Extranuclear inheritance is a non-Mendelian form of heredity that involves genetic information located in cytoplasmic organelles, such as mitochondria and chloroplasts, rather than on the chromosomes found in the cell nucleus.
Extranuclear genes, also known as cytoplasmic genes, are located in mitochondria and chloroplasts of a cell rather than in the cell’s nucleus on the chromosomes. Both egg and sperm contribute equally to the inheritance of nuclear genes, but extranuclear genes are more likely to be transmitted through the maternal line because the egg is rich in the cytoplasmic organelles where these genes are located, whereas the sperm contributes only its nucleus to the fertilized egg.
Therefore, extranuclear genes do not follow genetic pioneer Gregor Mendel’s statistical laws of segregation and recombination. Cytoplasmic genes are of interest in understanding evolution, genetic diseases, and the relationship between genetics and embryology.
Since the discovery of Mendel’s principles, research in genetics has been guided by the belief that the fundamental units of inheritance are located on chromosomes in the cell nucleus. T.H. Morgan, one of the founders of modern genetics, declared that the cytoplasm could be ignored genetically. However, some biologists resisted the concept of a “nuclear monopoly” over inheritance.
Embryologists, in particular, argued that nuclear genes, identical in every cell, could not explain how cells differentiated from one another in the course of development. They argued that differences among cells in the developing embryo must have a basis in the cytoplasm, the part of the cell outside the nucleus.
Trying to formulate a compromise, some biologists suggested that Mendelian genes play a role in determining individual characteristics, while cytoplasmic determinants are responsible for more fundamental aspects of plants and animals. The discovery of a wide variety of cytoplasmic entities seemed to support the concept that cytoplasmic factors played a role in development and heredity.
In the 1940’s, Boris Ephrussi’s work on “petite” mutants in yeast suggested that inheritance of this trait depended on some factor in the cytoplasm rather than the nucleus.
Yeast cells with the petite mutation produce abnormally small colonies when grown on a solid medium, with glucose as the energy source. Petite mutants grow slowly because they lack important membrane-bound enzymes of the respiratory system.
Similar studies have been made of slow-growing mutants of the bread mold Neurospora. Inheritance of the trait known as “poky” shows a non-Mendelian pattern. Microinjection of purified mitochondria from poky strains into normal strains has been used to demonstrate the cytoplasmic inheritance of this trait.
As early as 1909, geneticists were reporting examples of non-Mendelian inheritance in higher plants, usually green and white variegated patterns on leaves and stems.
These patterns seemed to be related to the behavior of the chloroplasts, photosynthetic organelles in green plants. Because of the relatively large size of chloroplasts, scientists have been able to study their behavior in dividing cells with the light microscope since the 1880’s.
Like mitochondria, chloroplasts contain their own deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Although chloroplast DNA (cpDNA) contains many of the genes needed for chloroplast function, chloroplasts do not seem to be totally autonomous; nuclear genes are required for some chloroplast functions.
Another interesting case of extranuclear inheritance in plants is that of cytoplasmic pollen sterility. Many species of plants seem to produce strains with cytoplasmically inherited pollen sterility.
Advances in experimental methods made in the 1960’s allowed scientists to demonstrate that organelles located in the cytoplasm contain DNA. This finding came as a great surprise to most biologists.
In 1966 the first vertebrate mitochondrial DNA (mtDNA)was isolated and characterized. Like bacterial DNA, mtDNA generally consists of a single double helix of “naked,” circular DNA. The mitochondrial genome is usually smaller than that of even the simplest bacterium.
Most of the proteins in the mitochondrion are encoded by nuclear genes, but mtDNA contains genes for mitochondrial ribosomal RNAs, transfer RNAs, and some of the proteins of the electron transport system of the inner membrane of the mitochondrion.
The DNA found in chloroplasts and mitochondria is chemically distinct from the DNA in the nucleus. Moreover, the extranuclear genetic systems behave differently from those within the nucleus.
Even more surprising is the finding that mitochondria have their own, slightly different version of the genetic code, which was previously thought to be common to all organisms, from viruses to humans. In general, because of its greatly smaller size, the DNA found in cytoplasmic organelles has a limited coding capacity.
Thus, by identifying the functions under the control of mitochondrial or chloroplast genes, all other functions carried on by the organelle can be assigned to the nuclear genome. Coordinating the contributions of the organelle and the nuclear genomes is undoubtedly a complex process.
In addition to the genes found in mitochondria and chloroplasts, extranuclear factors are found in various kinds of endosymbionts (symbiotic organisms that live within the cells of other organisms) and bacterial plasmids. Some biologists think that all organelles may have evolved from ancient symbiotic relationships. Endosymbionts may be bacteria, algae, fungi, protists, or viruses.
Unlike the mitochondria and chloroplasts, some endosymbionts seem to have retained independent genetic systems. The “killer” particles in paramecia, discovered by T.M. Sonneborn in the 1930’s, provide a historically significant example. After many years of controversy, the killer particles were identified as bacterial symbionts.
These cytoplasmic entities are not vital to the host cell, as the paramecia are capable of living and reproducing without them. Certain peculiar non-Mendelian conditions found in fruit flies also appear to be caused by endosymbionts.
Although bacteria lack nuclei, their circular DNA is usually referred to as bacterial chromosomes. Some bacteria also contain separate DNA circles smaller than the bacterial chromosome. In the 1950’s Joshua Lederberg proposed the name “plasmid” for such extrachromosomal hereditary determinants.
Some of the most interesting examples of these entities are the F (fertility) factor, the R (resistance transfer) factors, and the Col (colicin) factors. Resistance transfer factors can transmit resistance to antibiotics between bacteria of different species and genera.
Col factors, toxic proteins produced by bacteria that kill other bacteria, were studied as toxins for many years before their genetic basis was discovered. Because of their simplicity, the bacterial systems are better understood and can serve as models for the kinds of studies that should be performed for extranuclear genes in higher organisms as techniques improve.
Evolutionary Advantages and Uses
The recognition of extranuclear genetic systems raises important questions about their possible evolutionary advantage. In contrast to the remarkable universality of the nuclear genetic system, extranuclear genetic systems are quite diverse in function and mechanisms of transmission.
Although extranuclear genes control only a small fraction of the total hereditary material of the cell, in eukaryotic organisms the genes found in mitochondria and chloroplasts are clearly essential for maintaining life.
Although organelle DNAs clearly play an important part in cell organization, it has been difficult to pinpoint the essential roles of organelle DNA and protein-synthesizing systems. Many technical difficulties, and the traditionally low priority of this field, meant that adequate techniques for studying organelle genomes emerged slowly.
Studies of cytoplasmic genetics will doubtless have significant applications in medical science and agriculture as well as an impact on understanding of the evolution of genetic control mechanisms.
For example, M. M. Rhoades’s work on corn in the 1940’s forced American geneticists to take note of research on cytoplasmic genes, while plant breeders began to use cytoplasmically inherited pollen sterility in the production of hybrid seed.
Cytoplasmic pollen sterility is a useful trait to incorporate into commercial inbred lines because it ensures cross-pollination and thus simplifies seed production.
Unfortunately, a toxin-producing fungus to which the major corn cytoplasmic gene for pollen sterility was susceptible destroyed more than 50 percent of the corn crop in certain areas of the United States in 1970. This disaster prompted a return to hand-detasseling.