The plot thickens: haploid and triploid-like thalli, hybridization, and biased mating type ratios inLetharia

Abstract

AbstractThe 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 inLetharia. 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 inLetharia. We found that the read count variation in the fourLethariaspecimens, including the pure culture derived from a single sexual spore ofL. lupina, is consistent with haploidy. By contrast, theL. lupinaread 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 forL. vulpinaandL. lupina, suggesting widespread sexual reproduction, but highly skewed in Europe forL. vulpina, consistent with predominant asexuality. Taken together, we propose thatLethariafungi are heterothallic and typically haploid, and provide evidence that triploid-like individuals are rare hybrids betweenL. lupinaand an unknownLetharialineage, reconciling classic systematic and genetic studies with recent genomic observations.