Chapter 28, pages 573-588
Basic Botany-2 9:35-20:16
Five Kingdoms-3 7:27-11:25
I. Life Cycles of Selected Fungi
A. Saprolegnia - Division Oomycophyta
1. Characteristics
2. Life Cycle
(1) Types of asexual reproduction
(a) Increase in
hypha
length by simple growth of the
filament
and
mitosis
of
nuclei
(b) Chlamydospores
(c) Diaplanetism - two generations of zoospores
i) Zoosporangium forms
a) Cross wall forms near tip of a multinucleated hypha
b)
Each
nucleus
takes some
cytoplasm
and differentiates into a
zoospore
c)
Each zoospore has two
flagella
at the
anterior
end and is called a
primary
zoospore
ii) Primary zoospores liberated
a) Hole opens up at the end of the zoosporangium
b) All the zoospores flow out
iii) Primary zoospores swim and encyst
a) Swim for a time
b) Drop to the bottom (substrate) with flagella down
c) Develop cellulose walls (encysting)
d) Rests (remains in that situation for a while)
iv) Release of secondary zoospore
a)
Secondary
zoospore is formed out of the
cytoplasm
in the
encysted
primary
zoospore
b)
This
secondary
zoospore is released when the
encysted
cell
stops resting
c) Secondary zoospore has two flagella attached laterally
v) Secondary zoospore swims and encysts
a) Swims for a while
b) Falls to the bottom
c) Encysts
vi) Somatic thallus develops from encysted secondary zoospore
a)
Grows a new
hypha
(germ
tube) from the
encysted
secondary
zoospore
b) Germ tube eventually grows into a complete new mycelium
(2) Fertilization
(a) Oogonia formation
i) Oogonium is the female gametangium
ii)
Cross
wall separates the
oogonium
from the
hypha
as a side
branch off the hypha
iii) Cytoplasm becomes more dense in this area
iv) From 1 to 20 eggs form within the oogonium
v)
Each egg has one
nucleus
and is 3 to 4 times the diameter of the
hypha
(b) Antheridium formation
i) Antheridium is the male gametangium
ii)
Cross
wall separates the
antheridium
from the
hypha
as a side
branch off the hypha
iii) Usually curved and only slightly larger in diameter than the hypha
iv) Produces several non-motile gametes inside
(c) Fertilization
i) Antheridium grows partially around the oogonium
ii)
Fertilization tubes grow from the side of the
antheridium
into the
oogonium
through its
cell
wall
iii) Each
fertilization tube grows until it contacts the
eggs
in the
oogonium
iv)
Nucleus
from
antheridium
migrates
down the
fertilization
tube to
egg
nucleus which results in
fertilization
(1) Development of the Oospore
(a) Zygote forms from the union of the egg and male nucleus
(b) Zygote differentiates into an
oospore
that will not
germinate
under any
circumstances for
several months
(2) Germination
(a) Meiosis occurs to form four haploid nuclei
(b) Three of the nuclei disintegrate and one survives
(c) Single haploid nucleus grows into a coenocytic filament
(3) Chromosome numbers
(a) Vegetative hypha is haploid (N)
(b) Oospores and zygote are diploid (2N)
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