Nuclei—the Core Values in a Mushroom’s Life

© Else C .Vellinga
Lab
Original publication: Mycena News, April 2009

Birth, copulation, and death. That’s all the facts when you come to brass tacks” (T.S. Elliot), but not when you are a mushroom. Mushrooms are unique in many ways but no other organism has a life cycle like a fungus. As if the standard fungal life cycle were not fascinating enough, the exceptions are even more mind boggling.

Let’s recapitulate the standard life cycle (see Fig. 1), starting with the spores. Fungal spores are single cells, each with one nucleus containing one set of chromosomes—that’s called haploid. When conditions are right and a spore germinates, it forms a network of threads, made up of cells each still with one nucleus: the monokaryon (mono = 1, karyon = nucleus). This is a rather short-lived affair, and it depends on the following event for survival: meeting up with a mate. Either a spore with a different mating type (think sex) lands on it, or it meets another monokaryon with a different mating type, then the two merge and the newly formed cells contain two nuclei, one from each parent. This phase of the life cycle is called either a dikaryon (with two distinct haploid nuclei) or a heterokaryon (when there are two or more different nuclei). This phase does not occur in animals and plants, where following the merging of two cells with single nuclei from different parents, the nuclei fuse and the genetic material is mixed to form an offspring that differs from the parents.

A mating type can be compared to gender as we know it in animals, but in many cases there are not just two mating types in equal proportions, but many, increasing the possibility of meeting a mate.

And a second note to the story above, the cells of the monokaryon are separated from each other by a simple wall; those of the dikaryon have a special structure that makes sure that with every cell division a copy of both nuclei gets moved on to the new cell, this is the clamp (see Fig. 2 below). Monokaryons lack clamps.

When two compatible monokaryons meet, the cells merge, and the nuclei of one of them moves into the mycelium of the other, cell walls crumble, and the ‘invading’ nucleus divides and moves quickly along, a speed of two mm per hour has been noted, until all the cells are provided with two nuclei, and a truly dikaryotic mycelium continues to grow, with the proper use of clamp connections to move the two nuclei into new cells. (And don’t be awkward and ask me what happens when two mating events happen at different parts of a monokaryotic mycelium and propagating dikaryotic waves meet in the middle.)

The two nuclei in the cells of the dikaryon work together to get the cell’s machinery going. This dikaryotic mycelium is long lived, and can form the fruitbodies – our mushrooms – when the conditions are right. And that is where genetic recombination – a reshuffling of the genetic material of the parents – takes place. Place of action is in the basidia, the special cells on the gills of your Amanita, inside the tubes of the porcini or red belted conk, or on the branches of a Ramaria species. The two nuclei of the dikaryon fuse to form one nucleus with two sets of chromosomes—now we can call the cell diploid —recombination happens, and two new nuclei are formed. Another nuclear division, this time without changes in the contents of each nucleus, produces a total of four nuclei which are moved to the four budding spores each on top of a little prong (sterigma) of the basidium. The life cycle is complete.

For many species we do not know the details of this life cycle; it is not too hard to count nuclei in the cells of a fruitbody, but mycelium is harder to find, and in many cases hard to grow. And, what it actually means when there are two (or three or ten) nuclei in a stipe cell is another issue.

We do know, just from observations, that there are quite a few exceptions to the standard life cycle described above. Even the ‘lab rat’ Coprinopsis cinerea has variations on this life cycle, as it can form asexual spores on the monokaryon and the dikaryon, it can also sit out bad times as a sclerotium, an asexual resting structure, or a thick-walled resting spore, again sprouted from both stages, and, the four spores on the basidium have two identical nuclei each.

Honey mushrooms have not read the text books either and follow strange and different paths (see Brian Perry’s article on genetic mosaics); one aberrant road they follow is that the nuclei can fuse already in the mycelium phase, making the mycelium diploid (the spores of ‘normal’ mushroom species are haploid, and the mycelium is provided with two separate haploid spores per cell), long before the formation of fruitbodies, the place where diploidization takes place in other mushroom species.

A meeting between a monokaryon and a dikaryon can also result in a happy ending—with one of the two nuclei of the dikaryon moving into the monokaryon, but migration in the opposite direction has also been observed, resulting in a trikaryon.

Just like Coprinopsis cinerea, most Lepiota species have an identical nuclei are present in each spore. This is a widespread phenomenon among mushroom species. The situation in which two different nuclei per cell are present happens too, but is much rarer.

The button mushroom, Agaricus bisporus, with only two spores per basidium, has spores with four nuclei each, of two different mating types; it produces mycelium with four nuclei per cell (but three or five nuclei also happen), and skips the mating of the mycelium part of the cycle as an unnecessary and messy procedure, as each cell has already two different nuclei. The cells just under the basidia and the basidia themselves have 2 nuclei, one of each type, and with a meiotic and two mitotic (normal) nuclear divisions, two spores each with four nuclei are formed. Clamp connections are absent—all Agaricus species lack them as a matter of fact—nevertheless, nuclei divide and are moved from one cell to the next without any problem.

Recent research on a species of Annosum root rot (Heterobasidion parviporum) shows that there can be even more going on. Monokaryons were grown in the lab and different types were mated in a Petri dish.

The heterokaryotic stage of this fungus, after the mating of the two monokaryons, has cells with a multitude of nuclei each. Furthermore, these nuclei do, in many cases, not occur in 1:1 proportions, but with one type dominant over the other. If one of the parents is a monokaryon that is sick and old, its nuclei dominate the heterokaryon heavily, and can outnumber the other type of nuclei nine to one. Asexual, monokaryotic spores (conidia) are formed on the heterokaryons, but the number of nuclei of one type in a heterokaryon is not a predictor for the type of nucleus that will make it into these conidia. Different growth media also may influence which nucleus type will be most numerous in a mycelium. And on top of all that, there is no correlation between the presence of clamp connections (which varies from 5 to 100%) and the skewedness in the proportions of nucleus types.

In fact, many fungi have a heterokaryotic phase, with cells with more than one copy of each nuclear type, but in most cases, they grow out of it, and when it comes to the formation of a fruitbody, the organism is back to the normal situation of two nuclei per cell.

All evidence in the Heterobasidion example indicates that selection does not work on the organism as a whole but at the level of the nucleus.

Of course, lots of questions are still open—how widespread is this phenomenon, what is the role of clamps, are these imbalanced proportions maintained throughout the mycelium and other parts of the lifecycle, how do the nuclei operate the cell’s daily life, is there a division of labour, would it be possible to follow the different nuclei in the mycelium?

And, still, this is only one aspect of the intriguing life cycle of mushrooms.