The genome of a marine creature carries the secret of the unicellular ancestor of human beings

The newly established genome of a plankton, which consists of only one living sea cell, was published on Feb. 14 in Nature. This genome provides scientists with information about the evolution that comes with a jump from single-celled organisms to multicellular organisms like we do today.

The Nature paper and the Science journal supplement were released this week, biologists Nicole King, Daniel Rokhsar and colleagues at the University of California (Berkeley) have published the first gene diagram of a choanoflagellate. name Monosiga brevicollis . For the first time, they also made their comparisons with the genome of multicellular animals (also known as metazoan).

Genetic sequencing and analysis is carried out by the Walnut Creek (California) Department of Energy Joint Genome Institute (JGI) in collaboration with researchers at the University of California (Berkeley) and eight other research institutes.

According to Ms. King, biologists have almost no information about these organisms in addition to they are an important source of mollusks (this is the favorite of the horned whale). In addition, they also know that with the consumption of a large number of bacteria, choanoflagellates play an important role in the ocean cycle in the ocean. But since choanoflagellates and animals share common ancestors from 600 million to about 1 billion years ago, choanoflagellates hold the key to understanding the origins and evolution of animals .

Choanoflagellate are fully-submerged nucleated bacterial cells characterized by a whip above the head (green) used for swimming and feeding; Around is a group of sentiments (red) to catch prey. Their kernels are green. (Photo: Nicole King Laboratory, UC Berkeley)

Ms. King - assistant professor of combined biology, molecular biology and cell, and winner of the MacArthur award in 2005, said: "Choanoflagellate is the closest single-celled relative to the so it can help us understand human history as well as the history of life on earth - where unicellular organisms once dominate, they bring light to biology and secrecy. The genome of unicellular organisms from which we evolved '.

By establishing the gene sequence, scientists have discovered that choanoflagellate has many genes that in animals produce essential proteins in signaling between cells as well as in deciding which cells go into a group together. According to King, because Monosiga does not live as a crowded group as other choanoflagellates, the role of these proteins remains in the mysterious curtain.

King said: 'In animals, some of these proteins called cadherin have evolved to connect cells with each other. They are like a glue that prevents the cell group from detaching. Although choanoflagellates do not carry evidence of multicellular organisms, they have up to 23 genes that produce cadherin proteins - equal to genes found in fruit flies or mice '.

In Science , King, along with graduate student Monika Abedin, wrote that some of the proteins described above were found to be choanoflagellate cells, where it clings to surfaces, around the capture and digestion of bacteria. .

They argued that perhaps the last single-celled ancestor of all animals (including humans) used ancient cadherin proteins to catch and eat bacteria, while multicellular animals have More complex structures use cadherin to bind cells and become larger multicellular organisms. In the Science article, it says: 'The process of transforming into multicellular organisms seems to rely on the ability to incorporate diverse transmembrane proteins to form new functions in signaling between cells and binding. link '.

Dan Rokhsar, professor of molecular biology and cell biology at the University of California (Berkeley) and director of the computer genome program at JGI, said: ' Choanoflagellate is indeed a unique time window. leading to the origin of animals and humans. They are the best way to help us establish a triangular grid of the last unicellular ancestor of an animal without ever having fossils '. King and Rokhsar are also members of Integrative Genomics Center (University of California, Berkeley).

Choanoflagellate are organisms that feed on bacteria that grow in saltwater and freshwater areas around the world. At about 10 micrometers across, they are just the size of another complete human cell, yeast. While yeast is very familiar to gene researchers, choanoflagellate is not. King hopes to change this when the choanoflagellate genome has been sequenced.

These creatures are egg-shaped with a long tail at one end (also called whip); Surrounding the root is a tentacle that helps catch bacteria. Choanoflagellate is derived from ' collar ' (Greek means 'round'). Its whip makes choanoflagellates swim in water and pushes bacteria towards the fringes. Because choanoflagellates are similar to sponges (among the most primitive animals), 165 years ago biologists thought it was the distant ancestor of multicellular animals.

King and Rokhsar succeeded in suggesting the sequencing of choanoflagellate a few years ago. It is also part of the Department of Energy's Microbial Genome Program. After that, King began to separate the non-infected DNA to set up the sequence. Genetic diagram completed and annotated in 2007, including 9,200 genes. It is about the size of fungi and diatoms but is much smaller than the genome of multicellular animals. For example, we humans have about 25,000 genes.

Interestingly, choanoflagellate has quite a number of intron fragments on the gene (the area that does not carry the encoded information known as ' discarded ' DNA) is similar to humans and is on similar points . Intron segments need to be removed before the gene is used to make a protein-making blueprint. They are also related to higher organisms.

According to King, the choanoflagellate genome, like the genome of many simple organisms sequenced in recent years, has shown surprising levels of complexity. For example, many genes involved in the central nervous system of higher organisms have been found in simple organisms; It is worth mentioning that these creatures have no nerve center.

Similarly, choanoflagellate has 5 immunoglobulins although they do not have an immune system. They have regulated gene segments for collagen production, integrin and cadherin although there are no bones or molds to bind cells. At the same time, they have proteins of tyroxine kinase that play a key role in signaling between cells, although Monosiga is known to be unable to communicate, or at least not live in a multi-member group.

These findings help King and her colleagues assemble a picture of a common ancestor of humans and choanoflagellate-like creatures; At the same time, look for the first clues about animals.

Rokhsar said: 'This is equivalent to our level of understanding of how the ancestors of animals connect and communicate with each other. At least since then we have been able to make initial theories about the appearance of animal-like creatures today. '

However, it is not easy to determine which genes are present in the last common ancestor of choanoflagellate and humans; What genes are only available? Choanoflagellate and humans have evolved over a period of time, so genetic differences can reflect genes that are lost in choanoflagellate as well as genes in humans. Comparing Monosiga's genome to other organisms, including other choanoflagellate individuals called Proterospongia (group living organisms sequenced by the National Institutes of Health) can answer this question.

King hoped that Monosiga's genome could give answers to many questions about animal evolution and illuminate poor biology with information about this aquatic creature.

King while noting a case similar to the sea anemone Nematostella vectensis (established in 2007 sequencing) said: 'This is a new era where we start with a genome to understand the Biological characteristics of an organism. That genome is our advantage. '