Fig. 22.8. Reproduction of Sac Fungi

Sexual Reproduction

Sexual reproduction involves the formation of tiny finger-like sacs called asci (singular: ascus). When hyphae of two different “sexes” become closely associated in the more complex sac fungi, male antheridia may be formed on one and female ascogonia on the other, although in many species, antheridia and ascogonia may be produced on the same mycelium. Hyphae grow and connect an antheridium and an ascogonium to each other; male nuclei then migrate into the ascogonium. There, the male nuclei pair with the female nuclei present but do not unite. New hyphae (ascogenous hyphae), whose cells each contain one male and one female nucleus, grow from the ascogonium, the cells dividing in a unique way so that each cell has one of each kind of nucleus. This special cell division begins with the cell at the tip of an ascogenous hypha forming a hook called a crozier. The two nuclei in the crozier divide simultaneously, and crosswalls then form so that there are now three cells, the middle cell containing one male and one female nucleus. This middle cell becomes an ascus in which the two haploid nuclei unite and become a diploid zygote nucleus. The fingerlike, tubular asci develop in a layer (referred to as the hymenium) at the surface of a structure called an ascoma (previously called an ascocarp). The zygote nucleus undergoes meiosis, and the resulting four haploid nuclei usually divide once more by mitosis so that there is a row of eight nuclei in each ascus.

These nuclei become ascospores as they are walled off from one another with a little cytoplasm. Thousands of asci may be packed together in an ascoma, which often is cup shaped but also may be completely enclosed or flask shaped with a little opening at the top (perithecium). Truffles actually are enclosed ascomata (apothecia). Cup-shaped ascomata may be several centimeters (2 to 3 inches) in diameter and may be brilliantly colored on the inside.

When ascospores are mature, they are often released with force from the asci. If an open ascoma is jarred at maturity, it may appear to belch fine puffs of smoke consisting of thousands of ascospores that are dispersed to new locations by air currents.

When an ascospore lands in a suitable area, it may germinate, producing a new mycelium, and then repeat the process. In many instances, however, a number of asexual generations involving conidia are produced between the sexual cycles. Sexual reproduction in yeasts is somewhat streamlined in that individual yeast cells function as ascogonia and antheridia. When two haploid cells unite, they become a diploid fusion cell that may serve as an ascus in which meiosis takes place and ascospores are formed - all within the original cell wall. In some yeasts, however (including the common baking and brewing yeast Saccharomyces cerevisiae), the fusion cell may multiply asexually into a diploid colony, with all the cells eventually undergoing meiosis.

Date: 2016-01-03; view: 1131