Mating-type inheritance in Paramecium

Paramecium is a genus of unicellular Ciliate protozoa, which widespread in freshwater, brackish and marine environments. There are two distinct mating types E begets, named E and O. Mating types has been discovered nearly 70 years, however, It is still unknown how mating type is inherited during sexual reproduction in Paramecium. The O/E alternative is not random, but maternally inherited; experiments showed that mating-type determination in the developing zygotic MAC is controlled through the cytoplasm by the maternal MAC.

Our aim is to identify the putative mating-type-determining gene, by using a whole-genome microarray to compare the transcriptomes of sexually reactive cells of both mating types. We choose the gene mtA, for it has the largest expression ratio between E and O. This gene encodes a protein with signal peptide and should be a trans-membrane protein. Besides, mtA expresses only in E cells. During sexual reactivity, when using RNA interference (RNAi) to mediated silencing of mtA in E cells, all the cells developed into the default O phenotype. This data shows that mtA is needed for E expression.

To summary, we have identified the mtA as the different expressed gene. In the next part, we will explore the molecular mechanism of mating-type-E-specific expression.

In the part I, we have identified the mtA as the gene responsible for determining mating type. Mutational analyses showed that several genes are specifically required for expression of type E. Here, we explore the molecular mechanism of mating type selection. Previously studies have indicated that small RNAs commonly function to mediate transgenerational inheritance.

This study shows that Paramecium has adapted this pathway to regulate its sex determination. E cells are attractive to O cells is the E cells express a cell-surface protein called mtA. In E cells of P. aurelia species, the mtA protein could inhibit the synthesis, processing or transport of the O-specific receptor. In the sibling species Paramecium septaurelia, mating type O is determined by coding-sequence deletions in a different gene, mtB, which is specifically required for mtA expression.

As known previously, the common practice in modeling cancer is to reveal mutations by sequencing its genomes and then to discover the role of these mutations in the course of breeding certainly mutated mice. And now, researchers can rapidly model cancer by using CRISPR, a gene-editing system introducing cancer-causing mutations into the livers of adult mice.

The presence or absence of the mtA promoter is communicated between parent and progeny genomes by homologous RNAs, which effectively block or promote DNA deletion, respectively, thereby ensuring that mating type is propagated to the next generation. The exaptation of the scnRNA pathway is a general mechanism for transgenerational epigenetic inheritance of differentiated states. To summary, this discovery will speed up the identification of other examples of cell-intrinsic gene regulation mediated by homologous RNAs.