Figure 2.

A putative model for the evolution of meiosis from archaeal conjugation. A. Ancestral archaeal conjugation (as described in H. volcanii) involving cell fusions, bidirectional flow of plasmids, and recombination between parental chromosomes (dark blue and green, respectively) [51,52]. B and C. Chromosome linearization permitted efficient pairing of homologues and resolution of crossovers [11,67]. Telomeres (orange) evolved to protect chromosome termini and to nucleate the pairing of homologues [11,67]. A centromere (orange region in the centre of chromosomes) served as a connection between sister chromatids and as an attachment site, via kinetochores, for the meiotic spindles [11,13,99]. This consisted of a network of microtubules (red fibers) radiating from a microtubule-organizing center (red circle) that guided chromosome movement [11,13,99]. The proto-ER progressively (B - F) differentiated into the NE [26] by wrapping segments of chromosomes to scaffold chromosome pairing (B - E) and to constrain diffusion of broken chromosome segments (C). D. Spindle-mediated movements approximate parental chromosomes during mating [10,12]. E. Incipient karyogamy mechanics evolved to fuse proto-NE segments associated with chromosomes to create a common membrane platform to assemble, via clustering of telomeres, the meiotic bouquet [11,13,67]. F. Cytokinesis based on an actomyosin contractile ring (red) facilitated splitting of the fusion partners (i.e., reductional meiotic division) [99]. NE enclosed the nuclear compartment when nuclear pores (yellow cylinders) evolved to ensure nucleo-cytoplasmic traffic of proteins and RNA [26].