Parasitic castration by a stinking smut regulates sex determination and inflorescence architecture in dioecious buffalograss
Open Access
- Author:
- Chandra, Ambika
- Graduate Program:
- Agronomy
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 20, 2007
- Committee Members:
- David Robert Huff, Committee Chair/Co-Chair
Dawn S Luthe, Committee Member
Paula Mc Steen, Committee Member
Barbara Jane Christ, Committee Member - Keywords:
- Sexual reproductive allocation
nLSU-rDNA
Tasselseed2
Salmacisia
Buchloe - Abstract:
- Buffalograss is a dioecious grass with male and female inflorescences separated onto two different individuals. Buffalograss presumably evolved its present day dioecious breeding system from hermaphroditic ancestors. Pistil smut infection phenotypically alters unisexual florets of dimorphic male and female buffalograss making them hermaphrodites which are morphologically indistinguishable from one another. In this way, pistil smut infection allows the phenotypic expression of a hermaphroditic ancestor, which existed back in time, in the form of a retrospective phenotype or simply a retrophenotype. Underdeveloped stamens (male sex organs) and fungal sporulation within ovaries of male and female plants renders hermaphroditic florets of infected buffalograss reproductively sterile, i.e. parasitically castrated. Parasitic castration is a disease affecting animals, mollusks, insects, and plants where host gonads are sterilized preventing evolution of host resistance and redirecting resource allocation. How and why fungi induce hermaphroditism and cause parasitic castration in their hosts is unknown. Here I show that pistil smut induces hermaphroditism in male buffalograss by down-regulating a female suppressor gene homologous to Tasselseed2 (Ts2). This molecular mechanism is supported by temporal and spatial expression analyses performed using quantitative real time PCR and in situ hybridization, respectively, on a cloned full length buffalograss Ts2 homolog. In addition to inducing hermaphroditism, pistil smut infection enhances overall sexual reproductive allocation (SRA) in both male and female sex forms of buffalograss. In female plants, the fungus induces a 12.6-fold increase in ovary production while in male plants the fungus induces a 2.3-fold increase in floret number. Furthermore, phylogenetic analyses of morphology and nuclear 28s large subunit of ribosomal DNA (nLSU-rDNA) sequences show that pistil smut is clearly distinct from any other species of Tilletia suggesting that it may not even be a species of Tilletia. The unusually large genetic distance of pistil smut from Tilletia clade indicates an accelerated rate of evolution possibly due to pistil smut’s long term coevolutionary struggle with its host culminating in induced hermaphroditism. Therefore, a new genus Salmacisia is proposed and described to accommodate pistil smut.