Full Genomes of the Platypus and Echidna Decoded--Separated 54.6 million years ago, differences found in genes for smell and pheromones
Evolution of "egg laying mammals," or monotremes, explained--Hokkaido University, others

An international joint research group, comprised of Assistant Professor Takashi Hayakawa of the Graduate School of Environmental Science, Hokkaido University, Associate Professor Masato Nikaido of the School of Life Science and Technology, Tokyo Institute of Technology, digzyme Inc., the University of Adelaide (Australia), and the University of Copenhagen (Denmark), has announced that it has successfully decoded the full genomes of the platypus and echidna to a high degree of precision.

Both the platypus and echidna are egg laying mammals that together comprise the entirety of the order of monotremes. While their appearance is different, they exhibit several similar characteristics in both biological and anatomical terms. In both cases, the females lay eggs but nurse their newborn with milk. Platypus are found around rivers and lakes in eastern Australia, while echidna are found throughout Australia and New Guinea. The platypus lives in the water, and consumes shrimp and crabs with its duck-like beak. Meanwhile, the echidna lives on land and dines on ants and termites by digging in the earth and destroying ant nests. The genome of the platypus was originally decoded in 2008, but the accuracy at the time was not high due to limitations in technology.

This time, the research group used a next generation genome sequencer to sequence all of the genome sequences of the platypus in detail, then doing the same for the echidna. They then compared the genomes of the two monotremes with other amniotes. Amniotes are animals which develop amniotic membranes at the early developmental stage, a group in which monotremes are included. Specifically, the group compared placental mammals (humans, mice), marsupials (opossum), birds (chickens), and reptiles (lizards).

The comparison enabled an estimation to be made of when the monotremes separated from placental mammals in evolutionary terms. The overall ancestor of mammals split from that of birds and reptiles 300.5 million years ago, while the shared ancestor of monotremes and viviparous mammals split 187.6 million years ago. Furthermore, they learned that the platypus and echidna monotremes separated 54.6 million years ago after entering the Cenozoic era.

They also learned that the platypus possesses five each of the X and Y sex chromosomes, while the echidna has 5 X and 4 Y chromosomes. In comparison, the majority of mammals, including people, have two sex chromosomes, either XX or XY. This suggests that the origins of the monotreme sex chromosomes differ from that of viviparous mammals. Also, because the common ancestor to all mammals had 30 pairs of chromosomes, it was found that the chromosomes of the platypus have undergone complex restructuring 156 times.

Meanwhile, the Japan team led by Hayakawa focused on chemoreceptor genes. Chemoreceptor genes are genes that govern the creation of receptors, such as in the tongue or nose, that send electrical signals to the brain to signal taste and smell, and it was found that there was a major difference in the chemoreceptor genes of the platypus and echidna. Specifically, the platypus has 7 chemoreceptor genes for bitterness, while the echidna has 3. For chemoreceptor genes governing smell, the platypus has 299, while the echidna has 693. In comparison, humans have about 400. And there was a major difference in chemoreceptor genes for pheromones, with 262 for the platypus versus only 28 for the echidna.

These differences suggest that the chemoreceptors for smell developed to enable the echidna to efficiently search for colonies of ants and termites on land, while those for pheromones developed in the platypus for defending territory and for efficiently finding mates on the river banks.

Also, while it was known that monotremes were resilient in low oxygen conditions, with echidna burrowing in the soil, while the platypus swims in water, the team found a unique evolutionary change in both in the genes related to blood hemoglobin. The team also identified the gene proteins that govern the venom in the hind claws of adult male platypus, as well as genes related to oviparity and breastfeeding.

Hayakawa explains, "The platypus and echidna are extremely interesting animals that teach of the evolution of mammals and serve as a link between mammals on the one hand and birds and reptiles on the other. While the genome has been decoded, there are still many mysteries remaining, such as the way those genes function in cells and individuals, so the research is only just beginning. The platypus and echidna are facing risk of extinction because they only exist in a small part of the southern hemisphere, and their habitat has further been eroded by forest fires. I next hope to carry out field research in Australia to learn more about their ecology."