Macronuclear Development During Conjugation in Blepharisma sinuosum and B. stoltei : Insights Into the Diversity of Macronuclear Formation in Ciliates

Sexual reproduction is one of the crucial driving forces of evolution. Ciliates, single-celled eukaryotes with nuclear dimorphism, undergo a unique sexual process, conjugation, during which the macronucleus (MAC) and micronucleus (MIC) are extensively reorganized. Previous studies have shown that macronuclear development in Blepharisma can involve functionally differentiated micronuclei. Here, we investigated the conjugation process in two multi-micronucleate species, Blepharisma sinuosum and B. stoltei. In B. sinuosum, approximately two-thirds of MICs enter meiosis, but only one meiotic MIC completes the divisions that generate pronuclei; the remaining MICs persist as somatomicronuclei and develop into secondary MAC anlagen. The new MICs undergo variable additional mitoses, producing different MIC numbers among exconjugants. In both B. sinuosum and B. stoltei, primary MAC anlagen derived from the synkaryon and secondary MAC anlagen derived from somatomicronuclei show coordinated late development, supporting a model in which both nuclear sources contribute to the developing MAC. Compared with B. sinuosum, B. stoltei exhibits greater individual variation in the number of meiotic MICs, new MICs, and primary and secondary MAC anlagen. These results expand the species-level cytological basis for understanding the diversity of macronuclear development during ciliate conjugation.