Since 1996 the Mushroom Research Unit at Applied Plant Research has been working on the development of a sporeless oyster mushroom. Research has now progressed to the stage where the first sporeless prototypes can be tested at the farm. Whereas in the 1980s a sporeless variety was unsaleable because of its unattractive appearance, the current study has paid a great deal of attention to the look of the mushroom.

A sporeless oyster mushroom can prevent many problems. The enormous amounts of spores produced by the current varieties cause allergic reactions and lung-related health problems among employees working daily in the oyster mushroom cultivation sector. These problems can only be avoided by wearing over-pressure respirators with good air filters. If sporeless varieties are used for large-scale cultivation, these lung problems can become a thing of the past.

Valuable mistake

When developing a new version of the sporeless oyster mushroom, Applied Plant Research used a strain of sporeless oyster mushroom from the American Type Culture Collection. Experiments in crossing the strains showed that this feature was easy to introduce into a commercial variety.

The missing spores in the ATCC variety were caused by mistakes in the genetic material. In the mycelium of most moulds, genetic material is divided over two nuclei. Each originates from a different parent. The 'mistake' that caused the sporelessness, occurs in both nuclear types.

The selection strategy is then to successfully introduce the mistakes into both nuclear types of a commercial variety, to achieve a sporeless variety. In a first cross between a sporeless culture and a commercial strain, it is naturally not only the mistakes that are transferred to the commercial variety. Repeatedly crossing the progeny of the first cross with the commercial variety is used to try to restore as much as possible of the original genetic material from the commercial strain.

The decision over which commercial variety was to be made sporeless, was taken together with the advisory commission for the project. Two representatives from substrate companies, one representative from a spawn company and four growers form this commission. HK35 and P24 were the choices. HK35 is a variety that produces relatively large mushrooms, but is quite sensitive to bacterial infections. P24 is a variety that crops relatively quickly, is less sensitive to bacterial infections, but produces smaller mushrooms.

Original parent material 

Contrary to the researchers who developed the early sporeless oyster mushrooms, Applied Plant Research can use much more efficient technology to improve the strains. A method was used which isolated both nuclear types from a commercial oyster mushroom variety as two mycelium cultures.  This is precisely the parent material used to create the commercial variety. These two ‘parent cultures’ are the perfect starting point to create a commercial sporeless variety.

In the past, spores from one variety were used as parent material. The disadvantage is that they cannot be used to reconstruct a commercial strain.

A second advantage in comparison to the previous attempts is DNA technology, which can be used to identify the traits a spore has inherited from both parents. This technique is first used to decide which piece of genetic material from the sporeless variety must be placed in a commercial variety to create a sporeless variant.

Actual crossing can then take place. The descendents of these first crosses (spores) have each inherited a mosaic of the material from both parent cultures. DNA techniques make it possible to distinguish which part of the genetic material originates from one or the other parent. This is very useful if after a cross only the mistake causing sporelessness is to be retained in a culture, while the rest must have as much as possible original genetic material restored from the commercial variety.

The reconstruction process is achieved by back crossing with the commercial parent used in the beginning. During this cross attempts are made to exchange the part of the genetic material of the sporeless ATCC-culture that doesn't need to be retained, with the genetic material of the commercial parent.

This stage of the reconstructive crossing process has been completed and maybe a second round is necessary. These reconstructive crosses happen quite quickly. The Applied Plant Research unit expects to have a number of ‘prototypes’ of a sporeless oyster mushroom at the end of this year.

After the reconstructive crosses the original genetic material has been restored as far as possible. An unknown factor however is the importance of that part of the genetic material that was unable to be restored. This could possibly negatively affect the cultivation traits and quality for the new variety. For this reason, the research unit will test the prototypes in special trials. The new varieties will probably not differ greatly from the current HK35 or P24, except of course that they produce no spores.

Sporeless oyster mushroom

A sporeless oyster mushroom variety has multiple advantages. The most obvious one concerns labour conditions when harvesting and grading oyster mushrooms. There are however, other benefits. Cultivation of a sporeless variety is expected to be more energy efficient, as air can be recirculated without obstructing the filters. The exactly extent of energy saving should become obvious during the trials. Calculations indicate that a 40% reduction in energy consumption should be possible. New to be built growing facilities - if cultivating sporeless varieties only - could use a smaller climate installation - which will save on construction costs.

It will be interesting to see the effect of sporelessness on long-term yields. At the moment the incubation of oyster mushroom bags for the colonization of the substrate is kept strictly separated from oyster mushroom bags producing mushrooms. The idea behind this (not proven by research) is that spores from oyster mushrooms can transmit diseases from one crop to another. This is supposed to be the reason for the phenomenon that oyster mushroom growers have an excellent yield with a new variety, which rapidly decreases after a few months, until they start growing new varieties again.

If the spores are in fact the cause, then it is logical that a sporeless variety will not produce such fluctuating yields.