Conjugation in Euplotes songi, a phylogenetically basal species in the genus Euplotes: Nuclear events and new pheromone gene variants

Ciliate conjugation represents a fundamental biological process that involves complex nuclear reorganization and genetic exchange. In this study, we employed an integrated set of cytological and molecular approaches to investigate Euplotes songi-a phylogenetically basal species of the genus Euplotes-with the aim of determining the entire cascade of nuclear events during conjugation and obtaining new information on the macronuclear pheromone-coding genes. As is common in Euplotes species, the micronucleus of mating cells enters meiosis following a mitotic duplication; two of the resulting eight haploid nuclei undergo a further mitotic division to generate four haploid products, from which the migratory and stationary gametic nuclei are ultimately selected. Unlike phylogenetically more recent Euplotes species, in which mating cells completely resorb the parental macronucleus during conjugation, E. songi only resorbs the anterior fragment of the old macronucleus, while the posterior portion is likely to be functionally integrated into the primordium of the new macronucleus. Furthermore, we identified novel pheromone gene variants via crosses between wild-type strains, revealing that most polymorphisms occur in the 3' trailer region, one occurs in the 5' leader region, and only three are located within the coding region without altering the amino acid sequences of the known E. songi pheromones. Phylogenetic comparison of these pheromone sequences with homologous sequences from other Euplotes species, based on the well-established Euplotes SSU rDNA phylogenetic analysis, supports the conclusion that E. songi forms the earliest-diverging Euplotes clade together with E. petzi, E. sinensis, and E. huizhouensis. These findings provide valuable insights into the ancestral biological and genetic traits of Euplotes.