Cytoplasmic Inheritance.pdf
- 1. CYTOPLASMIC OR EXTRA-NUCLEAR INHERITANCE By Dr. Sadguru Prakash In Eukaryotes, all the genes are not located on chromosomes in nucleus but certain genes are found in cytoplasm also. The genes which are located in cytoplasm are called Cytogenes or plasmagenes and the phenomenon of their inheritance are called Cytoplasmic inheritance or Extranuclear inheritance. Thus it is the transmission of genes that occur outside the nucleus. In cytoplasmic inheritance, transmission of traits takes place through self- replicating cytoplasmic particles (non-chromosomal) such as mitochondria, chloroplast or through infectious particles or endosymbionts (like kappa particles, sigma particles etc.). These cytoplasmic particles or bodies or elements are non chromosomal or extra nuclear genetic elements are hereditary units and called plasmagenes or cytogenes. All the plasmagenes in an organism is referred to as plasmon. These are located in DNA of organelles (mitochondria or chloroplast), therefore, this type of inheritance is often referred to as Organellar inheritance. Since the plasmagenes are transmitted only by means of the cytoplasm, the zygote receives them only from the eggs not from the sperm. This is because the egg or ovum or female gametes alone contributes cytoplasm of fertilized eggs or zygote. Hence, this type of inheritance is also called as maternal inheritance. Note: In nuclear inheritance both parents contribute equally towards the inheritance of the offspring and results of reciprocal crosses are same i.e. ‘A’ type male X ‘B’ type female will produce the same kind of offsprings as ‘A’ type female X ‘B’ type male. On the other hand in Cytoplasmic inheritance transmission of traits takes place through cytoplasmic particles ( non chromosomal) only from mother to offsprings so the result of reciprocal crosses are not similar. Characteristics of cytoplasmic inheritance: 1. Reciprocal differences: Reciprocal crosses show marked differences for the characters controlled by plasmagenes. In most cases, plasmagenes from only one parent, generally the female parent are transmitted, so this phenomenon is known as uniparental inheritance. 2. Lack of segregation: In general, F2, F3 and the subsequent generations do not show segregation for a cytoplasmically inherited trait. This is because the F1 individuals generally receive plasmagenes from one parent only.
- 2. 3. Irregular segregation in biparental inheritance: In some cases, plasmagenes from both the parents are transmitted to the progeny, this is known as biparental inheritance. 4. Somatic segregation: Plasma genes generally show somatic segregation during mitosis, a feature of rare occurrence in the case of nuclear genes. 5. Association with organelle DNA: Several plasma genes have been shown to be associated with cp-DNA or mt-DNA. 6. Mutagenesis: Some mutagens eg: Ethidium bromide are highly specific mutagens for plasma genes while nuclear genes are not affected by them. Induction of mutation by such agents in a gene indicates it to be a plasmagene. 7. Lack of chromosomal location: In many organism, extensive linkage maps of nuclear genes are available. If a gene is shown to be located in one of these linkage groups, it cannot be a plasma gene. Failure to demonstrate the location of a gene in one of the linkage groups of an organism is indicative of its cytoplasmic location, but this is highly tentative. 8. Lack of association with a parasite, symbiont or virus: In many cases, a cytoplasmically inherited character is associated with a parasite, symbiont or virus present in the cytoplasm of the organism. Such cases cannot be regarded as cases of cytoplamic inheritance. Only those cytoplasmically inherited characters which are not associated with parasites, symbionts or viruses can be regarded as governed by plasma genes. The known cases of true cytoplasmic inheritance are concerned with either choloroplast or mitochondrial traits and are usually associated with their DNA. Such cases are therefore often referred to as organellar inheritance, plastid inheritance and mitochondrial inheritance. Types of cytoplasmic inheritance: The cytoplasmic inheritance is of three types: Maternal inheritance. Organellar inheritance. Inheritance involving infectious particles.
- 3. 1. Maternal inheritances: In this, generally, the character of only one of the two parents (usually female) is transmitted to the progeny. Hence such inheritance is usually referred to as extra-nuclear or extra-chromosomal or maternal or uniparental inheritance. In most animals, paternal mitochondria enter the oocyte cytoplasm after fertilization, their mt-DNA is never transmitted to the offspring. This pattern of mt- DNA inheritance is well known as “ maternal inheritance:”. Example: Shell coiling in Limnaea (a gastropods) In a freshwater snail Limnaea peregra (gastropods) the shell is spirally coiled. The coiling of the shell is two types: (1) Dextral (Clockwise) : Shell coiled to right is dextral. (2) Sinistral (Anticlockwise): Shell coiled to left is Sinistral. Both type of coilings are produced by two different types of genetically controlled cleavages namely, dextral cleavage and sinistral cleavage. In Limnaea, dextral coiling is normal and sinistral coiling is a mutant character. Direction of coiling is determined by a pair of nuclear genes, D(dextral) and d (sinistral). The gene for dextral (D) being dominant over sinistral coiling (d). Boycott and Driver (1923) showed that the character of coiling is determined by the gene of the mother and not by the individual’s own genes. Fig. Reciprocal crosses in Limnea peregra showing maternal influence on the shell coiling.
- 4. In Fig. a dextral snail provides the eggs and a sinistral snail provides the sperm. The offsprings are all dextral (Dd), in the F1 generation. When the F1 heterozygous dextral individual (Dd) were self crossed the F2 generation showed dextral coiling with genotype of 1DD, 2Dd and 1dd (Fig). When a reciprocal cross is made (Fig. ) The F1 individuals have Dd genotype but are coiled sinistrally, as in the female parent. In both the crosses the F1 are phenotypically similar to the female parent, though the offsprings in both crosses have the same genotype Dd. This is because the genotype of the maternal parent determines the phenotype of the offspring. When the F1 sinistral individuals were self crossed, the shell coiling in the F2 generation, were all dextral (Fig. right). This is because the genes do not segregate in the F2 generation. Only in the F 3 generation segregation occurs in the ratio of 3 dextral : 1 sinistral. Why does this pattern occur? The type of cleavage depends on the organization of the egg which is established before the maturation division of the oocyte nucleus and by the influence of the maternal genotype. The direction of coiling of the shell depends upon the orientation of the mitotic spindle during the first cleavage. Obviously, maternal control affects only one generation. In each generation the coiling is dependent on the maternal genotype. 2. Organellar inheritances: In this type of cytoplasmic inheritance, transmission of traits takes place through DNA of cell organelles (mitochondria or chloroplast). Thus cytoplasmic inheritance is again of two types, viz., plastid inheritance and mitochondrial inheritance. Plastid inheritance: The inheritance pattern of plastid characters due to plasma genes located in plastid is known as plastid inheritance. Plastid inheritance was first case of cytoplasmic inheritance to be discovered independently by Correns and Baur in 1908. Variegation refers to the presence of white or yellow spots of variable size on the green back ground of leaves. Variegation may be produced by some environmental factors, some nuclear genes and in some cases, plasma genes. Example: Inheritance of plastids in Mirabilis jalapa: The inheritance of plastids in Four ‘O’ clock plant Meiabilis jalapa was first described by Correns (1908). In M. Jalapa, some of the branches may have normal green leaves, while in the same plant, some other branches may have only pale green or white leaves and
- 5. still others may have variegated leaves. Flowers on branches with normal green leaves produce seeds that grow into plants with normal green leaves irrespective of whether they are pollinated by pollen from branches with normal green variegated or pale green leaves. Progeny of a variegated four ‘O’ clock plant: Type of branch from which flowers are chosen for pollination Type of branch from which pollen was obtained Type of leaf in the progeny grown from seed Green Green, variegated, pale green Only green Only green Only green Variegated Green Green, variegated, pale green Green, Variegated, or Pale Green, Variegated, or Pale Green, Variegated, or Pale Pale green Green, variegated, pale green Green, Pale green Green, Pale green Green, Pale green It is clear that variegation is determined by agencies transmitted through the female and that it is not influenced by the type of pollen used. These agencies are the chloroplast. They are capable of self-duplication and are transmitted from generation to generation through the cytoplasm of the egg. Seeds borne on a green branch have three gene only green plastids, seeds borne on a pale green branch have three gene only pale green plastids and seeds borne on a variegated branch have green or pale green or a mixture of the two types of plastids. Variegation is thus a heredity character determined by stable, self-duplicating, extra nuclear particles called plastids. Neither the nucleus of the female gamete nor the male gamete is involved in the control of this type of heredity character. Mitochondrial inheritance: The inheritance pattern of some characters is governed by plasma genes located in mitochondrial DNA is known as mitochondrial inheritance. The examples of mitochondrial inheritance includes cytoplasmic male
- 6. sterility in plants (Maize, Sorghum, cotton etc.), pokiness in Neurospora, petite in Yeast, etc. There are three type of male sterility in crop plants, viz., genetic (controlled by nuclear genes), cytoplasmic (controlled by plasma genes) and cytoplasmic genetic (controlled by both nuclear and plasmagenes). The cytoplasmic male sterility is controlled by plasmagenes associated with mt-DNA (mitochondrial DNA). 3. Inheritance involving infectious particles: In some cases, cytoplasmic inheritance is associated with infective particles like parasite, symbiont or viruses which are present in the cytoplasm of an organism. There are cases, where cytoplasmic inheritance depends on extra- chromosomal particles which are not essential for cell function and therefore, may be present or absent. Such dispensable particles are not only inherited but are also infective, since they can be introduced into new hosts without the need of actual process of reproduction. Example: Inheritance of Kappa particles in Paramecium T.H. Sonneborn (1943) described the inheritance of some cytoplasmic particles known as Kappa and their relation to ‘nuclear gene’ in Paramecium. The kappa particles are cytoplasmic symbionts occurring in some strains of the ciliated Paramecium. In Paramecium aurelia, two strains of individuals have been reported. One is called as ‘ Killer’ which secretes a toxic substance ‘ paramecin’ which is lethal to other strain called “sensitive” and is killed if it comes in contact with the ‘paramecin’. In the cytoplasm of the killer strain the kappa particles (cytoplasmic – DNA) are present kappa particles are absent in sensitive strains. The transmission of kappa particles is through cytoplasm but maintenance of kappa particles and production of paramecin is controlled by ‘K’ we assume that the killer strains carry dominant allele ‘KK; and that sensitive ‘kk’. Conjugation can occurs between the killer strain (K/K or K/k) and the sensitive strain (k/k) by placing them in fresh water free from ‘paramecin’. When a killer Paramecium “KK” conjugates with sensitive “kk”, the exconjugants are all heterozygous for Kk genes. The Kk genotype suggest that both exconjugants should be killers. But this is not seen because generally conjugation
- 7. occur less than 3 minutes than only nuclear material is exchanged and there is no exchange of cytoplasm between the two Paramecia resulting in both killers (Kk) and sensitives. However in rare case prolonged conjugation takes place and there will be mixing / exchange of cytoplasm along with nuclear material takes place between both the conjugants resulting the inheritance of kappa particle by both and ec-conjugants which produces killers strain only. This confirms that the killer trait is determind cytoplasmically. Dominant chromosomal genes are required to maintain the cytoplasmic kappa particles. Without a dominant gene this particle would disappear from the cytoplasm of the host. Fig. A. Short term conjugation: Result of cross b/w a Killer (K/K) and a sensitive strain (k/k) strain of Paramecium, when no cytoplasmic exchange takes place B. Long term conjugation: Result of cross b/w a Killer (K/K) and a sensitive strain (k/k) strain of Paramecium, when cytoplasmic exchange takes place A B
- 8. Significance of Cytoplasmic Inheritance 1. Development of cytoplasmic male sterility several crop plants like maize. Pearl millet, sorghum, cotton etc. 2. Role of mitochondria in the manifestation of heterosis. 3. Mutation of chloroplast DNA and mitochondrial DNA leads to generation of new variation. Difference between Nuclear and Cytoplasmic inheritance Nuclear Inheritance Cytoplasmic Inheritance Here egg and sperm contributed equally i.e. it is biparental. Here the contribution of egg is greater i.e. it is usually uniparental or maternal, The unit of nuclear inheritance is genes (DNA) present inside the nucleus The unit of inheritance is plasmagenes (DNA) present in cytoplasm. The result is the same in reciprocal cross except for sex linked character. Due to dependence on maternal cytoplasm the result of reciprocal cross is different. The nuclear gene are present at particular loci on chromosomes. The plasmagenes are present inside the cytoplasm in random fashion. Number of nuclear genes is fixed in every cell. Plasmagenes are indefinite in number in each cell.