Researchers from the University of Oregon have identified the oldest known animal sex chromosome, the Z chromosome, which emerged in the ancient ancestor of cephalopods and persists today. The discovery places the origin of this chromosome well before many other known systems and offers new insight into how sex is determined in creatures like octopuses and squids. The finding is reported in a high profile scientific journal and underscores the deep history of genetic systems in marine life.
According to evolutionary geneticists involved in the research, the Z chromosome represents the earliest example of a chromosome that carries sex-related genes in animals. This marks a significant step forward in understanding how sex is assigned in cephalopods, a group that includes octopuses, squids, and nautiluses. The team notes that while sex chromosomes play a major role in many animals, the exact mechanisms by which cephalopods determine sex have remained mysterious until now. This study provides the first concrete genetic evidence that genes contribute to sex choice in these intelligent mollusks. — Nature
In many vertebrates and some invertebrates, sex is guided by a distinct pair of sex chromosomes. Humans, for example, typically carry two X chromosomes in females and one X paired with a Y in males. In contrast, cephalopods have a different and less understood system for assigning sex, which may involve a combination of chromosome behavior and gene activity. The new research helps clarify that a Z chromosome exists within some octopus and squid species, while the nautilus does not display the same pattern. This suggests a stepwise evolution of sex-linked genetic control within the cephalopod lineage. — Nature
The researchers propose that the Z chromosome originated in a lineage that eventually gave rise to contemporary squid and octopuses, with this lineage diverging from the nautiloid group that kept a different genetic setup. This interpretation aligns with a timeline in which the Z chromosome appeared tens of millions of years ago and has endured across diverse cephalopod lineages. The study implies that the Z chromosome has remained a stable genetic feature through long periods of evolutionary change, helping explain why sex-related genes have persisted in certain cephalopod groups. — Nature
From the dating of the chromosome, it appears the Z chromosome first appeared far in the past and continues to be a part of cephalopod genetics today. This finding reconfigures previous ideas about how ancient and how persistent sex-determining systems can be in marine animals. The research also highlights how the genetic architecture of sex can vary between related lineages, offering a window into how early cephalopods might have navigated reproduction and population dynamics over time. — Nature
Earlier estimates placed the oldest known sex chromosome in sturgeon at about one hundred eighty million years ago, illustrating how the understanding of sex chromosome history has evolved with new data. The current cephalopod discovery pushes that timeline back and broadens the scope of inquiry into the evolution of sex determination in the animal kingdom. This shift invites further comparative studies across different marine species to explore how widespread early sex-linked chromosomes might be and what roles they played in shaping biodiversity. — Nature
Additionally, recent paleontological work has refined the timeline of ancient cephalopod diversity, including descriptions of longstanding varieties that lived hundreds of millions of years ago. One notable finding concerns an ancient vampire squid lineage estimated to date from roughly the same distant period, expanding the context for how cephalopod genetics and morphology have evolved over deep time. These lines of evidence together paint a richer picture of the evolutionary forces that shaped sex determination in ocean life. — Nature