Abstract
Recognition of self and nonself is important for outcrossing organisms, and different mating types establish the barrier against self-mating. In the unicellular ciliate T. thermophila, mating type determination requires complex DNA rearrangements at a single mat locus during conjugation to produce a type-specific gene pair (MTA and MTB) for 1 of 7 possible mating types. Surprisingly, we found that decreased expression of the DNA breakage-repair protein Ku80 at late stages of conjugation generated persistent selfing phenotype in the progeny. DNA analysis revealed multiple mating-type gene pairs as well as a variety of mis-paired, unusually arranged mating-type genes in these selfers that resemble some proposed rearrangement intermediates. They are found also in normal cells during conjugation and are lost after 10 fissions but are retained in Ku mutants. Silencing of TKU80 or TKU70-2 immediately after conjugation also generated selfing phenotype, revealing a hidden DNA rearrangement process beyond conjugation. Mating reactions between the mutant and normal cells suggest a 2-component system for self–nonself-recognition through MTA and MTB genes.
Highlights
Sexual reproduction is widespread throughout nature, providing opportunities for organisms to increase genetic variety and purge deleterious mutations
Selfing was observed frequently (>50%) even after subcloning at later growth (76 fissions). These results indicate that depletion of TKU80 from the zygotic nuclei during conjugation and the subsequent growth leads to persistent selfing in their progeny, suggesting that zygotic function of TKU80 is important for mating type determination in Tetrahymena
We found that abolishing Ku protein gene function perturbs mating type determination and generates selfing clones in Tetrahymena
Summary
Sexual reproduction is widespread throughout nature, providing opportunities for organisms to increase genetic variety and purge deleterious mutations. It can occur by outcrossing (combining gametes from 2 mating partners) or self-fertilization (combining gametes from the same individual). The mechanistic distinction between these processes rests largely on the compatibility between mating partners, for which a variety of mechanisms have been analyzed [1,2,3,4,5,6,7], though many remain unclear. Growth is interrupted intermittently by the sexual process of conjugation, which occurs between cells of different mating types (Fig 1B). Selfers—cells that form intraclonal mating pairs upon starvation—have been reported occasionally [8,9], but the mechanism behinds it remains largely unknown
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