Cervantes MD, Hamilton EP, Xiong J, Lawson MJ, Yuan D, Hadjithomas M, Miao W, Orias E. Selecting one of several mating types through gene segment joining and deletion in Tetrahymena thermophila.
PLoS Biol 2013;
11:e1001518. [PMID:
23555191 PMCID:
PMC3608545 DOI:
10.1371/journal.pbio.1001518]
[Citation(s) in RCA: 70] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2012] [Accepted: 02/12/2013] [Indexed: 12/18/2022] Open
Abstract
In Tetrahymena, a multi-sexed single-celled organism, the sex of the progeny is randomly determined by site-specific recombination events that assemble one complete gene pair and delete all others.
The unicellular eukaryote Tetrahymena thermophila has seven mating types. Cells can mate only when they recognize cells of a different mating type as non-self. As a ciliate, Tetrahymena separates its germline and soma into two nuclei. During growth the somatic nucleus is responsible for all gene transcription while the germline nucleus remains silent. During mating, a new somatic nucleus is differentiated from a germline nucleus and mating type is decided by a stochastic process. We report here that the somatic mating type locus contains a pair of genes arranged head-to-head. Each gene encodes a mating type-specific segment and a transmembrane domain that is shared by all mating types. Somatic gene knockouts showed both genes are required for efficient non-self recognition and successful mating, as assessed by pair formation and progeny production. The germline mating type locus consists of a tandem array of incomplete gene pairs representing each potential mating type. During mating, a complete new gene pair is assembled at the somatic mating type locus; the incomplete genes of one gene pair are completed by joining to gene segments at each end of germline array. All other germline gene pairs are deleted in the process. These programmed DNA rearrangements make this a fascinating system of mating type determination.
Tetrahymena thermophila is a single-celled organism with seven sexes. After two cells of different sexes mate, the progeny cells can be of any one of the seven sexes. In this article we show how this sex decision is made. Every cell has two genomes, each contained within a separate nucleus. The germline genome is analogous to that in our ovaries or testes, containing all the genetic information for the sexual progeny; the somatic or working genome controls the operation of the cell (including its sex). We show that the germline genome contains a tandem array of similarly organized but incomplete gene pairs, one for each sex. Sex is chosen after fertilization when a new somatic genome is generated by rearrangement of a copy of the germline genome. One complete sex gene pair is assembled when the cell joins DNA segments at opposite ends of the array to each end of one incomplete gene pair; this gene pair is thus completed and becomes fully functional, while the remaining sex gene pairs are excised and lost. The process involves programmed, site-specific genome rearrangements, and the physically independent rearrangements that occur at opposite ends of the selected gene pair happen with high reliability and precision.
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