EVOLUTION OF VEGETATIVE INCOMPATIBILITY IN FILAMENTOUS ASCOMYCETES. I. DETERMINISTIC MODELS

Area of Science:

• Evolutionary biology
• Mycology
• Population genetics

Background:

• Ascomycetes fungi exhibit vegetative incompatibility, restricting somatic genetic exchange between conspecifics.
• The frequent observation of numerous vegetative compatibility groups (VCGs) in natural fungal populations suggests a significant evolutionary role for this phenomenon.

Purpose of the Study:

• To investigate two primary selective hypotheses for the evolution of vegetative incompatibility in asexual fungi: nuclear parasitic gene selection and harmful cytoplasmic element selection.
• To evaluate whether these frequency-dependent selection mechanisms can explain the high diversity of VCGs observed in natural populations.

Main Methods:

• A population-genetic approach was employed.
• A deterministic model was developed, assuming random spatial distribution of VCGs in an infinitely sized population.

Main Results:

• Neither nuclear parasitic gene selection nor harmful cytoplasmic element selection, as modeled, adequately explains the large number of VCGs found in natural fungal populations.
• The selective pressure for increased VCG diversity diminishes once a limited number of VCGs are established, due to decreased compatible interaction frequencies.

Conclusions:

• The studied selective mechanisms, particularly frequency-dependent selection driven by parasitic genes or cytoplasmic elements, are insufficient to account for the high prevalence of VCGs in asexual fungi.
• Selection by a harmful cytoplasmic element appears to be a more plausible evolutionary driver for vegetative incompatibility than selection by a nuclear parasitic gene.