SEXUAL REPRODUCTION: CHARACTERISTICS, TYPES, PLANTS, ANIMALS - BIOLOGY - 2024

The sexual reproduction is the multiplication of individuals from two parents of different sexes: male and female - except when referring to sexual reproduction in bacteria or protozoans, where there is no distinction of sex. It is a widely distributed process in eukaryotic organisms.

Each individual that participates in sexual reproduction produces a type of specialized germ line cells: sperm and eggs. These are caused by a type of specialized cell division, called meiosis. This event is the crucial difference between asexual and sexual reproduction.

Source: By Manojiritty, from Wikimedia Commons

The process begins with the union of two gametes that give rise to a zygote. Later, the zygote gives rise to a new individual with characteristics of both of its parents and with certain unique characteristics.

Due to the ubiquity of the process, we infer that sexual reproduction involves a series of advantages over asexual reproduction. However, the possible disadvantages of sexual reproduction are more noticeable: the time and energy invested in the search for mates, the competition for females, the cost of production of gametes that are not fertilized, among others.

The costs appear to be very high, so they must have substantial advantages to help offset it. The benefits of sexual reproduction have been a subject of controversy and debate among evolutionary biologists.

One hypothesis suggests that sexual reproduction is beneficial since it produces varieties that, in times of environmental changes, could be beneficial for the species. In fact, the production of genetic variability is one of the advantages attributed to sex.

On the other hand, some researchers propose that sexual reproduction, specifically recombination, has been selected as a DNA repair mechanism. However, the prevalence of sex, despite its costs, is still unknown.

General characteristics

Sex is a complex phenomenon that varies widely among eukaryotic taxa. In general, we can understand it as a process that involves three steps: the fusion of two haploid nuclei, the phenomenon of recombination that produces novel genotypes, and the division of diploid cells to form haploid nuclei.

From this point of view, sex in eukaryotes depends on a life cycle, in which diploid cells must divide by meiosis. This meiotic division process is responsible for distributing the genetic material of future gametes.

Meiosis aims to separate homologous chromosomes, in such a way that each gamete has half of the somatic chromosomes. In addition to reducing the genetic load, in meiosis the exchange of material between non-sister chromatids also occurs, producing totally novel combinations.

Gametes

Gametes are the sex cells of organisms that are generated by meiosis and contain half of the genetic load, that is, they are haploid.

Gametes vary in both plants and animals, and are classified into three fundamental categories depending on their size and relative mobility: isogamy, anisogamy, and oogamy.

Isogamy is a form of sexual reproduction where the gametes that fuse to give rise to the new individual are identical in size, mobility and structure. Isogamy is represented mainly in plants.

In contrast, anisogamy consists of the union of two gametes that differ in size and structure. One particular type of anisogamy is oogamy, where the male gametes are relatively small in size and abundant in number. The female ones are much more conspicuous and are produced in fewer numbers.

Sexual reproduction in animals

In the animal kingdom, sexual reproduction is a widely distributed phenomenon among group members.

Almost all invertebrates and vertebrates possess the sexes in separate organisms - that is, we can distinguish a male and a female individual in a species. This condition is called dioecious, a term derived from the Greek roots “two houses”

In contrast, there are certain less numerous species whose sexes are present in the same individual called monoecious: “a house”. These animals are also known as hermaphrodites.

The distinction between the sexes is not given by morphological characteristics of size or coloration, but by the type of gametes that each sex produces.

The females produce the ovules, characterized by their large size and their immobility. Sperm, on the other hand, are produced by males in greater quantity, they are much smaller and have special structures to move and fertilize the ovum.

Next we will describe the typical sexual organs of animals and then we will detail the process of reproduction in each animal group.

Structures associated with reproduction

The specialized cells for sexual reproduction - eggs and sperm - are produced in specific tissues called gonads.

In males, the testes are responsible for the production of sperm, while the female gametes are formed in the ovaries.

The gonads are considered the primary sexual organs. Accessory sexual organs are present in an important group of metazoans that are responsible for receiving and transferring eggs and sperm. In females we find the vagina, the uterine tubes or fallopian tubes and the uterus, while in males there is the penis.

Poriferous

Porifers are commonly known as sponges and can reproduce both sexually and asexually. In most species, the production of male and female gametes occurs in a single individual.

Choanocytes are a particular type of cell of this lineage, which can transform into sperm. In other groups the gametes may be derived from archeocytes.

Many species are viviparous, which indicates that after the fertilization phenomenon the zygote is retained by the parental organism until the release of a larva occurs. In these species the sperm is released into the water and taken up by another sponge.

Cnidarians

Cnidarians are marine organisms that include jellyfish and the like. These animals have two morphologies: the first is the polyp and is characterized by a sessile lifestyle, while the second is the jellyfish that is capable of moving and floating.

Polyps generally reproduce asexually by budding or fission processes. The jellyfish is dioecious and reproduces sexually. The life cycle in this group is highly variable.

Acelomorphs and flatworms

Flatworms, such as planarians, are known primarily for their ability to regenerate and produce multiple clones asexually from a single individual.

Most of these vermiform animals are monoecious. However, they are looking for a partner to carry out the cross-fertilization.

The male reproductive system includes several testes and a papilla-like structure similar to the penises of complex vertebrates.

Mollusks and annelids

Most mollusks are dioecious and their reproduction gives rise to a freely swimming larva called the trocófera (very similar to the larvae present in annelids) and varies according to the species of mollusks.

Similarly, annelids have separate sexes and in some they have gonads that appear temporarily.

Arthropods

Arthropods are an extremely diverse group of animals, characterized by an exoskeleton composed of chitin and jointed appendages. This lineage includes the myriapods, chelicerates, crustaceans, and hexapods.

The sexes are generally separated, the organs specialized in reproduction appear in pairs. Most of the species have internal fertilization. They can be oviparous, ovoviviparous, or viviparous.

Echinoderms

Echinoderms include starfish, sea cucumbers, sea urchins, and the like. Although there are some hermaphrodite species, most are characterized by having separate sexes. The gonads are large structures, the ducts are simple, and there are no elaborate copulatory organs.

Fertilization occurs externally and a bilateral larva develops that can move freely in the body of water. Some species have direct development.

Chordates

Most of the sexes are separate. In this group we find more complex organs for reproduction. Each sex has gonads with ducts that direct the products of these to a cloaca or to some special opening located near the anus. Depending on the group, fertilization can be external or internal.

Parthenogenesis in animals

Parthenogenesis is a phenomenon widely represented in the animal kingdom, mainly in invertebrates and some vertebrates, which allows the generation of a new individual with only one parent. Although it is a form of asexual reproduction, certain types of parthenogenesis are considered types of sexual reproduction.

In meiotic parthenogenesis, an egg is formed by meiosis and may or may not be fertilized by sperm from a male.

In some cases, the eggs must be activated by the male sex gamete. In this case, there is no fusion of both nuclei, since the genetic material from the sperm is discarded.

However, in some species the ovum can develop spontaneously without the need for the activation process.

Sexual reproduction in plants

Analogous to the case of animals, plants can undergo sexual reproduction. It consists of the union of two haploid gametes that will give rise to a new individual with unique genetic characteristics.

The plant can have the male and female organs in a single individual or they can be separated. In cucumber and milky the sexes are separated, while in roses and petunias the sexes are together.

The flower

The organ in charge of the processes of sexual reproduction are the flowers. These specialized structures have regions that do not participate directly in reproduction: the calyx and corolla, and sexually active structures: the androecium and gynoecium.

The androecium is the male reproductive organ composed of a stamen, which in turn is divided into a filament and an anther. This last region is responsible for the production of pollen grains.

The gynoecium is the female flower organ and is made up of units called carpels. The structure is similar to an elongated “drop” and is divided into the stigma, the style and finally the ovary.

Pollination

The process of sexual reproduction in plants occurs mainly through pollination, which consists of the transport of pollen grains from the anther to the stigma.

Pollination can occur in the same flower (the pollen grains go to the female organ of the same plant) or it can be crossed, where the pollen grains fertilize a different individual.

In most plants, the intervention of an animal is necessary to carry out pollination. These can be invertebrates such as bees or other insects or vertebrates such as birds and bats. The plant offers the pollinator the nectar as a reward and they are in charge of dispersing the pollen.

The floral structures that do not participate directly in reproduction are the corolla and the calyx. These are modified leaves, in many cases with bright and vibrant colors, which are responsible for visually or chemically attracting the potential pollinator.

Similarly, some plants do not require animal pollinators and use wind or water to disperse pollen.

Fertilization, seeds and fruit

The process begins with the arrival of the pollen grains to the stigma of the flower. These travel by style until they find the ovary.

Double fertilization is typical of flowering plants and unique among all organisms. The phenomenon occurs as follows: a nucleus of a sperm unites with an ovum, and another nucleus of sperm fuses to a diploid embryo of the sporophyte.

The result of this unusual fertilization event is a trioploid endosperm that will act as a nutritive tissue for the development of the organism. Once the successful maturation of the ovules occurs, they transform into the seeds. The fruit, on the other hand, is formed by the mature ovaries.

The fruit can be classified as simple if it comes from a mature ovary and be added if it develops from several ovaries, like the strawberry, for example.

Sexual reproduction in bacteria

Bacteria are known primarily for their ability to reproduce asexually.

In this prokaryotic lineage an individual is capable of dividing in two by a process called binary fission. However, there are a series of mechanisms in bacteria that are reminiscent of sexual reproduction since there is an exchange of genetic material.

Until the mid-1940s, it was thought that bacteria reproduced exclusively asexually. However, researchers Joshua Lederberg and Edward Tatum disproved this belief through an ingenious experiment using the E. coli bacteria with different dietary requirements as a model.

The experiment consisted of a strain A growing in minimal medium with methionine and biotin, and a strain B that only grew in environments with threonine, leucine and thiamine. In other words, each strain carried a mutation that prevented it from synthesizing these compounds, therefore they had to be synthesized in the culture medium.

When the colonies were in contact for a few hours, the individuals acquired the ability to synthesize nutrients that they previously could not. Thus, Lederberg and Tatum demonstrated that there was a DNA exchange process similar to sexual reproduction and called it conjugation.

Conjugation

The conjugation process occurs through a bridge-like structure called the sexual pili, which physically binds two bacteria together and allows them to exchange DNA.

As bacteria do not have sexual dimorphism, we cannot speak of males and females. However, only one type can produce pili, and they have special DNA fragments called factor F, for "fertility." Factor F possesses the genes for pili production.

The DNA involved in the exchange is not part of the single bacterial chromosome. Instead it is an isolated circular portion called a plasmid, which has its own replication system.

Transformation

In addition to conjugation, there are other processes where the bacteria can obtain extra DNA and are characterized by being simpler than conjugation. One of them is transformation, which consists of taking naked DNA from the external environment. This exogenous DNA fragment can be integrated into the bacterial chromosome.

The transformation mechanism enters into the concept of sexual reproduction. Although the bacteria took free DNA, this genetic material had to come from another organism - for example a bacterium that died and released its DNA into the environment.

Transduction

The third and last known mechanism in bacteria to obtain foreign DNA is transduction. This implies the participation of a virus that infects bacteria: bacteriophages.

In transduction, a virus takes a portion of bacterial DNA and when it infects a bacterium, a difference can pass this fragment to it. Some authors use the term "parasexual events" to refer to these three mechanisms.

Evolutionary perspective

The ubiquity of sexual reproduction in organisms is a remarkable fact. Therefore, one of the biggest questions in evolutionary biology is why sex is spread across so many lineages if it is an energetically expensive activity - and in some cases even dangerous.

It is suspected that the selective forces that originated sexual reproduction in eukaryotes are the same ones that maintain the parasexual processes described for bacteria.

Sex costs

In the light of evolution, the term "success" refers to the ability of an individual to pass their genes on to the next generation. Paradoxically, sex is a process that does not fully meet this definition, since a series of costs associated with reproduction.

Sexual reproduction involves finding a mate and in most cases this task is not trivial. An enormous amount of time and energy must be invested in this endeavor which will determine the success of the offspring - in terms of finding "the ideal mate."

The animals display a series of rituals to woo their potential mates and in some cases they must fight exposing their own life to achieve copulation.

Even at a cellular level, sex is expensive, since division by meiosis takes much longer than mitosis. So why do most eukaryotes reproduce sexually?

There are two fundamental theories. One is related to cell fusion as a mechanism for horizontal transmission of a "selfish" genetic element, while the second theory proposes recombination as a DNA repair mechanism. Below we will describe the pros and cons of each theory:

Sex benefits

To answer this question we must focus on the possible benefits of sexual reproduction in the first eukaryotes.

The fusion of gametes to form a zygote leads to a combination of two different genomes that are capable of compensating for possible defective genes in one genome with a normal copy of the other.

In humans, for example, we inherit one copy from each parent. If we inherit a defective gene from our mother, the normal gene from our father can compensate for it (in such a case that the pathology or disease only presents as homozygous recessive).

A second theory - not as intuitive as the first - proposes that meiosis acts as a repair mechanism in DNA. Damage to genetic material is a problem that all organisms must face. However, there are organisms that only reproduce asexually and their DNA is not particularly damaged.

Another hypothesis states that sex may have evolved as a parasitic adaptation between selfish genetic elements, in order to be distributed to other genetic lineages. A similar mechanism has been evidenced in E. coli.

Although there are possible explanations, the evolution of sex is a subject of heated debate among evolutionary biologists.

Sexual selection

Sexual selection is a concept introduced by Charles Darwin that is only applicable to sexually reproducing populations. It is used to explain the presence of behaviors, structures, and other attributes whose existence cannot be conceived by natural selection.

For example, the very colorful and somewhat "exaggerated" plumage of peacocks does not provide direct benefits to the individual, since it makes it more visible to potential predators. Furthermore, it is only present in males.

References

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