Sei whales (Balaenoptera borealis) are part of the group of toothless whales Myticeti or baleen whales. Fast swimming (up to 50 km/h (31 mph)) Sei whales live across all oceans traveling long distances alone or in small groups and sometimes foraging in large herds. R.C. Haldane called Sei whale the “most graceful of all whales”. These slender giants with dark grey back can reach 20,000 kg and up to 20 meters in size and live up to 70 years. The baleen whales became such giants recently, about 3 millions years ago, due to favorable ocean conditions. Sei whales do not like to dive deep, only to about 300 meters. They mostly are sinking below the water surface and not arching the back like many other whales. A small group of baleen whales - Balaenoptera, including Sei whale, developed a large throat pouch that is decorated by nice throat grooves that can be seen on the pale bottom side of the whale when the whale jumps out of the water.
For a long time, cetaceans have been thought to have either an extremely limited sense of smell or none at all. More recently some members of baleen whales have been shown to have the necessary "hardware" to smell. They did loose their teeth though (the teeth buds do form in utero but do not develop). Instead baleen whales acquired complex filter-feeding mechanism. Baleen plates (formed by highly flexible keratin, think our nails and hair) are hanging from the roof of the mouth and retain the plankton, small fish, squids efficiently filtering the water gulped by whales and assembling tasty ocean-food for swallowing. Sei whales enjoy skim-feeding by traveling near the water surface with the open mouth. When the luck brings the school of fish the Sei whale happily engulfs large volumes of fishy water at once and swallows fish without chewing. Interestingly the tongue in baleen whales is greatly reduced. Sei whales consume 900 kg (2,000 lb) of food every day to nourish their giant body!
Sei whales were almost hunted to the extinction with over 300,000 whales killed by humans in 20th century. At some point the population of Sei whale was estimated to be only few thousands in each hemisphere. Multiple conservation efforts and protection laws allowed to stop the decline. Currently Sei whale is an endangered species and the commercial whaling of Sei is prohibited worldwide since 1986. Mass beaching (up to 300 whales) of Sei whales occasionally happens presumably due to toxic algae growth in subtropical waters. Killer whales, damaging ship encounters, diseases and internal parasites are other causes of Sei whale deaths. Current population of Sei whales is estimated to be about 65,000 - 80,000 and not showing bottleneck effects.
The migration patterns and reproduction are topics where we have very little information about Sei whales. Sei whales “speak” low-frequency pulse language. Lucky observers may hear Sei whale singing during breeding season.
Today, we release the chromosome-length genome assembly for the Sei whale, Balaenoptera borealis! This assembly follows the short-read data model, for more information please see our Methods page. The assembly was done from primary fibroblasts established from a skin sample provided by Charlie Potter, Smithsonian Institution, Washington, DC, US, in May 1989 from a whale stranded on the Atlantic coast of the USA. The primary fibroblast cell line (BBS-1) was established by Mary Thompson at the Laboratory of Genomic Diversity led by Dr. Stephen O’Brien. We acknowledge Drs. Melody Roelke-Parker, Carlos Driscoll, Christina Barr, David Goldman and Stephen Lindell for preservation of the cell line collection. The passage 4 was used to make the sequencing libraries.
Browse the 22 assembled chromosomes of the Sei whale in the interactive Juicebox.js session below. The count is consistent with the established karyotype (2n=44, W. Nash, Atlas of Mammalian chromosomes, 2020, p. 729).
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