The gametophytes of Plocamium
dioecious, which means that there are separate male and female individuals.
These produce gametes, which fuse to create the second life history stage
is tiny and lives inside the mother. This carposporophyte produces spores
that grow up into the tetrasporophyte,
which looks just like the gametophytes. The tetrasporophyte then produces
tetraspores in sets of four, two of which grow up into male gametophytes,
and two of which become female gametophytes. This whole process is described
in much greater detail below.
|Floridean Life History: (which includes that of Plocamium)|
(After Kylin 1925.)
|Floridean red algae in general:
The first of of the three stages in the the Floridean life cycle is the gametophyte, which produces gametes (eggs and spermatia). The eggs are called "carpogonia" and look like bulbs with a long hair (trichogyne) that sticks out of the thallus, or body of the alga. A sperm, in the red algae, is called a spermatium because it lacks a tail (flagellum) with which to swim. Therefore, the spermatium needs to rely on water motion to carry it to the trichogyne (hair) of the carpogonium (egg cell). When the spermatium lands on the trichogyne of the carpogonium, the cell walls fuse, and the nucleus of the spermatium travels down the tube of the trichogyne. Karyogamy, which is the fusion of the two nuclei, occurs in the lower bulb of the female carpogonium. This carpogonial cell can now be thought of as a zygote, which is the first cell of a new thallus. This zygote is also diploid (black cells in the picture at left), which means it has twice as much DNA in its nucleus than either of its parents did (those were haploid, and are white in the picture).Plocamium in particular:
Sexual individuals are infrequent. When they do occur, they are more often found growing on other algae (epiphytic) or on other animals (epizoic), than on rock (epilithic) (Kain 1984.) Spermatangia (structures that produce spermatia) are not useful in identifying a species. Carpogonia, containing the carpogonium (egg) and the trichogyne (hair on egg) are in bulges of ~1mm diameter on female gametophytes. (South & Adams 1979). The carpogonium is at the end of a carpogonial branch, which is composed of 3 cells and an auxiliary cell. In Plocamium it is this so-called auxiliary cell, not the carpogonium, that becomes the zygote. This can happen because the two nuclei that have fused are transferred to this auxiliary cell before cell divisions start creating the carposporophyte. (Chapman & Chapman,1973).
Cystocarp surrounding carposporophyte on Plocamium.
(After Kylin 1925).
|Floridean red algae in general:
The next phase of the Floridean life cycle is unusual compared to normal plant life cycles. Two things occur simultaneously. While the female gametophyte grows more tissue around the carpogonium, embedding it, the zygote itself undergoes many cell divisions, creating a filamentous structure composed on thousands of zygotes, all strung together. This tiny filamentous diploid structure, living within the haploid tissue of the female gametophyte, is what is referred to as the carposporophyte. The cells at the tips of the individual filaments of the carposporophyte can be converted into carpospores that are released into the water. The next cell down on the filament then undergoes the same conversion, and is also released into the water. In this way, one fertilization event can produce thousands and thousands of carpospores. This is important for an organism that relies on random water motion to carry a spermatium to the trichogyne of a carpogonium (which can be a rare event).Plocamium in particular:
Once the diploid nucleus enters the auxiliary cell, that cell fuses with neighboring cells (from the female gametophyte which carried the egg) to form a large fusion cell containing several nuclei. The fusion cell divides to form the filaments of the carposporophyte (which is the diploid phase living in, and dependent on the female gametophyte). Except for a few cells at the center of the carposporophyte (called a placenta), all other peripheral cells become carpospores. (Chapman & Chapman, 1973)
Detail of branched stichidium showing freshly emerged zonate tetraspore of Plocamium.
|Floridean red algae in general:
These carpospores, like all spores, can grow up into adult individuals without fusing with other spores. This is normally how asexual reproduction works. However, in this case, the carpospores don't grow up to become adult carposporophytes (which live in the mother, remember). Instead, they grow into tetrasporophytes, which are free living adults that look just like the adult gametophytes (which we started with). These tetrasporophytes produce tetraspores via meiosis, which is a process of cell division that creates four cells, each with half as much DNA as the individual that created them. Tetraspores can be either zonately, tetrahedrally or cruciately divided (see below). These tetraspores, once again, don't grow into the same form as the individual they came from. Instead, half the tetraspores grow up into female gametophytes (which later bear carpogonia - eggs), and the rest of the tetraspores grow up to be male gametophytes (which later release spermatia - non-motile sperm).
Plocamium in particular:
Tetrasporophytes grow from carpospores, which were released by the carposporophyte that was living in the female gametophyte.
Each disk of the zonate tetraspore is really a single spore cell. Two are female, and the other two are male. These tetraspores can each grow into a full-fledged gametophyte, which completes the life history cycle.
Bangean life history:
|The Bangean life history is usually diphasic, which means
it has two stages. These alternating generations can either be produced
sexually or asexually. Porphyra is a good example. The large fleshy
phase is free living, and is used as nori in sushi. The other phase is
a very small filamentous form that lives within the walls of shells, and
is therefore called the conchocelis (conch = shell). The fleshy stage is
composed of a parenchymatous sheet
which is one cell thick. These cells can do either of two things. They
can clone the large plant by converting themselves into monospores which
grow back into a genetically identical adult, or they can reproduce sexually.
In the sexual cycle, some of the cells in the big fleshy stage are converted
into spermatia, which are the male reproductive cells. These spermatia
then fertilize the normal, general cells of the parenchymatous sheet (not
egg cells), or they fertilize general cells that have been converted into
carpogonial cells (egg cells). In either case, these fertilized cells,
called carpospores, settle on shells and bore down under the surface where
they then live as the tiny, filamentous conchocelis phase.
Finally, the conchocelis phase bears conchospores (which are really tetraspores)
that grow into the large fleshy stage again. This completes the cycle.
At first these two phases were thought to be different species. The discovery of this life cycle has revolutionized the seafood industry in Japan. Now high quality stocks of the conchocelis stage can be stored. The "nori farmers" can induce conchospore production at will. They let the conchospores settle on (attach to) nets that are later placed in the ocean until harvested. In Japan, a single sheet of quality nori can fetch $50 (Goff, Lectures 1999).