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Evolution of the fungal self-fertile reproductive life style from self-sterile ancestors
S H Yun1, M L Berbee, O C Yoder
1Department of Plant Pathology, Cornell University, 334 Plant Science Building, Ithaca, NY 14853, USA.
Abstract:
In most fungal ascomycetes, mating is controlled by a single locus (MAT). Fungi requiring a partner to mate are heterothallic (self-sterile); those not requiring a partner are homothallic (self-fertile). Structural analyses of MAT sequences from homothallic and heterothallic Cochliobolus species support the hypothesis that heterothallism is ancestral. Homothallic species carry both MAT genes in a single nucleus, usually closely linked or fused, in contrast to heterothallic species, which have alternate MAT genes in different nuclei. The structural organization of MAT from all heterothallic species examined is highly conserved; in contrast, the organization of MAT in each homothallic species is unique. The mechanism of conversion from heterothallism to homothallism is a recombination event between islands of identity in otherwise dissimilar MAT sequences. Expression of a fused MAT gene from a homothallic species confers self-fertility on a MAT-null strain of a heterothallic species, suggesting that MAT alone is sufficient to change reproductive life style.
Insights
Heterothallism is ancestral in fungi, with homothallism evolving through recombination of mating type (MAT) genes. A single fused MAT gene can confer self-fertility, altering fungal reproductive strategies.
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.