Autogamy

Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species of protists have also been observed using autogamy as a means of reproduction. Flowering plants engage in autogamy regularly, while the protists that engage in autogamy only do so in stressful environments.

Paramecium aurelia is the most commonly studied protozoan for autogamy. Similar to other unicellular organisms, Paramecium aurelia typically reproduce asexually via binary fission or sexually via cross-fertilization. However, studies have shown that when put under nutritional stress, Paramecium aurelia will undergo meiosis and subsequent fusion of gametic-like nuclei. This process, defined as hemixis, a chromosomal rearrangement process, takes place in a number of steps. First, the two micronuclei of Paramecium aurelia enlarge and divide two times to form eight nuclei. Some of these daughter nuclei will continue to divide to create potential future gametic nuclei. Of these potential gametic nuclei, one will divide two more times. Of the four daughter nuclei arising from this step, two of them become anlagen, or cells that will form part of the new organism. The other two daughter nuclei become the gametic micronuclei that will undergo autogamous self-fertilization. These nuclear divisions are observed mainly when the Paramecium aurelia is put under nutritional stress. Research shows that Paramecium aurelia undergo autogamy synchronously with other individuals of the same species.

Similar to Paramecium aurelia, the parasitic ciliate Tetrahymena rostrata has also been shown to engage in nuclear division and autogamy when placed under nutritional stress. Due to the degeneration and remodeling of genetic information that occurs in autogamy, genetic variability arises and possibly increases an offspring’s chances of survival in stressful environments.

Allogromia laticollaris is perhaps the best-studied foraminiferan amoeboid for autogamy. Allogromia laticollaris can alternate between sexual reproduction via cross-fertilization and asexual reproduction via binary fission. The details of the life cycle of Allogromia laticollaris are unknown, but similar to Paramecium aurelia, Allogromia laticollaris is also shown to sometimes defer to autogamous behavior when placed in nutritional stress. As seen in Paramecium, there is some nuclear dimorphism observed in Allogromia laticollaris. There are often observations of macronuclei and chromosomal fragments coexisting in Allogromia laticollaris. This is indicative of nuclear and chromosomal degeneration, a process similar to the subdivisions observed in Paramecium aurelia. Multiple generations of haploid Allogromia laticollaris individuals can exist before autogamy actually takes place. The autogamous behavior in Allogromia laticollaris has the added consequence of giving rise to daughter cells that are substantially smaller than those rising from binary fission. It is hypothesized that this is a survival mechanism employed when the cell is in stressful environments, and thus not able to allocate all resources to creating offspring. If a cell was under nutritional stress and not able to function regularly, there would be a strong possibility of its offspring’s fitness being sub-par.

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Wikipedia