Lecture 5

 Gametogenesis and syngamy in Plants- identical and fraternal twins

Gametogenesis in plants (Angiosperms): 

     In plants male gametes are produced in stamen and female gametes are produced in pistil. The production of gametes involves two steps, i.e. sporogenesis and gametogenesis.

Sporogenesis: 

     Production of microspores and megaspores is known as sporogenesis. Microspores are produced in anthers (microsporogenesis), while megaspores are produced in ovule (megasporogenesis).

Microsporogenesis:

      Production of microspores in the anthers is known as microsporogenesis. Pollen sac on the anther contains numerous pollen mother cell (PMC) or microsporocyte. Each PMC on microspores (n). This process is known as microsporogenesis. The microspores mature into pollen grains mainly by thickening of their cell walls.

Megasporogenesis:

        Megasporogenesis occurs in ovules which are present in the ovary. A single cell in each ovule differentiates in to Megaspore Mother Cell (MMC) or megasporocyte. The MMC undergo meiosis to produce four haploid megaspores. Three of the megaspores’ degenerates having one functional megaspore per ovule.

Gametogenesis: The production of male and female gametes in microspores and the megaspores, respectively, is known as gametogenesis.

Micro gametogenesis:

       This refers to the production of male gamete or pollen. During maturation of pollen, the microspore undergoes a mitotic division of the chromosomes without cytoplasmic division to produce one generative nucleus and one vegetative or tube nucleus. Pollen grains are usually shed in this binucleate stage. Upon germination of the pollen grain on the stigmatic surface, the generative nucleuses divide mitotically without cytokinesis to produce two sperm nuclei or male gametes. The pollen, along with the pollen tube, is known as microgametophyte. The pollen tube finally enters the ovule through small pore, micropyle and discharges the two sperm nuclei into the embryo sac.

Megagametogenesis:

       The nucleus of a functional megaspore divides mitotically to produce four or more nuclei. In most of the crop plants, megaspore nucleus undergoes three mitotic divisions produce eight nuclei. Three of these nuclei move to one pole and produce a central egg cell and two synergid cells situated on either side of the egg cell. Another three nuclei migrate to the opposite to give rise to antipodal cells, the two nuclei remaining in the centre, the polar nuclei, fuse to form a secondary nucleus. The megaspore thus develops into a mature megagametophyte or embryo sac. The development of an embryo sac from a megaspore is known as mega gametogenesis. An embryo sac generally contains one egg cell, two synergids, three antipodal cells (All haploid), and diploid secondary nucleus.

Fertilization or Syngamy:

   After gemination of the pollen grain on the stigma, pollen tube grown down the style at the direction of tube nucleus and enters into the embryo sac through micropyle. Two germ nuclei are released into the embryo sac. Pollen degenerate. In the phenomena, one female gamete unites with two male gametes. One of the male gametes fertilizes the egg resulting in the formation of a zygote and the other unites with 2 polar nuclei for the formation of an endosperm. The outermost layer of the endosperm cells is called aleurone. The embryo either may be surrounded by endosperm tissue or in some cases it is enclosed by diploid maternal tissues called pericarp. In the process of fertilization, since one sperm nucleus fertilizes egg nucleus and the other sperm nuclei fertilizes fusion nucleus, it is termed as double fertilization.

Identical and Fraternal Twins:

   Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic (identical), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic (fraternal), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell.

    If twins are a boy and a girl, clearly, they are fraternal twins, as they do not have the same DNA. A boy has XY chromosomes and a girl has XX chromosomes. Girl-boy twins occur when one X egg is fertilized with an X sperm, and a Y sperm fertilizes the other X egg. Identical twins have the same DNA; however, they may not look exactly identical to one another because of environmental factors such as womb position and life experiences after being born.

Fraternal twins (2egg and 2 sperm):

      Fraternal twins are derived from two different zygotes resulting from fertilization of two different eggs by separate sperms. As a result, such twins have different genotypes and may be of different sexes.

Identical twins (1 egg and 1 sperm):

      Identical twins are derived from a single zygote: an abnormal division of the embryo very early in the development divides a single embryo into two parts each of which develops into a complete individual. Identical twins therefore are always of the same sex and have identical genotypes.

Concordance between fraternal and identical twins:

   There are two approaches for estimating the effects of environment on various human traits each suited for a different situation.

• Comparison of concordance between fraternal and identical twins.

• Comparison between identical twins reared apart.

   When a character is present in both the individuals of a twin pair: the pair is classified as concordant and the condition is known as concordance

    When the two individuals of a twin pair show different forms of the character: the pair is classified as discordant and the situation is referred to as discordance

    The numbers of twin pairs showing concordance and discordance may be recorded and expressed as per cent of the total number of pairs studies this is known as the frequency of concordance and discordance.