Evolution of the fungal self-fertile reproductive life style from self-sterile ancestors

Area of Science:

• Mycology
• Fungal Genetics
• Evolutionary Biology

Background:

• Mating in fungal ascomycetes is typically controlled by a single mating type (MAT) locus.
• Heterothallic (self-sterile) fungi require a partner, while homothallic (self-fertile) fungi do not.
• The evolutionary relationship between these mating systems is a key question in fungal biology.

Purpose of the Study:

• To investigate the evolutionary origins of homothallism from heterothallism in Cochliobolus species.
• To analyze the structural organization of mating type (MAT) genes in homothallic and heterothallic fungi.
• To determine the genetic basis for the transition between self-sterile and self-fertile reproductive strategies.

Main Methods:

• Comparative sequence analysis of MAT loci from homothallic and heterothallic Cochliobolus species.
• Examination of the structural organization and gene fusion within MAT regions.
• Functional analysis by expressing fused MAT genes in MAT-null heterothallic strains.

Main Results:

• Structural analyses support heterothallism as the ancestral state in fungal mating.
• Homothallic species possess fused or closely linked MAT genes within a single nucleus, unlike heterothallic species with alternate MAT genes.
• MAT gene organization is conserved in heterothallic species but unique in homothallic species.
• Recombination between conserved sequence islands facilitates the transition from heterothallism to homothallism.
• Expression of a fused MAT gene from a homothallic species restored self-fertility in a heterothallic MAT-null strain.

Conclusions:

• Heterothallism is the ancestral mating system in fungal ascomycetes.
• The evolution of homothallism involves genetic recombination within the MAT locus.
• The MAT locus alone is sufficient to determine the self-fertility or self-sterility of a fungal species.