5.9 Meiotic nuclear division

Meiosis is the division of a diploid nucleus in which chromosomes reassort, producing four haploid daughter cells. This step in meiosis is what generates the genetic diversity of sexual reproduction.

The majority of fungi are haploid for most of their life cycles, diploidy being limited to a short period immediately prior to meiosis. This is a major difference with animals and plants. The main biological impact of this arrangement in fungi has been the evolution of processes to bring together two haploids so that genetically different nuclei can co-exist in the same cytoplasm. These processes are the incompatibility mechanisms regulating cytoplasmic and nuclear compatibility. Through their action hyphae from different haploid parental mycelia can safely approach each other, undergo hyphal fusion to create a channel between themselves and then exchange cytoplasm and nuclei, so that a heterokaryotic mycelium is formed. All these processes we will describe in detail in Chapter 7. Once formed, the heterokaryon grows normally as a vegetative mycelium until conditions are right for sexual reproduction to take place. At this stage, cells of the heterokaryon go through karyogamy (nuclear fusion) to produce the diploid nucleus that can undergo meiosis.
 
Meiosis is often called 'reductional division' because in its first stage the number of chromosomes in the daughter nucleus is reduced by half. Meiosis I is the reductional division, achieving its reduction in chromosome number by sending paternal and maternal kinetochores to opposite poles. Meiosis II, the second meiotic division is an equational division because it does not reduce chromosome numbers; it shares the same machinery with mitosis. Unlike most plants and animals, fungi carry out meiosis with the nuclear membrane remaining intact through prophase I.

Meiosis in heterothallic fungi goes through stages fairly typical for eukaryotes. In particular, the major round of DNA replication precedes the start of meiosis I. Indeed, it was research with the filamentous ascomycete Neotiella (in 1970) that first demonstrated this aspect of meiosis. Because the haploid nuclei are in different (but adjacent) cells it could be demonstrated that DNA replication was completed before karyogamy established the diploid nucleus. Other aspects of preparation for meiosis became evident as the molecular events were established in yeast. In Saccharomyces cerevisiae, mating cells respond to each other by modifying their shape into pear-shaped cells termed shmoos (because of their similarity in shape to a character in Al Capp’s “Li’l Abner” cartoon strip). In the first stage of mating, and before the cells fuse, the SPB forms a cluster of microtubules (called astral microtubules) that move the haploid nucleus into the tip of the shmoo. As the nuclei move toward the shmoo tips of the two mating cells, the tips fuse, and the shmoo tip microtubule clusters fuse to form a bundle, which progressively shortens as the two nuclei and their SPBs come together and fuse (karyogamy).

The first (diploid) bud emerges adjacent to the fused SPB (Maddox et al., 1999), and then reproduces by mitosis. In fission yeast (Schizosaccharomyces pombe) the nuclear locations of centromeres and the ends of the chromosomes (called telomeres) change during vegetative growth and in the early stages of meiosis telomeres cluster near the SPB. This telomeric cluster then leads the chromosomes in a rapid oscillating movement that appears to aid synapsis. When conditions are right the diploid progeny enter into meiosis. In filamentous fungi the meiocytes, the cells in which meiosis takes place, are the zygosporangia, ascus mother cells or basidia, of the zygomycetes, Ascomycota and Basidiomycota respectively.

During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent as chromosomal condensation allows these to be viewed in the light microscope. Note that the bivalent has two chromosomes, one chromosome from each parent, each chromosome has one centromere, but each has two chromatids. One kinetochore forms on each chromosome (rather than one per chromatid) and the chromosomes attach to spindle fibres and begin to align at the metaphase plate. During anaphase I the chromosomes move to separate poles of the spindle, completing this in telophase. Each of the daughter nuclei so formed is now haploid, although each chromosome they contain is composed of two chromatids. Following the completion of telophase I the daughter nuclei continue to meiosis II.

Updated January 18, 2017