Spores, sexual and asexual cycles and mating systems

A spore needs a life-cycle context

A fungal spore is a propagule that can contribute to dispersal or establishment. Sexual cycles and asexual reproduction can both produce spores. Some fungal spores are moved passively; some fungal lineages have motile zoospores. The word spore therefore does not specify whether meiosis occurred, whether mating is required next, or how long the stage survives.

Asexual production can involve mitotic division followed by packaging or separation of propagules. Sexual reproduction involves a route that combines nuclear contributions and includes meiosis. To classify a particular stage, trace the events before and after it. A small round object is insufficient evidence of its reproductive origin.

Fungal spores are also different from the bacterial endospores introduced earlier. The bacterial survival structure develops within an existing cell and is not a general mechanism for multiplying that cell into numerous offspring. A shared name should prompt comparison of mechanisms rather than an assumption of equivalence.

Plasmogamy precedes karyogamy in the selected route

Plasmogamy is the joining of cells or cytoplasm. Karyogamy is fusion of nuclei. Those events can be separated in time. A dikaryotic compartment has two separate haploid nuclei, commonly represented as n + n. Some fungi maintain such an organisation for an extended phase; other fungal routes differ. The selected route is an accounting model, not the timetable of all fungi.

Here n means one complete assigned set of two chromosomes. Each initial unreplicated haploid nucleus has two chromosomes, two chromatids and 1C of DNA. After plasmogamy, a shared compartment contains two such nuclei: four chromosomes and 2C in total, still distributed between separate nuclei. After karyogamy, one diploid nucleus contains four chromosomes, four chromatids and 2C.

Replication changes the copies of DNA rather than the number of complete chromosome sets. The diploid nucleus then has four chromosomes, eight chromatids and 4C. At completion of meiosis I, each of two nuclei is haploid with two replicated chromosomes, four chromatids and 2C. At completion of meiosis II, each of four nuclei has two chromosomes, two chromatids and 1C. These are completed stages, with no anaphase centromere-counting ambiguity.

Count nuclear divisions before counting spores

The four haploid nuclei produced by this meiosis are not automatically four final spores in every fungal route. A subsequent mitotic division can produce eight haploid nuclei before packaging. Other routes have different packaging or survival. The model includes an optional postmeiotic branch to show why a count of meiotic products and a count of released spores answer different questions.

If four 1C haploid nuclei each replicate, the total DNA rises from 4C to 8C. Their mitotic divisions can then yield eight 1C haploid nuclei. The total remains 8C after division. Producing eight nuclei without the preceding DNA copying would fail the material account. This option is not an extra round of meiosis and does not halve ploidy again.

The separate asexual branch begins with one unreplicated haploid nucleus. Replication raises its DNA from 1C to 2C; mitosis produces two nuclei, each haploid and 1C. Mutation can occur during asexual lineages, so mitotic reproduction does not guarantee forever-identical descendants. It does avoid the particular joining and meiotic-segregation route represented in the sexual branch.

Mating type is a compatibility system

Mating types often regulate recognition and compatibility among fungal partners. They do not imply a universal pair of visibly male and female fungal bodies. Systems vary, including the number of compatibility factors and whether partners are needed in the same way. Compatibility under one rule is also not a complete species-delimitation test.

The four-isolate model assigns A and B to type I, and C and D to type II. Its explicit rule allows only unlike types, excludes self-pairs and counts unordered pairs once. The compatible pairs are A–C, A–D, B–C and B–D. These are possibilities, not observed mating frequencies. A–B and C–D fail this particular rule.

There are six unordered distinct pairs among four isolates: 4 × 3 ÷ 2. Four meet the rule and two do not. That fraction, 4/6, describes the list of eligible pairs. It becomes a probability only after specifying a sampling rule, such as choosing uniformly from those six pairs. It is not a universal fungal mating success rate.

Worked example

Check your understanding

1. Why is an n + n compartment not the same as a 2n nucleus?

   Show reasoning

   The n + n compartment retains two separate haploid nuclei; the 2n nucleus contains both complete sets within one nuclear boundary after karyogamy. Their total DNA can be equal while their nuclear organisation differs.

2. At completion of meiosis I in the model, what does each nucleus contain?

   Show reasoning

   Each is haploid, with two replicated chromosomes, four chromatids and 2C. Homologues have separated; sister chromatids remain together at this completed stage. Meiosis II then produces nuclei with two chromosomes, two chromatids and 1C each.

3. Uniformly choose one of the six unordered distinct pairs in the compatibility model. What fraction is eligible, and what does that establish?

   Show reasoning

   Four of six pairs are eligible, giving 2/3 under the specified uniform sampling rule. It establishes compatibility by the assigned rule, not actual mating success, reproductive output or a universal rule for species boundaries.

What to carry forward

Follow cytoplasm, nuclear boundaries, chromosome sets and DNA copies separately; interpret a spore or mating type within its specific reproductive route.