The research led by Youbin Xiang, Ph.D., a senior research specialist in the Hawley Lab, found that the chromosomes of female silkworms, which do not crossover, are instead held together by a very different and elaborate protein structure. The team assigned a name for this structure—the Bivalent Bridge.

“It was thought to be just a glob,” said Hawley. “We have identified its components, its structure, and provided real insights into its function, or how it actually works. And, we have now brought silkworms into the fold.”

During early meiosis, female silkworms initially form a synaptonemal complex, which primes their chromosomes for linkages or crossovers. When crossing over does not occur, the complex is dismantled and many of its proteins are recycled and repurposed to construct the Bivalent Bridge. This different meiotic process is likely common to all species of moths and butterflies for holding chromosomes together until they are ready to segregate.

“Forming the synaptonemal complex is a good way to stabilize the initial chromosome pairing,” said Hawley. “When crossovers are supposed to take over to link and orient the pairs, Nature had to evolve quickly to avoid a catastrophic failure of meiosis.”

Although crossovers increase genetic diversity in offspring—chromosomes at the end of meiosis have unique combinations of DNA—organisms with a large number of chromosomes can achieve diversity through independent assortment, or the random segregation of chromosomes to each new cell.

When caterpillars become moths, their lifespan is limited solely to reproduction. After emerging from their silk-spun cocoons, they have no mouth or digestive system, just a reproductive system. “Their only responsibility is to mate and lay those eggs,” said Hawley. And because silk is a valuable material in Asian countries like China and Japan, many scientists and labs there are dedicated to studying the biology of silkworms.

“In China there are entire institutes devoted to silkworms,” said Hawley. “Some study their biology to find ways to increase silk production. Others study foundational biology for comparative or evolutionary insight, yet only two or three labs do so in the U.S.”

Silkworms have found a way to recycle proteins to build a structure that can ensure their chromosomes are able to segregate properly. This bridge-like structure is complex, dynamic, and a fundamentally different mechanism for meiosis.

Hawley recounted a quote from renowned biologist Rochelle Esposito, Ph.D.: “Evolution is not going for a pretty or even sensible solution; all it cares about is that it works.” He continued, “The only question evolution is asking is, ‘Can I make progeny that survive?’”

Additional authors include Dai Tsuchiya, Ph.D., Zulin Yu, Ph.D., Xia Zhao, Ph.D., Sean McKinney, Ph.D., Jay Unruh, Ph.D., Brian Slaughter, Ph.D., and Cathleen M. Lake, Ph.D.

This work was funded by institutional support from the Stowers Institute for Medical Research.