Data from: The scope for nuclear selection within Termitomyces fungi associated with fungus-growing termites is limited

Background: We investigate the scope for selection at the level of nuclei
within fungal individuals (mycelia) of the mutualistic Termitomyces
cultivated by fungus-growing termites. Whereas in most basidiomycete fungi
the number and kind of nuclei is strictly regulated to be two per cell, in
Termitomyces mycelia the number of nuclei per cell is highly variable. We
hypothesised that natural selection on these fungi not only occurs between
mycelia, but also at the level of nuclei within the mycelium. We test this
hypothesis using in vitro tests with five nuclear haplotypes of a
Termitomyces species. Results: First, we studied the transition from a
mixture of five homokaryons (mycelia with identical nuclei) each with a
different nuclear haplotype to heterokaryons (mycelia with genetically
different nuclei). In vitro cultivation of this mixture for multiple
asexual transfers led to the formation of multiple heterokaryotic mycelia,
and a reduction of mycelial diversity over time. All heterokaryotic
mycelia contained exactly two types of nucleus. The success of a
heterokaryon during in vitro cultivation was mainly determined by spore
production and to a lesser extent mycelial growth rate. Second,
heterokaryons invariably produced more spores than homokaryons implying
that homokaryons will be outcompeted. Third, no homokaryotic ‘escapes’
from a heterokaryon via the formation of homokaryotic spores were found,
despite extensive spore genotyping. Fourth, in contrast to most studied
basidiomycete fungi, in Termitomyces sp. no nuclear migration occurs
during mating, limiting the scope for nuclear competition within the
mycelium. Conclusions: Our experiments demonstrate that in this species of
Termitomyces the scope for selection at the level of the nucleus within an
established mycelium is limited. Although ‘mate choice’ of a particular
nuclear haplotype is possible during mating, we infer that selection
primarily occurs between mycelia with two types of nucleus
(heterokaryons).