Snail genome duplication offers look at evolution in transition

A tiny freshwater snail from New Zealand may be further revealing what scientists know about how animals evolve.

University of Iowa biologists have traced the snail's evolutionary history through its genome and discovered that the species recently duplicated all of its genetic material - a finding that could further reveal how major evolutionary shifts in animals occur.

The Iowa team chose to study this snail species because individuals can reproduce either sexually or asexually (meaning females can produce offspring without males). While piecing together the snail's genome, the biologists discovered duplicated genes and doubled regions of DNA, evidence that the organism once carried more than two complete sets of chromosomes, a condition known as polyploidy.

Most animals, including humans, are diploids, meaning they have only two copies of their genome - the full set of instructions needed to make every cell, tissue, and organ in the body.

Asexual reproduction is often associated with polyploidy, as females may more easily manage extra chromosomes by cloning offspring. Biologists have long wondered whether these extra chromosomes help or hinder species over time.

To investigate, the Iowa biologists assembled the snail's genome by reconstructing some 20,000 genes from 30 individuals in the same lineage, much like completing a master puzzle despite the challenge of sorting through multiple pieces from several nearly identical puzzles. Their reconstruction revealed that the snail had copied all its genetic materials, a process known as whole-genome duplication, within the past 1 million to 2 million years.

"Having more than two genome copies is something that's breaking the rule, but it seems to be a rule that when it's broken, it's corrected over time," says Kyle McElroy, co-corresponding author on the study, who earned a doctorate from Iowa in 2019 and is now a postdoctoral research associate at Iowa State University. "And we don't know why being diploid is the rule."

They also found that the snail was still fairly early in its evolutionary return to a diploid state, the two-copy genome standard common in sexual animals, including humans.

"What we're looking at today is a mosaic," says Joseph Jalinsky, visiting assistant professor in the Department of Biology, who earned a doctorate from Iowa in 2022. "Some genes have two copies, some have three, some have four. So, the simplest explanation for how you have so many genes with more than two copies is a whole-genome duplication event."

"It is this discovery that is perhaps the most exciting," adds Maurine Neiman, professor in the Department of Biology and the study's senior author. "We hardly ever see organisms, and especially animals, in this transitory state."

The discovery raises new questions about how and why whole-genome duplication occurs and what role it plays in the emergence of new traits. Neiman notes that such events may mark evolutionary turning points by creating the raw genetic material for "evolutionary innovations."

"This could include advanced animal cognition or flowers developing seeds - basically almost any interesting traits might have been enabled by whole-genome duplications," Neiman says.

The study, "Whole-genome sequence of Potamopyrgus antipodarum - a model system for the maintenance of sexual reproduction - reveals a recent whole-genome duplication," was published online Nov. 5 in the journal Genome Biology and Evolution.

Contributing authors from Iowa include John Logsdon Jr., Laura Bankers, Joel Sharbrough, Chelsea Higgins, and Cynthia Toll. Other contributing authors are Peter Fields, from the University of Basel, in Switzerland; and Jeffrey Boore, from the University of California-Berkeley.

The U.S. National Science Foundation, the Carver Biomedical Trust at Iowa, the Iowa Office for Undergraduate Research Funding, and the Iowa Science Foundation funded the research.

Neiman is the principal investigator, and Boore and Logsdon are co-principal investigators on the original grant funding the research.