“Multilevel selection in the filamentous ascomycete Neurospora tetrasperma”
Cécile Meunier, Sara Hosseini, Nahid Heidari, Zaywa Maryush, and Hanna Johannesson (Mar 2018)
This paper experimentally shows an ongoing transition in individuality in a fungus
Insiders’ divergence: threat or bonus?
An individual defines a single, separate organism distinguished from others of the same kind. Throughout the evolution of life, there have been transitions in individuality when autonomous entities have grouped together to form new, higher-level individuals. Examples of such transitions are when genes grouped to form genomes, cells to form bodies, and organisms to form societies. Theoretical expectations during transitions in individuality are well defined. Most importantly, grouping has to yield benefits, arising e.g. from cooperation amongst lower levels of the group. At the same time, conflicts between members of the group may arise and should be minimized, e.g. in ensuring genetic homogeneity inside of the group. Divergence from the inside would mean that selfish variants, deleterious to the group-level could invade and take over, which is what happens, for example, in cancers. The researchers, all female scientists from Uppsala University in Sweden, investigated pros and cons of a transition in individuality in Fungi, a group where individuality has been a long-standing issue. They postulated that their model, Neurospora tetrasperma, is undergoing a transition in individuality. In this species, two different levels of individuals exist: the nucleus and the mycelium. Cooperation and/or conflicts may be expected among differing nuclei that could benefit or harm the mycelium, respectively. The researchers investigated how the nuclear ratio between the two types varied in different conditions, and how fitness was impacted by changes in nuclear ratios. The nuclei had complementary traits, consistent with cooperation and division of labor. However, they also verified the existence of conflicts at the nuclear level: in one lineage, one type of nucleus replicated and transmitted better, yet had a negative impact on the mycelium fitness. Heterokaryosis in N. tetrasperma thus exemplifies a genetic system where transition in individuality seems at the same time advantageous and incomplete.
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.