Monotremes display a unique mix of mammalian and reptilian features and form the most distantly related, and least understood, group of living mammals. Their genetic blueprint provides fundamental insights into their unique biology and into the evolution of all mammals.
This milestone in genomics research was the result of an international collaboration involving Australian scientists from the Universities of Adelaide, Sydney, Melbourne and La Trobe University, as well as researchers from China, Japan, USA and Denmark. The 40 researchers brought together expertise in bioinformatics, cytogenetics and molecular biology to produce and analyse the first ever echidna genome and a greatly improved, high quality platypus genome sequence.
“The new genomes are a hugely valuable public resource for research in mammalian biology and evolution, with applications in wildlife conservation and even human health,” said the University of Adelaide’s Professor Frank Grutzner, who co-led the study.
“More than 15 years ago we discovered that monotremes have different sex chromosomes to all other mammals. This finding revolutionised our understanding of mammalian sex chromosome evolution but also raised fundamental new questions. With the new genomes we can now tackle important questions about how sex chromosomes control monotreme sexual development and reproduction, and how they evolved.”
La Trobe University Professor Jenny Graves said: “The egg-laying monotremes, unique to Australia, help us to answer some of our deepest questions of mammal evolution. We last shared a common ancestor with the platypus and echidna about 180 million years ago, so comparing the monotreme and human genomes can tell us about our common ancestor, and what has changed during the 180 million years in humans as well as monotremes.”
Comparisons with other mammals can date many evolutionary innovations. For instance, co-first author of the study Dr Linda Shearwin-Whyatt, University of Adelaide, said: “The system for safe removal of the oxygen carrier, haemoglobin from blood was thought to be common to all mammals. We were surprised when we discovered the system was missing from monotremes, implying that it arose quite recently in the ancestor of all other mammals.”
Professor Marilyn Renfree, University of Melbourne, said the platypus and echidna are the only egg-laying mammals, and so provide the key to understanding the change in reproductive strategy from egg-laying to the production of live young in all other mammals.
During their short in egg incubation, they have kept one of the three major egg proteins that is used to make the yolk in chickens, but after hatching both platypus and echidna have a complex milk like other mammals to support their young during their long lactation.
Knowledge of monotreme reproduction is also relevant for conservation and captive breeding programs, and University of Sydney Professor Katherine Belov said: “The new genome sequences also provide a roadmap for genetic management of threatened echidna populations and exciting new leads for drug development via the discovery of novel peptides in platypus venom.”
Professor Grutzner said: “These animals are much-loved by people in Australia and around the world. Our new understanding of their genomes will provide novel insights into their biology, a foundation for a deeper understanding of mammalian biology and a basis for conservation of these iconic animals for future generations.”