Origin Of Sexual Reproduction Research Articles

E Pluribus Unum: The Fungal Kingdom as a Rosetta Stone for Biology and Medicine.

THE Genetics Society of America's (GSA's) Edward Novitski Prize recognizes a single experimental accomplishment or a body of work in which an exceptional level of creativity, and intellectual ingenuity, has been used to design and execute scientific experiments to solve a difficult problem in genetics. The 2019 recipient is Joseph Heitman, who is recognized for his work on fungal pathogens of humans and for ingenious experiments using yeast to identify the molecular targets of widely used immunosuppressive drugs. The latter work, part of Heitman's postdoctoral research, proved to be a seminal contribution to the discovery of the conserved Target of Rapamycin (TOR) pathway. In his own research group, a recurring theme has been the linking of fundamental insights in fungal biology to medically important problems. His studies have included defining fungal mating-type loci, including their evolution and links to virulence, and illustrating convergent transitions from outcrossing to inbreeding in fungal pathogens of plants and animals. He has led efforts to establish new genetic and genomic methods for studying pathogenesis in Cryptococcus species. Heitman's group also discovered unisexual reproduction, a novel mode of fungal reproduction with implications for pathogen evolution and the origins of sexual reproduction.

On the origin of sexual reproduction: a hypothesis

SUMMARY Alternation of haploid and diploid is a common feature of the sexual reproduction. Before establishment of such sexual haploidization through meiosis and sexual diploidization through fertilization, I hypothesize the asexual cycle alternating asexual haploidization and asexual diploidization, which deserves to be called primitive sexual reproduction.

Complete Sequence and Analysis of Plastid Genomes of Two Economically Important Red Algae: Pyropia haitanensis and Pyropia yezoensis

Background Pyropia haitanensis and P. yezoensis are two economically important marine crops that are also considered to be research models to study the physiological ecology of intertidal seaweed communities, evolutionary biology of plastids, and the origins of sexual reproduction. This plastid genome information will facilitate study of breeding, population genetics and phylogenetics.Principal FindingsWe have fully sequenced using next-generation sequencing the circular plastid genomes of P. hatanensis (195,597 bp) and P. yezoensis (191,975 bp), the largest of all the plastid genomes of the red lineage sequenced to date. Organization and gene contents of the two plastids were similar, with 211–213 protein-coding genes (including 29–31 unknown-function ORFs), 37 tRNA genes, and 6 ribosomal RNA genes, suggesting a largest coding capacity in the red lineage. In each genome, 14 protein genes overlapped and no interrupted genes were found, indicating a high degree of genomic condensation. Pyropia maintain an ancient gene content and conserved gene clusters in their plastid genomes, containing nearly complete repertoires of the plastid genes known in photosynthetic eukaryotes. Similarity analysis based on the whole plastid genome sequences showed the distance between P. haitanensis and P. yezoensis (0.146) was much smaller than that of Porphyra purpurea and P. haitanensis (0.250), and P. yezoensis (0.251); this supports re-grouping the two species in a resurrected genus Pyropia while maintaining P. purpurea in genus Porphyra. Phylogenetic analysis supports a sister relationship between Bangiophyceae and Florideophyceae, though precise phylogenetic relationships between multicellular red alage and chromists were not fully resolved.ConclusionsThese results indicate that Pyropia have compact plastid genomes. Large coding capacity and long intergenic regions contribute to the size of the largest plastid genomes reported for the red lineage. Possessing the largest coding capacity and ancient gene content yet found reveal that Pyropia are more primitive multicellular red algae.

Looking to Bacteria for Clues

In his News Focus story “On the origin of sexual reproduction” (5 June, p. [1254][1]), C. Zimmer highlights the importance of the phylogenetic perspective championed by John Logsdon, but by considering only eukaryotes he overlooks an important bacterial clue to the evolution-of-sex puzzle.

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A selfish origin for recombination

Recent findings of molecular biology show that recombination is initiated by interactions between homologous chromosomes and that an allele can induce the initiation of recombination on the homolog. Since gene conversion at the site of initiation is strong enough to promote the transmission of that allele, recombination may be a way for a self-promoting element to spread, even if it gives no advantage to the individual or to the population. I develop a simple model and discuss available molecular evidence in support of this hypothesis. A consequent argument is that with asexual reproduction the evolution of recombination leads to an intragenomic conflict, and a possible outcome of this conflict may be the origin of sexual reproduction.

Origin, age and diversity of clones

The origin of sexual reproduction may well have been a unique evolutionary event, or series of events, but the maintenance of taxonomically widespread sex is not: it consists of many local skirmishes between sexual lineages and their asexual offshoots. Neither sexually reproducing populations nor asexual lineages are uniform in their characteristics. They vary in many ways that might in uence the outcome of their evolutionary interaction, such as their ecological role, genome size and complexity, level of genetic variation, mutation rate and, for sexual populations, freedom of recombination. Although sexual reproduction predominates in animals and plants, and asexual lineages are typically short-lived and taxonomically isolated, these general patterns should not be allowed to obscure the true diversity. There are long-lived, widespread, genetically and even taxonomically diverse asexual lineages at one extreme and sexual populations with no asexual descendants at the other. In between, there are sexual species with high levels of inbreeding and asexual lineages that hybridize with sexual relatives. A complete understanding of the evolution of reproductive modes will encompass these extremes as well as the typical pattern. It seems to us that only a pluralist approach is likely to be successful in the sense that there are complex patterns to explain, not a simple dichotomy. However, this is not equivalent to the approach advocated by West et al. (1999) who apparently wish to abandon the search for a single mechanism capable of explaining the predominance of sexual reproduction but at the same time ignore the variety of reproductive modes found in nature. West et al. (1999) draw much needed attention to the dynamics of the interaction between sexual species and asexual lineages. Initially, a new clonal lineage is very susceptible to parasites as it becomes abundant but, as it accumulates genetic diversity, this risk declines. Although individual asexual lineages may persist for short periods of time, asexual reproduction might persist if new clones originate with suf®cient frequency from the sexual population. It may be that building frequent origination of clones into the models of Howard & Lively (1994, 1998) would increase the parameter space in which asexual reproduction displaces sexual reproduction. On the other hand, West et al. (1999) argue (p. 19) that maintenance of clonal diversity and maintenance of sex are `somewhat separate issues'. While we agree that clonal diversity may be explained in part by resource partitioning, this is not readily separable from the outcome of interactions between sexual populations and asexual lineages: clonal diversity maintained by resource partitioning can make the asexual lineages more resistant to displacement by the sexual population because clones are better adapted to environmental conditions, because diversity in parasite resistance is maintained incidentally through linkage disequilibrium, and because resource partitioning allows higher population size and thus retards the ratchet. The diversity of clones, their modes of origin and their rates of turnover are empirical issues that need to be addressed. The standing diversity of clones is clearly a product of origination and extinction rates but these are very hard to separate. In nonmarine ostracods, for example, clonal diversity is highly variable, as detected by allozyme electrophoresis: from seven clones in Darwinula stevensoni to 211 clones in Eucypris virens with comparable sampling efforts across Europe (Rossi et al., 1998). However, the reasons for this variation are largely unexplored. Clonal diversity may be generated in at least three ways (Butlin et al., 1998): mutation within existing clones (including autopolyploidy), separate origin of clones from a sexual ancestor, or hybridization between asexual females and males of the same or related species (usually generating triploid offspring). Only the ®rst process is available to D. stevensoni, which lacks sexual relatives, but multiple origins of asexual reproduction and hybridization have both been demonstrated in ostracod species with sexual populations or closely related sexual species, including E. virens (Turgeon & Hebert, 1995; SchoEn & Butlin, 1998). It has been suggested that species with sexual congeners tend to have higher clonal diversity than those that do not (Havel & Hebert, 1989). On the other hand, very little is known about rates of turnover. Grif®ths & Butlin (1995) found that asexual species were less abundant, and more variable in abundance, than sexual species in Holocene fossil sequences. Note that the term `asexual species' here refers to a set of morphologCorrespondence: Dr R. K. Butlin, School of Biology and Centre for Biodiversity and Conservation, The University of Leeds, Leeds LS2 9JT, UK. E-mail: r.k.butlin@leeds.ac.uk

Press Perturbation in Lattice Ecosystems: Parity Law and Optimum Strategy

Press perturbation, in which the reproduction rate r of a target species is altered and held at higher or lower levels, is studied by computer simulation. We apply a lattice Lotka-Volterra model to idealized ecosystems, and obtain the optimum strategy roptdefined as follows: the steady-state density of the target species takes a maximum value at r=ropt. It is found that ropttakes a finite value, and it usually locates at a phase transition point. Moreover, we find that the long-term response to applied perturbation can be significantly predicted by parity law, where the parity is defined by whether the system contains an even or odd number of species. If the parity is even (odd), the density of the target species increases (decreases) with increasing r. An evolutionary meaning of roptis discussed in connection with the origin of sexual reproduction. Copyright 1999 Academic Press.

The origin and evolution of sexual reproduction up to the evolution of the male-female phenomenon

Sexual reproduction is a composite, not a singular, phenomenon and as such can be subdivided into a number of componentsi.e. fusion, recombination, fission, and the male-female phenomenon. These components can evolve independently, though any evolutionary change in one component is likely to influence the future evolution of the other components. The ambiguity that surrounds the term ‘sex’ due to a failure to recognise the composite nature of sexual reproduction has led to considerable confusion in past discussions of the evolution of the phenomenon. This paper considers the possible chronological interaction of the components of sexual reproduction both with each other and with the sequence of selective pressures that seem likely to have acted. This chronological approach is used to consider: the origin of sexual reproduction; the evolution of sexual reproduction in the common ancestor of the procaryotes and eucaryotes; the modification of the ancestral system in the procaryote line following the procaryote-eucaryote dichotomy; and the modification of the ancestral system in the eucaryote line up to the origin of the male-female phenomenon.