Extranuclear inheritance or cytoplasmic inheritance is the transmission of genes that occur outside the nucleus. It is found in most eukaryotes and is commonly known to occur in cytoplasmic organelles such as mitochondria and chloroplasts or from cellular parasites like viruses or bacteria.
Mitochondria are organelles which function to transform energy as a result of cellular respiration. Chloroplasts are organelles which function to produce sugars via photosynthesis in plants and algae. The genes located in mitochondria and chloroplasts are very important for proper cellular function, yet the genomes replicate independently of the DNA located in the nucleus, which is typically arranged in chromosomes that only replicate one time preceding cellular division. The extranuclear genomes of mitochondria and chloroplasts however replicate independently of cell division. They replicate in response to a cell's increasing energy needs which adjust during that cell's lifespan. Since they replicate independently, genomic recombination of these genomes is rarely found in offspring, contrary to nuclear genomes in which recombination is common. Mitochondrial diseases are inherited from the mother, not from the father: mitochondria with their mitochondrial DNA from the mother's egg cell are incorporated into the zygote and passed to daughter cells, whereas those from the sperm are not.
Extranuclear transmission of viral genomes and symbiotic bacteria is also possible. An example of viral genome transmission is perinatal transmission. This occurs from mother to fetus during the perinatal period, which begins before birth and ends about 1 month after birth. During this time viral material may be passed from mother to child in the bloodstream or breastmilk. This is of particular concern with mothers carrying HIV or Hepatitis C viruses. Symbiotic cytoplasmic bacteria are also inherited in organisms such as insects and protists.
Three general types of extranuclear inheritance exist.
- Vegetative segregation results from random replication and partitioning of cytoplasmic organelles. It occurs with chloroplasts and mitochondria during mitotic cell divisions and results in daughter cells that contain a random sample of the parent cell's organelles. An example of vegetative segregation is with mitochondria of asexually replicating yeast cells.
- Uniparental inheritance occurs in extranuclear genes when only one parent contributes organellar DNA to the offspring. A classic example of uniparental gene transmission is the maternal inheritance of human mitochondria. The mother's mitochondria are transmitted to the offspring at fertilization via the egg. The father's mitochondrial genes are not transmitted to the offspring via the sperm. Very rare cases which require further investigation have been reported of paternal mitochondrial inheritance in humans, in which the father's mitochondrial genome is found in offspring. Chloroplast genes can also inherit uniparentally during sexual reproduction. They are historically thought to inherit maternally, but paternal inheritance in many species is increasingly being identified. The mechanisms of uniparental inheritance from species to species differ greatly and are quite complicated. For instance, chloroplasts have been found to exhibit maternal, paternal and biparental modes even within the same species.
- Biparental inheritance occurs in extranuclear genes when both parents contribute organellar DNA to the offspring. It may be less common than uniparental extranuclear inheritance, and usually occurs in a permissible species only a fraction of the time. An example of biparental mitochondrial inheritance is in the yeast Saccharomyces cerevisiae. When two haploid cells of opposite mating type fuse they can both contribute mitochondria to the resulting diploid offspring.
Poky is a mutant of the fungus Neurospora crassa that has extranuclear inheritance. Poky is characterized by slow growth, a defect in mitochondrial ribosome assembly and deficiencies in several cytochromes. The studies of poky mutants were among the first to establish an extranuclear mitochondrial basis for inheritance of a particular genotype. It was initially found, using genetic crosses, that poky is maternally inherited. Subsequently, the primary defect in the poky mutants was determined to be a deletion in the mitochondrial DNA sequence encoding the small subunit of mitochondrial ribosomal RNA.
- C. W. Birky, Jr. (1994). "Relaxed and stringent genomes: why cytoplasmic genes don't obey Mendel's laws". Journal of Heredity. 85 (5): 355–366.
- Sangeeta Jain; Nima Goharkhay; George Saade; Gary D. Hankins; Garland D. Anderson (2007). "Hepatitis C in pregnancy". American Journal of Perinatology. 24 (4): 251–256. doi:10.1055/s-2007-970181.
- Patrick Duff (1996). "HIV infection in women". Primary Care Update for OB/GYNS. 3 (2): 45–49. doi:10.1016/S1068-607X(95)00062-N.
- Jan Sapp (2004). "The dynamics of symbiosis: an historical overview". Canadian Journal of Botany. 82 (8): 1046–1056. doi:10.1139/b04-055.
- C. William Birky, Jr.; Robert L. Strausberg; Jean L. Forster; Philip S. Perlman (1978). "Vegetative segregation of mitochondria in yeast: estimating parameters using a random model". Molecular and General Genetics. 158 (3): 251–261. doi:10.1007/BF00267196.
- Marianne Schwartz; John Vissing (2003). "New patterns of inheritance in mitochondrial disease". Biochemical and Biophysical Research Communications. 310 (2): 247–251. doi:10.1016/j.bbrc.2003.09.037.
- C. W. Birky, Jr. (1995). "Uniparental inheritance of mitochondrial and chloroplast genes: mechanisms and evolution". Proceedings of the National Academy of Sciences USA. 92 (25): 11331–11338. doi:10.1073/pnas.92.25.11331. PMC 40394. PMID 8524780.
- A. Katie Hansen; Linda K. Escobar; Lawrence E. Gilbert; Robert K. Jansen (2007). "Paternal, maternal, and biparental inheritance of the chloroplast genome in Passiflora (Passifloraceae): implications for phylogenic studies". Botany. 94 (1): 42–46. doi:10.3732/ajb.94.1.42. PMID 21642206.
- Lambowitz AM, Bonner WD (May 1974). "The mitochondrial beta-cytochromes of the wild type and poky strains of Neurospora crassa. Evidence for a component reduced only by dithionite". J. Biol. Chem. 249 (9): 2886–90. PMID 4275176.
- Mitchell MB, Mitchell HK (May 1952). "A Case of "Maternal" Inheritance in Neurospora Crassa". Proc. Natl. Acad. Sci. U.S.A. 38 (5): 442–9. PMC 1063583. PMID 16589122.
- Akins RA, Lambowitz AM (June 1984). "The [poky] mutant of Neurospora contains a 4-base-pair deletion at the 5' end of the mitochondrial small rRNA". Proc. Natl. Acad. Sci. U.S.A. 81 (12): 3791–5. PMC 345306. PMID 6233613.