Zygomycetes I plant biology 371

ZYGOMYCETES I

• PLANT BIOLOGY 371

• GENERAL MYCOLOGY

• LECTURE 27

• 4 DECEMBER 2003

• Kingdom - Mycota

• Phylum - Zygomycota

• Class 1 - Zygomycetes - ~ 867 species in 125 genera, 30 families and 7 or 10 orders.

• Class 2 - Trichomycetes - 189 species in 48 genera, 7 families and 4 orders.

ZYGOMYCOTA

• DISTINGUISHING FEATURES:

• SEXUAL SPORE = ZYGOSPORE

• ASEXUAL SPORE = NON-MOTILE SPORANGIOSPORES

ZYGOMYCOTA

• No motile cells

• Hyphae without septa

• Cell walls contain chitin

• Asexual spores formed in sporangia (except Trichomycetes)

• Sexual spore (zygospore) formed by gametangial fusion

• Life cycle is haploid with restricted diploid

ZYGOMYCETES

• 1818 - Christian Gottfried Ehrenberg named a new genus (a fungus he found growing on the cap of Agaricus aurantius) as Syzgites megalocarpus.

• 1820 - Ehrenberg described the stages by which conjugation of 2 filaments occurred and a black fruit body was formed. He regarded this as a sexual process similar to the one that occurred in the alga, Spirogyra.

ZYGOSPORE

• 1864-81 - de Bary (deBary & Woronin) re-examined S. megalocarpus and emphasized the sexual nature of the conjugation and named the fusion spore a zygospore (zygos = yoke, spora = seed; spore). Two years later, they described the zygospore in R. stolonifera.

ZYGOSPORE

ZYGOMYCOTA

• Saprobes of dead plant and animal material, mostly in terrestrial habitats although they can be isolated from water.

• Some pathogenic species on humans and other animals, especially invertebrates.

• Some species symbiotic with invertebrates or plants.

• Several commercially important species.

BASIC LIFE CYCLE

ASEXUAL REPRODUCTION

• Non-motile, spores are cleaved out of cytoplasm within a multinucleate sporangium.

• Each sporangiospore consists of a haploid nucleus, cytoplasm, organelles and a cell membrane and cell wall.

• Spores are resistant to unfavorable environmental conditions and allow for dispersal over distance and time.

ASEXUAL REPRODUCTION

• The form of the sporangium can vary considerably among taxa.

• Because many zygomycete species have not been observed to reproduce sexually or do so within a rather narrow range of environmental conditions or only when the appropriate mating strains are present, asexual reproductive morphology is used to identify taxa.

ASEXUAL REPRODUCTION

• Stolons

• Rhizoids

• Sporangiophores

• Columella

• Sporangia

• Sporangiola

• Merosporangia

• Unispored sporangiola

Columellate Sporangium

Columellate Sporangium

Sporangium with Persistent Wall

SPORANGIA & SPORANGIOLA

MEROSPORANGIA

MUCORALES

• Asexual reproduction by multi-spored or few- (to one) spored sporangia (sporangiola).

• Sexual reproduction by zygospores.

• Cosmopolitan saprobes.

• A few facultative parasites of plants or animals (including humans.

MUCORALES

• Contains commercially important species - synthesize certain organic acids (citric, succinic, oxalic, fumaric, lactic), unusual alcohols, -carotene, transform steroids from one form to another (R. arrhizus/progesterone), etc.

• Some species (eg. Rhizopus stolonifera) attack fruits and vegetables in transit or storage (strawberry leak, soft rot of sweet potatoes)

• Cause mucormycoses in humans (Rhizopus arrhizus, R. oryzae, species of Mucor, Rhizomucor, Absidia, Cunninghamella) - four kinds of systemic infection: rhinocerebral, thoracic, gastro-intestinal, and cutaneous. Some species also cause abortion in cattle and swine.

MUCORALES

• Most mucors are mesophilic. Two species, M. miehei & M. pusillus, however, are thermophilic, i.e., they grow best at 50 C or above and will not grow below 20C.

• Thermophilic fungi are important decomposers in insulated piles of plant debris such as composts, hay and grass piles, peat, herbivore dung. Unfortunately both thermophilic mucors have been reported as causative agents of animal mycoses which limits their use as rapid decomposers.

MUCORALES

• In some countries, species of Mucor and Rhizopus are used in the production of alcohol (e.g., Sake) from grain mash. Corn mash is cooked to 40 C and cooled and inoculated with fungus. The mash is cooled to 32 C and yeast is added. The Mucors break down the starch (amylase) to simple sugars which are then utilized by yeast to produce alcohol. Fungal amylase is more efficient in converting starch to simple sugars than germinating malt seedlings, which are used in this country.

MUCORALES

• Tempeh, a kind of soybean cheese which is an important food in Indonesia, is made by inoculating cooked soybeans with Rhizopus oligosporus. The fungus helps digest the protein and imparts flavor. The same type of food is made in China but it is called sufu and the fungus is Actinomucor elegans.

FAMOUS MUCORALES

• Pilobolus (hat-thrower, cannon ball or shot-gun fungus.

• Spores are forcibly discharged as a single unit in the sporangium.

• Sporangiophores are positively phototropic. (Why?)

• Fungus exhibits a biorhythm in spore formation and discharge.

PILOBOLUS

SPORANGIOSPORE DISPERSAL

• Wind

• In sticky material

• Active discharge