Name:                                                                                                            

KEY Quiz #2 – The Ascomycota and Zygomycota

 

1.

Character

Zygomycota

Ascomycota

Has septate hyphae

N

Y

Most members are filamentous, some are dimorphic

Y

Y

Has the ability to exploit complex food sources (e.g. cellulose, lignin)

N

Y

Has a growing dikaryotic phase

N

Y

Produces sporangia for its asexual reproduction mode

Y

N

Lacks a motile (flagellated) stage in the life cycle

Y

Y

Forms sexual spores inside a sac-like structure

N

Y

 

2a.       Describe why we say that during sexual reproduction, the Zygomycota dies not have males and females?

            Members of the Zygomycota are sexually undifferentiated: sexual reproduction occurs by gametangial fusion between morphologically identical gametangia. There is no donor (male) and recipient (female) since both parents contribute equally during plasmogamy.

 

2b.      Describe why we say in contrast, that the Ascomycota do (generally) have males and females during their sexual reproduction cycle. What do we mean when we say ÒmalesÓ and ÒfemalesÓ? How do they interact?

            Because for most species, strains form morphologically different gametangia (eg. ascogonium – female structure, antheridium, spermatium, conidium – male structures) and the contribution of cytoplasm towards formation of the zygote is unequal.

            Male    – donor of nuclei / fertilizing strain

            Female – recepient strain of the male nucleus

            The two strains interact in plasmogamy by fusion through gametangial fusion / spermatization / gametangial contact / somatogamy. Karyogamy between the male and the female nuclei occur later.

 

3a.       What is an ascocarp? How do they form?

            An ascocarp is the fruiting body that contains the asci (the sacs that contains the sexual spores) and paraphyses of the Ascomycota.

            The ascocarp forms as a result of haploid maternal tissue growing from the base of the ascogonium, to enclose the asci.

 

3b.      Do all members of the Ascomycota have ascocarps? Explain.

            No. Members grouped together as Hemiascomycetes (e.g. Taphrina, Saccharomyces cerevisiae) produce asci that are NOT enclosed in an ascocarp. These are called naked asci.

 

4a.       Explain why we say the Ascomycota has a very short diploid phase. How is Saccharomyces cerevisiae exceptional when it comes to this feature?

            In general, the Ascomycota have a very short diploid phase because the only diploid nucleus is that in the ascus mother cell (penultimate cell of the Crozier) which after it forms by karyogamy, begins to undergo meiosis. S. cerevisiae is exceptional in that after plasmogamy, it does not have a dikaryon stage, but instead immediately undergoes karyogamy to produce diploid yeast cells. These cells then undergo an extended (unlimited) free-living diploid phase. The diploid phase grows is not dependent on maternal tissue for nutrition and can freely divide as long as nutrients are available.

 

4b.      Where does the diploid phase exist in the Zygomycota life cycle?

            In the zygospore.