A brooding nest of the California two-spot octopus: the sex of the offspring that emerge from the egg capsules (white) depends on the number of sex chromosomes. Photo: ImageSelect
Research reveals: female octopuses have only one sex chromosome, whilst males have two. This sex-determination system is not only unique, but also turns out to be ancient.
Chromosomes determine sex in octopuses and other cephalopods, concludes a team of American evolutionary biologists following an extensive genetic analysis prompted by an unexpected discovery in the DNA of a female California two-spot octopus (Octopus bimaculoides). They came across a chromosome with only half the usual amount of genetic material, of which the octopus possesses just a single copy (Current Biology, 3 February).
The Americans deduce that this is a sex chromosome. Although female octopuses all carry only one copy, males turn out to possess a matching pair. The researchers find no evidence for a separate chromosome occurring exclusively in females. There is therefore no ZZ-ZW sex-determination system – as seen in birds, where scientists designate males as ZZ and females as ZW – but rather a ZZ-Z0 system. Comparative genomic analysis further reveals the same chromosome in other octopus species, squids, cuttlefish, and even nautiluses, which the authors suggest hints at a single evolutionary origin dating back to the early history of cephalopods, approximately 480 million years ago.
‘A unique finding’, says Leo Beukeboom, evolutionary geneticist at the University of Groningen. ‘Sex-determination mechanisms are fascinating from an evolutionary perspective because they are so diverse, whilst the end product is always the same: males and females. In some animal groups the system is fairly fixed, such as the XX-XY system in mammals and the ZZ-ZW system in birds. In other groups there is enormous variation between species, and chromosomes are far from always responsible for determining sex.’ Even within chromosomal sex-determination systems there can be variation in the genes involved, Beukeboom explains. ‘Drosophila has an XX-XY system, but there is no dominant masculinising gene on the Y chromosome, as there is in humans; instead, the number of X chromosomes is counted: males have one, females have two. It would be very interesting to investigate further the counting mechanism of Z chromosomes proposed by the authors, because apart from Drosophila almost nothing is known about how such systems work.’
Beukeboom agrees with the possibility of an early origin of the Z chromosome in cephalopods: ‘The composition of this chromosome, with a conserved gene order and a large amount of repetitive DNA, does indeed point to an early origin.’ In his view, this need not imply that a universal sex-determination system exists within the Cephalopoda. In many animal groups, after all, the sex-determination system is so evolutionarily unstable that rapid transitions can occur.
The American team identified nineteen genes present on the Z chromosomes of all the species examined. A large number of these are also found in humans and are expressed in reproductive tissue. Further information on the genetic regulation of sex determination is still lacking, however, and will require additional research. ‘All in all, this is an interesting contribution to our knowledge of the diversity of sex-determination systems – a study of an animal group about which very little is known’, Beukeboom concludes.