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| STAFF |
Professor : Norihiro OKADA, Associate Prof. : Masaki KAJIKAWA
Assistant Prof. : Hidenori NISHIHARA, Yohei TERAI |
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| Website of the OKADA Lab >> http://www.evolution.bio.titech.ac.jp/index_e.html |
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| Research |
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It is considered that the diversity of living things arises from the repeated differentiation and formation of species. Essential to this process are the concepts of adaptation and reproductive isolation. At Okada Laboratory we are studying cichlid fishes from the Lake Victoria basin in Africa to elucidate the mechanisms underlying speciation at the molecular level.
Lake Victoria, situated in the Great Rift Valley in Africa, harbours 500 endemic cichlid species, which exhibit morphological and biological diversity. Geological evidence suggests that Lake Victoria dried up at the end of the Pleistocene and refilled only 15,000 years ago. Therefore, it is believed that the current cichlid varieties have arisen during very short evolutionary period after the re-formation of the lake. Accordingly, the different species of cichlid are genetically extremely closely related, and the process of speciation is just completed or now on going. This makes this fish as a model system for our research. Our recent research has shown that visual adaptation has driven speciation in cichlids. This research is outlined below.
We studied cichlid species living along the rocky shoreline with differing degrees of water clarity. This research revealed the following:
- Firstly, the cichlids have adapted their visual system to the light environment in which they are living. The light component in the water varies considerably according to the water turbidity, and species have adapted their visual system to the light environments.
- Secondly, the mating colouration of the male cichlid has evolved to a colour that can be easily regognized by the adapted visual system. After the adaptation of perception and divergence of mating colouration, the recognition of mating signals between two populations becomes weaker. Finally, reproductive isolation occurs when the individuals of one population no longer recognize those of another as potential mates.
This speciation mechanism was hypothesized as speciation by sensory drive. Our research shows the mechanism of speciation at the molecular level. |
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There is a multitude of retroposons in the eukaryotic genome. Retroposons are a type of transposon that replicates itself via reverse transcription of an RNA transcript (see diagram at right). A significant proportion of the eukaryotic genome (≥40% of the human genome) is made up of retroposons. Moreover, it is considered that retroposons are not merely the building blocks of the eukaryotic genome, but that they also play a significant role in its complexity and evolution. How is such a large number of retroposons amassed? What effect does this multiplication of retroposons have on the eukaryotic genome?
Our current research aims to explain the mechanisms underlying the amplification of so-called LINE (Long interspersed element) and SINE (Short interspersed element) retroposons. In recent years we have discovered that host cell DNA repair pathways are involved in LINE amplification.
In investigating the mechanisms underlying the amplification of retroposons, we will tackle the question of how the DNA on our eukaryotic genome was formed, which will help us understand its evolution and origins. |
![€ΰe}2@g|]Μ@\](../../images/seitaisystem/okada_1d_e.jpg) |
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In the genome, there are many conserved regions across species other than protein-coding regions, but the function of most of them is still unknown. At Okada Laboratory, we previously discovered many SINEs present in mammalian-specific conserved regions. This discovery suggested that each SINE which has no functionality evolved to acquire a particular function, and thus had a significant impact on mammalian evolution.
In our current research we are analysing functions of SINEs in conserved regions to find out how SINEs actually acquire functions and how they contributes to evolution in mammals. For example, we have proven that one SINE (AS071) is an enhancer responsible for Fgf8 gene expression in the diencephalon (see diagram) and it is involved in the structural formation of mammalian brain. We have also shown that another SINE (AS021) may enhance expression of Satb2 in the telencephalon, which is highly developed in mammals. It is thought that these SINEs acquired the function of enhancer in a common ancestor of all mammals and have played a role in the evolution of the brain structure. Through further functional analysis, we expect to identify the underlying molecular mechanisms of other mammalian evolutionary processes in which SINEs play a part. |
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| Papers |
| 1) |
Seehausen O., Terai Y., Magalhaes I.S., Carleton K. L., Mrosso H.D.J., Miyagi R., van der Sluijs I., Schneider M.V., Maan M., Tachida H., Imai H.Okada N.Speciation through sensory drive in cichlid fish.Nature(article)455, 620-626(2008) |
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Sasaki T., Nishihara H., Hirakawa M., Fujimura K., Tanaka M., Kokubo N., Kimura-Yoshida C., Matsuo I., Sumiyama K., Saitou N.,Shimogori T.and Okada N.Possible involvement of SINEs in mammalian-specific brain formation.
Proc. Natl. Acad. Sci. USA. 105, 4220-4225(2008) |
| 3) |
Nishihara H, Hasegawa M, Okada N.Pegasoferae, an unexpected mammalian clade revealed by tracking ancient retroposon insertions.
Proc. Natl. Acad. Sci. USA. 103,9929-9934(2006) |
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Sugawara T., Terai Y., Imai H., Turner G.F., Koblmuller S., Sturmbauer C., Shichida Y.and Okada N.
Parallelism of amino acid changes at the RH1 affecting spectral sensitivity among deep-water cichlids from Lakes Tanganyika and Malawi.
Proc. Natl.Acad. Sci. USA 102,5448-5453(2005) |
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Kajikawa M, and Okada N.LINEs Mobilize SINEs in the Eel through a Shared 3' Sequence. Cell 111,433-444(2002) |
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Shimamura M., Yasue H., Ohshima K.,Abe H., Kato H., Kishiro T., Goto M., Munechika I, and Okada N.
Molecular evidence that whales form a clade within even-toed ungulates. Nature 388,666-670(1997) |
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