In this article we will discuss about the meaning of cytoplasmic inheritance.
Genes in the chromosomes have undoubtedly been proved to be responsible for the transmission of the various hereditary characters which in turn are located in the nucleus.
But in recent years there have been found some evidences which suggest that inheritance of characters occur by some self-perpetuating or replicating bodies like plastids and mitochondria which possess their own equipment for synthesizing for DNA and proteins present in the cytoplasm and not in the chromosomes present in the nucleus.
This particular type of mechanism in which cytoplasmic particles or inclusions take part in transmission of characters from generation to generation constitutes cytoplasmic inheritance.
The total self reproducing hereditary material of cytoplasm is termed as Plasmon like the genome (which refers to the total gene complement of an haploid set of chromosomes) of chromosomes and such units of cytoplasmic hereditary material are called cytoplasmic genes or plasma-genes or Plasmon’s.
The cytoplasmic hereditary units are denoted by Greek letters-alpha, beta, gamma, sigma and so on, in contrast to the chromosomal genes which are denoted by Roman letters- a, b, c, d and so on. While considering various examples of cytoplasmic inheritance it has been assumed that the cytoplasm in the sperm cell is present in very minute quantity and mostly it is in larger quantity in the egg.
So, we could expect that plasma-genes mostly will transmit only through the egg cytoplasm rather than minute sperm. Thus, plasma-genes for various sizes and characters are likely to be supposed only in the egg cytoplasm.
Extra nuclear inheritance has two distinct features:
(i) It is more or less maternal inheritance i.e., only female contribute towards inheritance and therefore.
(ii) The result of reciprocal crosses are not the same. In these features, extra-nuclear inheritance contrasts sharply from nuclear inheritance.
Since the rediscovery of Mendel’s laws in 1900, only a few geneticists believed that inheritance of characters take place through genes. Johannsen thought that genes determines only the superficial characters while fundamental characters of the organisms are transmitted through the cytoplasm.
Advances of our knowledge of heredity, however, left no doubt that genes in the chromosomes account for the specific transmission of most of the individual, racial and other characters.
Only in few cases, mostly in plants, evidences have been found that some traits or characters are transmitted through cytoplasm. Such traits are transmitted through the female line only. Each set of nuclear genes is sometimes designated as genome and all the hereditary materials transmitted through the cytoplasm is referred to as plasma-genes, cytoplasmic genes, cytogeneses, extra nuclear genes or extra chromosomal genes.
The sum total of the genes present in cytoplasm of a cell is known as Plasmon while all the genes present in a plastid are said as plastom. All available evidences indicate that plasma genes are found in the DNA present in mitochondria abbreviated as mt-DNA and in chloroplast shortened as Cp- DNA.
Evidence for cytoplasmic inheritance was first presented by Correns (1908) in Mirabilis jalapa and by Baur in Pelargonium zonale. Both of these geneticists studied the inheritance of variegation in the respective plant species.
Correns in his experiment found that only the character of female parent was observed in to the progeny. But the examples of such a role were so few that Morgan in 1926 concluded that “the cytoplasm may be ignored genetically.”
Examples of Cytoplasmic Inheritance:
1. Maternal Inheritance:
(a) In Ephestia moth:
A clear case in which individual differences are known to depend on the extra nuclear transmission has been studied in the colour of the larval skin and eye of the moth Ephestia.
The colour depends upon the presence of pigments. The synthesis of pigment is controlled by a hormone like substance known as Kynurenine. The hormone like substance is controlled by allele ‘A’ and the absence of the substance is due to the presence of allele ‘a’ in a homozygous condition.
In the progeny of cross I and II, individuals of the constitution aa, should not show any pigment but the situation is some what different. The progeny of cross I with the constitution ‘a’ develop pigments in the skin and eyes in the larval stage; while those of cross II shows no pigment in the larval stage.
So, it became very clear that the presence or absence of pigment seems to depend upon the type of mother and on the cytoplasm, which offspring got through the ovum. Having got it through the cytoplasm, it may be cited as an example of cytoplasmic inheritance.
The real explanation is different. Allele ‘A’ elaborates the hormone like substance, kynurenine and this finds place in both type of ova ‘A’ and ‘a’ at the time of oogenesis. The small amount in the ovum ‘a’ is responsible for the development of pigments in the larva with the constitution aa.
But the pigments thus developed is only temporary and is lost in the adult. Since this is not permanent and is not transferred to subsequent generations. Some authors consider that this is not example of cytoplasmic inheritance at all.
(b) In snail Limnaea:
An example of inheritance determined by female (mother) eggs is described in water snail Limnaea by Boycott Diver and Garstang on its coiling. The dextral or right coiling is represented by dominant (DD) while its recessive is sinistral or left coiling (dd).
Now the direction in which the developing egg coils is not determined by its own genes but by the genes of its mother. Suppose mother is left turning (sinistral) designated as dd, its eggs will also be left turning coils while father is right turning (dextral) DD.
When a cross occurs between sinistral female and dextral male, the eggs will develop in to sinistral females like the mother even if fertilized by sperms of dextral males. This maternal influence is probably due to presence of some cytoplasmic bodies in the mother’s egg cell. This phenomenon is called maternal determination.
2. Kappa Particles in Paramecium:
Some varieties of Paramecia produce a toxic or lethal substance called paramecin that kills other varieties. In a series of investigations, T.M. Sonneborn and others found that this characteristic involves a kind of extra-nuclear inheritance. Parmecia that produce the toxic or lethal material are called killers and those individuals that are killed are known as sensitives or normal.
The killer race contains certain bodies called as kappa bodies which are directly responsible for the production of killing substance. The normal or sensitives lack these particles. The Kappa particles pass from one generation of Paramecia to the next in the process of cell division.
The continued reproduction or existence of kappa in a Paramecium depends on the presence of the dominant gene ‘K’ in the nucleus and the sensitives have the recessive allele ‘k’.
These genes are strictly transmitted as nuclear genes and kappa particles are transmitted through the cytoplasm. When once kappa is lost, it can not be regained, unless more kappa is introduced from another cell i.e., gene ‘K’ can only maintain the kappa particles and can not initiate its production.
When killers KK conjugate with sensitives kk, the ex-conjugants are Kk and should be killers, but if the conjugation is brief and little or no cytoplasm is exchanged, then a killer clone and a sensitive clone are produced. A more prolonged conjugation results in exchange of cytoplasm and all descendent clones are killers and have kappa particles.
So it is clear that these particles are clearly vehicles of cytoplasmic transmission and it has been shown that maintenance of these particles and the killer phenotype depends upon the presence of gene K in the nucleus. Thus a ‘Kk’ individual may be either a killer or a sensitive depending on whether or not it has received kappa particles.
Certain killer clones ‘KK’ or ‘Kk’ can be converted in to sensitives by making them undergo very rapid fissions (amitosis cell division as in bacteria or prokaryotes) at high temperature, during which the reproduction of kappa fails to keep distance or pace with the cell divisions.
As a result of this, the number of kappa per Paramecium becomes fewer and fewer in the course of the cell division and eventually some paramecia are left without any kappa particles and become sensitives. Thereafter, the gene ‘K’ can not initiate the production of the particles but can again maintain them if they are introduced during conjugation.
The exact nature of kappa has not been known. Some biologists consider that kappa is an symbiont organism and others think that it might be a gene present in the cytoplasm. So at present it is impossible to decide between the two theories.
In any case, kappa is capable of specific self duplication; it is found in the cytoplasm of the cell and it is normally transmitted from generation to generation. It has also been shown that kappa particles contain DNA.
Until the true nature of kappa is definitely settled, kappa can not be considered as an evidence for cytoplasmic genes. E. Altenburg suggests that kappa was derived from Zoochlorellae and in the course of evolution had lost their chlorophyll and assumed additional properties like killing etc.
3. CO2 Sensitivity in Drosophila:
This is another example to show the cytoplasmic inheritance in animals. Normally Drosophila can be anaesthetized by CO2 and they can tolerate higher concentration of the gas without injury.
But Heritier and Teissier discovered a race of Drosophila which is much more sensitive to CO2 than are the normals and they become anaesthetized on exposure to CO2 in much less time than do the normals or sometimes they are even killed.
This feature is called CO2 sensitivity, and is normally transmitted through the egg. Since the type of the offspring entirely depends upon the types of female parents in the cross; it has been cited as an example of cytoplasmic inheritance. Heritier was also able to produce sensitivity in to normal fly either by the grafting the organs of sensitive fly in to normal fly or by injecting extracts of sensitive fly in to normal.
But recent observations have led to the conclusion that CO2 sensitivity is due to a virus. All the cells or almost of the cells of body, both somatic and germinal contain the virus, but the eggs with their larger amount of cytoplasm contain more virus particles than do the sperm cells.
Therefore, the eggs, transmit the infection much more efficiently than do the sperm. Since CO2 sensitivity depends upon a virus, this can not be considered as a case of cytoplasmic inheritance.
4. Plastid Inheritance:
The cytoplasm of plants contain minute bodies called plastids. Some of them contain chlorophyll or green pigment produced by chloroplastids. Others are colourless and produced by leucoplastids and contain starch. Plastids resemble genes in two respects (i) they are capable of multiplying by division and (ii) they are capable of mutation.
The best known cases of continuous transmission of stable elements through the cytoplasm come from the plastid inheritance of plants. Correns studied the albomaculatus type of leaf variegation in the four O’ clock plant, Mirabilis jalapa in which the normal green tissue irregularly spotted with patches of pale green or white.
Flowers on wholly green branches produce seeds which grow in to normal plants. Flower of variegated plants yield offspring of 3 kinds viz., green, pale and variegated in various proportions. Flowers from wholly white give progeny without chlorophyll. In each case the pollen has no influence on the offspring. Inheritance is wholly maternal.
The colour of the offspring (Pale, green, pale-green and variegated) is determined by the egg and what the egg produces, depends upon the characters of its plastids. Since plastids are found in the cytoplasm, we can cite this as an example of cytoplasmic inheritance. Moreover, plastids are plasma genes since they have the capacity of self duplication and mutation like the genes.
5. Male Sterility in Plants:
Besides numerous examples where the plastids have been found as definite means of extra nuclear transmission of characters, there are also many other cases where cytoplasm influences the offspring but what exactly the cytoplasm has property is still not understood.
Certain types of male sterility have been found in corn, onion etc. This feature seems to depend upon a considerable degree on cytoplasmic control, but no definite cytoplasmic elements like plastids or any other elements have been found to be responsible for this character.
This character finds expression in the total abortion of all pollen and never the ovules. The exact mode of inheritance of this character seems to be entirely through the mother.
A classical example of maternal inheritance of male sterility was discovered by MM Rhoades in Maize. He successfully replaced all the chromosomes of the male sterile plant with those of the normal and still found male sterility persisting in plants and this indicates that male sterility as a character or trait is resident in the cytoplasm and hence this may be cited as an example of cytoplasmic inheritance.
6. Merogonic Hybrids:
Merogonic hybrids are those where the egg cytoplasm of one species is fertilized by the sperm nucleus of another species. Theodor Boveri removed the nucleus from the egg of sea-urchin of one species and artificially fertilized them by sperms of a species with different characters.
The merogonic hybrids seemed to resemble the species that furnished the sperm. This demonstrates that the nucleus controlled the development even when surrounded by cytoplasm from a very different source.
The work of Hadorn, however, rules out this monopoly of the nucleus over all traits in the offspring. Two species of Triton differ clearly in the character of the epidermis. Hadorn fertilized the egg of Triton palmatus with the sperm of Triton cristatus and succeeded in removing the T. palmatus nucleus before nuclear fusion. Development proceeds up to the blastula stage and stops.
If the presumptive epidermis from the blastula is grafted on to the normally developing larva of Triton alpestris, it maintains its identity and develops into the adult stage. Here it resembles the epidermis of the parent. T. palmatus contributed the cytoplasm where as T. cristatus contributed the nucleus. This shows that at-least in this trait e.g., the nature of the epidermis is under cytoplasmic control and to this extent the nucleus does not hold monopoly.
7. Milk Factor in Mice:
In outcrossing mice from a long inbred strain in which nearly all females developed a particular type of mammary cancer. J.J Bittner (1940) found that cancer susceptibility was maternally transmitted.
Moreover, when baby mice from lines showing respectively high and low incidence of cancer were taken from their mothers at birth, and allowed to nurse on foster mothers of the opposite type, it was shown that the substance that induces cancer susceptibility is transmitted through mother’s milk i.e., mother mice from lines showing a high incidence of the disease transmit through their milk a substance that later causes mammary cancer to develop in mice that have nursed from them.
These results established a maternal inheritance dependent on a factor carried in the milk. More recent studies have shown that the factor is transmitted also by body fluids other than milk including saliva and semen. Recent evidences suggest that this disease is probably due to virus. So this offers no support for the theory that the cytoplasm contains genes.
Predetermination and Dauermodifications:
Mostly maternally transmitted characters are due to cytoplasmic inheritance. However, there are certain cases where the maternally transmitted cases are not governed by cytoplasmic inheritance. The egg cell which develops within mother’s body, its cytoplasm is well affected by the mother’s gene as well as by the substances produced within mother’s body itself.
All of these affect some of the characters of embryo. This effect of the maternal genes on the cytoplasm of egg cell is called predetermination and the characters affected are said to be predetermined. Example of this type is the direction of coiling in snails and development of cancer in the breast of female mice at maturity stage. The gene for this case is transmitted to the young ones through the mothers milk.
Occasionally phenotypic changes are produced as a result of the environmental factors or chemicals and are transmitted through the cytoplasm of the egg cell. The changes so far produced insist for a few generations but gradually comes to declinity and finally get disappeared. These are called dauermodifications. These are never gene controlled characters and disobey Mendelian traits.