All the white button mushrooms (Agaricus bisporus) grown nowadays are hybrid varieties. This means they are crosses. Most varieties descend directly or indirectly from U1 and U3 or crosses of both. The present day commercially grown varieties are nearly all so-called intermediate hybrids such as Sylvan varieties 130 737 A15 and Amycel varieties 2100 and Delta. Other companies also offer similar varieties.
The various strains appear alike and the differences are so minute that growers and/or composters sometimes even mix the spawns. However, this is a risky business. For example, if compost has been inoculated with Delta and A15 is then used as CI (casing inoculum) the results may be good - but what happens if the mushrooms shown disparities? Who or what is the culprit? It’s preferable to stick to a single variety per cycle.
John Peeters, C point