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Origins of Eukaryotic Sexual Reproduction Downloaded from http://cshperspectives.cshlp.org/ on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press Origins of Eukaryotic Sexual Reproduction Ursula Goodenough1 and Joseph Heitman2 1Department of Biology, Washington University, St. Louis, Missouri 63130 2Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, North Carolina 27710 Correspondence: [email protected]; [email protected] Sexual reproduction is a nearly universal feature of eukaryotic organisms. Given its ubiquity and shared core features, sex is thought to have arisen once in the last common ancestor to all eukaryotes. Using the perspectives of molecular genetics and cell biology, we con- sider documented and hypothetical scenarios for the instantiation and evolution of meio- sis, fertilization, sex determination, uniparental inheritance of organelle genomes, and speciation. he transition from prokaryote to protoeu- tion of ploidy via cell–cell fusion and meiosis; Tkaryote to the last eukaryotic common an- (2) mating-type regulation of cell–cell fusion cestor (LECA) entailed conservation, modifica- via differentiation of complementary haploid tion, and reconfiguration of preexisting genetic gametes (isogametic and then anisogametic), circuits via mutation, horizontal gene transfer a prelude to species-isolation mechanisms; (3) (HGT), endosymbiosis, and selection, as de- mating-type-regulated coupling of the dip- tailed in previous articles of this collection. Dur- loid/meiotic state to the formation of adap- ing the course of this evolutionary trajectory, the tive diploid resting spores; and (4) mating- LECAbecame sexual, reassorting and recombin- type-regulated transmission of organelle ge- ing chromosomes in a process that entails regu- nomes. Our working assumption is that lated fusions of haploid gametes and diploid ! the protoeukaryote ! LECA era featured nu- haploid reductions via meiosis. That the LECA merous sexual experiments, most of which was sexual is no longer a matter of speculation/ failed but some of which were incorporat- debateasevidenceofsex,andofgenesexclusively ed, integrated, and modified. Therefore, this involved in meiosis, has been found in all of the list is not intended to suggest a sequence of major eukaryotic radiations (Brawley and John- events; rather, the four innovations most like- son 1992; Ramesh et al. 2005; Kobiyama et al. ly coevolved in a parallel and disjointed fash- 2007; Malik et al. 2008; Phadke and Zufall ion. 2009; Fritz-Laylin et al. 2010; Lahr et al. 2011; Once these core sexual-cycle themes were in Peacock et al. 2011; Vanstechelman et al. 2013). place, the evolution of eukaryotic sex has fea- We propose that the transition to a sexual tured countless prezygotic and postzygotic var- LECA entailed four innovations: (1) alterna- iations, the outcome being the segregation of Editors: Patrick J. Keeling and Eugene V. Koonin Additional Perspectives on The Origin and Evolution of Eukaryotes available at www.cshperspectives.org Copyright # 2014 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a016154 Cite this article as Cold Spring Harb Perspect Biol 2014;6:a016154 1 Downloaded from http://cshperspectives.cshlp.org/ on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press U. Goodenough and J. Heitman panmictic populations into distinct species cell–cell fusion entailed the engulfment of one with distinctive adaptations. protoeukaryote by another, with the internal- For additional reviews on the evolution of ized cell membrane then either digested or fused sex, the interested reader is referred to Goode- with the host cell membrane from within. nough (1985), Dacks andRoger (1999), Schurko Increases in ploidy confer indubitable ben- et al. (2009), Wilkins and Holliday (2009), Gross efits: The resultant redundancy allows novel se- and Bhattacharya (2010), Lee et al. (2010), quences/functions to arise in duplicate genes Perrin (2012), and Calo et al. (2013). without compromising existing pathways, and recessive nonadaptive alleles, masked but car- ried through time, may prove to be adaptive ALTERNATION OF PLOIDY VIA CELL–CELL in future contexts. Another potential benefit FUSION AND MEIOSIS of cell–cell fusion is hybrid fitness: If, as seems likely, there existed a variety of fledgling proto- The Benefits and Challenges of Increased eukaryotes in the population that eventually Genome Size and Ploidy gave rise to the LECA, then their fusion would Modern bacteria, and presumably their fore- be expected to have yielded more gene-rich and bears, are adept at taking up DNA from outside occasionally more successful lineages. sources (Narra and Ochman 2006). The proto- Increases in genome size/ploidy are expect- eukaryote, probably at some early stage in its ed to eventually become deleterious, however, evolution, was also phagocytic, at least occasion- one challenge being to organize a successful mi- ally, salient evidence being its engulfment of a tosis, another to regulate appropriate levels of proteobacterium that was then domesticated as gene expression. Hence, there would presum- the mitochondrion. Phagocytosis in modern ably have been positive selection for the acqui- cells is a complex process involving hundreds sition of mechanisms to maintain copy number of proteins (Boulais et al. 2010), but early ver- at a manageable size. sions can be assumed to have been less sophisti- In recent studies, a broad panel of Saccha- cated, and cell membranes capable of engaging romyces cerevisiae and Saccharomyces paradoxus in phagocytosis would presumably also have strains were tested as haploids and diploids un- been capable of engaging in cell–cell fusions, der diverse conditions (Zorgo et al. 2013). For as is the case for wall-less mutants of modern about half of the conditions tested in which bacteria (Errington 2013). It has been suggested there was a difference (such as growth with ra- (Rose 1983; Hickey 1982, 1993) that early cell– pamycin or phleomycin), haploids were more cell fusions might have been promoted by “self- fit, whereas for the other half (such as exposure ish” transposons and plasmids that incur repli- to heat or ethanol), diploids had increased fit- cative advantage in novel genomiccontexts; pos- ness. Which ploidy state was less fit was highly sible evidence for this hypothesis (Keeling and correlated between the two species that are di- Roger 1995) is the use of the HO endonuclease verged over several million years. These findings or a transposase (Barsoum et al. 2010; Rusche suggest that the ability to interconvert from and Rine 2010), conscripted from ancestral haploid to diploid and back again might be transposable elements, to effect mating-type beneficial when conditions shift from those fa- switching in several yeasts. voring the haploid state to those favoring the Cell–cell fusion generates an increase in diploid state (or vice versa). Thus, sexual repro- chromosome number, and although the large duction might confer benefits for organisms size of modern eukaryotic genomes could also just by enabling these rapid ploidy transitions, have been the consequence of endomitosis, it independent of any role in shuffling genetic seems likely that cell–cell fusions contributed composition by recombination, with endorepli- to genome expansion during protoeukaryote cation being another avenue. That said, the abil- evolution. An alternative possibility, leading to ity to toggle from haploid to diploid and back the same outcome, is that the earliest versions of again is dependent on a mechanism for ploidy 2 Cite this article as Cold Spring Harb Perspect Biol 2014;6:a016154 Downloaded from http://cshperspectives.cshlp.org/ on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press Origins of Eukaryotic Sexual Reproduction reduction, which, in modern eukaryotes, entails Parasexuality in Candida albicans meiotic or parasexual processes. Wealso know about an unusual parasexual cycle that is extant in the most common human fun- PARASEXUALITY gal pathogen, Candida albicans (Bennett et al. In the following sections, in which we consider 2005; Sherwood and Bennett 2009; Berman the origins and evolution of parasexual and 2012). In this species, mating occurs between meiosis-based sexual cycles, we use as examples diploids when the mating-type locus is homo- modern organisms whose mating-type-based zygous (a/a and a/a) and the cells switch to a sexual differentiation is already established. In specialized mating cell type called opaque (Mil- subsequent sections we will consider how sexual ler and Johnson 2002). Cell–cell fusion then differentiation itself might have originated and generates tetraploid cells, and adverse media evolved. conditions stimulate random chromosome loss to return to a diploid, or near diploid, state via a parasexual process (Bennett and Johnson Parasexuality in Aspergillus 2003; Forche et al. 2008). Given that Candida is As one must walk before one can run, the fusion embedded among sexual fungi (Butler et al. of cells may have led to early parasexual cycles 2009), this is presumably an example of a de- from which true meiotic sexual cycles evolved at rived parasexual state, but one can envision a later time. We know a great deal about extant analogous earlier versions of genetic exchange parasexual cycles from the classic work of Pon- involving cell–cell fusion followed by parasexu- tecorvo on the filamentous fungus Aspergillus al ploidy change from which true sexual cycles nidulans in the
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