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The genome of a close relative of invertebrates, the sea squirt, has been sequenced, which gives scientists an opportunity to study the origin and evolution of vertebrates by comparative genomics.
The research results of the genome of ascidian, published in the journal Science, were completed by an international team. It is the seventh animal whose genome has been sequenced. It is mainly led by the Genome Research Institute under the US Department of Energy, the Department of Molecular and Cell Biology of the University of California, Berkeley, the Department of Zoology of Kyoto University and the National Institute of Genetics of Japan. It is carried out in cooperation with more than 20 other research institutions, with 87 participants from 5 countries.
With the genome of ascidian, scientists can further understand the origin and evolution of human brain, heart, nerve and immune system, and better understand the general development of chordates and vertebrates by comparing with humans or other animals.
Mike Levine, a professor of genetics and development at Berkeley, said that sea squirts have a special place in the minds of biologists because they provide an evolutionary connection between invertebrates and vertebrates. When you see its adult, you will think it is a simple animal, and Aristotle thinks it is a mollusk. But when you see its embryonic development, it is obviously a complex higher animal, and Russian biologist Alexander Kowalevsky recognized the notochord of its larval tail. They are our very old cousins.
More than 500 million years ago, animals began to evolve rapidly, becoming 30 to 40 categories of different systems. So far, the living animal kingdom can be divided into 35 phyla. Among them, 34 phylum without backbones (or chordates) are collectively called invertebrates, and the rest are chordates. Chordata animals have four main characteristics: notochord, dorsal cord, hollow nerve cord, pharyngeal suture and muscular tail behind anus. Chordata are divided into three subphylums: one is Cephalopoda, also known as Acrania, represented by amphioxus; Second, caudate animals, also known as tunicates, such as sea squirts, are primitive chordates without spines; The third is vertebrate, one of the most familiar cephalopods, including fish, amphibians, reptiles, birds and mammals.
However, vertebrates took a different path. Compared with cephalochordates and caudates, the front end of vertebrates' nervous system is specialized into a complex brain, and the sensory organs related to the brain, such as eyes, ears and nose, are more sensitive to the environment. Motor organs are also more refined to match the changes of sensory organs; The skull is protected outside the brain, the notochord evolves into a jointed spine, and the internal skeleton protects the internal organs. All these characteristics make vertebrates more adaptable to the environment than other chordates, making vertebrates the most developed branch of chordates.
According to the fossil record, it is recognized that the earliest vertebrate was the Ordovician Ostracoda 475 million years ago. They live in the ocean like benthos, wearing heavy exoskeletons, without jaws, pectoral fins, ventral fins and other movable lateral fins. The vertebrates closest to OSTRACODA are toothless grouper and lamprey. Their spines are still cartilaginous, and they are very primitive vertebrates.
By the end of Silurian 395 million years ago, a group of fishes with upper and lower jaws appeared on the earth, which was a very remarkable change, which made vertebrates who had been filtering their lives become lively predators and evolved into movable lateral fins, making their activities more flexible. These fish include extinct plate-shell fish, some existing cartilaginous fish, such as sharks and rays, and bony fish that dominate modern oceans and fresh water. One kind of teleost fish slowly developed to land and evolved into various vertebrates on land today.
In the past, many zoologists thought that the ancestors of vertebrates probably evolved from caudates. Although there is no obvious notochord in the adult of sea squirt, there is obviously notochord in the tail of its larva. Some people think that in the early CAMBRIAN, the larvae of urochordates had embryonic development, and the reproductive organs were mature before metamorphosis. Through natural selection, the urochordates' larvae develop segmented muscles and strong bones to support their bodies, and because the formation of the head has considerable advantages in adapting to the environment, vertebrates with heads and endoskeletons have evolved.
Eddy Rubin, director of the Genome Research Institute under the US Department of Energy, believes that although they look very different from us, they still share many characteristics with vertebrates. Through the comparison of genomes, we can understand the relationship between them at the molecular level and study how these similar systems and genes evolved from the same ancestor more than 500 million years ago.
As a kind of coccidia, sea squirts are distributed in shallow seas all over the world. Barrel-shaped sea squirts are attached to rocks, breakwaters, ships and seabed, and a basket-shaped food filter is used to filter plankton. One day after fertilization, the ascidian egg developed into a small larva with only 2500 cells, and soon found a place to settle down and become an adult.
The genome of ascidian is composed of1600,000 bases, which is only about one twentieth of that of human beings and only half of that of the vertebrate puffer fish with the smallest genome. Among them,1170,000 bases are composed of about10.6 million genes, 80% of which can also be found in humans and other vertebrates. However, its gene number is only half that of human. Daniel Rokhsar, director of computational genomics at the Genome Institute under the US Department of Energy, said that the ascidian genome is so slender because it has several shellfish genes in vertebrates, but only one in ascidians. Additional shellfish may mutate or disappear, or evolve to be responsible for other functions. From the genome of ascidian, we can see the new progress of human genealogy. For example, some specific genes of immune system and theological system are not found in sea squirts, which is obviously a new device for vertebrates. The complexity of vertebrates may also stem from a large number of gene replication.
There are not many important Hox genes in ascidians that determine the body structure of vertebrates. Some Hox genes exist alone, while others disappear. However, sea squirts also have photosensitive genes similar to vertebrates, as well as other genes that form the heart and thyroid. However, sea squirts use hemocyanin instead of red blood cells to transport oxygen, and lack genes to make cholesterol and histamine. In some respects, the similarity between sea squirts and bacteria, fungi and plants is higher than that between sea squirts and vertebrates: the genes that make fiber membranes to support eating straws are very similar to those that make cellulose, but such genes are not found in other animals and may be transferred from plants and other transverse genes.
Levin thinks that the analysis results of the genome of the sea squirt are completely consistent with Darwin's opinion in 187 1 year "The Origin of Man". Darwin claimed that vertebrates and sea squirts had the same ancestor, one evolved into sea squirts, and the other blossomed and bore fruit in vertebrates.