Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2024 / Paper 1 / Section E / 15 / Solution

In ciliates, sex often renews and reorganizes nuclei without immediately increasing cell number, and its outcomes depend on both germline DNA and the old somatic nucleus. nuclear dimorphism is central. A usually diploid micronucleus preserves the germline genome, while a transcriptionally active, often highly polyploid macronucleus controls vegetative cell function. Repeated binary fission can increase cell number without a sexual event; mitotic micronuclear inheritance and often amitotic macronuclear division maintain the two nuclear systems differently. Sex is therefore separable from routine population growth, although some species or strains undergo clonal ageing and benefit from, or eventually require, nuclear renewal for continued vigorous growth.

In the standard conjugation sequence of Tetrahymena thermophila, appropriately mature, compatible cells pair under suitable conditions, often starvation. The micronucleus undergoes meiosis; selected haploid products give stationary and migratory pronuclei after an additional division. Partners exchange migratory pronuclei, and each fuses its incoming and stationary pronucleus to establish a diploid zygotic nucleus. Postzygotic divisions generate new micronuclear and macronuclear lineages, while the old macronucleus is eliminated. The immediate products are two reorganized exconjugant cells, rather than a new single cell created by fusion of the entire parents. Details and product numbers vary across ciliates, so this sequence should not be imposed unchanged on every species.

Compatibility is also more varied than a male/female distinction. Tetrahymena thermophila has seven mating types, with compatibility between different types rather than differentiated sperm and eggs. Mating type is established during formation of the new somatic genome by gene-segment joining and deletion at the mating-type locus; the germline retains genetic potential not all expressed together in the macronucleus. The available outcomes are constrained by the inherited mating-type allele. Primary experiments connecting somatic rearrangements to mating-type expression demonstrate why a simple two-sex Mendelian account is inadequate.^[ref]

Some Paramecium undergo autogamy, in which meiosis and fusion of pronuclei derived from one cell renew the nuclear apparatus without a partner. This can generate pronounced homozygosity rather than introduce new alleles through outcrossing. The potential benefits of recombination and nuclear renewal must therefore be distinguished from the acquisition of genetic diversity from another individual. Sex is not invariably equivalent to outbreeding.

Formation of the new macronucleus also involves DNA elimination, amplification and extensive genome reorganization. Maternal somatic information can influence which sequences survive. In Paramecium tetraurelia, small-RNA-guided excision of the mtA promoter contributes to mating-type inheritance: the old macronuclear state can guide development of the new one.^[ref] This is an experimentally defined epigenetic pathway, not evidence that all acquired traits become heritable. After conjugation, exconjugants can share a newly combined micronuclear genotype yet develop distinct somatic states; later macronuclear assortment can further alter phenotype. Ciliate sex combines meiotic inheritance with somatic-genome reconstruction, and can serve nuclear renewal without immediate numerical reproduction or obligatory outcrossing.