The secotioid form of Lentinus tigrinus: genetics and development of a fungal morphological innovation

Secotioid fungi resemble gasteromycetes, but are presumably closely related to agaricoid fungi. Lentinus tigrinus is a wood-decaying mushroom that has both a secotioid and an agaricoid form. We examined ontogeny and heritability of the secotioid phenotype in L. tigrinus with a combination of formal genetic crosses, scanning electron microscopy, and macroscopic observation of cultured sporocarps. For F1 analysis, we crossed single-spore isolates (SSIs) representing four mating types derived from a secotioid dikaryon and four mating types from an agaricoid dikaryon. All F1 sporocarps had typical agaricoid morphology. For F2 analysis, 200 SSIs from one F1 sporocarp and 100 SSIs from another F1 sporocarp were backcrossed to tester SSIs from sporocarps produced by the parental secotioid dikaryon. Ratios of secotioid to agaricoid dikaryons thus produced were 47:49 and 84:109, which confirms previous reports that the secotioid phenotype is conferred by a recessive allele at a single locus (χ² = 0.0417, P > 0.05, χ² = 3.2383, P > 0.05, respectively). Early ontogeny of the secotioid form is indistinguishable from that of the agaricoid form. Later, the hymenophore is obscured by a weft of hyphae that proliferates from the margins of the developing lamellae. Longevity of the sporocarps and rate and duration of sporocarp growth are approximately equal in the secotioid and agaricoid forms. Developmental evolution of the secotioid form is interpreted as an example of von Baerian differentiation, rather than paedomorphosis, which has been implicated in evolution of other secotioid taxa.