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Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two cell nucleus. Before karyogamy, each haploid cell has one complete copy of the organism's genome. In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process known as plasmogamy. Once within the joined cell membrane, the nuclei are referred to as pronucleus. Once the cell membranes, cytoplasm, and pronuclei fuse, the resulting single cell is diploid, containing two copies of the genome. This diploid cell, called a zygote or zygospore can then enter meiosis (a process of chromosome duplication, recombination, and division, to produce four new haploid cells), or continue to divide by mitosis. Mammalian fertilization uses a comparable process to combine haploid sperm and egg cells (gametes) to create a diploid fertilized egg.
The term karyogamy comes from the Ancient Greek karyo- (from κάρυον karyon) 'nut' and γάμος gamos 'marriage'.
Thus, karyogamy is the key step in bringing together two sets of different genetic material which can recombine during meiosis. In haploid organisms that lack sexual cycles, karyogamy can also be an important source of genetic variation during the process of forming somatic diploid cells. Formation of somatic diploids circumvents the process of gamete formation during the sexual reproduction cycle and instead creates variation within the somatic cells of an already developed organism, such as a fungus.
When subjected to environmental stress, such as nitrogen starvation in the case of Chlamydomonas, cells are induced to form . Gametogenesis in single-celled haploid organisms such as yeast is called Spore, resulting in many cellular changes that increase resistance to stress. Gamete formation in multicellular fungi occurs in the Gametangium, an organ specialized for such a process, usually by meiosis. When opposite meet, they are induced to leave the vegetative cycle and enter the mating cycle. In yeast, there are two mating types, a and α. In fungi, there can be two, four, or even up to 10,000 mating types, depending on the species. Mate recognition in the simplest eukaryotes is achieved through pheromone signaling, which induces shmoo formation (a projection of the cell) and begins the process of microtubule organization and migration. Pheromones used in mating type recognition are often peptides, but sometimes trisporic acid or other molecules, recognized by cellular receptors on the opposite cell. Notably, pheromone signaling is absent in higher fungi such as mushrooms.
The cell membranes and cytoplasm of these haploid cells then fuse together in a process known as plasmogamy. This results in a single cell with two nuclei, known as Pronucleus. The pronuclei then fuse together in a well regulated process known as karyogamy. This creates a diploid cell known as a zygote, or a zygospore, which can then enter meiosis, a process of chromosome duplication, recombination, and cell division, to create four new haploid gamete cells. One possible advantage of sexual reproduction is that it results in more genetic variability, providing the opportunity for adaptation through natural selection. Another advantage is efficient recombinational repair of DNA damages during meiosis. Thus, karyogamy is the key step in bringing together a variety of genetic material in order to ensure recombination in meiosis.
The Amoebozoa is a large group of mostly single-celled species that have recently been determined to have the machinery for karyogamy and meiosis. Since the Amoeboza branched off early from the eukaryotic family tree, this finding suggests that karyogamy and meiosis were present early in eukaryote evolution.
Microtubule movement is mediated by a family of known as , such as Kar3 in yeast. Accessory proteins, such as Spc72 in yeast, act as a glue, connecting the motor protein, spindle pole body and microtubule in a structure known as the half-bridge. Other proteins, such as Kar9 and Bim1 in yeast, attach to the plus end of the microtubules. They are activated by pheromone signals to attach to the shmoo tip. A shmoo is a projection of the cellular membrane which is the site of initial cell fusion in plasmogamy. After plasmogamy, the microtubule plus ends continue to grow towards the opposite pronucleus. It is thought that the growing plus end of the microtubule attaches directly to the motor protein of the opposite pronucleus, triggering a reorganization of the proteins at the half-bridge. The force necessary for migration occurs directly in response to this interaction.
Two models of nuclear congression have been proposed: the sliding cross-bridge, and the plus end model. In the sliding cross-bridge model, the microtubules run antiparallel to each other for the entire distance between the two pronuclei, forming to each other, and each attaching to the opposite nucleus at the plus end. This is the favored model. The alternative model proposes that the plus ends contact each other midway between the two pronuclei and only overlap slightly. In either model, it is believed that microtubule shortening occurs at the plus end and requires Kar3p (in yeast), a member of a family of kinesin-like proteins.
Microtubule organization in the cytoskeleton has been shown to be essential for proper nuclear congression during karyogamy. Defective microtubule organization causes total failure of karyogamy, but does not totally interrupt meiosis and spore production in yeast. The failure occurs because the process of nuclear congression cannot occur without functional microtubules. Thus, the pronuclei do not approach close enough to each other to fuse together, and their genetic material remains separated.
There are, however, a few fungi that exist mostly in the diploid state. One example is Candida albicans, a fungus that lives in the gastrointestinal tracts of many warm blooded animals, including humans. Although usually innocuous, C. albicans can turn pathogenic and is a particular problem in immunosuppressed patients. Unlike with most other fungi, diploid cells of different mating types fuse to create tetraploid cells which subsequently return to the diploid state by losing chromosomes.
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