Mechanisms of Sexual Reproduction among the Rhizophydiales

Resting spore The end product of sexual reproduction is a thick-walled resting spore which contains copious amounts of lipids and glycogen stored food for energy.

What are the two basic schemes described for sexual reproduction among the Rhizophydiales?

Scheme 1:The stationary thallus on the host becomes a receptive thallus and resting spore. The contributing cell is a cyst (may or may not contact host cell) or a thallus (with or without rhizoids).

Sexual Reproduction Scheme 1
From: Sparrow 1935. Rhizophydium sp.

A. Zoospores swim to a host cell.
B. On the host cell, zoospores produce a wall around themselves and may penetrate the host with a germ tube. The receptive cell (right) produces a rhizoidal system. The contributing thallus (left) only has a germ tube.
C. Contents from an adjacent contributing thallus (left) are transferred through a fertilization tube into the receptive thallus (right).
D. The receptive thallus enlarges into a thick-walled resting spore, with attached rhizoids (right). Sometimes the expanding resting spore lifts up the attached contributing cell (left), which persists as an appendage.

Examples:
Rhizophydium couchii Sparrow
R. columnaris Canter
R. ephippium
Canter
R. goniosporum
Scherffel
R. hyalobryonis
Canter
R. planktonicum
Canter
R. sphaerocystidis
Canter
R. tenue
(Sparrow) Karling
sometimes also R. granulosporumScherffel

Scheme 2:The stationary thallus on the host becomes the contributing thallus. A motile spore encysts on the surface of the contributing thallus and becomes the receptive thallus and resting spore.

Sexual reproduction scheme 2
From: Couch 1935. Rhizophydium ovatum

A. A zoospore swims to a host cell (the green alga, Stigeoclonium sp.).
B. This zoospore encysts and penetrates the host with a germ tube and becomes the contributing thallus (labeled male).
B-D. Another zoospore (labeled female) attaches to the surface of the contributing (stationary) thallus,retracts its flagellum, and forms a cyst wall around itself.This will become the receptive thallus.
E-H. Contents of the contributing (stationary) thallus are transferred into attached receptive thallus (upper cell), which continues to enlarge.
I. The end result is the resting spore (upper cell labeled female) is connected to the contributing thallus (lower cell labeled male) and is attached to the host cell by way of the contributing thallus' rhizoidal system.
J. After a dormant period, the resting spore releases zoospores and appears as an empty cell.

Examples:
R. anomalum Canter
R. androdioctes Canter
R. ovatumCouch
sometimes also R. granulosporum on Tribonema

To learn more about the various reports of sexual reproduction among Rhizophydium species, go to the reference section.

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