The following points highlight the top four examples of cytoplasmic inheritance in eukaryotes. The examples are: 1. Sigma Virus of Drosophila 2. Milk Factor in Mouse 3. Maternal Sex Ratio Condition in Drosophila 4. Cytoplasmic Male Sterility (CMS).
Cytoplasmic Inheritance: Example # 1. Sigma Virus of Drosophila:
Normally, Drosophila is anesthetized by CO2 without any after effect. But certain strains of this fly are very sensitive and become permanently paralyzed by CO2. Reciprocal crosses between CO2 sensitive and normal strains of Drosophila reveal sensitivity to be transmitted maternally.
Further, when an extract is prepared by crushing the CO2 sensitive flies, and this extract is injected into the normal flies, they also become CO2 sensitive. However, the sensitivity induced by this method is not stabilized. The CO2 sensitivity has been shown to be caused by a cytoplasmic particle called “sigma” factor, which is an RNA virus.
Cytoplasmic Inheritance: Example # 2. Milk Factor in Mouse:
In mouse, some lines were found to develop breast cancer around the age of 18 months. Through careful studies, it was shown that the transmission of breast cancer occurred through milk. When the litters from cancerous females were fed, from just after their birth, by normal foster mothers, they did not develop any cancer.
The cytoplasmic factor responsible for the breast cancer was discovered by Bittner in 1938. This factor has been identified as a virus called “mouse mammary cancer virus” (MTV). The presence of MTV even in the gametes of mice was demonstrated by Bentivelzen and coworkers in 1970.
Cytoplasmic Inheritance: Example # 3. Maternal Sex Ratio Condition in Drosophila:
In certain strains of many species of Drosophila, e.g., robusta, willistonii, paulistorum, nebulosa and bifasciata, there occurs a greater proportion of females in the progeny. In these strains, the zygotes or early embryos developing into males die and only daughters are produced.
Such strains are called sex ratio (SR) strains, and the condition shows maternal/cytoplasmic inheritance. In most such cases, the sex ratio effect is related with the presence of a “mycoplasma” that selectively kills the male zygotes. Mycoplasma are very small bacteria-like organisms that do not possess a cell wall, but have circular DNA molecules as their genetic material.
Cytoplasmic Inheritance: Example # 4. Cytoplasmic Male Sterility (CMS):
Male sterility in plants is characterized by nonfunctional pollen grains. It may be (i) genetic, (ii) cytoplasmic or (iii) cytoplasmic-genetic in determination. The genetic male sterility is generally governed by a single recessive nuclear gene. Cytoplasmic male sterility (CMS), on the other hand, is determined by the cytoplasm.
Since, the cytoplasm of a zygote comes mainly from the egg, the progeny of a cytoplasmic male sterile (CMS) plant is always male sterile (Fig. 19.3). Cytoplasmic-genetic male sterility is governed by both the cytoplasmic and nuclear genes. In such case, usually a single dominant nuclear gene restores male fertility of the cytoplasmic male sterile line; therefore, the gene is called restorer gene. (Fig. 19.3).
Cytoplasmic-genetic male sterility is widely used for the production of hybrid seeds in several crops such as, maize, Sorghum, Pennisetum etc.
The possible sources of male sterile cytoplasm are:
(i) Spontaneous mutation,
(ii) Inter-generic and interspecific hybridization and
(iii) Mutagenesis.
In 1970, Edwardson stated that reports of cytoplasmic male sterility were available in 6 families, 25 genera and 80 species of plants. CMS shows a typical cytoplasmic inheritance. Such plants are male sterile but they produce seeds after pollination with a fertile pollen.
CMS can be transferred easily to a given strain by using the concerned strain as a pollinator in 6-8 repeated backcrosses with the CMS line; as a consequence, the genotype of the CMS line becomes virtually similar to that of the pollinator strain (Fig. 19.4). This pollinator strain is used to maintain the new CMS line, therefore, it is called the maintainer line.
CMS has been the most widely investigated in maize; it was first discovered by Rhoades in 1933. In maize, three distinct types of CMS are known; they are designated as CMS-C (Charrua), CMS-S (USDA) and CMS-T (Texas). Inbred lines showing male sterility when placed in the S-cytoplasm do not necessarily become male sterile in the T-cytoplasm.
Causal Factors of CMS:
CMS is produced by cytoplasmic genes. Relationship between plasmid-like DNA present in mitochondria and CMS has been reported in many plant species.
CMS genes have been studied in several plant species with the following conclusions:
(1) They are a part of the mitochondrial DNA (mtDNA), e.g., in maize wheat, Sorghum, Petunia, sugar beet, tobacco, soybean, sunflower.
(2) They are a part of chloroplast DNA (cpDNA), e.g., in tobacco, cotton, maize, barley, Brassica napus. It is generally regarded that such cases arose due to movement of DNA from mitochondria into the chloroplasts.
(3) They are located in plasmid-like elements mainly present in mitochondria, for example, in maize, sugar beet, radish wheat, broad bean, Sorghum, Teosinte, Petunia, B. napus, B. campestris, Oenothera, lucerne.
(4) In some cases, CMS may be due to virus-like agents (double-stranded RNA virioides), e.g., in broad bean, Vigna unguiculata (compea).
In maize, the three cytoplasms, viz., CMS-T, CMS-S and CMS-C are characterized by the presence of specific nuclear genes called “Rf” genes which restore fertility (Table 19.1). These restorer genes are: Rfl, Rf2, (for CMS-T), RJ3 (for CMS-S) and Rf4 (for CMS-C). The genes Rf1, Rf2 and Rf4 act at the sporophytic level, while the gene Rf3 acts at the gametophytic level.
The degree of pollen fertility is influenced both by the environment as well as the genetic background (modifying genes). There is a high frequency of reversion to male fertility in the case of CMS- S. This instability of CMS is associated with the two plasmid-like DNA elements, S1 and S2, discovered by Pring and coworkers in 1977.
These plasmids are linear DNA elements of 6.2 kb (S1) and 5.2 kb (S2). The S1 and S2 plasmids are absent from the mitochondria of the male fertile revertants of CMS-S. Mitochondria of CMS-C contain two small circular DNA elements of 1.55 and 1.42 kb. But CMS-T lacks plasmid-like elements ordinarily found in the mitochondria of the normal maize cytoplasm (N-cytoplasm).
Further, some specific polypeptides (not found in the normal fertile lines) are produced in the mitochondria of CMS lines, while certain polypeptides found in the normal lines are absent in the CMS lines (Table 19.1). The plasmid-like DNA elements may carry genes for CMS in many plants, e.g., Sorghum, sugar-beet, Vicia faha etc.
There is a possibility that the CMS genes of higher plants may be mobile (transposable) in nature and they may move to chloroplast DNA or even to the nuclear DNA.
In 1984, Sehardl and coworkers demonstrated that the S1 and S2 plasmids found in CMS- S behave like episomes and become integrated into the mitochondrial chromosomes; the copies of S1 and S2 episomes are located at the ends of these linear molecules. The S1 and S2 episomes are not only absent from the male fertile revertants of CMS-S but their mtDNA also becomes circular.