At 03:46 PM 7/8/98 +1000, Donald Howes wrote:
>I see, I guess it would be a huge advantage to have a more varied gene
>pool, but I still don't understand the mechanism that would initiate sexual
>reprodution in the first place. How did sexual reprodution develope?
Gilbert writes:
"Sexual reproduction is one other invention of the protists that has had
profound effect on more complex organisms. It should be noted that sex and
reproduction are two distinct and separable processes. Reproduction
involves the creation of new individuals. Sex involves the combining of
genes from two different individuals into new arrangements. Reproduction
in the absence of sex is characteristic of those organisms that reproduce
by fission; there is no sorting of genes when an amoeba divides or when a
hydra buds off cells to form a new colony. Sex without reproduction is
also common among unicellular organisms. Bacteria are able to transmit
genes from one individual to another by means of sex pili. This
transmission is separate from reproduction. Protists are also able to
reassort genes without reproduction. Paramecia, for instance, reproduce by
fission, but sex is accomplished by conjugation. When two paramecia join
together, they link theiry oral apparatuses and form a cytoplasmic
connection. Each macronucleus (which controls the metabolism of the
organism) degenerates while each micronucleus undergoes meiosis followed by
mitosis to produce eight haploid micronuclei, of which all but one
degenerate. The remaining micronucleus divides once more to form a
stationary micronucleus, thereby creating a new diploid nucleus in each
cell. This diploid nucleus then divides to gie rise to a new micronucleus
and a new macronucleus as the two partners disengage. No reproduction has
occurred, only sex.
The union of these two distinct processes, sex and reproduction, into
SEXUAL REPRODUCTION, is seen in unicellular eukaryotes. Figure 13 shows
just one copy of each chromosome (like a mammalian gamete). The
individuals of each species, however, are divided into two mating groups:
plus and minus. When these meet, they join their cytoplasms together and
their nuclei fuse to form a diploid zygote. This zygote is the only
diploid cell in the life cycle, and it eventually undergoes meiosis to form
four new Chlamydomonas. Cells. Here is sexual reproduction, for
chromosomes are reassorted during the meiotic divisions and more
individuals are formed. Note that in this protist type of sexual
reproduciton, the gametes are morphologically identical-the distinction
between sperm and egg has not yet been made.
In evolving sexual reproduction, two important advances had to be
achieved. The first is the mechanism of MEIOSIS (Figure 14), where by the
diploid complement of chromosomes is reduced to the haploid state. (This
will be discussed in Chapter 22.) The other advance is the mechanism
whereby the two different mating types recognize each other. Recognition
occurs first on the flagellar membranes (Figure 15: Goodenough and Weiss,
1975; Bergman et al., 1975). The aggulutination of flagella enables
specific regions of the cell membranes to come together. These specialized
sectors contain mating type-specific components that enable the cytoplasms
to fuse. Following agglutination, the plus individuals initiate the fusion
by extending a 'fertilization tube.' This tube contacts and fuses with a
specific site on the minus individual. Interestingly, the mechanism used
to extend this tube-the polymerization of the protein actin-is also used to
extend processes of sea urchin eggs and sperm. In the next chapter, we
shall see that the recognition and fusion of sperm and egg occur in a
manner amazingly similar to that of these protists.
"Unicellular eukaryotes appear to have the basic elements of the
developmental processes that characterize the more complex organisms of
other phyla: cellular synthesis is controlled by transcriptional,
translational, and posttranslational regulation; a mechanism for processing
RNA through the nuclear membrane exists; the structures of individual genes
and chromosomes are as they will be throughout eukaryotic evolution;
mitosis and meiosis are perfected; and sexual reproduction exists,
involving cooperation between individual cells. Such intercellular
cooperation becomes even more important with the evolution of multicellular
organisms.
Colonial eukaryotes: The evolution of differentiation
"One of evolution's most important experiments was the creation of
multicellular organisms. There appear to be several paths by which single
cells evolved multicellular arrangements; we will discuss only two of them.
The first path involves the orderly division of the reproductive cell and
the subsequent differentiation of its progeny into different cell types.
This path to multicellularity can be seen in a remarkable series of
multicellular organisms collectively referred to as the family Volvocaceae,
or the 'Volvocaceans.'
"The simpler organisms among the Volvocaceans are collections of numerous
Chlamydomonas-like cells; but the more advanced members of this group have
developed a second, very different, cell type. A single organism of the
Volvocacean genus Oltmannsiella contains four Chalmydomonas-like cells in a
row, embedded in a gelatinous matrix. In the genus Gonium (Figure 16), a
single cell divides to produce a flat plate of 4 to 16 cells, each with its
own flagellum. In a related genus -Pnadorina-the 16 cells form a sphere;
and in Eudorina, the sphere contains 32 or 64 cells arranged in a regular
pattern. In these organisms, then, a very important developmental
principle has been worked out: the ordered division of one cell to generate
a number of cells that are organized in a predictable fashion. Like most
animal embryos, the cell divisions by which a single Volvocacean cell
divides to produce an organism of 4 to 64 cells occur in very rapid
sequence and in the absence of cell growth.
"The next two genera of the Volvocacean series exhibit another important
principle of development: the differentiation of cell types within an
individual organism. The reproductive cells become differentiated from the
somatic cells. In all the Vovocacean genera mentioned above, every cell
can, and normally does, produce a complete new organism by mitosis (Figure
17A,B). In the genera Pleodorina and Volvox, however, relatively few cells
can reproduce. In Pleodorina californica, the cells in the anterior side
can reproduce. In P. californica, a colony usually has 128 or 64 cells,
and the ratio of the number of somatic cells to the number of reproductive
cells is usually 3:5. Thus, a 128-cell colony has 48 somatic cells, and a
64-cell colony has 24.
"In Volvox, almost all the cells are somatic, and very few of the cells
are able to produce new individuals. In some species of Volvox,
reproductive cells, as in Pleodorina, are derived from cells that
originally look and function like somatic cells before they enlarge and
divide to form new progeny. However, in other members of the genus, such
as V. carteri, there is a complete division of labor: the reproductive
cells that will create the next generation are set aside during the
division of the reproductive cells that are forming a new individual. The
reproductive cells never develop functional flagella and never contribute
to motility or other somatic functions of the individual; they are entirely
specialized for reproduction. Thus, although the simpler Volvocaceans may
be thought of as colonial organisms (because each cell is capable of
independent existence and of perpetuating the species), in V. carteri we
have a truly multicellurlar organism with two distinct and interdependent
cell types (somatic and reproductive), both of which are required for
perpetuation of the species. (Figure 17C). Although not all animals set
aside the reproductive cells from the somatic cells (and plants hardly ever
do), this separation of germ (reproductive) cells from somatic cells early
in development is characteristic of many animal phyla and will be discussed
in more detail in Chapter 7.
Although all of the Volvocaceans, like their unicellular relative
Chlamydomonas, reproduce predominantly by asexual means (as described
earlier) they are also capable of sexual reproduction. This involves the
production and fusion of haploid gametes. In many species of
chlamydomomonas, including the one illustrated in Figure 13, sexual
reproduction is ISOGAMOUS, since the haploid gametes that meet are similar
in size, structure, and motility. However, in other species of
Chlamydomonas-as well as many species of colonial Volvocaceans-swimming
gametes of very different sizes are produced by the different mating types.
This is called HETEROGAMY. But the larger Volvocaceans have evolved a
specilized form of heterogamy, called OOGAMY. This involves the production
of large, relatively immotile eggs by one mating type and small, motile
sperm by the other. Thus, the Volvocaceans include the simplest organisms
that have distinguishable male and female members of the species and that
have distinct developmental pathways for the production of eggs or sperm."
~Scott F. Gilbert, Developmental Biology, (Sinauer, Mass.: Sinauer
Associates, Inc., 1991), p. 13-18
Gilbert continues:
"Although V. carteri reproduces asexually much of the time, in nature it
reproduces sexually once each year. When it does, one generation of
individuals passes away, and a new and genetically different generation is
produced. The naturalist Joseph Wood Krutch (1956) put it more poetically.
And finally, Gilbert relates:
'The amoeba and the paramecium are potentially immortal·But for Volvox,
death seems ot be as inevitable as it is in a mouse or in a man. Volvox
must die as Leeuwenhoek saw it dies because it had children and is no
longer needed. When its time comes it drops quietly to the bottom and
joins its ancestors. As Hegner, the Johns Hopkins zoologist, once wrote"
This is the first advent of inevitable natural death in the animal kingdom
and all for the sake of sex." And he asked: "Is it worth it?"'
"For Volvox carteri, it most assuredly is worth it. V. carteri, lives in
shallow temporary ponds that fill with spring rains, but that dry out in
the heat of late summer. During most of that time, V. carteri swims about,
reproducing asexually. But these asexual volvoxes would die in minutes
once the pond dried out. V. carteri is able to survive by turning sexual
shortly before the pond dries up, thereby producing dormant zygotes that
survive the heat and drought of late summer and the cold of winter. When
rain fills the ponds in spring, the zygotes break their dormancy and hatch
out a new generation of individuals to reproduce asexually until the pond
is about to dry up once more. How do these simple organisms predict the
coming of adverse conditions with sufficient accuracy to produce a sexual
generation just in time, year after year?
"The stimulus for switching from the asexual to the sexual mode of
reproduction in V. carterii is known to be a 30,000-DA SEXUAL INDUCER
protein. This protein is so powerful that concentrations as low as 6 x
10^-17 M cause gonidia to undergo a modified pattern of embryonic
development that results in the production of eggs or sperm, depending on
the genetic sex of the individual. The sperm are released and swim to a
female where they fertilize eggs to produce the dormant zygotes (Figure 22).
"What, then is the source of this sexual inducer protein? A few years
ago, Kirk and Kirk (1986) discovered that the sexual cycle could be
initiated by heating the dishes of Volvox to temperatures that might be
expected in a shallow pond in late summer. When this was done, the
somatic cells of the asexual volvoxes produced the sexual inducer protein.
Since the amount of sexual inducer protein secreted by one individual is
sufficient to initiate sexual development in over 500,000,000 axexual
volvoxes, a single inducing volvox can convert the entire pond to
sexuality." Scott Gilbert, Developmental Biology, ~Scott F. Gilbert,
Developmental Biology, (Sinauer, Mass.: Sinauer Associates, Inc., 1991), p.
20-22
**
"Sexual reproduction in Volvox carteri. Males and females are
indistinguishable in their asexual phase. When the sexual inducer protein
is present, the gonidia of both mating types undergo a modified
embryogenesis that leads to the formation of eggs in the females and sperm
in the males. When the gametes are mature, sperm packets (containing 64 or
128 sperm each) are released and swim to the females. Upon reaching the
female, the sperm packet breaks up into individual sperm, which can
fertilize the eggs. The resulting zygote has tough cell walls that can
resist drying, heat, and cold. When spring rains cause the zygote to
germinate, it undergoes meiosis to produce haploid males and females that
reproduce asexually until heat induces the sexual cycle again. "~ Scott F.
Gilbert, Developmental Biology, (Sinauer, Mass.: Sinauer Associates, Inc.,
1991), p. 21
glenn
Adam, Apes and Anthropology
Foundation, Fall and Flood
& lots of creation/evolution information
http://www.isource.net/~grmorton/dmd.htm