What is the Difference Between Homothallic and Heterothallic

The main difference between homothallic and heterothallic is that homothallic is the condition of having both male and female reproductive structures on the same thallus whereas heterothallic is the condition of having male and female reproductive structures in different thalli. 

Homothallic and heterothallic are two conditions of sexual reproduction found in fungi and some algae. In a homothallic condition, the resources for sexual reproduction occur in the same organism while in a heterothallic condition, sexual reproduction requires the presence of two compatible partners. 

Key Areas Covered 

1. What is Homothallic
     – Definition, Characteristics, Importance
2. What is Heterothallic
     – Definition, Characteristics, Importance
3. What are the Similarities Between Homothallic and Heterothallic
     – Outline of Common Features
4. What is the Difference Between Homothallic and Heterothallic
     – Comparison of Key Differences

Key Terms 

Heterothallic, Homothallic, Reproductive Structures, Sexual Reproduction, Thallus

What is Homothallic 

Homothallic is a condition of fungi in which both reproductive structures occur in the same thallus. In other words, the thallus is bisexual. However, homothallism is considered as a long-term evolutionary cost due to the reduction of effective recombination rates and population size by their self-fertilization. Consequently, this reduces the efficacy of purifying selection, which makes the population more prone to genetic drift. Significantly, genetic drift results in genomic maladaptation such as the spread of selfish genetic elements, which are deleterious or accelerated rates of protein evolution or decay. 

Figure 1: Aspergillus nidulans Wildtype – A Type of Homothallic Fungi

Furthermore, homothallic condition is a neutral mutational event. However, mutations that occur in the homothallus, reducing the self-incompatibility, will not undergo immediate positive or negative selection. The mutations which reduce the self-compatibility will be fixed immediately. 

What is Heterothallic 

Heterothallic in another condition of fungi in which male and female reproductive structures occur in different thalli. Therefore, the main importance of this condition is the requirement of a compatible mating partner in order to undergo sexual reproduction. It brings a totally different genetic condition in the heterothallic organisms in contrast to those that are homothallic. Actually, sexual reproduction occurs when the two opposite mating partners come into contact. Therefore, the formation of the zygote brings the gametes of two genetically different partners together, increasing the genetic variation of the population. Also, the genetic drift is prevented in heterothallic condition and no genomic maladaptation can occur.

Figure 1: Heterothallic Reproduction

On the other hand, during the formation of gametes, it is possible to undergo genetic recombination as well as mutations. Thus, this results in the formation of genetically different gametes. However, heterothallic condition allows natural selection and helps to increase the size of the population.  

Similarities Between Homothallic and Heterothallic 

• Homothallic and heterothallic are two conditions of having reproductive structures in the thallus. 
• They occur in both fungi and some algae. 
• Also, both conditions increase the potential for breeding throughout the fungal kingdom. 
• Based on the reproductive condition, the requirements for reproduction and genetic variation of the offspring differ. 

Difference Between Homothallic and Heterothallic 

Definition 

Homothallic refers to the condition of having a haploid phase that produces two kinds of gametes capable of fusing to form a zygote while heterothallic refers to the condition of having two or more morphologically similar haploid phases or types of which individuals from the same type are mutually sterile but, individuals from different types are cross-fertile. Thus, this is the fundamental difference between homothallic and heterothallic.

Reproductive Structures 

In the homothallic condition, both male and female reproductive structures occur in the same thallus while in the heterothallic condition, male and female reproductive structures occur in different thalli. Therefore, the main difference between homothallic and heterothallic is the reproductive structures.

The requirement of a Mating Partner  

In the homothallic condition, a mating partner is not required; however, two compatible mating partners are required in heterothallic condition. 

Type of Sexual Reproduction 

Besides, bisexual reproduction occurs in the homothallus while unisexual reproduction occurs in the heterothallus. Hence, this is another difference between homothallic and heterothallic.

Type of Fertilization 

While homothallic organisms undergo self-fertilization, heterothallic organisms undergo cross-fertilization.  

Recombination Rates and Population Size 

Moreover, homothallism reduces the effective recombination rates and population size while heterothallism allows the recombination and increased population size. 

Efficacy of Purifying Selection 

Another difference between homothallic and heterothallic is that homothallism has a reduced efficacy of purifying selection while heterothallism has a high efficacy of purifying selection. 

Genetic Drift 

Furthermore, homothallism increases the risk of undergoing genetic drift while heterothallism reduces the risk of undergoing genetic drift. 

Occurrence of Mutations 

In addition, homothallism is a neutral mutational event while mutations can occur in heterothallism. 

Examples 

Homothallic fungi include Aspergillus species while heterothallic fungi include Saccharomyces cerevisiae, Aspergillus fumigatus, Aspergillus flavus, Penicillium marneffei, Neurospora crassa, etc.  

Conclusion 

Homothallic is a condition in which both reproductive structures occur in the same thallus. However, it is considered a long-term evolutionary cost due to the reduction of variation and population size. On the other hand, heterothallic is a condition in which male and female reproductive structures occur in different types of thalli. Here, two compatible mating partners have to be present for undergoing sexual reproduction. Also, it increases genetic variation. Therefore, the main difference between homothallic and heterothallic conditions is the type of reproductive structures present in the thallus and the effect of reproduction. 

References:

1. “Homothallism.” Homothallism – an Overview | ScienceDirect Topics, Available Here.

Image Courtesy:

1.”Aspergillus nidulans wildtype” By 222fjb – Photographed with a Panasonic DMC-FZ8 (CC BY-SA 3.0) via Commons Wikimedia   
2. “Yeast lifecycle” By pl.wiki: Masurcommons: Masurirc: [1] – Own work (Public Domain) via Commons Wikimedia