Data from: Multilevel selection in the filamentous ascomycete Neurospora tetrasperma

The history of life has been driven by evolutionary transitions in individuality, i.e., the aggregation of autonomous individuals to form a new, higher-level individual. The fungus Neurospora tetrasperma has recently undergone an evolutionary transition in individuality from homokaryosis (one single type of nuclei in the same cytoplasm) to heterokaryosis (two genetically divergent and free-ranging nuclear types). In this species, selection can act at different levels: while nuclei can compete in their replication and transmission into short-lived asexual spores, at the level of the heterokaryotic individual cooperation between nuclear types is required to produce the long-lived sexual spores. Conflicts can arise between these two levels of selection if the coevolution between nuclear types is disrupted. Here, we investigated the extent of multilevel selection in three strains of N. tetrasperma. We assessed the ratio between nuclear types under different conditions, and measured fitness traits of homo- and heterokaryotic mycelia with varying nuclear ratios. We show that the two nuclei have complementary traits, consistent with division of labor and cooperation. In one strain, for which a recent chromosomal introgression was detected, we observed the occurrence of selfish nuclei, enjoying better replication and transmission than sister nuclei at the same time as being detrimental to the heterokaryon. We hypothesize that introgression has disrupted the coevolution between nuclear types in this strain.