Notes on Cytoplasmic Inheritance :- 1. Introduction to Cytoplasmic Inheritance 2. Characteristic Features of Cytoplasmic Inheritance 3. Classes 4. Significance of Cytoplasmic Inheritance in Plant Breeding.
Note # Introduction to Cytoplasmic Inheritance:
The inheritance of most of the characters of an individual is governed by nuclear genes. But in some cases, the inheritance is governed by cytoplasmic factors or genes. When the transmission of characters from parents to offspring is governed by cytoplasmic genes; it is known as cytoplasmic inheritance or extra nuclear inheritance or extra chromosomal inheritance or non-mendelian inheritance or organellar inheritance.
The first case of cytoplasmic inheritance was reported by Conens in 1909 in four ‘o’ clock (Mirabilis jalapa) for leaf colour. Later on, cytoplasmic inheritance was reported by various workers in various organisms.
Note #Characteristic Features of Cytoplasmic Inheritance:
Cytoplasmic inheritance differs from Mendelian inheritance in several aspects (Table 11.1) and exhibits some characteristic features.
The important characteristic features of cytoplasmic inheritance are briefly described below:
1. Reciprocal Differences:
Characters which are governed by cytoplasmic inheritance invariably exhibit marked differences in reciprocal crosses in F1, whereas in case of nuclear inheritance such differences are not observed except in case of sex linked genes.
2. Maternal Effects:
In case of cytoplasmic inheritance, distinct maternal effects are observed. This is mainly due to more contribution of cytoplasm to the zygote by female parent than male parent. Generally ovum contributes more cytoplasm to the zygote than sperm.
3. Mappability:
Nuclear genes can be easily mapped on chromosomes, but it is very difficult to map cytoplasmic genes or prepare linkage map for such genes. Now chloroplast genes in Chlamydomonas and maize, and mitochondrial genes in human and yeast have been mapped.
4. Non-Mendelian Segregation:
The mendelian inheritance exhibits typical segregation pattern. Such typical segregation is not observed in case of cytoplasmic inheritance. The segregation when occurs, is different from mendelian segregation.
5. Somatic Segregation:
Characters which are governed by cytoplasmic genes usually exhibit segregation in somatic tissues such as leaf variegation. Such segregation is very rare for nuclear genes.
6. Infection-Like Transmission:
Cytoplasmic traits in some organisms exhibit infections like transmission. They are associated with parasites, symbionts or viruses present in the cytoplasm. Such cases do not come under true cytoplasmic inheritance.
7. Governed by Plasma Genes:
The true cases of cytoplasmic inheritance are governed by chloroplast or mitochondrial DNA. In other words, plasma genes are made of cp-DNA or mt-DNA.
Note # Classes of Cytoplasmic Inheritance:
There are three different classes of cytoplasmic inheritance or non mendelian inheritance, viz., 1. maternal effects, 2. inheritance due to infective particles, and 3. cytoplasmic inheritance. These are briefly described below with examples.
1. Maternal Effects:
When the expression of a character is influenced by the genotype of female parent, it is referred to as maternal effect. Such characters exhibit clear-cut differences in F1 for reciprocal crosses. Maternal effects are known both in plants and animals. Some examples of maternal effects are briefly presented below.
(i) Coiling Pattern of Shell in Snail:
The effect of maternal genotype on the coiling behaviour in water snail was studied by Sturtevant. There are two types of coiling pattern of shell in snail (Limnaeaperegra), viz., right handed (dextral) and left handed (sinistral).
The coiling behaviour is controlled by a single gene. The dextral coiling behaviour is governed by dominant allele D and sinistral by recessive allele d. When a cross is made between dextral female and sinistral male, it produces dextral snails in F1 as well as in F2.
However, in F3 a segregation ratio of 3 dextral and 1 sinistral is observed. Similarly, when a reciprocal cross is made, i.e., sinistral as female and dextral as male, all the snails are sinistral in F1 and dextral in F2. Again in F3 a ratio of 3 dextral and 1 sinistral is observed (Fig. 11.1). This indicates that the inheritance of coiling direction in water snail depends on the genotype of female parent and not on its own genotype.
The maternal genotype affects the organization of egg cytoplasm. In other words, it affects the orientation of first cleavage plain in the zygote. If it is tilted to the left, successive cleavages will produce a spiral to the left. If it is tilted to the right a dextral pattern will follow (Suzuki and Griffiths, 1976).
(ii) Pigment in Flour Moth:
The maternal effect for inheritance of pigment in flour moth (Ephestia kuhniella) was reported by Caspari in 1936. In flour moth, production of pigment in the eye and skin is governed by single pair of dominant nuclear genes. Production of pigment is related to a substance known as kynurenine.
The production of kynurenine is controlled by a dominant gene A. This dominant gene results in darkening of the skin both in larvae and adults leading to the development of dark eyes. The recessive allele fails to produce pigment in the skin of larvae resulting in red eyes in the adult.
A cross between recessive female (aa) and heterozygous male (Aa) produced pigmented and un-pigmented larvae in F1 in the ratio of 1 : 1. However, the reciprocal cross viz., heterozygous female (Aa) x recessive male (aa) produced in F1 all larvae with pigments. But when they became adult half of them had dark eyes and half un-pigmented eyes (Fig. 11.2).
Both the larvae (Aa and aa) are pigmented because they have received cytoplasm from the pigmented female (Aa). When they become adult, the heterozygous larvae (Aa) are able to synthesize pigment due to the presence of one dominant allele and hence have dark eyes. The homozygous recessive larvae (aa) are not able to synthesize kynurenine which produces pigment and hence lose pigment when they become adult.
2. Inheritance Involving Infective 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. However, such cases are not considered as true examples of cytoplasmic inheritance.
Some examples of this type are given below:
(i) Kappa Particles in Paramecium:
There are two types of strains in Paramecium. One has kappa particles in its cytoplasm and other does not have such particles. The presence of kappa particles in the cytoplasm leads to production of a toxin known as paramecin. This toxin can kill the strain of Paramecium which lacks kappa particle. Thus, the strain with kappa particle is known as killer strain and that without kappa particle is called as sensitive strain.
Multiplication of kappa particles in the cytoplasm takes place by fission. However, their multiplication is governed by a dominant nuclear gene (K). They can multiply in the homozygous dominant (KK) or heterozygous (Kk) individuals.
Kappa particles cannot multiply in recessive (kk) individuals. Even if kappa particles are introduced into kk strains, they will gradually disappear due to their inability to multiply and the strain will become sensitive.
Though the multiplication of kappa particles is dependent on nuclear genes, their action is independent of nuclear gene. The inheritance of kappa particles can be studied by conjugation between killer and sensitive strains.
The conjugation may be of two types, viz:
a. Short duration conjugation and
b. Long duration conjugation.
The consequences of such conjugations are given below:
(a) Short Duration Conjugation:
Short duration conjugation leads to exchange of nuclear genes between the killer and sensitive strains. Exchange of cytoplasm does not take place in such conjugation. Thus, the ex-conjugants (resultant strains) will be heterozygous (Kk) for killer gene.
However, the strain with killer cytoplasm produces killer (KK) and sensitive (kk) strains by further division, whereas the sensitive stain produces only sensitive strains (kk) by further division. This clearly indicates that the killer character is not governed by nuclear gene.
(b) Long Duration Conjugation:
Such conjugation between killer and sensitive strains leads to exchange of both nuclear genes as well as cytoplasm. Here both the ex-conjugants are heterozygous (Kk) but killer. Autogamy of both the ex-conjugants produces killer and sensitive strains in 1 : 1 ratio. This has demonstrated that kappa particles have cytoplasmic inheritance (Fig. 11.3).
(ii) Sigma Particle in Drosophila:
In Drosophila, two types of flies are found with regard to C02 sensitivity. Some flies are sensitive to C02 exposure and some are fairly resistant. The sensitive flies become immobile when they are exposed to C02 concentration and may even die sometimes.
A cross between sensitive female and normal male produced all sensitive individuals in F1. The reciprocal cross (normal female x sensitive male) produced all normal offspring in F1 (Fig. 11.4). This suggested that CO2 sensitivity is inherited through the cytoplasm. Investigation using electron microscopes have demonstrated that a virus like particle called sigma is responsible for sensitivity to CO2 in Drosophila.
3. Cytoplasmic Inheritance:
The true cytoplasmic inheritance is one which involves plastids (chloroplasts) and mitochondria. Thus, cytoplasmic inheritance is again of two types, viz., 1. plastid inheritance and 2. mitochondrial inheritance. The former is applicable to plants only because plastids are found only in plants. The mitochondrial inheritance is common for both plants and animals.
The cytoplasmic inheritance is governed by genes which are found in chloroplasts and mitochondria. The genes which govern cytoplasmic inheritance are called plasma genes or cytoplasmic genes or cytogenes or extra nuclear genes. These genes are made of DNA found in chloroplasts (cp-DNA) and mitochondria (mt- DNA).
The important features of cytoplasmic DNA in comparison to nuclear DNA are briefly described below:
1. The cytoplasmic DNA is of two types, viz., chloroplast DNA (cp-DNA) and mitochondrial DNA (mt-DNA).
2. The cytoplasmic DNA is usually circular molecule except in ciliate protozoa where it is linear molecule. Nuclear DNA on the other hand is usually linear in eukaryotes and circular in prokaryotes.
3. The synthesis of chloroplast DNA and mitochondrial DNA continues throughout the cell cycle, whereas synthesis of nuclear DNA occurs only during interphase (S) of cell cycle.
4. Cytoplasmic DNA replicates both in chloroplasts and mitochondria in a semiconservative fashion, whereas nuclear DNA replicates in the chromosomes in semiconservative manner.
5. Both cytoplasmic and nuclear DNAs are liable to mutation. Mutation of cytoplasmic DNA leads to change in the behaviour of plasma genes and mutation of nuclear DNA leads to alteration in nuclear genes.
6. Cytoplasmic DNA is capable of coding for RNA and protein synthesis like nuclear DNA.
7. In vitro synthesis of cytoplasmic DNA is possible in the respective organelle. In vitro synthesis of nuclear DNA is also possible.
8. The synthesis of cytoplasmic DNA is blocked when treated with ethidium bromide, acriflavin and actinomycin-D. Similar is the case with nuclear DNA. Thus, cytoplasmic DNA differs from nuclear DNA in four main aspects (Table 11.2).
1. Plastid Inheritance:
Chloroplasts are the important plastids. Plastids have green pigments called chloroplasts. Plastids self-duplicate, have some amount of DNA and play an important role in cytoplasmic inheritance.
Some examples of plastid inheritance are given below:
(i) Mirabilis jalapa:
The first conclusive evidence of cytoplasmic inheritance was reported by Correns in 1909 for leaf colour in four ‘o’ clock plant (Mirabilis jalapa). This plant has three types of leaves, viz., green, white and variegated. Three types of results were obtained from crosses between these genotypes as given below.
1. When green was used as female and either green, white or variegated as male, all individuals in F1 were green.
2. When white was used as female and either green, white or variegated as male, all individuals in F1 were white.
3. When variegated was used as female and either green, white or variegated as male, various proportions of green, white and variegated individuals were obtained in F1 (Table 11.3).
The inheritance is governed by chloroplasts which are originated from proplastids. If the proplastids are normal, they will develop into normal chloroplasts and when proplastids are mutants, they will produce white chloroplasts. This suggests that green leaf branches have normal chloroplasts; white branches have mutant chloroplasts and variegated have a mixture of both normal and mutant chloroplasts.
Since cytoplasm is contributed to the zygote mainly by female parent, the plastids are transmitted to the zygote from the female parent. These plastids are responsible for variation in the crosses of green, white and variegated leaves.
(ii) Plastid inheritance in Oenothera:
Plastid inheritance in evening primrose (Oenothera) was reported by Renner. In some species of Oenothera, entire chromosome set of either pistillate or pollen parent of an individual is transmitted to the gametes. Thus gametes have either all chromosomes of female parent or of male parent. Such inheritance of chromosomes in block is due to complex series of reciprocal translocations.
Such inheritance is exhibited by the cross between two species, viz., Oenotheramuricata and O. hookeri. Cross between O. muricata female and O. hookeri male produced normal green plants in F1.
However, the reciprocal cross (O. hookeri female x O. muricata male) resulted in yellow plants in F1 which were unable to survive. This suggests that muricata plastids can develop normally in the presence of hookeri nucleus, but hookeri plastids cannot develop in the presence of muricata nucleus.
Sometimes, hybrid between O. muricata and O. hookeri exhibits variegation in course of time like reciprocal cross. These yellow patches in later stage were explained due to the presence of hookeri plastids. Renner assumed that some plastids of hookeri pollen are transmitted to the hybrid. After multiplication they resulted in somatic segregation of plastids in the later stages of hybrid producing yellow sectors.
(iii) Iojap in Maize:
In maize, three types of leaves are found, viz., green, iojap (green and white stripes) and white. Crosses between green female and iojap male produced all green individuals in F, and a single gene segregation ratio, i.e., 3 green and 1 iojap in F2. However, the reciprocal cross (iojap female x green male) produced individuals with all the three phenotypes, viz., green, white and striped in F1 (Fig. 11.5).
The iojap phenotype is governed by plastids. The green leaves have normal plastids; white leaves have mutant plastids and striped leaves have mixture of normal and mutant plastids. In a cross between iojap female and green male, there are three types of plastids, viz., green, white or both in the egg cell.
Depending upon the presence of these three types of plastids in the egg cell, a cross between iojap and green will produce three types of individuals, viz., green, white and striped in F1 because male parent does not contribute cytoplasm and thereby plastids to the zygote.
(iv) Zebrina:
Zebrina is common house plant known as wandering Jew, This plant exhibits green and white sectors in the leaves (Fig. 11.6). These sectors are due to the presence or absence of mutant chloroplasts. These mutants do not have green portion to express in the chloroplast. A flower from white sector will produces seeds which will produce plants with white leaves only.
But such plants die soon as the food supply in their cotyledons is exhausted after sometimes. The beautiful strips of Zebrina are produced due to mitotic division along the length of the leaf. Chloroplasts have their own DNA and it is assumed that cytoplasmic mutants in Zebrina result due to alteration in chloroplast DNA.
2. Mitochondrial Inheritance:
The inheritance of some characters is governed by mitochondrial DNA. The examples of mitochondrial inheritance include cytoplasmic male sterility in plants, pokyness in Neurospora, petite in yeast, etc.
(i) Cytoplasmic Male Sterility:
There are three types 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 plasma genes). The cytoplasmic male sterility is controlled by plasma genes associated with mtDNA or cpDNA.
The CMS lines are maintained by crossing them with a fertile line known as maintainer line. Three types of CMS lines, viz., CMS-T, CMS-C, and CMS-S have been studied in maize. It is believed that cytoplasmic male sterility is controlled by plasma genes which are part of mt-DNA.
In other words, in maize cytoplasmic sterility is governed by mitochondrial DNA. Cytoplasmic sterility is found in several other crop plants, viz., pearl millet, Sorghum, cotton, etc.
(ii) Pokyness in Neurospora:
Neurospora, a bread mould, has two strains, viz., wild and poky. The wild strain has normal growth and the mutant has very slow growth. The mutant strain is called poky. A cross between poky female and normal male produced poky progeny. In the reciprocal cross (normal female x poky male), all the progeny were normal (Table 11.4).
This suggests presence of cytoplasmic inheritance, because the only difference between the reciprocal crosses was in the contribution of cytoplasm. In Neurospora, female is the main contributor of cytoplasm.
Male gametes contribute very little cytoplasm as in animals and higher plants. The difference between reciprocal crosses indicated that pokyness is controlled by cytoplasm. In other words, the nuclear genotype has no effect on the phenotype.
Later on several other cytoplasmic genotypes were discovered in fungi and many of them were very bad growers or slow growing. In such mutants mitochondrial cytochrome contents were considered altered. This indicated involvement of mitochondrial DNA in such mutants.
The transmission of hereditary characters such as poky through only one parent is called uniparental transmission. It is a good criteria for identification of cytoplasmic inheritance in fungi.
(iii) Petite in Yeast:
Petite yeast or little yeast is Ascomycetes. Petite is a mutant from normal condition and is very small or little in size. This character is believed to be controlled by cytoplasmic factor. Petite mutant has defective cytoplasmic factor and is unable to grow on glucose.
Petite allele lacks cytochrome a and b and enzyme cytochrome oxidase. The mt-DNA in petite has altered composition. This suggests that petite character is controlled by plasma genes contained in mt- DNA.
(iv) Chlamydomonas:
This is a unicellular green alga. In this plant, many inherited chloroplast markers are available. Many of them are drug resistant alleles. Ruth Sager has demonstrated recombination between them and found a circular map.
Note # Significance of Cytoplasmic Inheritance in Plant Breeding:
1. Cytoplasmic inheritance has been useful in explaining the role of various cytoplasmic organelles in the transmission of characters in different organisms.
2. Studies of cytoplasmic inheritance have played key role in mapping of chloroplast and mitochondrial genome in several species, viz., yeasts, clamydomonas, maize, human, etc.
3. Development of cytoplasmic male sterility. CMS lines have been developed in several crops like maize, pearl millet, Sorghum, cotton, etc. Availability of CMS lines has facilitated the production of hybrid seed in these crops at a cheaper cost than with hand emasculation and pollination method.
The CMS cytoplasm can be easily transferred to various agronomic bases for their use in the development of superior hybrids. Since CMS based hybrids have danger of uniformity, it is desirable to utilize various CMS sources.
4. Role of mitochondria in the manifestation of heterosis is gaining increasing importance these days.
5. Mutation of chloroplast DNA and mitochondrial DNA leads to generation of new variants. Some of such variants are of special significance especially in ornamental plants.