Author Stieg Larsson, the second biggest-selling novelist in the world in 2008 (behind Khaled Hosseini), left three-quarters of an unfinished book on his laptop when he died in 2004.

It was the fourth book in his posthumously published “Millennium” series of crime novels, which has already sold some 30 million copies worldwide. Larsson also reportedly left synopses for a fifth and sixth “Millennium” book. Fans of the series that began with 2005’s “The Girl with the Dragon Tattoo” long to get their hands on anything more from the Swedish journalist and author.

Well, while I can’t promise sex crimes, serial murder or shadowy corporations, this week I can tell of a nature mystery set outside the city of Skelleftea in northern Sweden, where Larsson was born. It involves DNA testing, a valuable gourmet prize, and a feisty young Japanese female researcher who helped discover it.

In fact, this week’s column could be titled “The Girl Who Found the Golden Mushroom.”

Our heroine is Etsuko Harada, from the Iwade Research Institute of Mycology in Mie Prefecture, though I should admit right away that I don’t actually know if she is either young or feisty: That description was fanciful artistic licence.

What I do know is that, in 2008, Harada traveled to Skelleftea to meet Henrik Sundberg, a student at the Department of Plant and Environmental Sciences at the University of Gothenburg.

Harada was on the hunt for a rare fungus named Lyophyllum shimeji, the second-most-soughtafter and valuable mushroom in Japan. Hon-shimeji (literally, “true shimeji”), as it is known in Japan, is a delicacy thought to only grow in east Asia, and worth as much as $1,000 per kg.

Harada wasn’t blundering around in northern Scandinavia for no reason, but because, 10 years previously, some “Japanese” species of fungus had been discovered growing there.

With hindsight, perhaps it shouldn’t have been so unexpected: After all, Sweden and Japan are both temperate, northern hemisphere countries with several ecological similarities — as well as winds that no doubt sometimes blow between them, carrying such things as tiny spores.

Biologists had reported that a Swedish mushroom named Tricholoma nauseosum was identical to the Japanese species Tricholoma matsutake. “So what?” you might think, but this was no mere academic discovery: Matsutake is the first-most valuable and perhaps the most prized of all Japanese mushrooms, fetching up to $2,000 per kg. It has a distinct, spicy aroma and grows in an intimate ecological entwining with the roots of certain tree species.

The discovery of matsutake in Sweden led to what could be termed a “mushroom rush,” as mycologists and traders headed to Scandinavia in search of “fungal gold.”

Production in Japan of both species of gourmet mushroom — hon-shimeji and matsutake — has crashed over the last 50 years because of infection by a nematode worm, and because of changes in forestry management. Japanese domestic supplies are highly prized, but consumers are often forced to buy mushrooms imported from China and South Korea. Harada suspected that relief supplies of hon-shimeji might be found in the land of Pippi Longstocking.

Arriving in Sweden, Harada soon found a fungus on a pine heath near Skelleftea that she thought was similar to hon- shimeji. Mushroom experts Koji Iwase at Tottori University and Akiyoshi Yamada at Shinshu University in Nagano Prefecture were consulted, but a firm conclusion could not be reached until Sundberg carried out DNA tests on the suspected “true shimeji.”

“After getting a positive response from Japanese mycologists, we became more and more convinced that we were on the trail of a Japanese delicacy,” said Sundberg, who wrote up his results last year as part of his masters degree project.

“When we found more the following year, we started up a project to examine the fungus using molecular techniques. We were soon able to show that the Swedish and Japanese fungi are, without a doubt, identical.”

But just why are hon-shimeji and matsutake mushrooms so valuable in Japan?

Sundberg quotes Yamada as saying that the word “shimeji” denotes the perfect “mushroomish” appearance of the fungus. They do look like a mushroom a child might draw even without having seen a hon-shimeiji. Matsutake mushrooms are more phallic, frankly. Indeed, Japanese matsutake-hunters are told to pick mushrooms that look more like a penis than an umbrella: It’s better, in other words, if the cap of the mushroom hasn’t opened.

So: Hon-shimeji and matsutake mushrooms look like the Platonic ideal of a mushroom. They taste great when cooked, and can be used in a variety of dishes. They symbolize autumn and longevity — fittingly for ever-graying Japan — and are given as lavish gifts to business partners.

They’ve been revered for a long time, according to the the Japan Special Forest Production Promotion Association, citing clay artifacts in the shape of matsutake mushrooms that have been dated back to the Jomon Period (from 13,500 years ago to about 3,000 years ago). Some people claim they have medicinal, and even anticancer properties, but I’ve seen no evidence for that.

I don’t expect anyone to make a movie about the quest for the gourmet mushroom, but I do hope that the price of the things comes down, as I’d love to be able to eat them more often.

Follow Rowan Hooper on Twitter @rowhoop. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru” (“The Evolving Human”).

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