Mating Systems in True Morels (Morchella)

Abstract

True morels (Morchella spp., Morchellaceae, Ascomycota) are widely regarded as a highly prized delicacy and are of great economic and scientific value. Recently, the rapid development of cultivation technology and expansion of areas for artificial morel cultivation have propelled morel research into a hot topic. Many studies have been conducted in various aspects of morel biology, but despite this, cultivation sites still frequently report failure to fruit or only low production of fruiting bodies. Key problems include the gap between cultivation practices and basic knowledge of morel biology. In this review, in an effort to highlight the mating systems, evolution, and life cycle of morels, we summarize the current state of knowledge of morel sexual reproduction, the structure and evolution of mating-type genes, the sexual process itself, and the influence of mating-type genes on the asexual stages and conidium production. Understanding of these processes is critical for improving technology for the cultivation of morels and for scaling up their commercial production. Morel species may well be good candidates as model species for improving sexual development research in ascomycetes in the future.

Keywords: asexual reproduction; evolution; genome analysis; heterothallism; mating type; mitospore; pseudohomothallism; skewed distribution; spatial competition; unisexual reproduction.

FIG 1

Maximum likelihood (ML) phylogenetic analysis of 75 species in the genus Morchella based on a four-gene data set (ITS plus EF1-α plus RPB1 plus RPB2), with Verpa and Disciotis as outgroups. The species Morchella castaneae, M. vulgaris, and Morchella sp. Mes-28 were not included in this analysis because only ITS sequences for them are available from GenBank. The ML analyses were run in PhyML 3.0 using the GTR+I+G model of molecular evolution. The numbers by the nodes represent branch support >75%. Mating-type genes detected and sexual reproduction modes known in the species of Morchella are listed on the right. (Photo of M. rufobrunnea of the Rufobrunnea clade courtesy of Michael Loizides, reproduced with permission.)

FIG 2

Structure of the MAT locus of Morchella importuna (Elata clade) (A) and Morchella sp. Mes-20 (Esculenta clade) (B). Introns are represented inside the MAT genes with vertical black lines. At the MAT locus, either a MAT1-1 or MAT1-2 idiomorph is present, containing at least a MAT1-1-1 α-domain gene or a MAT1-2-1 MATA_HMG-box gene, respectively. In addition, further genes are present in the MAT1-1 idiomorph: MAT1-1-10 and MAT1-1-11 in M. importuna (A) and MAT1-1-10 in Morchella sp. Mes-20 (B). The genes flanking the MAT idiomorphs are fairly well conserved. The flanking region of the MAT1-1 idiomorph of Morchella sp. Mes-20 has not yet been revealed. APN2 encodes an abasic endonuclease/DNA lyase; COX13 encodes the cytochrome c oxidase subunit VIa; CPSF6 encodes cleavage and polyadenylation specificity factor subunit 6; TFA1 encodes transcription initiation factor IIE subunit alpha; ATP4 encodes mitochondrial membrane ATP synthase subunit 4; SDH2 encodes succinate dehydrogenase (ubiquinone) iron-sulfur subunit; MBA1 encodes mitochondrial inner membrane-associated mitoribosome receptor; EOS1 encodes N-glycosylation protein; and SLA2 encodes a protein that binds to cortical patch actin. Distances and sizes are not drawn to scale. Mating-type structures from M. importuna and Morchella sp. Mes-20 were summarized from references and our unpublished data.

FIG 3

Schematic illustration of distribution of mating types in mature ascomata of morels. (A) Three recognized types: I (both mating types equally distributed and present in the hymenium and stipe), II (the dominant type, MAT1-1 or MAT1-2 acting female, and opposite as male, with both mating types present in the hymenium while either of them is present in the stipe), and III (only one MAT idiomorph present in the ascomata, no mating, and ascospores not formed). (B) Distribution of mating types in the hymenium of the type II morels.

FIG 4

Schematic diagram of a conidiophore from Morchella, conidial nuclei, and conidial germination (A), chlamydospore formation from conidial hyphae (B), and primordia of morels and probable conidial hyphae at cultivation sites (C). Uninucleate (white arrowheads, A2), binucleate (white arrows, A2), and trinucleate conidia (purple arrowhead, A2). Nucleus mitosis in detached conidia (purple arrow, A2). Conidial germination (red arrows, A3 to A5). Terminal chlamydospores (black arrows, B1 and B3). Intercalary chlamydospores (black arrowheads, B2 and B4). Nuclei of chlamydospores (green arrowhead, B5). Much narrower conidial hyphae (yellow arrowheads, B1 and B4) than other hyphae (yellow arrow, B1). Primordia from morels (blue arrowheads, C1 to C3). Putative conidial hyphae at cultivation sites (blue arrows, C3 to C4). Bars, 5 μm (A and B) or 0.2 mm (C). (Panels A and B are modified from reference with permission from the British Mycological Society.)

FIG 5

Generalized life cycle of Morchella, with heterothallism, pseudohomothallism, unisexual reproduction, and asexual reproduction included. Of the known three types of ascocarps with different MAT distribution patterns (20), the ascocarps produced by heterothallism are likely to correspond to type II (Fig. 3A), needing two partners with opposite mating types to mate for sexual reproduction and being the dominant strategy. The ascocarps produced by pseudohomothallism may correspond to type I (Fig. 3A), developing from a single heterokaryotic ascospore and undergoing independent sexual reproduction. The ascocarps produced by unisexual reproduction should coincide with type III (Fig. 3A), independently undergoing sexual reproduction with only one mating type, with no ascospores observed (20). In the sexual process of heterothallism, presumably the specialized cell of the female hypha receives male nuclei from one conidium via spermatization or from a specialized cell of the male hypha via somatogamy to form a heterokaryotic cell. The heterokaryotic cell probably acts as an ascogonium and produces multinucleate ascogenous hyphae. Meiosis takes place almost immediately after karyogamy in the developing asci, followed by several postmeiotic mitoses, giving rise to eight plurinucleate ascospores. The heterokaryotic phase is short in the life cycle of morels and is restricted to the reproductive hyphae, being embedded in the homokaryotic haploid vegetative mycelium. The vegetative mycelia contribute to the main structure of the ascocarp (the nonascogenous sterile tissue), and the reproductive hyphae contribute to the ascospore pools. In asexual reproduction, mitospores (conidia and chlamydospores) are formed. The dashed lines represent hypotheses of fertilization pathways that require verification.