Mating Type Determination in Tetrahymena: Last Man Standing

Tetrahymena, a single-celled protist, is a eukaryote, just like you and me. But that’s about where the similarity ends. Each Tetrahymena cell contains not one, but two nuclei. The diploid germline nucleus remains transcriptionally silent throughout asexual reproduction, while the somatic nucleus is transcriptionally active. Most especially, Tetrahymena do sex differently from us—there are not two, but seven mating types (I to VII), and any Tetrahymena cell can mate with a cell of any type but its own. The existence of the seven mating types has been known since the 1950s, but the genetic basis of mating type determination has remained a mystery until now. In this issue of PLOS Biology, Marcella Cervantes, Wei Miao, Eduardo Orias, and colleagues show that incomplete gene pairs for each type are arranged in a linear array within the germline nucleus. During the mating process, completion of one gene pair by joining non-contiguous DNA segments, and stochastic elimination of all the others within the new somatic nucleus, fixes the mating type of the offspring. To search for genes that might be involved in mating, the authors starved the cells, a necessary step before conjugation (sex). During conjugation, gamete nuclei fuse and then divide mitotically to form both germline and somatic nuclei; the old somatic nucleus from each parent is then destroyed. The new somatic nucleus undergoes wholesale genomic rearrangements that result in, among other things, determination of mating type. The authors took RNA sequences from starved mating type V and VI cells and mapped them onto the Tetrahymena somatic genome. They found two adjacent genes that were not expressed during growth, and were expressed only in mating type VI (not V) during starvation, and showed that knocking out either one prevented conjugation, suggesting they were involved in mating. They called the genes MTA and MTB. In the somatic nucleus, MTA and MTB were arranged head to head, each containing an exon at its distal end that encodes a transmembrane domain. Since two Tetrahymena must contact each other to sense a mating type difference, it stood to reason they might employ membrane proteins to distinguish between self and non-self mating types, strengthening the case for the involvement of the two genes. Next, they searched the germline genome sequence for the type VI MTA and MTB sequences, and got a surprise. They found a 91-kilobase region of the genome in which the transmembrane portion of each gene had not one, but six separate matches, while the remaining portion matched only once. They tried MTA and MTB sequences from the remaining types, and found that the mating type–