it. He also hopes his research will help reduce the guilt and stigma people often feel about pregnancy loss. “The underlying biology of reproduction is sufficiently complicated; it’s sort of amazing it ever succeeds,” he says.
“People tend to think of natural selection as eventually perfecting organisms, but an organism can never be perfected because there are always tradeoffs,” Levitis adds.
The perspective he gained from his research helped ease the pain of the lost pregnancies for Levitis, as did the fact that he and his wife have gone on to have three healthy children, each reflecting a unique combination of his traits and his wife’s. That was all made possible by the exact same complicated process that likely caused the miscarriages: meiosis.
