A key metabolic gene for recurrent freshwater colonization and radiation in fishes.
Asano Ishikawa, Naoki Kabeya, Koki Ikeya, Ryo Kakioka, Jennifer N. Cech, Naoki Osada, Miguel C. Leal, Jun Inoue, Manabu Kume, Atsushi Toyoda, Ayumi Tezuka, Atsushi J. Nagano, Yo Y. Yamasaki, Yuto Suzuki, Tomoyuki Kokita, Hiroshi Takahashi, Kay Lucek, David Marques, Yusuke Takehana, Kiyoshi Naruse, Seiichi Mori, Oscar Monroig, Nemiah Ladd, Carsten J. Schubert, Blake Matthews, Catherine L. Peichel, Ole Seehausen, Goro Yoshizaki, and Jun Kitano.
Science 31 May 2019: Vol. 364, Issue 6443, pp. 886-889 DOI:10.1126/science.aau5656
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Fishes are present in not only marine but also freshwater environments including rivers, streams, lakes, and ponds because ancestral marine fishes have colonized freshwater multiple times and diversified in the freshwater environment during evolution. However, only few fish lineages could colonize freshwater habitats. What enable some lineages to colonize freshwater?
Dr. Asano Ishikawa and Dr. Jun Kitano of Ecological Genetics Laboratory at the National Institute of Genetics, Japan, collaborated with researchers from Japan, Switzerland, USA, and Spain and asked this question using stickleback fishes (Fig. 1) as a model. They identified a key gene important for freshwater colonization, Fatty acid desaturase 2 (Fads2), encoding an enzyme involved in the synthesis of docosahexaenoic acid (DHA), a polyunsaturated fatty acid. DHA is a component of the cell membrane and is essential for growth and survival of fish. In aquatic ecosystems, marine-derived diets are rich in DHA, while diets in freshwater environments comprise limited DHA. They found that sticklebacks that successfully colonized freshwater have higher copy numbers of Fads2 and higher abilities to synthesize DHA than those that failed to colonize freshwater (Fig. 2). Analysis of publicly available whole-genome sequences of 48 fish species revealed higher Fads2 copy numbers in freshwater fish than in marine fish across diverse fish taxa.
This research was supported by JSPS KAKENHI (15H02418, 23113007, 16H06279, 26870824, 16K07469), JSPS PD (11J04816, 16J06812), Asahi Glass Foundation, Sumitomo Foundation, SNF grant (31003A 175614), and SNSF (31003A_163338, PDAMP3_123135).
This was published in Science at 2:00 p.m. U.S. Eastern Time on Thursday, 30 May 2019.
Fig: 1．A photo of a Japan Sea stickleback (Gasterosteus nipponicus) male. Photo by Yasuyuki Hata.
Fig: 2．Freshwater colonization and increase in Fads2 gene Japan Sea sticklebacks that could not colonize freshwater have only one Fads2 copy on Chromosome 19, while three-spined sticklebacks that could colonize have additional copy of Fads2 on Chromosome 12. Some freshwater populations of three-spined sticklebacks further duplicated Fads2 genes on Chromosome 19.