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Are genetic crosses between asexual organisms possible?

Are genetic crosses between asexual organisms possible?


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To my knowledge (Please correct me if I am wrong), genetic basis is the key in defining species. When we encounter an unknown species, we can sequence it's genome and compare the genome with other known organisms from the genome data bank and see if there is a significant similarity. If no significance were found then we can call it a newly discovered organism. Also careful study of genetic crosses between species pairs and partially isolated sub species to identify general patterns in the genetics and origin of reproductive isolation helps to answer the question of speciation. Then how would we cross asexual organisms like many bacteria or protists? Is it possible to cross asexual organisms? If not, how do scientists study their genetics and define species of asexual organisms?


When it comes to viruses and bacteria, genetic cross is just as a vaguely defined concept as species. The latter is often defined on the percentage of the sequence similarity (for viruses) or the percentage of common genes (for bacteria).

If we speak about simply mixing genomes of different organisms, there are many possibilities, how this could happen. Just to give a couple of examples:

  • Some viruses integrate their genome into the host genome. In some cases these integrated genomes cease being functional viruses, and may evolve to be functional genes of the host. This is truly a genetic cross. See here, for example.
  • Bacteria and viruses routinely undergo horisontal gene transfer in which genes from one bacteria are integrated into the genome of the other one. A typical bacterial species has a few hundred core genes, and thousands of genes that are present only in some specimen or borrowed from other species, which significantly vary from one bacteria to another. Thus, every individual bacteria or bacterial community can be thought of as a genetic cross.

Hybridization promotes asexual reproduction in Caenorhabditis nematodes

Although most unicellular organisms reproduce asexually, most multicellular eukaryotes are obligately sexual. This implies that there are strong barriers that prevent the origin or maintenance of asexuality arising from an obligately sexual ancestor. By studying rare asexual animal species we can gain a better understanding of the circumstances that facilitate their evolution from a sexual ancestor. Of the known asexual animal species, many originated by hybridization between two ancestral sexual species. The balance hypothesis predicts that genetic incompatibilities between the divergent genomes in hybrids can modify meiosis and facilitate asexual reproduction, but there are few instances where this has been shown. Here we report that hybridizing two sexual Caenorhabditis nematode species (C. nouraguensis females and C. becei males) alters the normal inheritance of the maternal and paternal genomes during the formation of hybrid zygotes. Most offspring of this interspecies cross die during embryogenesis, exhibiting inheritance of a diploid C. nouraguensis maternal genome and incomplete inheritance of C. becei paternal DNA. However, a small fraction of offspring develop into viable adults that can be either fertile or sterile. Fertile offspring are produced asexually by sperm-dependent parthenogenesis (also called gynogenesis or pseudogamy) these progeny inherit a diploid maternal genome but fail to inherit a paternal genome. Sterile offspring are hybrids that inherit both a diploid maternal genome and a haploid paternal genome. Whole-genome sequencing of individual viable worms shows that diploid maternal inheritance in both fertile and sterile offspring results from an altered meiosis in C. nouraguensis oocytes and the inheritance of two randomly selected homologous chromatids. We hypothesize that hybrid incompatibility between C. nouraguensis and C. becei modifies maternal and paternal genome inheritance and indirectly induces gynogenetic reproduction. This system can be used to dissect the molecular mechanisms by which hybrid incompatibilities can facilitate the emergence of asexual reproduction.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1. Crossing C . nouraguensis females…

Fig 1. Crossing C . nouraguensis females to C . becei males results in sterile…

Fig 2. Asexually-produced F1 females are diploid.

Fig 2. Asexually-produced F1 females are diploid.

Fig 3. Asexually-produced fertile interspecific F1 inherit…

Fig 3. Asexually-produced fertile interspecific F1 inherit two randomly selected homologous chromatids from each maternal…

Fig 4. Sterile interspecific F1 inherit a…

Fig 4. Sterile interspecific F1 inherit a diploid C . nouraguensis genome and a haploid…

Fig 5. Dead interspecific F1 embryos inherit…

Fig 5. Dead interspecific F1 embryos inherit the C . becei X-chromosome and two maternal…

Fig 6. Cytological characterization of early embryonic…

Fig 6. Cytological characterization of early embryonic development in interspecies hybrids.

Fig 7. Diploid maternal inheritance can occur…

Fig 7. Diploid maternal inheritance can occur independently of interspecies hybridization.

Fig 8. Models for diploid maternal inheritance…

Fig 8. Models for diploid maternal inheritance and paternal genome loss in C . nouraguensis…


Types of Asexual Reproduction

There are many different ways to reproduce asexually. These include:

1. Binary fission. This method, in which a cell simply copies its DNA and then splits in two, giving a copy of its DNA to each “daughter cell,” is used by bacteria and archaebacteria.

2. Budding. Some organisms split off a small part of themselves to grow into a new organism. This is practiced by many plants and sea creatures, and some single-celled eukaryotes such as yeast.

3. Vegetative propagation. Much like budding, this process involves a plant growing a new shoot which is capable of becoming a whole new organism. Strawberries are an example of plants that reproduce using “runners,” which grow outward from a parent plant and later become separate, independent plants.

4. Sporogenesis. Sporogenesis is the production of reproductive cells, called spores, which can grow into a new organism.

Spores often use similar strategies to those of seeds. But unlike seeds, spores can be created without fertilization by a sexual partner. Spores are also more likely to spread autonomously, such as via wind, than to rely on other organisms such as animal carriers to spread.

5. Fragmentation. In fragmentation, a “parent” organism is split into multiple parts, each of which grows to become a complete, independent “offspring” organism. This process resembles budding and vegetative propagation, but with some differences.

For one, fragmentation may not be voluntary on the part of the “parent” organism. Earthworms and many plants and sea creatures are capable of regenerating whole organisms from fragments following injuries that split them into multiple pieces.

When fragmentation does occur voluntarily, the same parent organism may split into many roughly equal parts in order to form many offspring. This is different from the processes of budding and vegetative propagation, where an organism grows new parts which are small compared to the parent and which are intended to become offspring organisms.

6. Agamenogenesis. Agamenogenesis is the reproduction of normally sexual organisms without the need for fertilization. There are several ways in which this can happen.

In parthenogenesis, an unfertilized egg begins to develop into a new organism, which by necessity possesses only genes from its mother.

This occurs in a few species of all-female animals, and in females of some animal species when there are no males present to fertilize eggs.

In apomoxis, a normally sexually reproducing plant reproduces asexually, producing offspring that are identical to the parent plant, due to lack of availability of a male plant to fertilize female gametes.

In nucellar embryony, an embryo is formed from a parents’ own tissue without meiosis or the use of reproductive cells. This is primarily known to occur in citrus fruit, which may produce seeds in this way in the absence of male fertilization.


Difference Between Monoecious and Dioecious Organisms

Sexual reproduction implies the fusion of a female gamete with a male gamete to produce a fertilized zygote which will develop into an individual similar to its parental organism. Depending on the gametes produced, these organisms are classified as monoecious and dioecious. This article explains the difference between the two with suitable examples.

Sexual reproduction implies the fusion of a female gamete with a male gamete to produce a fertilized zygote which will develop into an individual similar to its parental organism. Depending on the gametes produced, these organisms are classified as monoecious and dioecious. This article explains the difference between the two with suitable examples.

Dual Nature
The common aquatic weed, Hydrilla, exhibits distinct biotypes, of which some are monoecious and some are dioecious, i.e., it has unisexual as well as bisexual strains/varieties.

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The system of sex-determination ascertains the development of specific sexual characteristics in an individual organism. This system is yet to be fully understood, but a few pathways have been partially deciphered. In some species, the gender can be determined by examining the genetic constitution of the organism. This holds true for organisms that show the presence of sex chromosomes. In other species, sex can be decided on the basis of the environmental conditions, the organism’s relative size, or the gender of other individuals within the population. Some species exhibit two genders, while some exhibit as many as seven genders like the Tetrahymena genus. Alternatively, some species exist that do not possess a permanent gender, rather they change their gender from one to the other during the course of their lifespan. This unique phenomenon is called sequential hermaphroditism.

In comparison, a majority of organisms show the presence of only two genders. However, there exists great diversity in the way these genders are exhibited by various species. Broadly, these organisms can be classified into two types – monoecious and dioecious organisms. The difference between these types are explained in the following table.


Materials and methods

Study species

Allium vineale L. is a weedy, bulbous perennial plant distributed over most of west and central Europe, extending northwards to central Scandinavia. It also occurs in, but is not native to, North America, Australia and New Zealand. In continental Europe, A. vineale prefers dry, open habitats such as old fields and roadsides. In Sweden it is mostly found along the coastline and in the steppe-like grasslands of the Baltic islands of Öland and Gotland.

The flowering season starts in May or June, when the scape, or flowering stalk, emerges and the spathe bursts open to expose the inflorescence. Each flowering plant carries only a single inflorescence. Flowers are strongly protandrous and are visited by insects, mainly bumblebees. The fruit is trilocular with two seeds in each loculus, but it is rare that all seeds develop ( 35 ). Alliumvineale is self-compatible however, the strong protandry and the observation that insect pollinators are required for seed-set (personal observation) suggest that outcrossing is the norm (see also 38 ). This conclusion is supported by allozyme data (A. Ceplitis, unpublished data).

Bulbils mature in August to September, and seeds usually mature 1–2 months later. Both bulbils and seeds are dispersed by gravity. After flowering the scape dies back and underground offset bulbs replace the parent bulb. As a means of vegetative propagation, the offset bulbs are presumably of minor importance compared with the numerically superior bulbils, in particular as the underground bulbs are known to have a high mortality rate as a result of fungal infections ( 35 13 ).

Sampling procedure

During a period of 4 years (1995–98) A. vineale plants were sampled from five geographical locations (sites) in southern Sweden (Fig. 1). Each year, sampling took place at two separate occasions. In the first sampling, carried out in July or August, inflorescences were collected from individual plants. For each inflorescence, three traits were scored: the number of flowers, the number of bulbils, and the proportion of flowers (the number of flowers divided by the sum of the numbers of flowers and bulbils). The number of flowers and the number of bulbils were chosen to represent the individual plant’s allocation of resources to sexual and aboveground asexual reproduction, respectively. The proportion of flowers was taken to represent the relative importance of sexual reproduction. These three traits are henceforth collectively referred to as allocation traits.

Map of southern Sweden showing the five sites from which samples of Alliumvineale were taken each year between 1995 and 1998. The insert map depicts the area using a larger scale.

In late September to October, the second sampling was carried out. At this point, inflorescences from flowering individuals were collected to obtain data on seed production. For each inflorescence, the average number of seeds per flower was determined. This quantity was defined as the total number of seeds produced by the inflorescence divided by the total number of flowers, and it is taken to represent the relative efficiency of sexual reproduction.

The reason for sampling at two different occasions each year is that many plants have lost some or most of their bulbils when seeds mature, thereby making it impossible to correctly determine the number of bulbils in the inflorescences. The average sample size per site and per year was 78 and 97, respectively, for the first sampling, and 84 and 106, respectively, for the second sampling. At each sampling occasion, plants were sampled at intervals of at least 2 m, while trying to cover the entire area taken up by the plant population. Final sample sizes were unequal, as plants that had lost bulbils (first sampling) or seeds (second sampling) were discarded.

In addition to collecting data directly from the field, asexually derived (bulbil) offspring of individuals sampled in 1995 (first sample) were grown under uniform conditions in a greenhouse. Bulbils from the mother plants were germinated in Petri dishes. The ‘seedlings’, one from each mother, were transferred to individual pots approximately one week after germination. The offspring plants were randomized in the greenhouse and subjected to cold treatment in order to induce flowering. The plants flowered in the following year when the number of flowers, the number of bulbils, and the proportion of flowers in the individual inflorescences were determined. As all offspring plants produced an inflorescence, sample sizes were equal to those for the 1995 sample (Table 1).

RAPD analysis

Multilocus genotypes were inferred from RAPD fingerprints for all individuals that were sampled on the first occasion every year, i.e. individuals that were scored for allocation traits (sample sizes are given in Table 1). As the RAPD analysis were performed on DNA extracted from bulbils which are asexually produced propagules, the genotype of the plant that produced the bulbils, and on which all allocation traits were scored, was assumed to be identical to that of the bulbil.

Five to ten bulbils from each individual chosen for RAPD-analysis were germinated in Petri dishes. One germinated bulbil per mother plant was used for DNA extraction. Total DNA was extracted from fresh leaf material using the method of 11 as modified by 32 .

One hundred and fourty random decamer primers (kit A-G, Operon Technologies, Inc., Alameda, CA, USA) were used in an initial screening procedure. Five primers (B05, C13, D08, E11, and F04) gave stable and reproducible band patterns, and these were chosen for the analysis of genet identity.

Polymerase chain reactions (PCR) contained 10 m M Tris–HCl, 2.0 m M MgCl2, 50 m M KCl, 100 μ M each of the four dNTPs, 0.4 μ M primer, and 1 unit of Taq polymerase (Boehringer, Mannheim, Germany) in a total of 25 μL reaction mixture. Polymerase chain reactions were carried out in a Perkin–Elmer Cetus Thermocycler with the following programme: 3 min at 94 °C, followed by 45 cycles of 1 min at 94 °C, 1 min at 36 °C and 2 min at 72 °C, ending with 5 min at 72 °C.

Two replicate PCR reactions were performed for each individual. Amplified fragments were visualized on ethidium bromide-stained 1.5-% agarose gels. Fragment patterns from replicate PCR reactions were identical and only clear and easily detected fragments were used. In a few cases, when the presence/absence of some fragments was uncertain, additional PCR reactions were performed. Individuals with identical fragment patterns were considered to belong to the same genet, whereas individuals with different fragment patterns were treated as belonging to different genets.

Analysis of variation in allocation traits

Using the information from the RAPD analysis, variation for each of the allocation traits was analysed using a partially nested model:

where Pijkl is the phenotypic value of the lth individual plant belonging to the kth genet at the jth site in the ith year, μ is the overall mean, yi is the contribution from the ith year, sj is the contribution from the jth site, g[s]jk is the contribution from the kth genet nested within the jth site, y·g[s]ijk is the contribution from the interaction between the ith year and the kth genet nested within the jth site, and ɛijkl is the error term. From this model the total phenotypic variation (σ 2 p) of each allocation trait was partitioned into variance components due to year (σ 2 y), site (σ 2 s), genet within site (σ 2 g[s]), and the interaction between year and genet within site (σ 2 y·g[s]).

The rationale of this design was the analysis of the relative contributions from different sources of variation, i.e. to estimate the variance components. Thus, all effects were treated as random in the analyses. Treating the main effects (year and site) as random can be justified by regarding them as samples from an underlying distribution of factors that cause variation in the mean phenotype (see 3 ). The among-year variation component was interpreted as reflecting short-term environmental fluctuations, whereas the among-site variation component was supposed to result from long-term environmental and/or genetic differences. The magnitude of the among-genet variation component is directly related to the amount of genetic variation found, on average, at a site. Finally, the interaction term indicates the extent to which differences between genets within sites vary over years, thus giving an estimate of the importance of genotype–environment interactions to the phenotypic variation (cf. 42 ).

Among-genet variance components for the allocation traits were also estimated separately for each population and year. These one-way analyses were performed to obtain a more detailed picture of the amount of within-site genetic variation that is expressed under field conditions and to see whether genet differences remain stable over years. The significance levels in these analyses were adjusted by a sequential Bonferroni technique ( 33 ) to account for the multiple simultaneous test performed on each trait.

Data from the greenhouse-grown bulbil-derived offspring plants were subjected to one-way analysis of variance ( ANOVAS ) to test to what extent site differences and/or genet differences for allocation traits found under natural conditions could also be detected in a uniform environment. To investigate whether the ranks of the site means remained similar in the greenhouse environment, Kendall’s coefficient of rank correlation, τ, was used to compare the site ranks of the bulbil-derived plants with the site ranks of their field mothers (from the 1995 sample).

Because of the unbalanced nature of the data, general linear model ANOVA was used to evaluate the statistical significance of different factors, whereas a maximum-likelihood method was used to estimate the variance components. The latter method has been shown to provide more accurate estimates of variance components from unbalanced data than other methods ( 48 15 ). To meet the assumptions of normality required in the analyses, the following transformations were used: number of flowers, X’=log(√(X + 1)) number of bulbils, Y′=√Y flower proportion, Z′=log(√Z + √(Z + 2)).

As the genealogical relationships within the samples were unknown and non-identical genets may show various degrees of relatedness, no attempt was made to partition the variation among genets into additive and non-additive components ( 29 12 ). However, if the allocation traits under study are to some extent genetically determined, closely related genets are expected to show a higher phenotypic resemblance than more distantly related ones ( 12 ). To explore this assumption, an analysis of the correlation between phenotypic similarity (estimated as the distance between clonal means, see, e.g. 10 ) and genetic similarity (estimated as the proportion of shared RAPD bands 24 ) was performed using a Mantel test ( 47 ). This procedure thus constitutes an additional test for phenotypic trait differences among genets.

Analysis of variation in seed production

The analyses of the average number of seeds per flower were similar to the analyses of allocation traits. Total variation was partitioned into components because of variation among years, among sites, and to the interaction between years and sites. As information on genet identity in this data set was available only for one of the 4 years (1996), a full partitioning into the variance components postulated for the allocation traits was not performed. A separate ANOVA was, however, carried out on the 1996 sample to examine whether there were any differences in seed-set among genets.

Statistical significance testing and variance component estimation procedures were the same as those for the analyses of the allocation traits. No normality transformation was required for the seed production data. All statistical analyses were carried out using the GLM, VARCOMP and FREQ procedures of the SAS statistical package ( 41 ).


Acknowledgments

We thank S. Glémin for discussion and for sharing his algorithm and B. Charlesworth, D. Charlesworth, P. Keightley, S. Otto, and an anonymous reviewer for discussion and comments on earlier versions of the manuscript. We are also grateful to S. Otto for pointing out the full extent of the analogy to the Hill–Robertson effect. This work was supported by the Swiss National Science Foundation, a Marie Curie Fellowship (C.R.H.), and a European Molecular Biology Organization Fellowship (D.R.).


Transformation

Frederick Griffith was the first to demonstrate the process of transformation. In 1928, he showed that live, nonpathogenic Streptococcus pneumoniae bacteria could be transformed into pathogenic bacteria through exposure to a heat-killed pathogenic strain. He concluded that some sort of agent, which he called the “transforming principle,” had been passed from the dead pathogenic bacteria to the live, nonpathogenic bacteria. In 1944, Oswald Avery (1877–1955), Colin MacLeod (1909–1972), and Maclyn McCarty (1911–2005) demonstrated that the transforming principle was DNA (see Using Microorganisms to Discover the Secrets of Life).

In transformation, the prokaryote takes up naked DNA found in its environment and that is derived from other cells that have lysed on death and released their contents, including their genome, into the environment. Many bacteria are naturally competent, meaning that they actively bind to environmental DNA, transport it across their cell envelopes into their cytoplasm, and make it single stranded. Typically, double-stranded foreign DNA within cells is destroyed by nucleases as a defense against viral infection. However, these nucleases are usually ineffective against single-stranded DNA, so this single-stranded DNA within the cell has the opportunity to recombine into the bacterial genome. A molecule of DNA that contains fragments of DNA from different organisms is called recombinant DNA. (Recombinant DNA will be discussed in more detail in Microbes and the Tools of Genetic Engineering.) If the bacterium incorporates the new DNA into its own genome through recombination, the bacterial cell may gain new phenotypic properties. For example, if a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and then incorporates it into its chromosome, it, too, may become pathogenic. Plasmid DNA may also be taken up by competent bacteria and confer new properties to the cell. Overall, transformation in nature is a relatively inefficient process because environmental DNA levels are low because of the activity of nucleases that are also released during cellular lysis. Additionally, genetic recombination is inefficient at incorporating new DNA sequences into the genome.

In nature, bacterial transformation is an important mechanism for the acquisition of genetic elements encoding virulence factors and antibiotic resistance. Genes encoding resistance to antimicrobial compounds have been shown to be widespread in nature, even in environments not influenced by humans. These genes, which allow microbes living in mixed communities to compete for limited resources, can be transferred within a population by transformation, as well as by the other processes of HGT. In the laboratory, we can exploit the natural process of bacterial transformation for genetic engineering to make a wide variety of medicinal products, as discussed in Microbes and the Tools of Genetic Engineering.

Think about It

  • Why does a bacterial cell make environmental DNA brought into the cell into a single-stranded form?

Clone and Asexual reproduction are two main methods of producing genetically identical offsprings from parent organisms or cells. Asexual reproduction takes place under natural conditions in prokaryotes and some plant cells. It is a natural phenomenon. Clone reproduction or cloning is an in vitro molecular technique that has the ability to produce clones of organisms under controlled conditions. Cloning is a widely used technique in recombinant DNA technology. This is the difference between clone and asexual reproduction.

Reference:

1.Editors. “Asexual Reproduction – Definition, Types, Advantages and Examples.” Biology Dictionary, 28 Apr. 2017. Available here
2.“Read “Scientific and Medical Aspects of Human Reproductive Cloning” at NAP.Edu.” National Academies Press: OpenBook. Available here

Image Courtesy:

1.�’by kennethr (Public Domain) via pixabay
2.’Binary Fission 2’By Ecoddington14 (CC BY-SA 3.0) via Commons Wikimedia