In this explainer, we will learn how to describe different strategies of sexual reproduction and give examples of organisms that use both sexual and asexual reproduction in their life cycle.
Reproduction is the process of generating offspring. Reproduction may be sexual or asexual. Each method of reproduction has advantages and disadvantages. There are some organisms that alternate between sexual and asexual methods and are able to take advantage of both. A basic diagram illustrating sexual and asexual reproduction is shown in Figure 1.
A key difference between sexual and asexual reproduction is that sexual reproduction requires the fusion of genetic material from two different mating types, which, in many species, are called the male and female sexes, while “asexual” reproduction does not require female and male gametes. Let’s look into these two types of reproduction in more detail.
Asexual reproduction is when just one parent generates offspring. The offspring of asexual reproduction are usually genetically identical to their parent and to each other. Organisms that can regularly use asexual reproduction include prokaryotes (like bacteria), some eukaryotes (like fungi such as yeast), and some single-celled protists (such as 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. Another advantage is that asexual reproduction does not require finding a mate, which can be dangerous.
However, the main 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. This can leave populations vulnerable to environmental changes.
Key Term: Asexual Reproduction
Asexual reproduction is a type of reproduction that only involves one parent and does not involve the fusion of gametes.
Sexual reproduction occurs when two parents combine their genetic material to produce genetically unique offspring. The offspring will not be identical to either parent and will likely also be different from each other. Sexual reproduction has the major advantage of increasing the genetic variation in a population, which can increase their resilience in the case of environmental change.
Some of the disadvantages of sexual reproduction include the fact that it is generally slower, more energy costly, and more time consuming. Although some sexually reproducing organisms can produce vast numbers of offspring, asexual reproduction typically produces far more offspring at a faster rate. Generally, organisms that reproduce sexually must grow to reach sexual maturity, which is otherwise called puberty, before they can reproduce. They usually must make gametes, or reproductive sex cells, which themselves will take time to mature. Furthermore, it often requires time to find and prepare an appropriate place to mate and, more importantly, to actually find a mate, a process that can sometimes be dangerous. For example, in some species like lions, pictured below, males have to fight or compete to gain the opportunity to reproduce.
Depending on the species, sexual reproduction sometimes also requires that a parent keeps and protects the developing embryo within their bodies until birth. It may even require parental care and protection after birth in certain species, which can be a costly long-term investment.
Key Term: Sexual Reproduction
Sexual reproduction is a type of reproduction that involves two parents combining half of each of their genetic material to produce offspring.
Example 1: Contrasting the Reproductions of Rabbits and Yeast
Organisms can reproduce sexually or asexually. Which of the following could threaten the continuity of a population of rabbits but not a population of yeast?
- Members of a species are isolated from each other.
- There is a disease present within the population.
- There is a decrease in nutrient availability.
- There are more breeding sites than there are breeding pairs.
- There is a sudden change in the environment.
All living things are able to reproduce in order to perpetuate their species. Reproduction may be described as either sexual or asexual.
Rabbits are animals that reproduce sexually. Sexual reproduction in rabbits requires two parents to each combine half of their genetic material to produce genetically unique offspring. These offspring will not be identical to either parent and will also likely be different from each other.
Yeast are unicellular fungi that can reproduce asexually or sexually. Asexual reproduction is when just one parent generates offspring. The offspring of asexual reproduction are usually genetically identical to their parent and to each other.
Sexual reproduction has the advantage of being able to increase the genetic variation in a population, which may increase their resilience in the case of environmental change. Asexual reproduction has the advantage of being a generally more rapid process than sexual reproduction. It also does not require finding a mate, which can be time consuming and dangerous.
Since rabbits reproduce sexually, they need to first find a mate. If the rabbits are isolated from each other, they may not be able to find a mate to produce offspring. However, since yeast can reproduce asexually, they do not always need a partner or mate to generate offspring. One single yeast cell, completely isolated, can generate an entirely new population of yeast.
Using this information, we can conclude that the population of rabbits could be threatened when members of a species are isolated from each other, while this would not threaten the continuity of a population of yeast.
Sexual reproduction can occur in several different ways. These include conjugation, external fertilization, internal fertilization, and alternation of generations (which includes both sexual and asexual reproduction). Let’s examine each of these methods more closely.
Conjugation occurs when two organisms exchange and recombine their genetic material. Many organisms including bacteria, fungi, algae, and some protists utilize conjugation.
Conjugation occurs when two organisms recombine their genetic material.
Spirogyra is a genus of free-floating green algae that is abundant in freshwater. The cylindrical cells of Spirogyra form chains that join together to form long filaments. It gets its name from the spiral arrangement of the chloroplasts within its cells.
Conjugation occurs in Spirogyra when environmental conditions become undesirable, making it difficult for Spirogyra to survive in them. By reproducing sexually, Spirogyra can increase the genetic variation of their offspring, which gives their offspring a better chance of survival. This is because genetic variation adds new traits to their genetic composition that might help them to adapt to their new environments.
Spirogyra conjugation occurs in two ways.
Individual Spirogyra cells are typically haploid (n), meaning that they normally possess only a single set of chromosomes. During conjugation, two adjacent filaments grow a tube called a conjugation tube that connects one cell to another in the different filament. 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 (2n). The diploid zygote has twice the number of chromosomes of the haploid cells that combined to create it.
Key term: Haploid Cell
A haploid cell is a cell that only has a single set of chromosomes (n).
Key term: Diploid
A diploid cell is a cell that has two complete sets of chromosomes (2n).
Key term: Zygote
A zygote is a diploid cell resulting from the fusion of two haploid gametes.
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 favorable. The diploid nucleus within the zygospore then divides using the process of meiosis, creating 4 haploid nuclei. Only one of the four haploid nuclei survives, and the zygospore grows into a new Spirogyra filament with genetic material from both the filaments that created it. This type of conjugation in Spirogyra is called scalariform conjugation. As you can see in Figure 3, the conjugation tubes that form between the two filaments resemble the rungs of a ladder, and the word scalariform actually derives from the Latin words 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 to form a zygote that develops into a zygospore. Just like in scalariform conjugation, the zygospore remains dormant until environmental conditions improve, then divides using meiosis before growing into a new Spirogyra strand. Lateral conjugation in Spirogyra is illustrated in Figure 4.
In complex multicellular organisms, sexual reproduction requires the production of gametes. The production of gametes involves the process of meiosis, which halves the genetic material of cells. Gametes are cells used for sexual reproduction. They usually come in two varieties, the male sperm and the female egg cell or ovum. The sperm is generally considered to be the gamete that travels toward the egg cell for fertilization to occur and the egg cell is generally the nonmotile gamete. In organisms that do not have pronounced differences between the biological sexes, the type of gamete produced can be used to characterize them!
Key Term: Meiosis
Meiosis is a type of cell division where one cell can produce four genetically different cells with half the number of chromosomes. Meiosis is required for the production of gametes.
Gametes are haploid (n) cells that are usually produced by meiosis. One male gamete and one female gamete combine in a process called fertilization to produce a diploid (2n) zygote. This zygote then grows through mitotic cell division into an offspring organism that will eventually develop into a sexually mature adult that is able to reproduce and begin the life cycle again.
Key Term: Mitosis
Mitosis is a type of cell division where one cell divides to produce two new cells that are genetically identical.
Key Term: Gamete
Gametes are an organism’s reproductive cells which contain half the genetic material of a normal body cell.
Humans are an example of an organism that reproduces sexually in this way, through internal fertilization.
The male gamete, or sperm, is produced in the male reproductive organs. Sperm are small cells with very little cytoplasm and a tail that allows them to swim toward the egg cell. The female gamete, also called the egg cell or ovum, is made in the ovaries. In general, egg cells are large spherical or ovoid cells that possess the organelles and nutrients necessary for the growth of the developing egg cell. In humans, many more sperm are produced than egg cells, a characteristic that is typical of gamete production.
The sperm is haploid: it possesses half of the genetic material found in the male parent’s typical somatic, or body, cells. The egg cell is also haploid, possessing half of the genetic material found in the female parent’s typical somatic cells. These two cells combine during internal fertilization to create a unique diploid zygote. This is 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 will divide, eventually develop into an embryo and then a fetus, and eventually be delivered as a baby. This baby will mature through childhood, adolescence, and adulthood, eventually becoming an adult who is able to reproduce and have children of their own. The human life cycle is illustrated in Figure 5.
External fertilization differs from internal fertilization as it occurs outside of either of the parents’ bodies. External fertilization occurs in most fish species, in particular bony fish, and also usually occurs in amphibians like frogs and toads. During this process, male gametes are often transported to the female via water and fertilization of the gametes takes place externally.
Example 2: Recalling the Steps of Zygote Production
What must occur for an organism to produce a zygote?
- Meiosis only
- Meiosis and gamete fusion
- Mitosis only
- Mitosis and gamete fusion
Often, sexual reproduction requires the production of gametes. The production of gametes involves the process of meiosis, which halves the genetic material of normal body cells.
Gametes are cells used for sexual reproduction. They usually come in two varieties, the male sperm and the female egg cell or ovum. The sperm is generally considered to be the gamete that travels toward the egg cell for fertilization to occur, and the egg cell is generally the nonmotile gamete.
Gametes are haploid (n) cells. One male gamete and one female gamete combine in a process called fertilization to produce a diploid (2n) zygote. This zygote then grows through mitotic cell division into an offspring organism.
So, in order for an organism to produce a zygote, meiosis and gamete fusion must occur.
Reproduction in humans is a good example of the benefits and drawbacks of sexual reproduction. One of the disadvantages of sexual reproduction is the time and energy involved. Two adult humans can typically only make one offspring at a time. There is a significant 40-week gestation period between fertilization and birth. Gestation is more commonly known as pregnancy, and it is the period of development of a baby inside its mother’s uterus, which is typically around 9 months in duration in humans. Then, the offspring must be cared for until they reach maturity.
The main benefit of sexual reproduction is the increase in genetic variation. Since gametes like sperm cells and egg cells are produced by meiosis, they are genetically unique cells. Each gamete possesses a different combination of genes. When these cells combine, they produce offspring with a random assortment of characteristics inherited from each parent. This genetic variation is a benefit to the overall population because, in the case of new environmental pressures or changes, some individuals will likely possess adaptations necessary to survive. They can then pass these adaptations on to the next generation.
So far, we have learned about sexual reproduction in organisms like Spirogyra, whose individual haploid cells within a filament fuse to produce diploid zygotes. We have also learned about primarily diploid organisms, like humans, that must make haploid gametes in order to carry out sexual reproduction. Next, we will learn about organisms that have a life cycle that includes both a diploid sexual phase and a haploid asexual phase. We commonly refer to this as “alternation of generations.”
Key Term: Alternation of Generations
Alternation of generations describes a pattern of reproduction that involves alternation between two distinct forms for the same individual organism. The generations are usually alternately sexual and asexual or haploid and diploid.
Plasmodia are a unicellular parasitic genus of protists (protozoa) that can infect humans and cause a disease called malaria. The Plasmodium life cycle is an example of alternation of generations. Sporozoites are haploid (n) Plasmodium cells that are found in the salivary glands of some mosquitoes. These spindle-shaped sporozoites enter the human body when a person is bitten by an infected female Anopheles mosquito.
The sporozoites infect the cells of the liver and reproduce through an asexual process called schizogony, which is similar in concept to binary fission; however, the number of daughter cells the two processes produce differs. While, in schizogony, a single parent cell divides into many daughter cells by multiple fission, in binary fission, the parent cell divides into two equal and identical daughter cells. There are usually between two to four replicative cycles of schizogony, and this number varies between Plasmodium species.
The sporozoites mature into schizonts, and the final cells produced by schizogony that are released from these schizonts are called merozoites, which are also haploid. The merozoites are typically released every 48 hours, and they spread to infect red blood cells, where the number of merozoites quickly increases. During this time, the human host experiences the symptoms of a malaria infection, such as fever, chills, and sweating.
As shown in Figure 6, some of the merozoites differentiate into sexual-stage haploid gametocytes, which means that there are both female macrogametocytes and male microgametocytes present in the infected human’s red blood cells. The blood cell stages are responsible for the symptoms that are displayed in an infected human with malaria. These gametocytes are picked up by another female mosquito when it bites the infected human host.
The gametocytes in the blood meal ingested by the mosquito soon develop into mature male and female gametes through a process called gametogenesis.
The sexual reproduction phase can then take place within the mosquito’s midgut. The microgametes (male) fertilize the macrogamete (female) to form a diploid zygote. The zygote divides many times and changes shape to transform into an ookinete that later 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 through a process called sporogony. The oocyst ruptures and releases sporozoites, and these sporozoites then infect the salivary glands of the mosquito, where they remain until the mosquito feeds again and infects another human. Then, the life cycle can begin again.
To summarize, a period of asexual reproduction in the infected human’s liver (schizogony) is followed by a sexual phase in their red blood cells, which forms gametes in the gut of a mosquito that has fed on the infected human’s blood (gametogony). After the fusion of these gametes in the mosquito, there is then a period of maturation (sporogony) within the female Anopheles mosquito,which requires nuclear division and multiple fission to produce sporozoites that can be spread to another human.
Example 3: Identifying the Stages of a Plasmodium Life Cycle
The protist Plasmodium is a parasite that lives in two hosts, humans and mosquitoes, as shown in the figure. An alternation of generations can also be seen in its life cycle.
At which stage in the Plasmodium life cycle does fertilization occur?
The diagram below shows a more detailed illustration of the life cycle of a Plasmodium.
Plasmodium is a genus of unicellular parasites, a species of which is responsible for causing the disease malaria in humans. The Plasmodium life cycle involves several different forms and occurs in two hosts, humans and mosquitoes.
Fertilization is the name of the process of two gametes fusing to form a zygote. Gametes are haploid cells, which means that they have half of the typical number of chromosomes for an organism, also referred to as n. Two gametes fuse to form a diploid zygote, which has the typical number of chromosomes for an organism, also referred to as 2n.
In the diagram, we can see that the stage of the Plasmodium life cycle where gametes are located is in the bottom right of the image in the section labeled “mosquito.” The sequence of arrows shows us that the haploid male microgamete fuses with the haploid female macrogamete to produce a diploid zygote. This process is called fertilization, and it occurs in the mosquito host after it has bitten an infected human.
Applying this information to the diagram in the question, we can determine that the stage of the Plasmodium life cycle in which fertilization occurs is stage 5.
The life cycle of a Plasmodium shows alternation of generations in a unicellular organism. It experiences an asexual phase in the human host and a sexual phase in the mosquito host. Alternation of generations is also commonly observed in plants, an example of which is the life cycle of a fern.
Ferns are nonflowering vascular plants of the class Polypodiopsida, like polypodium, for example, which is an ornamental plant.
An adult fern is also called a sporophyte. The sporophyte is diploid. On the underside of the leaves of a fern plant, there are structures called sori (singular: sorus). These sori have diploid cells called spore mother cells (2n) that will divide through meiosis to generate haploid spores (n). The spores are released and carried by the air. Spores that land on a suitable surface will begin to grow and germinate as shown in Figure 7.
The haploid spores grow into heart-shaped gametophytes. Gametophytes are much smaller plants with haploid cells. They have rhizoids that attach them to soil and facilitate the absorption of minerals and water to support the sporophytes in their early stages. Gametophytes are capable of generating both sperm and egg cells.
The gametophyte makes haploid egg cells in a structure called archegonium and haploid sperm cells in a structure called antheridium. The sperm from one gametophyte are able to travel to an adjacent or nearby plant via water droplets. The sperm are attracted to the entrance of the archegonium on another plant where they can fertilize the egg cells. Sometimes self-fertilization occurs when the sperm from a gametophyte fertilizes an egg cell on the same gametophyte.
Whether cross fertilization or self-fertilization occurs, the diploid zygote that is formed from this process will grow into a new diploid sporophyte to begin the life cycle again. The sporophyte becomes independent of its gametophyte when the fern’s first leaves and roots begin to grow.
Alternation of generations provides certain advantages to organisms that utilize it. For example, it allows rapid production of new organisms through the asexual phase but also provides genetic diversity through the sexual phase. This enables them to disperse widely and to also adapt to environmental changes.
Example 4: Recalling the Life Cycle of a Fern
During the life cycle of the ornamental fern (Polypodium), a zygote is produced. What plant form does it develop into?
- A diploid sporophyte
- A haploid gametophyte
- A diploid gametophyte
- A haploid sporophyte
- A haploid sporangium
This diagram shows a simplified illustration of the life cycle of a typical fern plant.
A zygote is the diploid cell that forms from the fusion of two haploid gametes. We can see in this diagram of the fern life cycle that the male and female gametes are made by the gametophyte, a small haploid offspring of the mature diploid fern plant. The gametophyte makes haploid gametes that fuse during fertilization to make a diploid zygote that develops into a new diploid sporophyte, which is the name of the mature fern plant we are familiar with.
This means that the plant form that a fern zygote develops into is a diploid sporophyte.
Let’s review some of the key points we have covered in this explainer.
- During sexual reproduction, two haploid gametes fuse to form a diploid zygote that will eventually develop into a mature offspring.
- Sexual reproduction increases genetic variation when compared with asexual reproduction.
- Sexual reproduction is generally slower and more time and energy costly than asexual reproduction.
- In sexual reproduction, haploid gametes are formed through meiosis.
- Conjugation and reproduction by gametes are two methods of sexual reproduction.
- Plasmodia and fern plants are examples of organisms that show alternation of generations.