Tutorials/Genetics/Smash Tek

Smash tek is a simple way to cross two strains: combine spores from both on the same agar plate, let them germinate and mate together, and then select the best of the new cultures that come out of it. It's how hobbyists create new strains without a lab.

This page explains how mushroom mating works, why smash tek produces crosses, and how to run one from start to finish.

Can I cross two species?

Almost always, no. Crossing works between strains of the same species: two varieties of oyster, two varieties of lion's mane, and so on. Different species generally can't mate; that inability is a large part of what makes them separate species. When two species meet on a plate, you'll usually see a barrage line, a visible ridge or gap where the two cultures stop and refuse to merge. Sectors along that line are almost always just the two cultures competing, not a hybrid.

There are rare reports of crosses between very closely related species in the same genus, and labs can force fusions with specialized techniques, but neither is something a hobby setup will produce. If you think you've made an interspecies cross, the most likely explanation is that one culture took over the plate.

How mushrooms mate

Understanding this makes the rest of the process make sense.

• A spore carries a single set of genes, one nucleus' worth.
• When a spore germinates, it grows into a monokaryon: mycelium with one kind of nucleus. Monokaryons grow, but they cannot produce mushrooms.
• When two compatible monokaryons meet, their hyphae fuse. Each cell of the new mycelium carries one nucleus from each partner. This is a dikaryon, and only dikaryons can fruit.
• Compatibility is controlled by mating type genes. Most cultivated mushrooms have many mating types, so most pairings from different parents are compatible, but not all.
• Under a microscope, many dikaryons show small loops called clamp connections at the cell walls. They confirm a culture is dikaryotic, but not which parents it came from.

A normal spore-started grow is already full of this: spores from one parent pair off with each other into many slightly different dikaryons. Smash tek just puts spores from two parent strains into the same pool, so some of the pairings end up with one nucleus from each strain. Those are the crosses.

The method

1. Pick your parents

Choose two strains of the same species, each with traits you want to combine: fast colonization, dense fruits, high yields, a particular look. Work from clean, fresh prints or freshly opened caps from each.

2. Combine the spores

Do this in a still air box or flow hood.

• From prints: scrape a similar amount of spores from each print into the same jar of sterile water. Let it sit 12–24 hours to hydrate, then mix well.
• From fresh caps: press gills from both caps together, or swab both with one sterile swab. This is the "smash."

3. Plate it

Put 1–2 drops of the mixed solution, or a light zigzag streak with the swab, onto several agar plates. Run more plates than you think you need; you want lots of variety to choose from.

4. Let it germinate

Spores germinate in 3–10 days. Early growth will be thin and patchy: that's monokaryons finding each other. Over the next week or two, distinct sectors appear as dikaryons form and outgrow the monokaryons.

5. Isolate sectors

Using the three-transfer method, take wedges from the leading edges of several different strong sectors and move each to its own plate. Label each one (cross name, sector letter, transfer number, date). Keep going until each plate grows out evenly with no sectoring.

At this point each plate is one candidate. You can't tell yet which ones are crosses and which are just one parent mated with itself; the only real test is fruiting them.

6. Fruit and compare

Take each clean isolate through a small grow: grain and a small tub. Keep notes on each one: colonization speed, time to pin, yield, size, density, appearance. Isolates that show a mix of both parents' traits, or something new, are your crosses.

7. Keep the winners

Clone the best mushroom from the best isolate and put a copy in long-term storage. That clone is your new strain.

Things to know

• Crosses don't breed true. Spores from your new strain will produce a wide mix of offspring. To keep a cross, propagate it by cloning, not spores. Stabilizing a line so its spores behave consistently takes many generations of selection.
• Most results will be ordinary. Expect to fruit several isolates to find one worth keeping. That's normal.
• Name it and write it up. If you develop a strain worth sharing, add a page for it here with its parents, how you selected it, and photos.

Advanced: di-mon crossing

A more controlled approach pairs a dikaryon from one strain with a monokaryon grown from a single isolated spore of the other. The dikaryon can donate a nucleus to the monokaryon, converting it into a new dikaryon that combines both. It takes more agar work and ideally a microscope to confirm the monokaryon is single-spored, but it gives you more control over parentage than a mixed spore pool.

Further viewing

• Smash Tek – How To Cross Mushrooms (YouTube)