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 spouts 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.