|MadSci Network: Genetics|
Dear Jeff, Mitochondria are fascinating organelles about which a great deal is known and a great deal is still to be learned. Iíll limit my answer to mammalian inheritance, since nature provides many variations in animals and plants on the theme of gender and fertilization, including hermaphroditic species, variable inheritance by different sexes, enormous variations in the size of mitochondrial genomes in different species, etc. You are correct in saying that mitochondrial DNA is inherited maternally in mammals. Iím not certain I understand from your question what the exact controversy is, but if you mean that sometimes, sperm mitochondrial DNA can be inherited by an embryo, I didnít find any reports of that in my computer search of the MEDLINE database. Of course, the DNA in mitochondria in all the cells of males and females is subject to mutation by all the environmental factors and just plain mistakes during replication that affect nuclear DNA - cosmic rays, UV light, chemical damage, and so on. At the end of this message I have pasted two references, including the summaries, from the recent scientific literature that discuss maternal inheritance of mitochondrial DNA - one experimental, one theoretical. Thereís a lot of cell biology terminology, but I think the gist of them will be helpful. In the first, there is a description of the fate of sperm mitochondria and re-organization of the cytoskeleton (protein filament systems) and other cell constituents observed under the microscope during in vitro fertilization in cattle. The sperm mitochondria appear to be partitioned in a defined way during the fist several cell divisions and then disappear, just as one might expect from what we know about the inheritance patterns. This constitutes visual confirmation of maternal inheritance. In the second, there is an interesting hypothesis presented which says that the energy required for a motile gamete (i.e., the swimming sperm) depends on a very high rate of oxidative phosphorylation, which puts at risk the male mitochondrial DNA. The free-radical chemical by-procucts of this burst of energy production can cause mutations, and the author proposes that the division of labor and inheritance between the gametes from male and female is evolutionarily favored to provide the fertilized embryo with the highest-fidelity mitochondrial DNA. This provides a theoretical basis for maternal inheritance. Thereís another very hot area of research concerning mitochondrial DNA and mutations to it that is currently underway in many laboratories, one that your students may find interesting. Some scientists have been examining the hypothesis that aging in an individual is due in large part to an accumulation of mutations in mitochondrial DNA that build up over a lifetime in the various cells and tissues of our bodies. As mutations occur and slip past the repair mechanisms, energy production decreases, and slowly cells lose their ability to carry out their normal functions. This is a very attractive idea, and so far it appears to be consistent with the data obtained from tissue samples. I hope this answers your question. If not, please feel free to get in touch. Paul Odgren, Ph.D. Dept. of Cell Biology University of Massachusetts Medical School Worcester, MA 01655 The two references follow: Biol Reprod 1996 Dec;55(6):1195-1205 Fate of the sperm mitochondria, and the incorporation, conversion, and disassembly of the sperm tail structures during bovine fertilization. Sutovsky P, Navara CS, Schatten G Department of Zoology, University of Wisconsin, Madison 53706, USA. Sperm incorporation and the conversion of the sperm-derived components into zygotic structures during in vitro fertilization of bovine oocytes was explored by combining ultrastructural studies with observations of the fertilizing sperm tagged with a mitochondrion-specific vital dye MitoTracker green FM. The zygotes fertilized by the MitoTracker-labeled sperm were fixed at various times after fertilization and then processed for immunocytochemistry to examine the distribution of DNA, microtubules, and sperm tail components, including the fibrous sheath and axonemal microtubules. We show here that the complete incorporation of the sperm, but not sperm-oocyte binding and oocyte activation, depends upon the integrity of oocyte microfilaments and is inhibited by the microfilament disrupter cytochalasin B. After sperm incorporation, the mitochondria are displaced from the sperm's connecting piece, and the sperm centriole is exposed to the egg cytoplasm. This event is followed by the formation of the microtubule-based sperm aster, which is responsible for the union of male and female pronuclei. Concomitantly, the major structure of the sperm principal piece, the fibrous sheath, disappears. After the first mitosis, the compact mitochondrial sheath can be seen in one of the blastomeres. An aggregate of the sperm mitochondria is observed at the entry of the second mitosis, although they remain in the vicinity of the nucleus and can later be seen at one pole of the metaphase spindle. The mitochondrial cluster is occasionally found in one of the blastomeres in the early-stage four-cell embryos, but it is no longer detected by the beginning of the third mitotic cycle. These data suggest that the disassembly of the sperm tail during bovine fertilization occurs as a series of precisely orchestrated events involving the destruction (fibrous sheath and mitochondrial sheath) and transformation (DNA, sperm centriole) of particular sperm structures into zygotic and embryonic components. J Theor Biol 1996 May 21;180(2):135-140 Separate sexes and the mitochondrial theory of ageing. Allen JF Department of Plant Cell Biology, Lund University, Sweden. An hypothesis is presented by which gamete specialization resolves a conflict between the function and replication of mitochondria. The function of mitochondria is to synthesize ATP by oxidative phosphorylation, which is coupled to respiratory electron transport. This requires a mitochondrial genetic system. However, "incorrect" electron transfers produce free radicals that cause mutation, and the frequency of these events is increased by mutation. Mitochondrial function is therefore detrimental to the fidelity of mitochondrial replication. Damage to somatic mitochondrial DNA may accumulate within, and indeed determine, the life span of individual organisms. Motility of one gamete is required for fertilization, and requires ATP. It is proposed that male gametes maximize energy production for motility by sacrificing mitochondrial DNA to electron transfer and its mutagenic by-products, while female gametes, whichare non-motile, repress mitochondrial oxidative phosphorylation, thus protecting mitochondrial DNA for faithful transmission between generations. Male gametes then make no contribution to the mitochondrial genome of the zygote: mitochondria are maternally inherited. This testable hypothesis may help to explain the evolution of separate sexes and a number of their characteristics. Maternal inheritance of chloroplasts may be explained in a similar way, and contribute to the maintenance of separate sexes in plants.
Try the links in the MadSci Library for more information on Genetics.