The duck-billed platypus (Ornithorhynchus anatinus) has long baffled evolutionary biologists with its bizarre blend of mammalian, reptilian, and avian physical traits. However, its most profound mystery lies hidden inside its genome. While humans and almost all other therian mammals possess a straightforward pair of sex chromosomes (XX for females, XY for males), the male platypus possesses ten sex chromosomes: five X chromosomes and five Y chromosomes.
Females possess five pairs of X chromosomes, totaling ten X chromosomes overall. During male meiosis—the specialized cell division process that produces sperm—these ten sex chromosomes do not align independently. Instead, they interact via tiny homologous regions at their tips to construct an extraordinary physical multi-chromosome chain.
As meiosis progresses, the chromosomes alternate along this chain in a precise X1-Y1-X2-Y2 pattern. When the chain segregates, all five X chromosomes move to one side of the dividing cell while all five Y chromosomes move to the opposite side. This ensures that male platypuses produce sperm containing either only the five X chromosomes or only the five Y chromosomes, preventing chaotic genetic mismatches in offspring.
Even more astonishing is the evolutionary origin of these chromosomes. Gene sequencing reveals that the platypus sex chromosome system shares virtually no homology with the therian mammal X chromosome, which evolved from the ancestral SOX3 gene. Instead, the platypus system shows strong genetic homology with the Z and W sex determination system found in birds, specifically containing the DMRT1 gene.
This proves that monotremes split off from the common mammalian ancestor before the modern mammalian XY system evolved, preserving an ancient genetic stepping stone between reptiles, birds, and mammals.