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CLONAL UNDER STEADY STATE. "Under more constant optimal conditions, relatively simple organisms with short generation times are selected for high numbers of progeny, which can most efficiently be achieved by asexual reproduction. Thus, clonal reproduction may last for an extended period. However, when adverse conditions or internal constraints are encountered, sex is induced." https://lnkd.in/deZ8DED View in LinkedIn

SEX IN EMERGENCIES. "It has also been pointed out that for a single cell the costs of meiosis are high due to the time and effort it has to invest and that, in protists, sexual reproduction is often associated with their survival of harsh periods." https://lnkd.in/deZ8DED View in LinkedIn

ROTIFERS AND CILIATES. "Although meiotic sex was lost in a few multicellular lineages millions of years ago, they constitute rare exceptions. These lineages represent easily amenable multicellular systems, and population genetic analysis showed that they do not suffer from the lack of genetic diversity, which is retained among different clones." https://lnkd.in/deZ8DED View in LinkedIn

CASE STUDY. "If we accept the view that any costly nonadvantageous mechanism is subject to eventual loss, sex retention proves its usefulness, and bdelloid rotifers and ciliates, possibly having done without sex over an extended time period, might enlighten us regarding that use." https://lnkd.in/deZ8DED View in LinkedIn

GENES, CELLS, AND NUCLEI. "In the excitement over the recognition of meiosis-associated genes all over the eukaryotic phylogenetic tree, one might forget that fusion of both cells and nuclei is also critical during the sexual cycle. However, gamete fusion (plasmogamy) and nuclear fusion (karyogamy) have been studied much less than meiosis." https://lnkd.in/deZ8DED 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

IMPORTED MACHINERY. “The enzymes and machinery for meiosis presumably evolved from a core set of DNA-manipulating enzymes brought in and modified as needed from prokaryotic forebears, including both bacterial and archaeal lineages which introduces the DNA double-strand breaks (DSBs) that provoke meiotic recombination.” https://lnkd.in/ebVhXPf View in LinkedIn

HIGH CERTAINTY. “During the course of this evolutionary trajectory, the last eukaryotic common ancestor became sexual, reassorting and recombining chromosomes in a process that entails regulated fusions of haploid gametes and diploid → haploid reductions via meiosis. That the last eukaryotic common ancestor was sexual is no longer a matter of speculation/debate as evidence of sex, and of genes exclusively involved in meiosis, has been found in all of the major eukaryotic radiations.” http://cshperspectives.cshlp.org/content/6/3/a016154.full View in LinkedIn

PURGING. “Like parasexuality, meiosis also serves as a mechanism to purge deleterious mutations from the genome and also reveals recessive alleles in haploid progeny of a heterozygous diploid progenitor.” http://cshperspectives.cshlp.org/content/6/3/a016154.full View in LinkedIn