HOMOTHALLISM
The condition in which one
individual originating from a single asexual spore is capable of forming zygospore (zygote) independently is
known as homothallism. A few species
of Achlya such as A. racemosa are
homothallic. In homothallic species, two types of sex organs ie., antheridia and oogonia are developed on the same mycelium.
In A. racemosa, the oogonia are developed at the tips either of short
lateral hyphae or on the main hypha. The tip of the female branch swells up to
form a sac-like oogonial initial. It
is filled with dense homogenous contents (cytoplasm and nuclei). A central vacuole
then appears in the oogonial initial. Subsiquently, the swollen structure is
cut off by a cross wall at its base. It is the oogonium.
The antheridia are developed on
thin hypal branches which arise from the female hypha. The tip of the
antheridial hypha enlarges. A number of nuclei and some cytoplasm migrated to
the inflated tip. The swollen portion is then cut off by a cross wall. It is
the antheredium.
HETEROTHALLISM
Heterothallism may be defined as the condition
in which zygospore (zygote) formation
takes place only when mycelia arising from asexual spores of two genetically
different mating types, (+) and the (–), are allowed to interact. The term
heterothallism was first coined by A.F. Blakeslee in 1904.
On the basis of nature of the
mating types, heterothallism may be of the following types –
1. Morphological
Heterothallism:-
Morphological heterothallism may be defined as the condition when the
morphologically different male and female sex organs are produced in two
closely associated mycelia. The two sex organs or gametes are so
morphologically dissimilar that it is easier to term one of them as male and
the other as female. Some of the examples of heterothallic fungi are – Achlya ambisexualis, A. bisexualis,
Phytophthora palmivora, Peronospora parasitica, etc.
2. Physiological
Heterothallism:-
In physiological heterothallism, the interacting thalli differ in mating type
or incompatibility irrespective of the presence or absence of sex organs or
gametes. This means that sexual reproduction takes place by two morphologically
similar but physiologically different hyphae. The gametangia or gametes do not
show morphological differentiation but physiologically they behave differently.
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