Video Transcript
In this video, we will learn the
key features of sexual reproduction and compare them to asexual reproduction. We will investigate how sexual
reproduction occurs through some fascinating examples and why special type of cell
division called meiosis is so vital to the process. Finally, we’ll look at a couple of
interesting examples of species whose life cycles involve both sexual and asexual
phases through alternation of generations.
Reproduction is the biological
process through which new offspring are produced, and it’s essential to the survival
of any species. There are two main types of
reproduction: sexual reproduction and asexual reproduction. Each method has its advantages and
disadvantages. There are even some organisms that
alternate between asexual and sexual methods and are able to take advantage of
both. Let’s take a look at these two
types of reproduction in more detail.
Sexual reproduction occurs when two
parents combine their genetic material to produce genetically unique offspring. Though they will inherit some
traits from their parents, the offspring produced will not be identical to either of
their two parents, and they will likely also be different to each other. Sexual reproduction has the major
advantage of increasing the genetic variation in a population. This can increase their resilience
in case of environmental changes. In the case of new environmental
pressures or changes, some of the genetically unique individuals that are produced
through sexual reproduction will likely possess adaptations necessary to
survive. They can then pass on these
adaptations to the next generation.
Some of the major disadvantages of
sexual reproduction include the fact that it is generally slower and also much more
energy costly and time-consuming. Although some sexually reproducing
organisms can produce large numbers of offspring, asexual reproduction tends to
produce far more offspring at a faster rate than sexual reproduction. Generally, organisms that reproduce
sexually must first grow to sexual maturity, which is otherwise known as puberty,
before they’re able to reproduce. They must also make reproductive or
sex cells called gametes, which themselves take time to mature, and the production
can be energy costly.
Furthermore, it often takes time to
find a mate and prepare an appropriate place to mate. Finding mates and gaining the
opportunity to reproduce sexually in some species is also very dangerous. For example, in some species, like
lions, males will compete and even fight to reproduce, which can be deadly. Depending on the species, sexual
reproduction sometimes also requires that a parent keeps and protects the developing
embryo within their bodies until birth. This is called the gestation period
and in some species can be very lengthy. The offspring might even require
parental care and protection after birth in certain species, which can also be a
costly long-term investment.
Asexual reproduction is when just
one parent produces offspring. The offspring produced through
asexual reproduction are usually genetically identical to their parent and to each
other. Organisms that can regularly use
asexual reproduction include prokaryotes, like bacteria, and some eukaryotes, for
example, fungi, like yeast, and some single-celled protists like the amoeba. Some of the advantages of asexual
reproduction include it being a generally faster process than sexual reproduction
because all members of the species can generate offspring on their own. This means that asexually
reproducing organisms don’t need to find a mate, which as we learned can be
dangerous.
However, the major disadvantage of
asexual reproduction is that it does not give rise to the same level of genetic
variation that occurs as a result of sexual reproduction as all of the offspring
produced by asexual reproduction are genetically identical clones of their parent
cells. This can leave populations
vulnerable to environmental changes as they are less able to adapt. In complex multicellular organisms,
sexual reproduction requires the production of gametes. The production of gametes involves
a special type of cell division called meiosis, which halves the genetic material of
cells.
Gametes are haploid cells, which
means that they only contain one single set of chromosomes, which is half the number
of chromosomes found in most other body cells and is often represented as n. Gametes usually come in two
varieties: the male sperm cell and the female egg cell, sometimes known as an
ovum. While the sperm cell is generally
considered to be the gamete that travels towards the egg cell for fertilization to
occur, the egg cell is generally the nonmotile gamete.
One haploid male gamete and one
haploid female gamete combine in a process called fertilization to produce a diploid
zygote. The diploid zygote has twice the
number of chromosomes of the original two haploid cells that combine to create
it. This is often represented as
2n. The zygote then goes through
mitotic cell divisions to grow into a fully developed organism that can reproduce
itself.
Sexual reproduction can occur in
several different ways. These include fertilization, which
can either take place internally or externally. It can occur through a process
called conjugation, or in some species, it can occur through alternation of
generations, which includes both sexual and asexual reproduction. Let’s examine each of these
processes more closely starting with internal fertilization in a species you’ll be
most familiar with, humans.
The male gamete sperm is produced
in the father’s reproductive organs, while the female gamete, the egg cell, is
produced in the mother’s reproductive organs. Remember that because both of these
gametes were formed through meiosis, they’re both haploid cells. These two cells combine during
internal fertilization to create a unique diploid zygote. It’s called internal fertilization
as this process occurs within the female parent’s body.
The zygote possesses genetic
material from both parents. The cells of the zygote can then
divide by mitosis, eventually developing into an embryo within the mother’s body and
then into a fetus, which after an approximately nine-month gestation period can be
delivered as a baby. This baby can then mature through
childhood and adolescence, eventually becoming an adult who might be able to have
children of their own.
External fertilization differs from
internal fertilization as it occurs outside of either of the parents’ bodies. It’s common in most fish species,
in particular bony fish, and also in amphibians, like frogs and toads. It usually occurs in water where a
female will release her egg cells into the water column. And then, the male will quickly try
to swim over and release his sperm cells into the water too. This allows fertilization to take
place externally. Many of these egg cells will
usually be fertilized. But as water is such a harsh
environment to live in, very few of them will usually survive. Let’s take a look at conjugation
next.
Conjugation occurs when two
organisms combine their genetic material through direct cell-to-cell contact, sort
of like building a bridge. Many organisms including bacteria,
fungi, and some protists, like this algae, can utilize conjugation in their
reproduction. Let’s look at the example of
Spirogyra, a genus of algae that grow in fresh water in long filaments of
cells. Spirogyra reproduces through
a mode of asexual reproduction through simple fragmentation. When environmental conditions
become difficult, for example, during drought in the summer or freezing temperatures
in winter, Spirogyra can increase its chance of survival in these harsh new
conditions by reproducing sexually.
Conjugation provides the
opportunity to create more genetic variation in offspring and potentially creates
traits better adapted to the new difficult conditions. Conjugation in Spirogyra
occurs in two ways. Individual Spirogyra cells
are typically haploid, meaning that they normally possess only a single set of
chromosomes. During conjugation, two adjacent
filaments from two different individuals grow a tube, called a conjugation tube. The conjugation tube connects one
cell to another in a different filament and a different organism. And a special type of gamete is
moved from the cell in the male filament to the other female filament through this
conjugation tube. There, the genetic material of the
two haploid cells combines into a diploid cell, called a zygote.
The zygote becomes surrounded by a
thick wall and waits out the winter in a dormant state, called a zygospore. The zygospore remains dormant until
environmental conditions become more favorable. The diploid nucleus within the
zygospore then divides, using the process of meiosis. This means that in conjugation, in
contrast to fertilization, the recombination of the parental genetic material occurs
after the formation of the zygote. This creates four haploid
nuclei. Only one of these four haploid
nuclei survives. And then, the zygospore, which we
can see in its tough external wall here, is set to germinate. This means that it grows into a new
Spirogyra filament with a genetic material from both filaments that
created it.
This type of conjugation in
Spirogyra is called scalariform conjugation. The conjugation tubes that form
between the two filaments resemble the rungs of a ladder. And the word “scalariform” actually
derives from the Latin word for ladder form. Spirogyra can also use a
mechanism called lateral conjugation. In this type of conjugation, the
genetic material from two adjacent cells in the same filament is combined. The male gamete will move into and
fuse with the female gamete in the adjacent cell. This means that the zygote also
forms in alternate cells to develop into a zygospore. Just like in scalariform
conjugation, the zygospore remains dormant until environmental conditions
improve. Then, it divides using meiosis
before growing into a new spirogyra strand.
Let’s learn about organisms that
have a life cycle that involves both the diploid sexual phase and a haploid asexual
phase. We commonly refer to this as
alternation of generations. Plasmodium is a unicellular
parasitic genus of protists, otherwise known as protozoa. Plasmodium can infect humans
and cause a disease called malaria. And the Plasmodium life
cycle is an example of alternation of generations.
Sporozoites are haploid
plasmodium cells that are found in the salivary glands of some
mosquitos. When an infected female
Anopheles mosquito bites a human, these spindle-shaped sporozoites enter
the human body and infect the liver cells. The sporozoites then reproduce
through an asexual process called schizogony. In schizogony, a parent cell
divides into multiple daughter cells through a process called multiple fission. There’re usually between two and
four replicative cycles of schizogony, and this number varies between different
Plasmodium species.
During this process, the
sporozoites mature into schizonts. And eventually, these schizonts
burst in the human liver cells. The final cells that are produced
by schizogony that burst out of the schizonts are called merozoites. These cells are also haploid. The merozoites are typically
released every 48 hours, and they quickly spread to infect the human’s red blood
cells. Once this occurs, the number of
merozoites quickly increases. The blood cell stages are
responsible for the symptoms that are displayed in an infected human with malaria,
such as fever, chills, and sweating.
The merozoites can then enter a
stage called gametogony, which will cause them to eventually develop into male and
female gametes. To begin this phase, some of the
merozoites differentiate in the human’s red blood cells into sexual-stage haploid
gametocytes. The female gametocytes are called
macrogametocytes, and the smaller male gametocytes are called microgametocytes, both
of which will be present in the human’s red blood cells. These gametocytes are picked up by
another female mosquito when it bites an infected human host. The gametocytes in the blood meal
ingested by the mosquitoes soon develop into mature female and male gametes. The transformation of gametocytes
into mature gametes is called gametogenesis, and it’s the final part of
gametogony.
Sexual reproduction can then take
place in the mosquito’s midgut, where the male microgamete fertilizes the female
macrogamete to form a diploid zygote. The zygote then divides many times
and changes shape to form an ookinete that then transforms into an oocyst as it
moves through the gut of the mosquito. The replication of the parasite
within the oocyst forms thousands of sporozoites in a process called sporogony. The oocyst eventually ruptures and
releases the sporozoites. The sporozoites can then infect the
salivary glands of the mosquito where they remain until the mosquito feeds again to
infect another human, and then the life cycle can begin once more. We’ve seen that the life cycle of
Plasmodium shows alternation of generations. It experiences an asexual phase
mainly in the human host and a sexual phase that mainly takes place in this mosquito
hosts.
Alternation of generations is also
commonly observed in plants, an example of which is in the life cycle of a fern. Ferns are nonflowering vascular
plants, and an adult fern is also called a sporophyte. The cells in a sporophyte are
diploid. On the underside of the leaves of a
fern plant, there are structures called sori or a singular sorus.
Each sorus has diploid cells called
spore mother cells that will divide by meiosis to generate haploid spores. The haploid spores that are
produced are stored in a structure called a sporangium. The spores will eventually be
released from the sporangium into the air where they can be carried by the wind. The spores that land on a suitable
surface will begin to grow and germinate. After germination, the haploid
spores will develop into heart-shaped structures, called gametophytes. The young gametophytes are far
smaller plants than the mature sporophytes, and they have haploid cells.
The gametophytes have rhizoids that
attach them to soil and facilitate the absorption of minerals and water to support
the plant in its early stages. Gametophytes are capable of
generating both sperm cells and egg cells. The gametophyte makes haploid egg
cells in a structure called an archegonium and haploid sperm cells in a structure
called an antheridium. The sperm from one gametophyte are
able to travel to an adjacent or nearby plant via water droplets. Sometimes, self-fertilization
occurs when the sperm from the gametophyte fertilizes an egg cell on the same
gametophyte. The sperm cells are attracted to
the entrance of the archegonium where they can fertilize the egg cells.
Whether cross-fertilization or
self-fertilization occurs, the diploid zygote that’s formed in the process will grow
into a new diploid sporophyte set upon the gametophyte to begin the lifecycle
again. The sporophyte becomes independent
of the gametophyte when its first leaves and roots begin to grow.
Alternation of generations provides
certain advantages to the organisms that use it. For example, it allows rapid
production of new organisms through the asexual phase, but it also provides genetic
diversity through the sexual phase. This enables them to disperse
widely but also to adapt to environmental changes.
Let’s wrap up by reviewing some of
the key points that we’ve covered in this video. In sexual reproduction, haploid
gametes fuse to form a diploid zygote. Sexual reproduction introduces
genetic variation to offspring; however, it’s generally slower and more energy
costly than asexual reproduction. Conjugation and reproduction by
gametes are two methods of sexual reproduction. Plasmodium and fern plants
are examples of organisms that show alternation of generations.
