Video Transcript
In this video, we will learn how to
distinguish between asexual and sexual reproduction. We will talk about some of the
advantages and disadvantages of each type of reproduction. We will then take a look at some of
the different ways that organisms reproduce asexually.
Reproduction is a biological
process by which offspring are produced from their parents. The ability to pass down genetic
information through reproduction is one of the key characteristics distinguishing
living organisms, like this cow, from nonliving biological entities, for example,
prions, which are infectious proteins that cause brain and central nervous system
diseases. Prions cause other normal proteins
to also become infectious, leading to conditions like mad cow disease. However, they do not pass down
genetic information through reproduction and are therefore not considered living
organisms.
The two major types of reproduction
are sexual and asexual. Many organisms, including the
simplest animals known as sea sponges, can undergo both types of reproduction. Sexual reproduction involves the
union of gametes, or sex cells. A male gamete called a sperm cell
fuses with the female gamete called an egg cell in a process called
fertilization. Fertilization creates a zygote,
which will have genetic material from both the male and female gametes represented
here as the blue and pink squiggly lines in the nucleus. The zygote will develop into a new
individual, the offspring. In the case of sea sponges, this
new offspring will begin its life as a free swimming larva, which will eventually
settle down and develop into an adult sponge. In humans, the zygote produced from
fertilization will develop into a human baby, like the one shown here.
Because of the way genes are
rearranged and inherited, the offspring produced during sexual reproduction are
genetically unique, meaning that their genetic makeup will be different from that of
their parents and from other offspring produced by the same parents. In asexual reproduction, offspring
are produced by a single parent with no union of gametes, with the result being that
the offspring are genetically identical to the parent as well as to any other
offspring created by the same parent. We often refer to these genetically
identical organisms as clones.
Sexual reproduction is typically
slower than asexual reproduction because it can take time to find a mate, for
fertilization to occur, and for the zygote to develop into offspring. However, sexual reproduction has
the advantage of increasing genetic variation, which refers to the differences in
the genetic makeup of individuals in the same population. Higher genetic variation makes
populations more resilient to environmental changes, for example, short- or
long-term climate changes or new pathogens. The increased resilience comes from
the fact that with higher genetic variation, at least some of the individuals are
likely to have adaptations that help them cope with the changes. For example, if faced with a
drought, a hypothetical plant population with high genetic variation is likely to
contain individuals with adaptations that enable them to survive.
Here, we can see that some of the
plants in the hypothetical population have deeper roots, an adaptation that enables
them to reach water that may be unavailable to plants with shallower roots. Other plants have smaller leaves,
an adaptation that reduces water loss through transpiration. The individuals that have both of
these adaptations are more likely to be able to survive the drought. Their survival and continued
reproduction ensures the survival of the species when environmental conditions
improve.
Asexual reproduction is usually
faster than sexual reproduction because no time is needed to find a mate, for
fertilization, or for zygote development. For example, some types of bacteria
can reproduce asexually in just 20 minutes, which means that in just 12 hours, a
single bacterium could produce 68 billion offspring. Now, that’s fast reproduction. A disadvantage of asexual
reproduction is that it does not increase genetic variation. So if the environment changes
rapidly, there might not be individuals with adaptations to cope with the
change. This leaves the population
vulnerable to size reductions or even extinction.
There are many methods of asexual
reproduction. Binary fission is the most common
method used by single-celled organisms. You may remember that the prefix
bi- means two and the word fission means to split, which tells us that binary
fission involves a parent cell splitting into two daughter cells. In prokaryotes like bacteria, the
parent cell first makes a copy of its DNA, which is typically found on a circular
chromosome that floats freely in the cytoplasm. The parent cell then undergoes
cytokinesis, the division of its cytoplasm, to produce two genetically identical
cells called daughter cells.
In eukaryotes such as amoeba, DNA
is found within a membrane-bound nucleus. So, the first step in binary
fission is mitosis, in which replicated chromosomes are divided between two new
nuclei. As with binary fission in
prokaryotes, the final step for single-celled eukaryotes is cytokinesis, and the end
result is two genetically identical daughter cells.
Another common method of asexual
reproduction is budding, in which new offspring arise from outgrowths on the parent
cell or organism. Budding is used by unicellular
organisms, such as the single-celled fungi known as yeast, and by simple
multicellular organisms like sea sponges. The process of budding begins with
a small bulge growing outward from the parent organism. This is the bud. In yeast cells, the parent then
undergoes mitosis to make a copy of its nucleus. One of the copies will move into
the bud, which develops to maturity while still attached to the parent. When it reaches maturity, the new
yeast cell may detach from the parent to continue its life independently. Or it may remain attached to the
parent after completing cytokinesis, forming branched chains of loosely attached
cells to make a colony.
Budding in unicellular organisms
appears similar to binary fission, but there is a key difference. In binary fission, one parent cell
splits into two immature daughter cells, which grow to maturity as separate
individuals. In contrast, during budding, an
immature daughter cell sprouts from a parent cell and reaches maturity while still
attached to the parent, with the end result being that after detachment, there’re
two fully mature cells. The daughter cell will continue to
grow in size either independently or as part of a colony.
Regeneration is the process of
regrowing damaged or missing body parts. Nearly all organisms can undergo
some form of regeneration, but the capacity to regenerate decreases as organisms
become more advanced. For example, simpler organisms like
lizards can regenerate tails or even legs, while more complex organisms, for
example, humans, can repair damaged skin and muscle cells and can even regenerate
the liver but typically can’t regrow entire limbs. In these examples and in most cases
of regeneration, no new offspring are created, so it is not a form of
reproduction.
However, there are cases where two
or more pieces or fragments of an organism each grow into a new individual. The diagram shows an example of
this process in a type of flatworm called a planarian. If the planarian is cut into two
fragments, each will regenerate the body part that is missing shown in the shaded
pink regions. The end result is two genetically
identical offspring. This reproductive form of
regeneration is often called fragmentation and can also be seen in fungi, sea stars,
sponges, and segmented worms.
A method of asexual reproduction
that is similar to regeneration is called vegetative propagation. It refers to the production of
plant clones from parts of the parent. These offspring can grow from
fragments that have broken off of the parent plant, as shown here in the prickly
pear cactus. Fruits and special stem segments
called cladodes fall to the ground and begin to grow their own roots, eventually
developing into new offspring next to their parent plant. If this process is repeated enough
times, it leads to large colonies of prickly pear clones, which can be quite
problematic for farmers and ranchers.
A rather amazing method of asexual
reproduction is called parthenogenesis. It refers to the production of an
embryo from an egg cell that has not been fertilized by a male gamete. Parthenogenesis occurs in plants;
invertebrates such as nematodes, insects, and crustaceans; and a surprising number
of vertebrate taxa including reptiles, amphibians, and fish, although it has not yet
been documented in mammals. The mechanisms of parthenogenesis
are complex and vary widely between species. Let’s take a look at one example in
insects called aphids.
These common garden pests exhibit
cyclical parthenogenesis, meaning they cycle through a sexual generation typically
produced in autumn and multiple asexual generations produced in spring and
summer. The process begins with an
unfertilized diploid egg contained inside a mature female aphid. The unfertilized egg develops into
an embryo and eventually hatches as an immature form called a nymph while still
inside the mother. The nymph contains its own
developing embryo, meaning when the mother gives live birth a few days later, the
nymph is already pregnant. Through parthenogenesis, a female
can produce up to 12 nymphs per day, which are genetic clones and usually all
female.
The cycle of parthenogenesis
continues throughout the spring and summer, with nymphs developing into mature
females that give live birth to more nymphs, resulting in at least 10 and up to 40
asexual generations per year. In autumn, male and female nymphs
are produced. Once they are mature, they will
mate with each other to begin the sexual phase of this life cycle. Cycling between asexual and sexual
phases is a beneficial strategy for aphids and many other organisms. They can rapidly grow their
population during the asexual phase and introduce genetic variation during the
sexual phase, allowing them to obtain the best of both worlds.
You may remember that spores are
special reproductive cells that under certain environmental conditions can produce
offspring. Spores are typically haploid,
meaning they have a single set of chromosomes, and they are produced through
sporogenesis. Spores can be used for sexual or
asexual reproduction in organisms like mosses and ferns, fungi, and certain groups
of protists like algae.
You may have had the unpleasant
experience of going to make your lunch and discovering white, grey, or black fuzz on
a piece of bread or fruit. The fuzz is actually a fungi
commonly called bread mold that belongs to the genus Rhizopus. Like many fungi, the main body of
Rhizopus is composed of branching hyphae, strands that grow into and
across the substrate, in this case your piece of bread. The hyphae form a network called
the mycelium, which anchors the fungus and allows it to absorb the sugar and starch
it needs for survival.
In asexual reproduction, bulbous
black structures called sporangia develop at the tips of special hyphae called
sporangiophores. Inside each sporangium, enlarged
here, spores are produced via mitosis. Most species release spores in
large numbers, and Rhizopus is no exception. Each sporangium can contain 50,000
spores, so a single piece of moldy bread will contain billions or even trillions of
spores. When the sporangia break open, the
spores are released to be dispersed through the air. Other spore-producing organisms
rely on animals or water to disperse their spores. Certain types of algae and bacteria
even have flagellated spores that can actively swim to new areas. If the spores land in a favorable
environment, they will germinate, producing new hyphae that are genetically
identical to the parent fungi.
An artificial method of asexual
reproduction is known as tissue culture, which involves taking a sample of cells
from living tissue such as a plant leaf. The cells are placed in a
controlled laboratory setting, typically on a special growth medium that contains
essential nutrients for survival and growth. Hormones may be added to the growth
medium to encourage the development of the cells. In some cases, tissue culture can
produce new individuals that are genetically identical to the sample cells, such as
these plantlets, which can be transplanted and used in research, thereby controlling
for genetic differences.
In such cases, tissue culture is
considered a form of asexual reproduction. Because tissue culture relies on
some of the same biological pathways as vegetative propagation and regeneration, the
production of whole new individuals is usually done with plant specimens, although
there are examples of microorganisms and simple animals that have been grown from
samples of cells.
Now, let’s review some of the key
points from the video. All forms of life undergo
reproduction, which can be asexual or sexual. Sexual reproduction involves the
union of male and female gametes, typically from two parents. It tends to be slower than asexual
reproduction but has the advantage of increasing the genetic variation within a
population because sexually produced offspring are genetically unique. Asexual reproduction only requires
one parent and does not involve fertilization. It’s usually faster than sexual
reproduction but tends to decrease genetic variation of a population because it
produces genetically identical offspring. Some of the methods of asexual
reproduction are binary fission, budding, regeneration, vegetative propagation,
parthenogenesis, the use of spores, and tissue culture.
