Black truffles (nicknamed black diamonds) have a reputation for romance. For thousands of years the rare and tiny fungus has been eaten as a delicacy, as medicine, an aphrodisiac even. They've been impossible to cultivate and their rare occurrence in the wild makes them a hard meal to find. At last, we may understand why: The fungus has trouble mating.

The fact that the mushrooms mate at all makes them unique among fungi (which usually reproduce asexually). It's long been a mystery as to how the truffles spread, but a recent article from the Telegraph reports that researchers have discovered male and female sex characteristics of the truffles. The article states that since the truffles only reproduce sexually and also tend to become isolated into single-sex colonies, their reproduction mojo needs a kick-start.

The story mentions "trufficulteurs" in France and Italy who try to make magic happen for the mushrooms by impregnating the roots of young oak trees. Even this intervention is not always successful — only 30 percent of the trees will bear fungus fruit. According to the story, Prince Phillip spent 5,000 pounds purchasing impregnated oak trees and still needed specialists to get the truffles shuffling.

All this resistance to mating makes the truffles expensive: The story says a single Tuber melanosporum can cost about $160. Enter the Plant Genetics Institute in Perugia, Italy. Dr. Francesco Paolocci, the lead researcher in the fungus sexuality research, told the Telegraph how his team observed the truffles.

He noted that even when his group started an oak tree with male and female truffles, "it quickly became dominated by one sex or the other." His task was to figure out how to get the "two different sexual strains meeting in some way." In the wild, he hypothesized, animals probably carry spores from one colony to another. The article goes on to describe how potential farmers would need to pay attention to the balance of male and female spores in the underground truffle colonies.

The Telegraph explains that the truffles grow in a "mutually-advantageous relationship with the roots of trees." They give the trees nutrients and minerals and take food back from the oak trees. The mature truffles can grow up to three inches long and apparently, it is only the female mushrooms' fruiting bodies that are so sought after by chefs worldwide.

The article says that the truffles aren't exactly male or female in the way other animals are, but that there are distinct mating types that Paolocci has discovered through genetic testing. The Plant Genetics Institute found that it is the female type truffles that tend to cling to the oak roots while the male types seem to live in the soil elsewhere.

The Telegraph says that cultivation attempts prior to this have "overlooked the sexuality of the truffles," inadvertently creating single-sex colonies that cannot hope to reproduce. Paolocci told the Telegraph his research could help increase truffle production and bring down the prices of the delicacies.

Truffle sex: Here's the problem
Researchers dig into why treasured fungi are so rare and finicky.