Questions: Fungal Reproduction: Sexual and Asexual Strategies

Question 1

Candida albicans reproduces primarily by asexual budding during human infections. A patient treated with an antifungal drug develops a resistant infection despite incomplete treatment. What does the primarily asexual reproductive mode predict about the likely origin of resistance?

• A. Resistance arose through meiotic recombination during a sexual cycle triggered by antifungal stress
• B. Resistance likely arose by mutation in a single surviving clone that then expanded clonally, because asexual reproduction copies the genome without recombination
• C. Resistance spread by horizontal gene transfer from a resistant Aspergillus strain co-colonizing the patient
• D. Resistance is unlikely because the clonal population has no mechanism to generate heritable variation

Answer: (B)

Asexual (clonal) reproduction copies the genome by mitosis, so all offspring are genetically identical to the parent except for new mutations. If a spontaneous mutation in one cell confers drug resistance, that cell survives and reproduces clonally, rapidly expanding under drug pressure. This is the standard model for resistance emergence in primarily asexual pathogens. Option D is wrong because mutation still occurs in asexual organisms — it's recombination that is absent, not variation entirely.

Question 2

In a Basidiomycete mushroom fungus, two compatible mating types (A and B) encounter each other and their hyphae fuse. What typically happens next?

• A. The two nuclei immediately fuse (karyogamy), followed immediately by meiosis to produce basidiospores
• B. The fused cell undergoes mitosis to produce a diploid mycelium that grows for the rest of the organism's life
• C. Nuclear fusion is delayed — the resulting dikaryotic mycelium (cells with two unfused nuclei) can grow for years before karyogamy occurs at fruiting
• D. Both nuclei are degraded and a new haploid nucleus is synthesized from combined genetic material

Answer: (C)

The extended dikaryotic stage is a defining feature of Basidiomycetes. After plasmogamy (cytoplasmic fusion), the two nuclei from compatible mating types coexist in a n+n (dikaryotic) state without fusing. This dikaryotic mycelium can persist as the dominant growth form for years — the bulk of what we call the mushroom's 'body.' Karyogamy and meiosis occur only when the fruiting body (the mushroom cap) forms, producing haploid basidiospores. This contrasts sharply with animal fertilization, where nuclear fusion follows plasmogamy immediately.

Question 3

Asexual reproduction in fungi can generate enormous numbers of genetically identical offspring rapidly, but the entire clonal population is vulnerable to any single environmental challenge that affects all clones equally.

• T. True
• F. False

Answer: True

This captures the fundamental trade-off. Asexual reproduction is fast and numerically prolific — Aspergillus can release millions of conidia per day — but all offspring are clones sharing identical genotypes. A new antifungal drug, a shift in host immune status, or an environmental change can eliminate the entire population if no clone happens to carry a pre-existing resistance mutation. Sexual reproduction sacrifices speed for genetic diversity, generating novel genotype combinations that ensure at least some individuals may survive novel challenges.

Question 4

Fungi have distinct male and female sexes, analogous to animal sexes, that determine which individuals are compatible for mating.

• T. True
• F. False

Answer: False

Fungi do not have male or female sexes. Instead, they have **mating types** determined by specific genetic loci (MAT loci). Compatible mating types can mate; incompatible types cannot. Some species have just two mating types (+/−); others (like Coprinopsis cinerea, a Basidiomycete) have thousands of mating types, so the vast majority of potential pairings are compatible. Mating type compatibility is determined by allelic differences at these loci, not by morphologically distinct reproductive roles analogous to sperm and egg.

Question 5

Why does the discovery of a cryptic sexual cycle in a primarily asexual fungal pathogen (like Aspergillus fumigatus) change clinical and epidemiological predictions about that pathogen?

Model answer: In a purely asexual population, drug resistance mutations can only spread if they arise independently in each lineage — they cannot be recombined into a single organism. A sexual cycle (even rare or cryptic) allows recombination to combine resistance alleles and virulence factors from different lineages, dramatically accelerating the spread of dangerous combinations. Sex also increases overall genetic diversity, making the population less homogeneous and harder to suppress with a single drug. Discovering a sexual cycle therefore changes the predicted rate of resistance evolution and the epidemiological models for managing the pathogen.

This is not merely theoretical: after the discovery that A. fumigatus has a cryptic sexual cycle, researchers had to reconsider assumptions about how genetic diversity — including azole resistance alleles — spreads through clinical populations. Cryptic sex and parasexual cycles (mitotic recombination without meiosis) are now recognized as mechanisms that can generate significant diversity even in 'asexual' pathogens.