By Robert Sanders, Media relations, UC Berkeley; FEBRUARY 4, 2022

A fire or flame jellyfish (Rhopilema esculentum) photographed at the Monterey Bay Aquarium. These jellyfish, a popular food in Japan, are native to the warm temperate waters of the Pacific Ocean. (Credit: Bill Abbott, Creative Commons License)

Many of today’s marine invertebrates, including sponges and jellyfish, have chromosomes with the same ancient structure they inherited from their primitive ancestors more than 600 million years ago, according to a new study.

The surprise finding is a reminder that evolution is conservative — it keeps things that work well, like the organization of genes on a chromosome — and provides a key link between creatures alive today, including humans, and our very distant ancestors.

“It emphasizes that even in something as fundamental as their chromosomes, diverse animals resemble each other,” said the study’s senior author, Daniel Rokhsar, the Marthella Foskett Brown Chair in the Department of Molecular and Cell Biology at the University of California, Berkeley. “That’s one of the reasons why we can learn so much about human biology from studying fruit flies, nematode worms, jellyfish and other ‘simple’ model systems — it’s because of the underlying unity of all animals. What we learn about animal diversity affects how we think about ourselves.”

The findings were published Feb. 2 in the journal Science Advances.

The lancelet, or amphioxus, is an invertebrate, but has a similar body plan to a vertebrate. (Credit: Vincent Moncorgé)

The new analysis predicts that the first multicellular animals carried their genes in 29 pairs of ancient chromosomal units. As the first animals arose in the oceans and evolved into diverse invertebrates, from sponges to worms to humans, many of these chromosomes have remained intact for half a billion years.

For comparison, humans now have 23 pairs of chromosomes, for a total of 46, the result of two duplications and multiple mergers and chromosomal rearrangements since the earliest animals.

The study, led by Rokhsar and Oleg Simakov of the University of Vienna in Austria, is the first to compare the chromosomal position of genes from diverse animals, such as sponges, jellyfish, sea scallops and other aquatic invertebrates, allowing the ancestral organization to be inferred and rare changes in chromosome organization to be studied. Though this kind of analysis has been done for fruit flies and many vertebrates, including humans, it is only recently that the chromosome-scale genomes of diverse invertebrates have been determined.

Evolution is conservative

Because of increasingly advanced techniques for identifying which genes are close to one another when the chromosome is curled up inside the nucleus, scientists over the past few years have begun assigning genes to chromosomes in several invertebrates: the Florida lancelet, Branchiostoma floridae, a dainty, quill-like sea creature also known as amphioxus; a scallop, Patinopecten yessoensis; a fresh water sponge, Ephydatia muelleri; and the fire jellyfish, Rhopilema esculentum, a cnidarian. Rokhsar, Simakov and their team extended this set by determining the chromosomal sequences of a fifth animal, a hydra, Hydra vulgaris, another type of cnidarian.

Hydra vulgaris is a freshwater species of cnidarians. (Credit: Courtesy of the Smithsonian)