SEA STARS SHED LIGHT ON HUMAN REPRODUCTION

Michael Hart and his colleague Daryn Stover sifted through mounds of data going back over a million years in their quest to find out whether humans carry the same allele differences as sea stars. Why? To find out more about evolutionary processes that could have important implications on fertility.

Hart explains that natural selection is at play in the fertilization process of sea stars, making some pairs of sea stars more likely to conceive than others. “We discovered that the fertility rate of mated sea stars depends on what forms of reproductive genes they have. Male and female sea stars with certain types of gene pairs might successfully produce many offspring, while sea stars with other combinations of gene pairs were much less successful,” says Hart.

Hart says, “this finding suggests that, over time, genetic incompatibility could cause populations of sea stars to gradually separate into different species.”  The concept of genetic incompatibility in these organisms led Hart to wonder if this might be true for humans as well.

To Hart’s surprize, he found that human populations are not evolving to become reproductively isolated from each other.  In other words, large molecular differences exhibited by sea stars preclude successful mating with each other, leading to the evolution of different species. But this is not the case in humans.

Instead, Hart explains, “human selection seems to be favouring the evolution of two different alleles or variants of each of these three genes (ZP2, ZP3 and C4BPA) that are necessary for reproduction and present in all human populations going back to the common ancestor we share with Neanderthals.”

“We think the cause of this pattern is selection that favours some pairs of mates with matching pairs of sperm and egg alleles: matching leads to higher fertility, mismatching leads to lower fertility. We don’t have direct evidence for this because it is hard to watch fertilization occur in mammals, but in other animals like sea stars we can directly observe interactions between eggs and sperm of females and males with different combinations of alleles, and we see a pattern where some pairs of male and female alleles have higher reproductive success than others” Hart says. 

Hart is excited that his research could potentially lead to understanding some causes of infertility in humans. He explains that, if (mis)matching of these three genes really does explain fertility variation in some human groups, then that might suggest screening for these three genes in couples that are seeking treatment for infertility.

Hart says, “We don’t know enough yet about how the ZP proteins interact with sperm in order to imagine how mismatching between them could be treated medically, but screening for these genes might be helpful as a way to rule out such mismatches as one possible cause of infertility.”

Hart accepts that there is much work to be done before firm outcomes for humans can ever be made, but he’s “delighted to find parallels between the genetics and evolution of marine invertebrates and the evolution of my own species.”

Hart presented this research at the 2016 annual meeting of the Society for Integrative and Comparative Biology in Portland, Oregon.