The platypus was genetically a mammal, a bird, and a reptile.
The genome (base sequence) map of the
Platypus and echidna genomes reveal mammalian biology and evolution | Nature
https://www.nature.com/articles/s41586-020-03039-0
Now We Know Why Platypus Are So Weird --Their Genes Are Part Bird, Reptile, And Mammal
https://www.sciencealert.com/platypus-genes-reveal-some-of-the-bizarre-traits-that-come-with-5x-and-5y-chromosomes
The platypus that live in Australia have the same oviparity as birds, but their children are breast-fed like mammals and are considered to be one of the mammals, as they are classified in the Monotreme. The research team used male platypus to analyze the genome map of the platypus sequenced in 2008.
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At the time of writing the article, mammals on Earth are divided into three groups: monotremes, marsupials, and eutheria, and we humans belong to the last group. Both marsupials and eutheria lay their offspring in the embryo, while monotremes (Montremes) lay eggs in the same way as birds and reptiles.
It is still unclear when these three groups began to divide, with some theories that 'monotremes first split and followed by marsupials and eutheria' and 'simultaneously evolved into three'. .. Therefore, analyzing the genome of the platypus will lead to the elucidation of the evolutionary trajectory of mammals.
Data collected from monotreme platypus and short-beaked echidna show that the entire monotreme appears to have diverged from marsupials and eutheria about 187 million years ago. Nevertheless, semi-aquatic platypus have remained unchanged for over 100 million years and, unlike many marsupials and mammals, are adapted to the Australian outback environment.
The platypus is the only animal with 10 sex chromosomes. The sex chromosomes of the platypus are composed of five types of X chromosomes and five types of Y chromosomes arranged in a circle, and it is thought that each sex chromosome was fragmented during the evolution of mammals. Comparing this chromosomal information with the genomes of humans, possums , Tasmanian devils , chickens, and lizards, we found that the sex chromosomes of platypus are more common to birds like chickens than mammals like humans.
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Platypus lay eggs like chickens, but platypus pups are breast-fed and raised. As a result of genome analysis, it was found that the platypus lost a gene that can express an egg protein that nourishes children in the egg about 130 million years ago. Instead, the platypus genome was found to contain genes that express casein , a component of breast milk, like marsupials and eutheria.
Guojie Zhang, a member of the research team and a researcher at the Kunming Zoological Institute of the Chinese Academy of Sciences, said, 'The milk production of all existing mammals is a gene of ancestors who lived in the same age as dinosaurs more than 170 million years ago. I found out that it came from. ' However, it is said that platypus breast milk is made differently from cows and humans who make breast milk derived from blood.
Also, platypus have a wide range of beaks like ducks, but they do not have teeth. Again, genome analysis revealed that the platypus lost four genes related to tooth development approximately 120 million years ago.
In addition, a 2008 study found that the toxin protein secreted by the platypus hind legs originated from the same genetic lineage as reptiles. In other words, the toxicity of platypus is evidence of relatives of reptiles. However, the echidna, which is the same monotreme as the platypus, seems to have lost the gene that expresses this toxin protein.
by Stowe Boyd
The research team said that platypus still have many mysteries, and hopes that 'analyzing the genome maps of platypus and echidna will provide further insights into biology and evolution.' I am.
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