Main Difference – Plasmogamy vs Karyogamy

Plasmogamy and karyogamy are two sequential stages of syngamy in fungi. Syngamy is a method of recombination, involved in the sexual reproduction of fungi. Plasmogamy is followed by karyogamy and karyogamy is followed by the mitotic division of the forming diploid nucleus. Plasmogamy in lower fungi occurs through the union of the two cytoplasms of fungal gametes. But in higher fungi, the two opposite mating types of the fungal thalli are also capable of fusing together, forming the diksryotic cell stages. Plasmogamy is immediately followed by karyogamy in lower fungi. In higher fungi, karyogamy is delayed for several generations, maintaining the dikaryotic stage of cells. The main difference between plasmogamy and karyogamy is that plasmogamy is the fusion of two hyphal protoplasts while karyogamy is the fusion of two haploid nuclei in fungi.

This article looks at,

1. What is Plasmogamy
     – Definition, Plasmogamy in Fungi
2. What is Karyogamy
     – Definition, Karyogamy in Fungi
3. What is the difference between Plasmogamy and Karyogamy

What is Plasmogamy

During the syngamy of fungi, the union of the two cytoplasms of haploid gametes is known as plasmogamy. The fusion of the two nuclei of the fused cells occurs later. But, by plasmogamy, the fusion of two haploid nuclei is facilitated by bringing them close together in the same cell. Plasmogamy is followed by a dikaryotic stage, which sometimes persists through several generations before undergoing karyogamy. Heterothallism is the fusion of thalli, belonging to different mating types. Heterothallism is exhibited by Basidiomycota.  In Basidiomycotina, the union of the two hyphal protoplasts of haploid parent cells occurs from the mycelia. A single cell consists of two haploid nuclei (dikaryon) for several generations. The growth and cell division occur in these cells while performing the dikaryon. Plasmogamy in Basidiomycota is shown in figure 1.

Figure 1: Plasmogamy in Basidiomycota

Plasmogamy in lower fungi occurs in three ways: planogametic copulation, gametangial contact and gametangial copulation. Planogametic copulation occurs in Chytridiomycetes and Plasmodiophoromycetes and one or both of the gametes are found motile. In gametangial contact, Oomycetes like fungi produce non-motile gametes called aplanogametes. In gametangial copulation, strictly terrestrial lower fungi like Mucorales accomplish the fusion of gametangia. Homothallism is an alternative mechanism for sexual reproduction of fungi, fusing one thallus with another thallus of the same organism. Uniflagellate planogametes in Chytridiomycetes is shown in figure 2.

Figure 2: Uniflagellate planogametes in Chytridiomycetes

What is Karyogamy

During the syngamy of fungi, the union of two haploid nuclei of a dikaryotic cell is known as karyogamy. Karyogamy is the second or the final step in the process of syngamy. During karyogamy, nuclear envelopes of the two haploid nuclei are fused in three steps. First, the outer membranes of the two nuclei are fused. Then, the two inner membranes are fused and finally, the fusion of spindle pole bodies occurs. After undergoing the karyogamy, the dikaryotic cell becomes diploid. The resultant diploid cells are known as  zygotes or zygospores. The zygote is the only diploid phase found in fungal life cycle. Karyogmy is followed by the meiosis of diploid nuclei. During meiosis, duplication of chromosomes occurs along with the recombination of genetic material and the division of the cell ultimately produces four daughter haploid cells. That means, karyogamy contributes to the genetic variations among fungal population. Producing daughter cells then undergo mitosis in order to increase the cell number. These daughter cells are called as spores. Ultimately, as a result of sexual reproduction of fungi, haploid spores are produced.  

In higher fungi like Ascomycetes and Basidiomycetes, karyogamy is delayed and dikaryokitc cells are maintained for several generations. The dikaryon is capable of dividing mitotically along with usual cytokinesis. This phase of the fungal life cycle is called dikaryotic phase. The development of the mycelium along with the two dikaryotic nuclei is followed by the simultaneous cell division, separating sister nuclei into two daughter cells. But in lower fungi like Phycomycetes, karyogamy occurs immediately after the plasmogamy. The production of ascospores in ascogonium by undergoing karyogamy is shown in figure 3.

Figure 3: Production of ascospores

Difference Between Plasmogamy And Karyogamy

Definition

Plasmogamy: Plasmogamy is the fusion of two hyphal protoplasts.

Karyogamy: Karyogamy is the fusion of two haploid nuclei in fungi.

Syngamy

Plasmogamy: Plasmogamy is the first step of syngamy in fungi.

Karyogamy: Karyogamy is the second step in syngamy of fungi.

Resulting Cell

Plasmogamy: Plasmogamy produces a dikaryotic cell.

Karyogamy: Karyogamy produces a cell containing a diploid nucleus.

Number of Nuclei

Plasmogamy: Plasmogamy generates a cell containing two haploid nuclei.

Karyogamy: Karyogamy generates a cell containing a single diploid nucleus.

Consequence

Plasmogamy: Plasmogamy is followed by karyogamy.

Karyogamy: Karyogamy is followed by meiosis.

Conclusion

Plasmogamy and karyogamy occur during syngamy in fungi. Syngamy is a type of recombination, considered as the sexual reproduction of fungi. Plasmogamy is followed by karyogamy. During plasmogamy, two protoplast of either gametes or different mating types of thalli are fused. Plasmogamy forms a cell, containing two haploid nuclei, which can also be called as a dikaryon. In higher fungi like Basidiomycota, this dikaryotic stage is maintained for several generations. But in lower fungi, plasmogamy is immediately followed by karyogamy. During karyogamy, the fusion of two haploid nuclei in the dikaryotic cell is observed. In Basidiomycetes, plasmogamy occurs between two mating types of thalli. Developing dikaryotic thallus form the basidiocarp, which is a characteristically large fruiting body. But in lower fungi like Oomycota, two gametes are fused during syngamy. Karyogamy of the two haploid nuclei produces a diploid nucleus which can undergo meiosis to produce spores. Spores germinate to produce haploid mycelium. The main difference between plasmogamy and karyogamy is their structures, which are susceptible to fusion.  

Reference:
1.Cole, Garry T. “Basic Biology of Fungi.” Medical Microbiology. 4th edition. U.S. National Library of Medicine, 01 Jan. 1996. Web. 29 Mar. 2017.
2.”Reproduction in Fungi- Part-3: Sexual Reproduction (Lecture Notes and PPT).” Easybiologyclass. N.p., n.d. Web. 29 Mar. 2017. <http://www.easybiologyclass.com/reproduction-in-fungi-part-3-sexual-reproduction-lecture-notes-ppt/>.
3.”Cycling Through Life.” Life Cycles. N.p., n.d. Web. 29 Mar. 2017. <http://plantphys.info/plant_biology/lifecycle.shtml>.

Image Courtesy:
1.”Figure 24 02 07″ By CNX OpenStax –  CC BY 4.0) via Commons Wikimedia
2. “Chytridiomycete” by AJC1 (CC BY-SA 2.0) via Flickr
3. Derived from “Brachymeiosis” By FourViolas – Own work (CC BY-SA 4.0) via Commons Wikimedia

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