Sexual Reproduction in Animals

Definition of Sexual Reproduction 

“Sexual reproduction is a biological process by which organisms create descendants that have a combination of genetic material contributed from two (usually) different members of the species.”

What is Sexual Reproduction?

  • Sexual reproduction is the process through which offspring are generated by the union of male and female gametes.
  • It is the most advanced form of reproduction found in all multicellular organisms.
  • With the exception of a few invertebrates and vertebrates, most higher species follow this pattern.

Characteristics of Sexual Reproduction

  • Sexual Reproduction involves the union of two gametes (sperms and ova) with haploid nuclei.
  • Fusion of gametes is called syngamy.
  • Each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes.
  • Most organisms form two different types of gametes.
  • Isogametes are the morphologically similar male or female gametes as in Monocystis.
  • In isogamous species the gametes are similar or identical in form, but may have separable properties and then may be given other different names. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as ciliates, have more than two kinds of gametes
  • Anisogametes are the morphologically dissimilar male or female gametes in size and shape.
  • In these anisogamous species, the two sexes are referred to as male (producing sperm or microspores) and female (producing ova or megaspores).
  • Male gamete is smaller in size, motile and deficient in stored nutrients called microgamete or sperms.
  • Female gamete is much larger in size, non-motile and laden with stored nutrients called macrogamete or ovum.
  • The union between two such dissimilar gametes is known as anisogamy.
  • Most animals (including humans) and plants reproduce sexually. Sexually reproducing organisms have two sets of genes for every trait (called alleles).
  • There is a specific reproductive period in plants. Some are annual or biennial or perennial.
  • Bamboo flowers once and Neelakurunji (Strobilanthus kunthiana) flowers once in 12 years.
  • An individual organisms may be homothallic/monoecious or heterothallic/dioecious.
  • Cucurbits and Coconut are monoecious plants while Papaya and Date Palm are dioecious plants.
  • Sexual reproduction of all organisms shows similarity, inspite of differences in the structures.

Stages of Sexual Reproduction

The events can be grouped into three distinct stages-

  • Pre-Fertilisation
  • Syngamy and Fertilisation
  • Post-Fertilisation

Pre-Fertilisation Events

“All the preparatory events which occur in the organism before fusion of gametes are called pre-fertilisation events.”

The two major pre-fertilisation events are Gametogenesis and Gamete Transfer.

Gametogenesis

“Gametogenesis is a process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes.”

  • Depending on the biological lifecycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes or by mitotic division of haploid gametogenous cells.
  • For example, plants produce gametes through mitosis in gametophytes.
  • The gametophytes grow from haploid spores after sporic meiosis.
  • The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as alternation of generations.
  • The gametes are usually haploid.
  • The gametes are called homogametes/isogametes, when the fusing gametes are similar in their morphology/appearance.
  • The gametes are said to be heterogametes, when the fusing gametes are morphologically distinct; in such case, the male gamete is called antherozoid or sperm and the female gamete is called the ovum.
  • If the parent body is haploid, gametes are formed by mitosis and if the parent body is diploid, gametes are formed by meiosis.

Gamete Transfer

  • In some Algae, both male and female gametes are motile.
  • In majority of organisms, the male gametes are motile, while the female gametes are stationary.
  • In Algae, Bryophytes and Pteridophytes, water is the medium for gamete transfer.
  • Since a large number of male gametes fail to reach the female gamete, the number of male gametes produced is several thousand times more than those of female gametes.
  • Pollination is the method of gamete transfer in higher plants, as pollen grains are the carriers of male gametes.
  • Dioecious/unisexual animals have evolved special mechanisms for transfer of gametes.

Fertilisation

“Fertilization (also known as conception, fecundation and syngamy), is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo.”

  • Syngamy results in the formation of a diploid zygote.
  • Fertilisation may be external fertilisation or internal fertilisation.
  • Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation.
  • The entire process of development of new individuals is called procreation, the act of species reproduction.

Fertilisation in Plants

  • After the pistil is pollinated, the pollen grain germinates in a response to a sugary fluid secreted by the mature stigma (mainly sucrose).
  • From each pollen grain, a pollen tube grows out that attempt to travel to the ovary by creating a path through the female tissue.
  • The vegetative (or tube) and generative nuclei of the pollen grain pass into its respective pollen tube.
  • After the pollen grain adheres to the stigma of the carpel (female reproductive structure) a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle.
  • The pollen tube does not directly reach the ovary in a straight line.
  • It travels near the skin of the style and curls to the bottom of the ovary, and then near the receptacle, it breaks through the ovule through the micropyle (an opening in the ovule wall) and the pollen tube "bursts" into the embryo sac.
  • After fertilisation, the ovary starts to swell and will develop into the fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule.

Fertilisation in animals

  • Most of the animals are unisexual with an individual only producing either male or female gametes.
  • The male gamete of one individual fertilizes with the female gamete of another individual to produce the zygote.
  • The fertilisation may be external or internal.
    • External fertilisation takes place in animals like the fish and frog where the eggs are released from the body of the females into the water outside. These eggs are then fertilised by the sperms produced by the male species. The fishes and frogs are oviparous, that is they lay eggs.Organisms like birds, insects and reptiles are also oviparous.
    • However, in these organisms the fertilisation is internal. Internal fertilisation occurs in mammals also. In internal fertilisation the sperms are released into the body of the females during copulation. The resulting zygote develops into a young one. Thus mammals are viviparous, that is, they give birth to young ones.

Post-Fertilisation Events

  • All those events in sexual reproduction after the formation of zygote are called post-fertilisation events.

The Zygote

“A zygote (from Greek zygōtos "joined" or "yoked", from “to join" or "to yoke"), or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction.”

  • A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development.
  • Zygotes are produced by fertilization between two haploid cells — an ovum from a female and a sperm cell from a male — which combine to form the single diploid cell.
  • Zygote is the vital link that ensures continuity of species between organisms of one generation and of the next.
  • Development of zygote depends upon-
    • The type of life cycle of the organism
    • The environment it is exposed to.
  • In many Algae and Fungi, the zygote develops a thick wall around it and undergoes a period of rest before germination.
  • In organisms showing haplontic life-cycle, zygote undergoes meiosis, while in organisms showing diplontic life-cycle, zygote undergoes mitosis.
  • DNA in such zygotes is derived from both the mother and the father, and this provides all the genetic information necessary to form a new individual.
  • The term zygote is also used more loosely to refer to the group of cells formed by the first few cell divisions, although this is properly referred to as a morula.
  • In mammalian reproduction, after fertilization has taken place the zygote travels down the fallopian tube, while dividing to form more cells without the zygote actually increasing in size.
  • This cell division is mitotic, and is known as cleavage. All mammals go through the zygote stage of life.
  • Zygotes eventually develop into an embryo, and then a fetus.
  • A human zygote exists for about four days, and becomes a blastocyst on the fifth day.
  • The egg cell (and hence the fertilized egg) is always asymmetric, having an "animal pole" (future ectoderm and mesoderm), two of three primitive tissue types, and a "vegetal pole" (future endoderm), it is also covered with different protective envelopes.

Embryogenesis

“Embryogenesis is the process by which the embryo is formed and develops.”

  • It starts with the fertilization of the ovum (or egg) which, after fertilization, is referred to as a zygote.
  • The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo.
  • It occurs in both animal and plant development, this article addresses the common features among different animals.
  • Cell divisions with no significant growth, producing a cluster of cells that is the same size as the original zygote is called cleavage.
  • The zygote develops into an embryo (embryogenesis).
  • Embryogenesis involves-
  • Cell division, to increase the number of cells.
  • Cell enlargement or growth, to increase the volume/weight of living matter.
  • Cell differentiation for the formation of different kinds of tissues.
  • Depending on whether the development of zygote occurs inside or outside the body of the female organism (parent), animals are categorized as oviparous and viviparous.
  • Oviparous: Animals that lay eggs. An egg that has been fertilised and is being placed outside has a hard calcareous shell. After some time of incubation, the newborn animal emerges. E.g. Reptiles, birds, etc.
  • Viviparous: Gives birth to young ones.. Within the female body, there is Embryogenesis, and young ones are born. The gestation period varies between species.
  • In flowering plants, the zygote is formed inside the ovule, where it develops into the embryo; simultaneously the ovule becomes the seed and the ovary becomes the fruit.
  • The seeds germinate after they are dispersed from the fruit; they produce new plants.

Parthenogenesis

  • Parthenogenesis is a modified form of sexual reproduction in which a female gamete develops into a new individual without being fertilized by a male gamete.
  • Development of egg without fertilization is called Parthenogenesis.
  • Natural Parthenogenesis occurs in honeybee. The queen and workers are developed from fertilized ova. Drones (males) are produced parthenogenetically.
  • Natural Parthenogenesis occurs during the regular reproduction in some insects (ants and aphids), some crustaceans and in rotifers.
  • Artificial Parthenogenesis can be induced in many animals by a wide variety of techniques.
  • Parthenogenesis is seen in rotifers, honeybees, some lizards and birds (Turkey).

Advantages of Sexual Reproduction

  • Results in genetic variation in progeny
  • The species has a survival edge because of its ability to adapt to new environments due to variation.
  • A disease has a smaller chance of affecting the entire population.

Disadvantages of Sexual Reproduction

  • It takes time and effort to find a partner.
  • A single isolated individual cannot reproduce.