CHLOROPHYTA

143

tive geotaxis by swimming against gravity. Such

general line of evolution occurring in the same

a feature would be selected for in evolution

direction. Usually gametes are specialized cells

because when the algal cell (which is heavier than

and not vegetative cells, although in the one-

water) is confronted with darkness, it must move

celled Volvocales the latter can occur. If the

up to the surface in order to obtain light for

species is isogamous or anisogamous, the gametes

growth and reproduction. In Chlamydomonas, neg-

are usually not formed in specialized cells

ative geotaxis is an energy-dependent response

although in the oogamous species, gametes are

that requires a horizontal swimming path of at

normally formed in specialized gametangia (e.g.,

least 200 m because the normal geotactic orien-

Coleochaete, Fig. 5.25). Whereas most Chlorophyta

tation maneuvers require long gradual turns. The

form motile flagellated gametes (zoogametes), in

rate of geotaxis is steady but slow relative to the

the Zygnematales aplanogametes or amoeboid

average swimming speed (Bean, 1977).

gametes are formed.

In some of the Chlorophyta, gametogenesis is

Asexual reproduction

induced by environmental changes, whereas in

others the presence of two sexually different

strains is necessary. In the latter, vegetative cells

There are a number of types of asexual reproduc-

of one sex secrete a substance that initiates sexual

tion, the simplest being fragmentation of colonies

differentiation in competent cells of the opposite

into two or more parts, each part becoming a new

sex. Such a situation is common in the Volvocales

colony. Zoosporogenesis commonly occurs, usually

(Starr, 1972; Kirk and Kirk, 1986) and is considered

induced by a change in the environment of the alga.

in more detail later. In Oedogonium (Fig. 5.95), sex

In the Chlorophyta, zoospores are normally pro-

organs form without the complementary strain,

duced in vegetative cells (e.g., Ulothrix; Fig. 5.31),

but subsequent fertilization is under a complex

and only in a few cases are they formed in special-

hormonal control. In other genera, a chemotactic

ized sporangia (e.g., Derbesia; Fig. 5.42). Zoospores

substance is sometimes produced by the egg that

are usually formed in the younger parts of fila-

attracts the spermatozoids. This does not gener-

ments, and the number of zoospores is generally a

ally happen in isogamous species. In isogamous

power of two in uninucleate genera. Aplanospores

species, sexually different gametes meet at

are non-flagellated and have a wall distinct from the

random and immediately adhere by means of an

parent cell wall (e.g., Trebouxia; Fig. 5.86(a)–(c)).

agglutination reaction. The agglutinative flagel-

Aplanospores are considered to be abortive

lar adhesion between gametes of different sex is

zoospores and have the ability to form a new plant

designated as the mating-type reaction. Initially

on germination. Autospores are aplanospores that

after mixing, the gametes of opposite sexes

have the same shape as the parent cell, and are common

adhere by their flagella tips in clusters of up to 25

in the Chlorellales (e.g., Chlorella; Fig. 5.83).

gametes. Soon the anterior ends of complemen-

Autospores are usually formed in a multiple of two

tary gametes fuse, and the flagella free them-

in the parent cell. Coenobia are colonies with a definite

selves. The motile zygote then swims for some

number of cells arranged in a specific manner (e.g.,

time before settling and secreting a thick wall.

Volvox; Fig. 5.69). Genera with colonies arranged in

The mating-type substances (responsible for

coenobia form daughter colonies with a certain

flagellar agglutination) are localized and function

number of cells. In maturation of the daughter

at the flagella tips. It is possible to isolate the

coenobia, there is enlargement but no division of

mating-type substances that still have the ability to

vegetative cells in the coenobia.

interact with the gametes of the opposite sex. When

added to the opposite gamete type, they cause

Sexual reproduction

isoagglutination (male gametes will clump with

each other when a female mating-type substance is

added to the culture). The mating-type substances

Sexual reproduction in the Chlorophyceae may be

are discussed in more detail for Chlamydomonas,

isogamous, anisogamous, or oogamous, with the

which has been most intensively studied.