Lesson Video: Types of Sexual Reproduction Biology

In this video, 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.


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.

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