“What we find is remarkable: If you compare those five species with each other, you find that there’s extensive conservation; in many cases, whole chromosomes or big pieces of chromosomes have stayed together. A whole chromosome in a sponge might correspond to a chromosome in a jellyfish,” he said. “They’re not organized in exactly the same way — the genes are in a different order in the various species — but over these long-time scales, a chromosome behaves like a bag of genes that has maintained its integrity since the beginning of animal life in the pre-Cambrian era.”
Once they discovered, in their sample of invertebrates, that genes tended to remain together on the same chromosome — something referred to as synteny, from the Greek for “on the same thread” — they predicted that the same would be true of other invertebrates, including sea urchins and various kinds of worms and mollusks. When they looked at the chromosomes of these organisms, they found similar conservation of DNA across chromosomes. All seemed to harken back to the same 29 chromosomal pairs that were present in the early animal ancestors.
What does this mean for humans and other vertebrates?
“If you compare amphioxus to scallops and then representatives of a lot of different vertebrates — different kinds of fish, like lampreys, chickens, and so forth — you can see that there are 18 different groups of genes that seem to always stick together,” said Rokhsar, who is also a Chan Zuckerberg Biohub investigator and a member of the Joint Genome Institute at the Lawrence Berkeley National Laboratory. “They always travel together on the same piece of DNA, and so the simplest interpretation is that there were 18 ancestral chromosomes in the proto-vertebrate ancestor.”
Rokhsar and his team have long suspected that chromosomes were more preserved than people thought. Over the past 20 years, he and his group have sequenced and analyzed the genomes of diverse animals, including a sea squirt, a placozoan, a species of lancelet and a different species each of sponge, choanoflagellate, sea anemone, octopus, acorn worm, leech, limpet and polychaete worm. While the early “draft” genomes were often fragmented, they nevertheless showed signs that there were anciently conserved groups of genes linked together across diverse animals. Newer technologies that allow whole chromosomes to be determined have confirmed those early hypotheses.
The fact that the genes of diverse invertebrates group together so faithfully, despite hundreds of millions of years of independent evolution, could indicate that for genes to jump around among chromosomes is a lot harder than scientists presumed from their studies of vertebrates, where genes have rearranged more frequently, likely because of genetic drift.