The branches are of three types deep green, pale green and variegated. 3. The flowers are of various colors on the same plant in different branches they are pink, yellow, or white with pink dots. The mechanism of inheritance in this plant was worked out by Correns, who opined that the flower and branch color inheritance is not through the nucleus, but through the plastids. The phenotype of the progeny depends from which part of the plant the seed is obtained. If a seed is obtained from a flower on a green branch, the progeny would always produce a green branch. Again a seed obtained from the same plant, but from a variegated branch would produce offspring with variegatci green and pale branches.
The same is true for flowers also. In these inheritances, the source of the pollen does not seem to have any effect on the phenotype of the progeny. The mechanism of inheritance is as follows- The color of the branch and flowers depend on specific plastids. The female gamete from a particular source has one or more types of plastids. The pollen carries no plastids at all as it has a limited cytoplasmic boundary. When the zygote divides, the plastids are distributed irregularly.
Some ‘cells may get plastids for green; some may get plastids for pink etc. These cells develop into branches, leaves flowers etc. Hence the color of the branch and flower depends on what types of plastids they have received during development.
2. Plastid inheritance in maize:
In certain maize strains, the leaves have peculiar type of striping of green and white on the leaves. Rhodes made a study of these characters and came to the conclusion, that the (striped) character is controlled by a gene located on chromosome number. The dominant gene would produce green plastids, and its recessive allele would produce the striping, is green and is striped. The plants would carry both green as well as abnormal white plastids in their cytoplasm, while plants would carry only the green plastids.
In a cross between and normal plant, if parent is used as female, the female gametes carry either only green plastids, only white plastids or both green and white plastids. Consequently when is the female parent and is the male parent, the offspring could be green, variegated or white. The White seedlings die. If a striped offspring from the F, is used as the female parent and the as the male parent and a cross carried out, two genotypes are obtained.
But phenotypically they are of three types – green, white or striped, i.e. each one has all the three .phenotypic expressions. The above experiments clearly show, that the iota character, once inherited through the female parent maintains itself in the cytoplasm, irrespective of the genotype of the plant. Hence the striped phenotype may have any one of the genotypes –
3. Male sterility in plants:
Sterility in male may be influenced by nuclear genes, cytoplasmic genes or by both cytoplasmic and nuclear genes acting in co-ordination. We shall briefly discuss all these three.
4. Male sterility by nuclear genes:
A single dominant gene on the chromosome controls the fertility, while its recessive allele brings about sterility. In a cross between the two, the F, individuals are heterozygous fertile. Selfing of the F1 individuals to obtain F2 shows a typical 3:1 segregation of fertile and sterile plants.
Cytoplasmic male sterility:
In many of the crop plants notably maize, pollen grains degenerate even though the ovules are normal and fertile. Marcus Rhodes made a detailed study of the male sterility in maize. He conducted breeding experiments extensively and was successful in replacing all the ten chromosomes of a male sterile strain with those of a male fertile strain. In spite of this replacement of all the chromosomes, the progeny of this plant continued to be male sterile. This indicated that the nuclear genes did not play any role in determining the male sterility.
Since male sterility was more when male sterile plant was used as the female parent in any cross, it is obvious, that the male sterility factor must be present in cytoplasm. A reciprocal cross in which the male sterile plant was used as the male parent and the male fertile pant as the female parent proved this tact (Although it may seem a paradox to use a male sterile plant as the male parent, some pollen grains are fertile even in a male sterile plant).
6. Cytoplasmic and nuclear male sterility:
In certain instances cytoplasmic induced male sterility may be reversed by a fertility restorer gene in the nucleus.
For example, if the female parent in a cross is male sterile, then the genotype (nuclear) of the male parent will decide the phenotype of the F, progeny. The male sterile female parent will have the recessive genes (rr) while the restorerone is dominant (R). If the male parent is homozygous dominant, the F, progeny Will be heterozygous (Rr) and fertile. On the other hand if the male parent is rr all the F, progeny will be also male sterile. A test cross between F, hybrid (Rr) and double recessive male parent will produce 50% male sterile and 50% fertile plants.
7. Cytoplasmic inheritance in Chlamydomonas:
Chlamydomonas is a fresh water green alga belonging to the order Volvocales. Genetic investigations in Chylamydomonas have shown the existence of three independent and operationally distinct genetic systems (Hudock and Rosen, 1976).
These are 1. A Mendelian system comprising 16 linkage groups and residing in the nucleus. 2 A non Mendelian uniparental transmission of genes through the mating type plus (mf) parent which is traccable to DNA in the chloroplast. 3. A non Mendelian biparental trasmission, the genes for which are thought to reside in the – mitochondrial DNA (Alexander et al, 1974). We will study the second category of inheritance which is cytoplasmic in nature. The pioneer in the field of plasma gene studies in Chylamydomonas is Ruth Sager (1954, 1964, and 1970) who has studied the inheritance of streptomycin resistance in C. reinhardtii.
Seven gene pairs have been identified on the chloroplast genome. Ruth Sager (1972) has proposed a circular model for the oroplast DNA Cytoplasmic inheritance in Chlamyomonas reinhardtii is led using mutations for various chemicals; such as ac (acetate requiringmutant), Sniz (streptomycin resistant), very (erythromycin resistant), time (temperature sensitive) etc. In C. rcinhardtii, sexual reproduction is isogamous, hence both the gametes are supposed to contribute equal amount of cytoplasm and as such it is difficult to assess the maternal origin for any cytoplasmic factor. In higher organisms, with oogamous reproduction, the female gamete always contributes more cytoplasm, and the male gamete very little or none.
Ruth Sager has identified two races of C. reinhardtii, which she designated as mf and mf. Of these, it was found out by radiolabelling that only the mf race can donate its chloroplast genome to the zygote and not the mf race. On the basis of this even though sexual reproduction is isogamous, the mf race was called maternal and race paternal. Hence chloroplast genes of only the mf race are inherited.
Petite character inheritance in yeasts:
Petite character in Yeasts refers to slow growth of mutant’s in. aerobic medium. The petite yeasts develop into small colonies when compared with the normal one, if the medium is oxygenated. The petite yeasts have a defective aerobic respiratory mechanism. Some of the characters of petite yeasts are as follows a) they are insensitive to cyanide poisoning (affecting the aerobic path way) b) Absence of cytochrome and cytochrome a-?) Incompletely formed mitochondria) non stain ability of petite mitochondria with Janus green etc. The genetic basis of petite character is a cytoplasmic factor, which is either defective or absent in petite yeasts. Thus in petite yeasts p is p or p’ (defective or suppressive).
They’ cytoplasmic factor is known to be transmitted through the mitochondrial DNA. Evidence for petite factor to be linked with mitochondrial DNA comes from the fact that ethidium bromide, inducing petite mutations disturbs the DNA. Mit DNA with suppressive petite factor has alteration in their base sequence when compared with non petite mit DNA (N. W. Gilham, 1974).
9. Kappa particles in paramecium:
There are two strains in Paramecium, the killer strain and the sensitive strain. T.M. Sonnebcrn, USA has conducted extensive studies on these strains in P.
aurelia. The killer strains have certain bodies in cytoplasm called kappa particles. These kappaparticles are composed of DNA. The killer strains produce a substance called paramecin, which is toxic to the sensitive strains. When this substance is released into the medium, the sensitive strains die. But the paramecin has no effect on the killer strain even though they are produced by it.
The killing capacity of the killer strain is influenced by temperature, food and repeated cell divisor. Low temperature, inadequate nutrition and frequent cell divisions would reduce the killing ability. This is due to the fact, that with rapid cell division, the kappa particles do not multiply at the same rate, and the daughter cells in subsequent generations receive less and less kappa particles, hence reduction of killing ability It has been estimated that at least 400 kappa particles are necessary to maintain the killing ability kappa particles are self reproducing, but their multiplication depends on the presence of the dominant gene K. But there is a peculiar relationship between the gene and the kappa particle.
K gene is necessary for the multiplication of the kappa particles, but if the cytoplasm lacks the kappa particles, K gene cannot produce them de now. Hence even though the genotype may be KK, the strain will not be a killer. Similarly, in the absence of K, kappa particles even if they are present in cytoplasm will not reproduce and gradually their number diminishes with every cell division. The genotypes of killer and sensitive strains are as follows. By repeated culturing, it is possible to induce a rapid cell division in the killer strains with either KK or Kk genotypes.
The kappa particles cannot keep pace with this division and consequently after several generations, the progeny would be without kappa particles, even though they are genotypically KK or Kk and their parents were killers. These progenies (KK or Kk) would now be sensitive strains. This is not possible if the killer character were to be located on the nuclear gene. For, with every cell division, nucleus also divides and even after repeated divisions, the killer strain should have retained its killer gene if it is in the nucleus. When the killer (KK) strain with kappa particles is placed along with the sensitive strain (kk) on a fresh medium without paramecin, conjugation occurs and the genotype of the exconjugants (products of conjugation) will be Kk. The phenotype (ie. killer or sensitive) however will depend on the duration for which conjugation takes place. If the conjugation is over in a short period of time the exconjugants will be killers and sensitive strains like their parents.
Killer strain will remain killer and sensitive strain will remain sensitive even after conjugation. This is not possible if the killer factor were to be located on the nucleus. All the heterozygotes (Kk) of a cross (KK and kk) should have been killers.
But if the conjugation persists for a long time, the sensitive exconjugant will now turn a killer for, with longer conjugation it would have received cytoplasm and together with it kappa particles also from the killer strain Recent discoveries have indicated that kappa particles could be parasites permanently housed in the cell of Paramecium.
10. Coiling of shells in Snails:
The coiling pattern in snails (Limnaeaperegra) has been studied in detail with reference to cytoplasmic inheritance (Boycott, Diver and Garstang). Two types of coiling have been observed in this species during the development.
In some strains the coiling is dextral (coiling is to the right) and in some it is sinisterly (coiling is to the left). The direction of the coiling is supposed to be controlled by the gene D for right hand coiling and its recessive allele d controls left hand coiling. But in crosses involving the dextral and sinistral snails, the inheritance pattern does not always follow the genotype. When dextral females are mated to sinistral males all the F offspring is dextral. In the F2 generation however the expected ratio of 3:1 for dextral and sinistral is not obtained. All the individuals -are dextral and there is no recessive trait. The phdiotypes of all the progeny resemble that of the mother. When the double recessive (dd) individuals are inbred they are all sinistral.
On the other hand when DD (dextral) was crossed with Dd (dextral), there appears to be no aggregation and all the offspring are dextral. When a sinistral (recessive) female is crossed to a dextral (dominant) male, the offspring are all like the mother – sinistral. Investigations have shown that, the coiling is influenced by the orientation the spindles formed during the cleavage of first metaphase. The spindle is S, fated to the right in dextral and to the left in sinistral. This is controlled by the maternal genes (not nuclear) and the paternal genes do not seem to have any influence.
11. Carbon dioxide sensitivity in drosophila:
Heritier and Teissier have described a strain of Drosophila, which is very sensitive to carbon dioxide than others.
Such strains get immobilized on exposure to a high concentration of carbon dioxide. By breeding experiments, Heriteir and Teissier obtained a true breeding strain for carbon sensitivity and established the trait to be heritable. They further proved that this heritable trait does not follow the chromosomal inheritance.
When sensitive female strains were crossed to normal male strains, all the progeny were sensitive. However one can assume this to be the dominance of sensitivity over normal. But a reciprocal cross soon disproved this.
When a sensitive male was crossed with a normal female, all the progeny were normal. This indicated that, sensitivity trait is maternal in transmission.
Merogenic hybrids in Urodeles:
Merogenic hybrids are obtained from a cross between two different species in which, the individuals have the cytoplasm of the egg, but the nucleus of the sperm. Obviously before fusion, the egg nucleus is removed, hence the zygote will be haploid, but has the cytoplasm of both egg and sperm. Hadom, conducted his experiments with two species of urodeles Triton palmatus and T.
cristatus which differed in the nature of the epidermal cells. The egg of T. palmatus was fertilized by the sperm of T. cristatus. The zygote was haploid as it had only the sperm nucleus and developed up to the blastula stage. When a portion of the tissue (which would develop into epidermis) from this embryo was grafted on to the larva of another species. The tissue developed into epidermis characteristic of T.
palmatus, which contributed cytoplasm, through the egg. There was no resemblance to the epidermis of T.cristatus, which contributed the nucleus.
13. Cytoplasmic inheritance in ephestia:
Ephestia is the common flour moth that occurs in old and stale flour. In one of the species, E.
kuhniella a clear- instance of maternal inheritance has been observed. The dark and the light skin colour are known to be transmitted through the cytoplasm of the mother. The dark skin pigmentation is known to be governed by the gene A, while its recessive trait a brings about light skin character homozygous AA individuals are dark while homozygous aa individuals are light. In a cross between, aa females with males, half of the progeny are dark and the other half are light skinned. In a reciprocal cross (with Aa female and aa males) however all the progeny irrespective of the genotype are dark, because they receive the precursors of the pigment Kynurenine from the egg cytoplasm. However, when the larvae molt, the progeny without the gene A would gradually become pigmentless, as the gene (A) is necessary for pigmentation. Transplantation experiments of ovary have confirmed that a diffusible substance from ovary influences pigmentation. The following chart gives the results of reciprocal crosses.
14. Other instances of cytoplasmic transmission:
Besides the above mentioned examples, which are studied in detail there are also a number of less known instances of cytoplasmic transmission of characters. Epilobium is a member of the family Onagraceae and has been studied in some detail by Lehmann, Renner, Michelis and others. There are two species Epilobium, E.
hirsutum and E.roseum, which differ in some morphological characters. Reciprocal crosses between these two species produces some interesting results. In a cross where E.hirsutum is the female parent and roseum, the male parent. The progeny are nearly sterile with reduced anther size in a reciprocal cross, where the E.hirsutum is the male parent and E.
roseum, the female parent, the offspring are normal with little or no sterility. Michelis produced a-progeny [by repeated back crossing of the F, female plant E.roseum (female) x E.hirsutum (male)] with E.hirsutum males, where the cytoplasm was derived from E. roseum, while all the chromosomes belonged to E.
hirsutum. When this type was crossed with a typical E. hirsutum, the results were same as those obtained between a pure roseum and hirsutum clearly indicating that the differences (sterility etc) are clearly cytoplasmic in origin. Lehmann explained this due to the changed behavior of the nucleus in a changed environment A similar case of maternal cytoplasmic inheritance has been reported by Wettstein in Funaria (a genus of mosses).
Reciprocal crosses between F.hygrometrica and F. mediterranea proved that many of the traits followed the maternal line i.e. Repeated the characters of the female parent.
The above instances of cytoplasmic inheritance will clearly indicate that while in a large majority of the cases, the nuclear genes no doubt play a predominant role in inheritance, a cytoplasmic environment is very essential for its proper functioning. Sometimes cytoplasmic factors may even mask or alter the effect of the nuclear genes.