The genetics of somatic incompatibility in tetrapolar Collybia fusipes was studied using eight dikaryotic isolates collected from the
wild and their experimentally derived progeny. Monokaryons from each isolate were all paired with the same unrelated monokaryon
and also paired together in all combinations. The somatic compatibility of the two resulting sets of dikaryons was studied. Two
different types of somatic incompatible interaction were observed, lightly or heavily pigmented lines developing between the two
isolates. The dikaryons that had one nuclear type in common and one coming from sibling monokaryons were compatible in
7–27% of the cases, incompatible with a lightly pigmented interaction in 30–93% and incompatible with a heavily pigmented
interaction in 0–53%. The results suggest that at least three to four loci control the somatic incompatibility in C. fusipes, one of
them alone controlling the heavily pigmented interaction.