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THE SHELTERED DIPLOID. “Aspergillus has a bona fide homothallic (selfing) sexual cycle, but also undergoes a parasexual cycle under laboratory conditions: Diploids are generated from haploid progenitors by hyphal fusion and the resulting diploid then loses chromosomes randomly to return to the haploid state. Recent studies have shown that this parasexual cycle provides access to a sheltered diploid state that can serve as a capacitor for evolution.” https://lnkd.in/ebVhXPf View in LinkedIn

TWO PHASES. “As one must walk before one can run, the fusion of cells may have led to early parasexual cycles from which true meiotic sexual cycles evolved at a later time.” https://lnkd.in/ebVhXPf View in LinkedIn

CUMBERSOME. “Increases in genome size/ploidy are expected to eventually become deleterious, however, one challenge being to organize a successful mitosis, another to regulate appropriate levels of gene expression. Hence, there would presumably have been positive selection for the acquisition of mechanisms to maintain copy number at a manageable size.” https://lnkd.in/ebVhXPf View in LinkedIn

FITNESS BENEFIT. “Another potential benefit of cell–cell fusion is hybrid fitness: If, as seems likely, there existed a variety of fledgling protoeukaryotes in the population that eventually gave rise to the last eukaryotic common ancestor, then their fusion would be expected to have yielded more gene-rich and occasionally more successful lineages.” http://cshperspectives.cshlp.org/content/6/3/a016154.full View in LinkedIn

UTILITY OF REDUNDANCY. “Increases in ploidy confer indubitable benefits: The resultant redundancy allows novel sequences/functions to arise in duplicate genes without compromising existing pathways, and recessive nonadaptive alleles, masked but carried through time, may prove to be adaptive in future contexts.” https://lnkd.in/ebVhXPf View in LinkedIn

ENGULFMENT. “An alternative possibility, leading to the same outcome, is that the earliest versions of cell–cell fusion entailed the engulfment of one protoeukaryote by another, with the internalized cell membrane then either digested or fused with the host cell membrane from within.” http://cshperspectives.cshlp.org/content/6/3/a016154.full View in LinkedIn

PROVENANCE. “That Spo11 was present in the last eukaryotic common ancestor can be inferred by its presence in all of the major eukaryotic lineages studied thus far, with the notable exception of Dictyostelium discoidium and closely related slime molds that have apparently lost Spo11 but undergo complete meiotic sexual cycles.” https://lnkd.in/ebVhXPf View in LinkedIn

GENOME EXPANSION. “Cell–cell fusion generates an increase in chromosome number, and although the large size of modern eukaryotic genomes could also have been the consequence of endomitosis, it seems likely that cell–cell fusions contributed to genome expansion during protoeukaryote evolution.” http://cshperspectives.cshlp.org/content/6/3/a016154.full View in LinkedIn

BOLT-ON. “Spo11 is derived from an ancestral archaeal topoisomerase VI homolog that became included in the protoeukaryote genome and was then adapted for meiosis. In essence, Spo11 is a topoisomerase that lost the ability to re-ligate DNA.” https://lnkd.in/ebVhXPf View in LinkedIn

PROK —> EUK: FOUR INNOVATIONS. “We propose that the transition to a sexual last eukaryotic common ancestor entailed four innovations: (1) alternation of ploidy via cell–cell fusion and meiosis; (2) mating-type regulation of cell–cell fusion via differentiation of complementary haploid gametes (isogametic and then anisogametic), a prelude to species-isolation mechanisms; (3) mating-type-regulated coupling of the diploid/meiotic state to the formation of adaptive diploid resting spores; and (4) mating-type-regulated transmission of organelle genomes.” https://lnkd.in/ebVhXPf View in LinkedIn