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PHYCOLOGICAL SOCIETY OF AMERICA
Chondracanthus exasperatus life history
The life histories of all red algae are fairly complex, and that of C. exasperatus is certainly no exception. It may be useful to look at a quick run down of the basic life history of the Rhodophyta, specifically those in the subclass Florideophyceae, before delving into the specifics of C. exasperatus'. If you would like to skip this introduction, please feel free to jump right to the life history of our star, C. exasperatus.
Simplified life history of a typical Florideophyceae
This
diagram depicts a simplified version of the Florideophyceae life history. It
is a triphasic life history, indicating three distinct phases:
the gametophyte, the carposporophyte, and the tetrasporophyte. The
male and female gamete-producing reproductive organs are the spermatangium and carpogonium respectively,
and both develop from the gametophyte. There are no flagellated
cells in any of the Rhodophyta, thus when the spermatia are
produced and released by the spermatangium, they are carried passively
by water currents to the trichogyne, or reproductive hair, located
on the carpogonium. The membranes fuse and the male nucleus
travels down to the base of the carpogonium, where it fuses with the
female nucleus. This type of fertilization is called oogamy,
because it involves the fusion of a larger, nonmotile egg cell with
a smaller, motile sperm cell. The fertilized carposporangia,
still on the female gametophyte, then produces gonimoblast filaments,
which in turn produce carposporangia and carpospores. These
carpospores, when released, settle and grow into the tetrasporophyte,
which in many cases is indistinguishable from the gametophyte. This
situation is termed an isomorphic life history. The
tetrasporophyte is the site of meiosis, more specifically the tetrasporangia where
haploid tetraspores are produced. These tetraspores
are released, settle, and grow into the gametophyte, completing the
basic red algal life history (Lee, 1980).
Life History of our star, C. exasperatus
Most of what is known about this story is based on general studies of the
Gigartina family, which C. exasperatus was originally a part
of. Species specific life histories have only been determined
for one member of this family, Chondrus crispus. The germination
of spores and their early stages of development has been investigated specifically
for C. exasperatus and other members of the old Gigartina family. Recent
work by the Hommersand group to reclassify the Gigartinaceae order have
led to new investigations of the specific life histories of its members. Not
only has G. exasperatus been reclassified as Chondrocanthus exasperatus,
but further specifics of this life history have been determined. So
lets go....
In the beginning, there were the gametes and, since C. exasperatus is dioecious, these were each located on their own male or female gametophyte.....
The spermatium are
spherical and oblong, and each is formed in its own spermatangia cell. These,
in turn, are produced on spermatangial mother cells, which branch
off from the male gametophyte. The spermatangia
are commonly colorless and found partially embedded in extensive patches
on the male gametophyte (Ho Kim, 1976)....
When the spermatium are released, they take with them a mucilage sheath
which will aid them later in adhering to the female trichogyne. As
these cells are unflagellated, they must rely on water currents to
passively carry them to the female egg. This condition of a small,
motile sperm traveling to a larger, nonmotile egg cell is known as oogamy (Dixon,
1973).
The female reproductive organ is the carpogonium. It consists of an elongated, gelatinous tip, called the trichogyne, and an inflated basal region. The carpogonium has two nuclei, one in the trichogyne, which disintegrates after fertilization, and the other in the base, which is the reproductive nuclei. The carpogonium is located at the end of the three-celled carpogonial branch, originally formed from inner cortical cells, which, in turn, arises from the supporting cell. This cell is very large and also serves as the auxiliary cell once the cell is fertilized. The carpogonial branch is curved bringing the auxiliary cell towards the carpogonium in a formation called a procarp, all of which is typically colorless (Dixon, 1973).
Fertilization
begins with the fusion of the spermatium to the trichogyne. The
cell walls of each dissolve, and the male nucleus moves down into the
basal portion of the carpogonium where it fuses with the female nucleus
(karyogamy). After fertilization,
pit plug develops separating the carposporangial base from the trichogyne. The
supporting cell becomes the auxiliary cell and serves a vegetative
role in development. In the base, the nucleus begins to differentiate
and produces about two dozen gonimoblast filaments "initials" extending
in all directions. Only those facing inward develop fully into
gonimoblast filaments, and these cells are round (Ho Kim, 1976). Carposporangia develop
out from almost all of the complete gonimoblast filaments and have
diploid carpospores at their tips. This is the carposporophyte stage.
An envelope of cells develops from the cortex and medulla to surround the
carposporophyte, and the entire structure is known as thecystocarp. It
is an interesting structure as the envelope is haploid and the internal
carposporophyte is diploid. These cystocarps are pushed out from
the thallus and housed in the wart-like papillae so characteristic of C.
exasperatus. There is only one cystocarp per papillae (Ho Kim, 1976).
The carpospores are released, settle, and develop into the diploid tetrasporophyte. It
is not possible to differentiate the nonreproductive tetrasporophyte
from the gametophyte except by very careful chemical analysis, and
so this life history is termed isomorphic.
The
tetrasporophyte develops tetrasporangia from cells in primary filaments
within the cortex, which lie adjacent to or embedded in the medulla. These
serve as the site of meiosis and produce four tetraspores. Each
of these is divided as if by a cross, a structure termed cruciate.
Finally, these tetraspores are released through pores in the cell wall (Ho Kim, 1976). They are distributed passively by water currents, settle, and grow into the gametophyte, beginning the life cycle all over again!
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Last modified: 3/18/99
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