Salient feat.,economic importance ,asexual and sexual reproduction ,economic importance of Rhizopus,Aspergillus & Saccharomyces

1. Theory
Salient features of Rhizopus

2. Rhizopus
 Rhizopus is a saprophytic organism that is found in damp warm weather
especially on dead organic matter.
 The mycelium during the vegetative phase is white fluffy mass of loosely
entangled hyphae.
 The hyphae are aseptate or coenocytic
 During the vegetative growth the mycelium consists of stolon hyphae and
rhizoidal hyphae. The former helps in spread of mycelium and the latter in
anchoring and absorption of nutrients from the substratum.
 When the reproductive phase sets in the mycelium becomes mouldy and
develops a third kind of hyphae called sporangiophore.
 They arise in the air from the stolon. They are unbranched.
 Reproduction takes place by formation of non-motile multinucleated
sporangiospores produced endogenously inside the sporangium. The
sporangium arise terminally at the tip of sporangiophore.

3. Life cycle of Rhizopus
Asexual reproduction
Each sporangium has a vacuolate central
portion known as columella, which is
surrounded by a zone containing
large number of sporangiospores.
The sporangiospores are released when
the sporangial wall breaks.
The sporangiospores are globose to oval
and multinucleate.
Under favourable conditions, the
sporangiospores germinate by a germ
tube to form white, aerial mycelium
and thus complete the asexual cycle.

4. Sexual reproduction
It requires the presence of two physiologically
distinct compatible mycelia (+) and (-).
When two opposite strains come in contact with
one another, copulating branches called
progametangia are formed.
Cytoplasm and nuclei flow to the contacting tips
which enlarge.
A septum then forms near tip of each
progametangium, separating it into two
cells, a terminal gametangium and the
adjacent suspensor cell.
The walls of two contacting gametangia dissolve
at the pint of contact.
After plasmogamy, the nuclei fuse in pairs, +
and -, to form diploid nuclei.

5. The unfused nuclei disintegrate and the young
zygosporangium enlarges with its wall
thickening, becoming black and warty. This
thick walled structure is the
zygosporangium that contains a single
zygospore.
After a rest period of 1-3 months, the
zygospore gets activated to germinate.
During germination, zygosporangium cracks
open, a sporangiophore emerges from the
zygospore and develops a germ sporangium
at its tip.
Meiosis takes place during the process of
zygospore germination
The germ sporangium may contain either +
spores or – spores.

6. Economic Importance
• Rhizopus stolonifer called common black bread mould grows and
spoils bread.
• Rhizopus stolonifer causes fruit rot on strawberry, tomatoes.
• Rhizopus oryzae is used for production of alcoholic beverages.
• Rhizopus oligosporus is used to make tempeh which is a
fermented food.

7. Theory
Salient features of Aspergillus
 Aspergillus colonies are usually fast growing white, yellow, brown or black in
color.
 It mostly consists of a dense collection of erect conidiophore.
 Conidiophore terminate in a vesicle covered with either a single layer of
phylides or a layer of subtending cells which bears small whorl of phylides
(uniseriate/ biseriate)
 Conidia are single celled, rough/smooth walled, hyaline or pigmented and are
borne basipetally forming long chains which maybe divergent or aggregated
in compact column.
 Asexual reproduction : While still young the mycelium of Aspergillus
produces conidiophores. The hyphae compartment or cell which give rise to
a conidiophore is called a foot cell.

8. Foot cell Foot cell

10. Asexual reproduction
It takes place by means of conidia ,which are formed in chains externally
on conidiophores.
1. When asexual reproduction takes place a certain cell of the hyphae
become larger and thick walled (foot cell).
2. From each foot cell a vertical, conidiophore which ends in a vesicle.
3. A large number of nuclei and cytoplasm migrate into the vesicle.
4. From each vesicle a tubular outgrowths are produced(sterigmata).
5. From the tips of sterigmata a chain of conidia are formed.

11. Sexual Reproduction
• Three perfect states of Aspergillus are :
– Eurotium, Emericella and Sartorya
• In Eurotium, the antheridium and ascogonium are produced close to each
other on the somatic hyphae.
• Both are multinucleate, elongate, often helical structures coiling around
each other.
• The antheridium may/maynot be functional but pairing of nuclei takes place
in the ascogonium.
• If antheridium is functional, its nuclei enter the ascogonium and pair with
the ascogonial nuclei but when the antheridium is non-functional, the
ascogonial nuclei themselves pair with each other.
• After pairing of the nuclei, the ascogonium produces a number of
ascogenous hypahe that branch within the developing ascocarp
(cleistothecium), which begins to develop as a single layer of cells.
• Mature cleistothecia are small, globose with smooth walls generally of
yellow color.
• The asci are formed at the tips of ascogenous hyphae at different levels.
• They are globose, ovoid orpear shaped, 8 spored, dissolving soon after
ascospore formation, leaving the ascospores free within the cleistothecium.
• The ascospores are broadly lenticular, colourless and without surface
ornamentations.
• On germination, the ascospores produce germ tubes, which gives rise to the
mycelium.

12. Economic Importance
• It is used for commercial production of enzymes and organic
acids.
• Aspergillus niger is used in the production of citric acid in large
scale.
• Aspergillus niger and Aspergillus oryzae are used in production
of amylase.
• Aspergillus oryzae is used in production of food products such
as soya sauce and koji.
• Aspergillus fumigatus and Aspergillus niger cause a group of
diseases called aspergillosis.
• Aspergillus clavattus and Aspergillus flavous produce mycotoxins
(Aflatoxin).

13. Saccharomyces
• Unicellular thallus and may produce pseudomycelium.
• They reproduce asexually by multilateral budding.
• Lifecycle is haplodiplobiontic in which both the haploid and diploid
phases are equally important and are perpetuated by budding.
• Sexual reproduction is initiated when two haploid cells of opposite
mating types fuse and give rise to diploid cells, which initiate diploid
phase by budding.
• After several generations of diploid cells, the diploid nucleus undergoes
meiosis and forms four haploid nuceli around which four ascospores are
developed.
• The ascal wall is thin and soon breaks, releasing the mature, hapoid
ascospores.
• These ascospores multiply by budding producing several generations
before copulation.

14. Gametangial copulation
Zygote