The edible mushroom Agaricus bisporus (Lange) Imbach var. bisporus, a microorganism belonging to the basidiomycete fungi, is widely cultivated around the world. Accordingly, development of novel hybrid mushroom strains or lines of this mushroom fungus is seen as highly desirable to the cultivated mushroom industry, particularly if those novel strains or lines can be developed to provide various desirable traits within a single strain, culture, hybrid or line.
Thus, various entities within the mushroom industry, including Sylvan America, Inc., have set up mushroom strain development programs. The goal of a mushroom strain development program is to combine, in a single strain, culture, hybrid, or line, various desirable traits. Strains currently available to the mushroom industry allow growers to produce crops of mushrooms successfully and profitably. Several factors exist that influence the degree of success and profitability achieved. Characteristics of strains that are factors that can improve producer profitability include increased productivity (higher yield or shorter cycle time), accelerated revenue capture (earlier harvest), reduced costs (for example, greater ease and speed of harvesting), reduced shrinkage (pre-sale weight loss), reduced overweighting of product in packages (extra weight of product packaged, due to particular sizes of individual mushrooms), improved consistency of crop performance responses to variations in raw materials, growing conditions and practices, superior crop performance in particular facilities, regions, etc., reduced losses to diseases including viral, bacterial and fungal disease agents, and/or reduced losses to insect and nematode pests of the crop. There also exist improvable properties of the mushroom product that increase demand in the distribution chain, and thus sales volume and/or sales price, such as improved visual appeal (more desirable coloration, shape, size, or surface texture), improved or distinct flavor characteristics, improved keeping qualities (longer persistence of desirable visual attributes), etc. Still other improvements may enhance the suitability of the mushroom crop for mechanical harvesting, canning, and/or food processing. Thus there are many characteristics by which a novel strain might be judged as superior in a particular production facility or sales market, or in the industry regionally or globally. All of these characteristics can be assessed using techniques that are well known in the art. Novel strains are most preferably and successfully developed from unique hybridizations between homokaryotic lines, including novel lines. Thus, the need continues to exist for new lines that can be used to produce new hybrid strains of Agaricus bisporus mushroom cultures and microorganisms that provide improved characteristics for producer profitability and for improved mushroom products over other previous strains of Agaricus bisporus. 
There is also a need for commercially acceptable A. bisporus strains with different genotypes, relative to the U1 derived lineage group, for two reasons. First, strains incompatible with strains of the U1 derived lineage group are known to retard the spread of viral diseases between cultivated strains. The incompatibility phenotype can be assessed using techniques that are well known in the art. Second, it is well understood that when an agricultural crop industry relies extensively on a single genetic lineage (i.e., creates a commercial monoculture as now exists for the white-capped U1 lineage of A. bisporus), there is an increased risk of unpredictable, catastrophic crop failure on a facility-wide or even industry-wide scale. Therefore from a risk management and food security perspective, it is highly desirable to simultaneously provide both genetic diversification and commercially acceptable performance and crop characteristics. The use of novel lines that incorporate DNA from non-cultivar stocks provides important genetic diversification of the strain pool used to produce crops of cultivated A. bisporus mushrooms.
Cultures are the means by which the mushroom strain developers prepare, maintain, and propagate their microorganisms. Cultures of Agaricus, like those of other microorganisms, are prepared, maintained, propagated and stored on sterile media using various microbiological laboratory methods and techniques. Sterile tools and aseptic techniques are used within clean rooms or sterile transfer hoods to manipulate cells of pure cultures for various purposes including clonal propagation and for the development of new strains using diverse techniques. Commercial culture inocula including mushroom ‘spawn’ and ‘casing inoculum’ are also prepared using large-scale microbiological production methods, and are provided to the end user as pure cultures contained within sterile packaging.
One use of such cultures is to produce mushrooms. Mushrooms are cultivated commercially within purpose-built structures on dedicated farms. While there are many variations on methods, the following description is typical. Compost prepared from lignocellulosic material such as straw, augmented with nitrogenous material, is finished and pasteurized within a suitable facility. Mushroom spawn, which comprises a sterilized friable ‘carrier substrate’ onto which a pure culture of one mushroom strain has been aseptically incorporated via inoculum and then propagated, is mixed with the pasteurized compost and is incubated for approximately 13 to about 19 days at a controlled temperature, during which time the mycelium of the mushroom culture colonizes the entire mass of compost and begins to digest it. A non-nutritive ‘casing layer’ of material such as peat is then placed over the compost to a depth of from about 1.5 to about 2 inches. Additional ‘casing inoculum’ incorporating the same mushroom culture may be incorporated into the casing layer to accelerate the formation and harvesting of mushrooms, and improve uniformity of the distribution of mycelium and mushrooms in and on the casing surface. Environmental conditions, including temperature and humidity, in the cropping facility are then carefully managed to promote and control the transition of the culture from vegetative to reproductive growth at the casing/air interface. In a further about 13 to about 18 days after casing, mushrooms will have developed to the correct stage for harvest and sale. A flush of mushrooms comprising the original culture will be picked over a 3 to 4 day period. Additional flushes of mushrooms appear at about weekly intervals. Commercially, two or three flushes of mushrooms are produced and harvested before the compost is removed and replaced in the cropping facility.
Agaricus bisporus has a reproductive syndrome known as amphithallism, in which two distinct life cycles operate concurrently. As in other fungi, the reproductive propagule is a spore. Agaricus produces spores meiotically, on a meiosporangium known as a basidium. In a first life cycle, A. bisporus spores each receive a single haploid postmeiotic nucleus; these spores are competent to mate but not competent to produce mushrooms. These haploid spores germinate to produce homokaryotic offspring or lines which can mate with other compatible homokaryons to produce novel hybrid heterokaryons that are competent to produce mushrooms. Heterokaryons generally exhibit much less ability to mate than do homokaryons. This lifecycle is called heteromixis, or more commonly, outbreeding. This life cycle operates but typically does not predominate in strains of Agaricus bisporus var. bisporus. 
A second, inbreeding life cycle called intramixis predominates in most strains of Agaricus bisporus var. bisporus. Most spores receive two post-meiotic nuclei, and most such pairs of nuclei consist of Non-Sister Nuclear Pairs (NSNPs) which have a heteroallelic genotype at most or all centromeric-linked loci including the MAT locus. That MAT genotype determines the heterokaryotic phenotype of these offspring, which are reproductively competent and can produce a crop of mushrooms. Unusually among eukaryotes, relatively little chromosomal crossing-over is observed to have occurred in postmeiotic offspring of Agaricus bisporus; empirically, very little heteroallelism (analogous to heterozygosity) is lost among heterokaryotic offspring of a heterokaryotic strain. Consequently, parental and offspring heterokaryotic genotypes and phenotypes tend to closely resemble each other, as noted above. For this reason, essential derivation, e.g., the production of Essentially Derived Varieties (EDVs), is a familiar strain development technique among commercial mushroom spawn producers.
Therefore, the need exists for the development of new Agaricus bisporus lines that meet the needs and desires of mushroom producers, marketers and consumers.