Researchers have discovered a sex chromosome in the California two-spot octopus, believed to have persisted for about 480 million years, dating back to a time before octopuses diverged from nautiluses on the evolutionary timeline. This positions it among the oldest known sex chromosomes in the animal kingdom. This discovery provides proof that octopuses, along with other cephalopods—such as squid and nautiluses—use chromosomes for sex determination, resolving a long-standing question in biology.
The octopus has unveiled yet another of its mysteries: the basis of its sex determination.
Researchers from the University of Oregon have pinpointed a sex chromosome in the California two-spot octopus. This chromosome has likely existed for 480 million years, tracing back to a period prior to the evolutionary split between octopuses and nautiluses. This discovery renders it one of the most ancient animal sex chromosomes known.
The finding also demonstrates that octopuses and other cephalopods, a group of marine creatures that includes squid and nautiluses, rely on chromosomes to establish their sex, answering a question that has intrigued biologists for many years.
“Cephalopods are fascinating organisms, and much about them remains to be uncovered, especially in the field of neuroscience,” remarked Gabby Coffing, a doctoral student at UO working in biologist Andrew Kern’s lab. “This is yet another intriguing aspect of their biology: They possess remarkably ancient sex chromosomes.”
Coffing, Kern, and their research team published their findings on February 3 in the journal Current Biology.
In humans and the majority of mammals, sex is primarily determined by chromosomes. However, “there’s an immense variety in how different species determine sex,” Kern noted. Consequently, it couldn’t be assumed that octopuses operate under the same rules.
For example, in turtles, sex is determined by the temperature during egg incubation. Certain fish have a specific gene responsible for sex determination, rather than an entire chromosome. Even within humans, the straightforward appearance of the X/Y chromosome system can be misleading; genetic mutations or the presence of extra sex chromosomes can result in variations that don’t neatly fit into male or female categories.
Additionally, cephalopods are not typical laboratory subjects like mice or fruit flies, which means they have not undergone extensive genetic study. While scientists have sequenced the genomes of several octopus species, they haven’t been able to connect genes to specific traits as easily as they can in mice or even humans.
During a recent analysis of a female California two-spot octopus’s DNA, UO researchers stumbled upon an unexpected finding: a chromosome with only half the quantity of genetic material compared to others. This chromosome appeared distinct and was absent in previously sequenced male octopus DNA.
“This particular chromosome had only half the sequencing data, suggesting it was represented by a single copy,” Coffing explained. “Upon further investigation, we concluded that we had identified a sex chromosome.”
To verify their discovery, the researchers reviewed other octopus genomic data from prior studies, noting that not all data was clearly categorized by male or female origins.
Eventually, they found another instance of the half-sized chromosome in a different octopus species. It was also identified in squid, which evolved separately from octopuses between 248 and 455 million years ago. Further analysis revealed the chromosome’s presence in the nautilus, a mollusk that diverged from octopuses approximately 480 million years ago.
The existence of this unique chromosome in these species implies its long-standing presence in some form.
“This suggests that their common ancestor possessed a similar mechanism for sex determination,” Coffing stated.
This situation is somewhat atypical for sex chromosomes, according to Kern. Because these chromosomes directly affect reproductive abilities, they are exposed to significant selective pressures, leading to rapid evolutionary changes. However, cephalopods appear to have successfully established a stable sex determination system.
Other ancient sex chromosomes have been found in plant groups such as mosses and liverworts, among the earliest plants to evolve. Insect sex chromosomes may date back 450 million years, yet they have also undergone considerable changes over time.
Initially, Kern and his colleagues speculated that octopuses might utilize a sex determination system akin to that of birds and butterflies, where males are identified as ZZ and females as ZW. (To avoid confusion, biologists use different letters to denote systems where males have two identical sex chromosomes, as opposed to the XX/XY system where females possess two copies of the same chromosome.)
However, the research team has not yet discovered a W chromosome in octopuses. Alternately, octopuses could employ a sex determination method that exclusively involves the Z chromosome, with males having a pair and females possessing a single chromosome. This matter remains unresolved, according to Coffing. For now, the octopus retains some of its enigmatic traits.
