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Welcome to the SDGs in the NEWS Podcast.
This is Susan Yoshimura, Managing Editor of Japan 2 Earth, coming to you from Tokyo.
Today, we bring you another English article on Japan and the SDGs.
You can find the full text on our website.
Just click the link in the episode notes.
Have a listen.
Groundbreaking research to produce affordable caviar with super female sturgeons.
A research team at Kinki University has produced super female sturgeons.
Sturgeons have been overfished for caviar and are on the brink of extinction.
Caviar, one of the world's three most prized delicacies, is the egg of the sturgeon.
Wild sturgeon face a serious threat of extinction.
Therefore, some countries, in particular China and Russia, are pursuing aquaculture.
However, the process is time-consuming,
and the increasingly unstable international security environment has caused the price to skyrocket.
Amid this situation, a team at Kinki University became the world's first to breed a super female
sturgeon. This would dramatically improve the efficiency of caviar aquaculture, and affordable
caviar from Japan could be available in the future. Caviar is made by loosening the mature
ovaries of sturgeon and then pickling them in salt. They are called black diamonds due to their
color and are prized for their rich and full taste. Caviar is not only a staple of Russian
court cuisine but also a must-have for grand parties. However, their popularity has led to
overfishing, and sturgeon are on the brink of extinction. In July, the International Union for
Conservation of Nature, IUCN, published its latest Red List of Threatened Species. Two
species of sturgeon are already extinct in the wild and 27 species are at severe risk of extinction.
In fact, according to a study by an international research team from Europe and the United States,
global natural caviar production peaked in 1984 at over 3,400 tons but dropped to a few
dozen tons by the early 2000s. One metric ton is around 2,205 pounds. Most of the caviar currently
on the market is farmed, with shipments beginning in earnest around 2004. In 2017,
China was the top producer of farmed caviar, with 100 tons per year. Russia was second with 49 tons,
followed by Italy, 43 tons, France, 37 tons, and Poland, 20.4 tons. Japan was 23rd with 2 tons.
The total production, however, was only about 400 tons.
Caviar aquaculture has not expanded much because sturgeon farming takes an extraordinary amount
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of time and effort. Since the goal is to collect the ovaries, male sturgeons are unnecessary.
But about half of the sturgeons are male, and it is impossible to distinguish the males from
the females at the time of hatching, just from their appearance. As a result, the sturgeon sex
is determined after three years of cultivation, when the fish have reached sexual maturity.
Their abdomens are opened one by one to examine the gonads. Males are then sold cheaply for meat
and the females are sutured and raised in aquaculture ponds. It takes more than 10 years
from hatching for large breeds and about seven years for medium-sized breeds to be able to
collect the ovaries. Therefore, a research team at Kinky University searched for a method to raise
only females in order to increase the efficiency of caviar production. By adding soy isoflavones
to the food of hatched fish, they succeeded in developing a method to change the sex of
males to females. But they sought a more fundamental solution. The team's research
resulted in super females, which only lay female eggs. For humans, two chromosomes,
X and Y, determine sex. The 20th chromosome pair makes a person female and the Z chromosome pair
makes a person male. Similarly, sturgeon have Z and W chromosomes, with ZZ making them male
and ZW making them female. This means that females require a W chromosome.
The chromosomes of each male and female gametophyte are reduced to one of the chromosomes
through meiosis. Sperm are always Z, and eggs are either Z or W. Therefore, in normal reproduction,
half are males and half are females. Fish eggs produced by meiosis contain either Z or W
chromosomes that are not normally used in addition to the original chromosomes. If the chromosomes
of a fertile sperm are broken, and external pressure and heat are applied to the eggs,
these egg-derived chromosomes will bond together in a form of gynogenesis.
At this point, there are three combinations of chromosomes, ZZ, ZW, and WW. The WW combination
doesn't normally occur. But if the next generation is bred using the eggs of a WW female,
the chromosomes will always be ZW, which theoretically means that all of its offspring
will be female. This WW female is the super female sturgeon that produces only females.
Although the existence of these super female sturgeons in the wild due to gynogenesis has been
suggested, it has long been difficult to prove the difference between WW females and ZW females.
Therefore, super females have been regarded as phantoms.
The reason why the existence of WW females could not be proven is that previous research
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focused on the analysis of the W chromosome, which females must have, instead of focusing
on getting rid of the unnecessary Z chromosome. To prove that a given sturgeon had WW chromosomes,
the genetic analysis must prove that the Z chromosome was not involved.
Ryuhei Konami, an assistant professor at Kinki University and a member of the research team,
analyzed the Z chromosome to clarify the characteristics of its genetic information.
He then used fertile sperm whose chromosomes were destroyed by ultraviolet treatment to
fertilize the egg. The egg was then placed in warm water to induce gynogenesis.
PCR genetic analysis of the 112 juveniles obtained revealed 21 to be ZZ males, 66 to be ZW females,
and 25 to be WW super females. For the first time ever, the team was able to successfully
acquire and confirm a super female. The super females acquired in this experiment did not
survive because they were used for genetic analysis. In the future, the team will be
In the future, the team plans to observe the super females for a long period of time to confirm
whether they will mature properly and produce only females. To select super females, they have
developed a method to distinguish the live fish by analyzing the genes contained in the mucus on
the surface of the bodies. For this experiment, they used medium-sized sturgeon of the Vestal
species, whose ovaries mature in about 7 years. In future studies, the team is considering using
smaller sturgeon, whose ovaries mature in about 3 years, in order to speed up the process to
check whether they will lay only female eggs. Toshinel Inino is an associate professor at
Kinki University who supervised the research. He said, if super females can be raised properly
to parentage so that eggs can be collected, we will be able to farm only females. Caviar
production will become dramatically more efficient. I hope to quickly establish a production system
and deliver inexpensive, delicious caviar from Japan to the world.
Until next time, this is Susan Yoshimura of Japan to Earth, signing off.
