The plot thickens: Haploid and triploid-like thalli, hybridization, and biased mating type ratios in Letharia

The study of the reproductive biology of lichen fungal symbionts has been
traditionally challenging due to their complex and symbiotic lifestyles.
Against the common belief of haploidy, a recent genomic study found a
triploid-like signal in Letharia. Here, we used genomic data from a pure
culture and from thalli, together with a PCR survey of the MAT locus, to
infer the genome organization and reproduction in Letharia. We found that
the read count variation in the four Letharia specimens, including the
pure culture derived from a single sexual spore of L. lupina, is
consistent with haploidy. By contrast, the L. lupina read counts from a
thallus' metagenome are triploid-like. Characterization of the
mating-type locus revealed a conserved heterothallic configuration across
the genus, along with auxiliary genes that we identified. We found that
the mating-type distributions are balanced in North America for L. vulpina
and L. lupina, suggesting widespread sexual reproduction, but highly
skewed in Europe for L. vulpina, consistent with predominant asexuality.
Taken together, we propose that Letharia fungi are heterothallic and
typically haploid, and provide evidence that triploid-like individuals are
rare hybrids between L. lupina and an unknown Letharia lineage,
reconciling classic systematic and genetic studies with recent genomic
observations.