Ascomycetes: Asexual & Sexual Reproduction

Asexual Reproduction

Ascomycetes reproduce asexually through fission, fragmentation, chlamydospores, or conidia, depending on the species and environment.

Fission & Budding in Yeast: Fission and budding are methods of propagation typically encountered in the yeasts and in a few other Ascomycetes. Spores produced by budding are often called blastospores (Gr. blasts = a bud + spora = seed, spore).

Fragmentation: Because all living portions of the thallus are potentially capable of growth, fragmentation, whether natural or artificial, under favorable conditions results in as many new individuals as there are fragments.

Conidia: Conidia are formed by a great many—perhaps the majority of—Ascomycetes. It is in fact, in this class of fungi that conidial development has reached its zenith as indicated by the great variety of conidial forms produced. Like in most of all other fungi, here too the asexual stages carry the burden of propagating and disseminating the species throughout the spring and summer, with several generations of conidia being produced during the growing season.

Briefly, conidia may arise either directly from the somatic hyphae or from specialized conidiogenous cells often borne on hyphal branches known as conidiophores. Conidiophores vary from short hyphal branches to those that are long and intricately branched. In some species the conidiophores may be produced free from each other without any evident organization, while in other species they are cemented together to form complex structures such as sporodochia and synnemata. In still other species, conidia are produced in definite fruiting bodies, either pycnidia or acervuli.

Sexual Reproduction

Sexual reproduction in the Ascomycetes, as in other living organisms, aims at the union of two compatible nuclei. These nuclei are brought together in the same cell by one of the many methods developed by the Ascomycetes during their evolution.

We have seen that in a great many of the simpler fungi such nuclei fuse (karyogamy) soon after they are brought together. In the Ascomycetes, this is not the case. Here, the two nuclei remain in close association and undergo successive divisions that usually result in a number of dikaryotic cells.

Nuclear fusion eventually takes place in the ascus mother cell, which develops into the ascus. Meiosis of the diploid, zygote nucleus occurs almost immediately after fusion, and four haploid nuclei are produced. These four nuclei divide once more mitotically, and form eight nuclei that become incorporated into the eight ascospores typically produced in the ascus. This is, in brief, the developmental process of the asci and ascospores, which are the characteristic structures of the Ascomycetes.

We are now ready to examine the methods As- comycetes employ to bring two compatible nuclei together. The most common of these are the following. 1. Gametangial Copulation. This method may be regarded as similar to that found in the Zygomycetes. Two similar gametangia touch at their tips or coil around each other and fuse, with the fusion cell developing into the ascus. No dikaryotic phase is developed in such species because karyogamy takes place immediately after plasmogamy. In the yeasts, which are unicellular, the somatic cells themselves act as gametangia; two of them fuse and form a unicellular zygote that becomes transformed directly into the ascus. as we shall see later. 2. Gametangial Contact (Gametangy). Some species produce morphologically differentiated uninucleate or multinucleate gametangia. These are designed as antheridia and ascogonia (sing, as- cogonium; Gr. askos = sac -I- genmio = I give birth), the latter being the female structures from which the asci eventually develop. The gametes are reduced to undifferentiated protoplasts; the

male nucleus passes from the antheridium into the ascogonium through a pore developed at the point of contact between the two gametangia. The as- cogonium is often provided with a trichogyne (Gr. thri.y = hair + gyiic = woman, female) that receives the male nucleus. In some Ascomycetes the antheridia, although formed, have lost their function; in others no antheridia are formed. Either the ascogonia of such species receive nuclei from their trichogynes, which pair with the ascogonial nuclei, or the as- colonial nuclei themselves form functional pairs. 3 . Spermatization. In some species that form no antheridia, nuclei reach ascogonia by means of spermatia, microconidia, or conidia. Spermatia are minute, spherical or bacillary, specialized, uninucleate, male sex cells that become attached to the receptive organs—whether trichogynes or so- Matic hyphae—and empty their contents into them. The spermatial nuclei migrate to the as- cogonium through the septal pores. Spermatia be- come detached from or slip out of the parent hy- phae and are carried by some means such as insects, wind or water to the receptive organs. The hypha that produces spermatia is called the sper- matiophore (Gr. spennation = little seed -f phoreiis = bearer). Spermatia are sometimes formed in pycnidium-like structures we call sper- mogonia (sing, spermogonium). Microconidia (Figure 11-2) are minute conidia that behave as spermatia but are also capable of germinating and giving rise to mycelium. Conidia and oidia may also function as spermatia by at- taching themselves to the receptive organs and emptying their contents into them. 4. Some Ascomycetes fuse somatic hyphae from two compatible mycelia, and the nuclei migrate to the ascogonia through septal perforations.