The Genetic Control of Apomixis: Apomixis asexual seed formation is the result of a plant gaining the ability to bypass the most fundamental aspects of sexual reproduction: Without the need for male fertilization, the resulting seed germinates a plant that develops as a maternal clone.
This dramatic shift in reproductive process has been documented in many flowering plant species, although no major "Accommodates asexual reproduction morphological markers" crops have been shown to be capable of apomixis.
"Accommodates asexual reproduction morphological markers" ability to generate maternal clones and therefore rapidly fix desirable genotypes in crop species could accelerate agricultural breeding strategies.
The potential of apomixis as a next-generation breeding technology has contributed to increasing interest in the mechanisms controlling apomixis. In this review, we discuss the progress made toward understanding the Accommodates asexual reproduction morphological markers and molecular control of apomixis.
Research is currently focused on two fronts. One aims to identify and characterize genes causing apomixis in apomictic species that have been developed as model species. The other aims to engineer or switch the sexual seed formation pathway in non-apomictic species, to one that mimics apomixis. Here we describe the major apomictic mechanisms and update knowledge concerning the loci that control them, in addition to presenting candidate genes that may be used as tools for switching the sexual pathway to an apomictic mode of reproduction in crops.
Asexual Reproduction in Holothurians. Aspects of asexual reproduction in holothurians are discussed. Holothurians are significant as fishery and aquaculture items and have high commercial value. The last review on holothurian asexual reproduction was published 18 years ago and included only 8 species. An analysis of the available literature shows that asexual reproduction has now been confirmed in 16 holothurian species.
Five additional species are also most likely capable of fission. The recent discovery of new fissiparous holothurian species indicates that this reproduction mode is more widespread in Holothuroidea than previously believed.
New data about the history of the discovery of asexual reproduction in holothurians, features of fission, and regeneration of anterior and posterior fragments are described here. Asexual reproduction is obviously controlled by the integrated systems of the organism, primarily the nervous system. Special molecular mechanisms appear to determine the location where fission occurs along the anterior-posterior axis of the body.
Alteration of the connective tissue strength of the body wall may play an important role during fission of holothurians. The basic mechanism of fission is the interaction of matrix metalloproteinases, their inhibitors, and enzymes forming cross-link complexes between fibrils of collagen.
The population dynamics of fissiparous holothurians are discussed. Genomic analyses of primitive, wild and cultivated citrus provide insights into asexual reproduction. The emergence of apomixis -the transition from sexual to asexual reproduction -is a prominent feature of modern citrus.
Here we de novo sequenced and comprehensively studied the genomes of four representative citrus species.
Additionally, we sequenced accessions of primitive, wild and cultivated citrus. Comparative population analysis suggested that genomic regions harboring energy- and reproduction -associated genes are probably under selection in cultivated citrus.
We also narrowed the genetic locus responsible for citrus polyembryony, a form of apomixisto an kb region containing 11 candidate genes.
One of these, CitRWP, is expressed at higher levels in ovules of polyembryonic cultivars. We found a miniature inverted-repeat transposable element insertion in the promoter region of CitRWP that cosegregated with polyembryony.
This study provides new insights into citrus apomixis and constitutes a promising resource for the mining of agriculturally important genes. Dynamics of asexual reproduction in flatworms.
Planarians flatworms are one of the simplest bilaterally symmetric organisms and famous for their extraordinary regenerative capabilities. One can cut a worm in pieces and after a few weeks one obtains new worms that have reconstructed their entire body, including a central nervous system.
This amazing regenerative capability is due to a population of stem cells distributed throughout the planarian body.
These stem cells do not only allow the worms to heal without scarring after wounding, they also allow for asexual reproduction: Planarians can split themselves in two, and then regenerate the missing body parts within about a week.
Naively, one would think that this kind of asexual reproduction could be captured by simple models that describe cell growth in bacteria or other lower organisms.
Surprisingly, we observe that reproduction decreases with increasing food supply, opposite to the relationship between food and reproduction in other asexually reproducing organisms e. Female parthenogenetic apomixis and androsporogenetic parthenogenesis in embryonal cells of Araucaria angustifolia: Cell fate, development timing and occurrence of reproductive versus apomictic development in gymnosperms are shown to be influenced by culture conditions in vitro.
In this study, female parthenogenetic apomixis fPAandrosporogenetic parthenogenesis mAP and progenesis were demonstrated using embryonal initials of Araucaria angustifolia in scaled-up cell suspensions passing through a single-cell bottleneck in darkness and in an artificial sporangium AS. In fPA, the nucleus of an embryonal Accommodates asexual reproduction morphological markers undergoes endomitosis and amitosis, forming a diploid egg-equivalent and an apoptotic ventral canal nucleus in a transdifferentiated archegonial tube.
Discharge of egg-equivalent cells as parthenospores and their dispersal into the aqueous culture medium were followed by free-nuclear conifer-type proembryogenesis. This replaced the plesiomorphic and central features of proembryogenesis in Araucariaceae. Protoplasmic fusions of embryonal initials were used to reconstruct heterokaryotic expressions of fPA in multiwell plates.
mAP, restitutional meiosis automixis was responsible for androsporogenesis and the discharge of monads, dyads, tetrads and polyads. In a display of progenesis, reproductive development was brought to an earlier ontogenetic stage and expressed by embryonal initials. Colchicine increased polyploidy, but androspore formation became aberrant and fragmented. Aberrant automixis led to the
Accommodates asexual reproduction morphological markers of chromosomal bouquets, which contributed to genomic silencing in embryonal initials, cytomixis and the formation of pycnotic micronucleated cells.
Dispersal of female and male parthenospores displayed heteromorphic asexual heterospory in an aqueous environment. Genetic mechanisms of apomixis. The introduction of apomixis to crops would allow desirable genotypes to be propagated while preventing undesirable gene flow, but so far there has been little success in transferring this trait from a natural apomict to another species.
One explanation is the sensitivity of endosperm to changes in relative maternal and paternal contribution owing to parental imprinting, an epigenetic system of transcriptional regulation by which some genes are expressed from only the maternally or paternally contributed allele. In sexual species, endosperm typically requires a ratio of two maternal genomes to one paternal genome for normal development, but this ratio is often altered in apomicts, suggesting that the imprinting system is altered as well.
Accommodates asexual reproduction morphological markers present evidence that modification of DNA methylation is one mechanism by which the imprinting system could be altered to allow endosperm development in apomicts. Another feature of natural apomixis is the modification of the normal fertilization programme. Sexual reproduction uses both sperm from each pollen grain, but pseudogamous apomicts, which require a sexual endosperm to support the asexual embryo, often use just one.
We present evidence that multiple fertilization of the central cell is possible in Arabidopsis thaliana, suggesting that pseudogamous apomicts may also need to acquire a mechanism for preventing more than one sperm from contributing to the endosperm. We conclude that strategies to transfer apomixis to crop species should take account of endosperm development and particularly its sensitivity to parental imprinting, as well as the mechanism of fertilization.
Functional characterization of an apple apomixis -related MhFIE gene in reproduction development. The products of the FIS genes play important regulatory roles in diverse developmental processes, especially in seed formation after fertilization. These results suggested that polycomb complex including FIE and CLF proteins played an important role in reproductive development by regulating the expression of its downstream genes.
In addition, it Accommodates asexual reproduction morphological markers found that MhFIE constitutively expressed in various tissues tested. Its expression levels were lower in apomictic apple species than the sexual reproductive species, suggested it was possibly involved into apomixis in apple. Furthermore, the hybrids of tea crabapple generated MhFIE transcripts at different levels. The parthenogenesis capacity was negatively correlated with MhFIE expression level in these hybrids.
These results suggested that MhFIE was involved into the regulation of flower development and apomixis in apple. Mendel used hawkweeds and other plants to verify the laws of inheritance he discovered using Pisum. Trait segregation was not evident in hawkweeds because many form seeds asexually by apomixis. Meiosis does not occur during female gametophyte formation and the mitotically formed embryo sacs do not require fertilization for seed development.
The resulting progeny retain a maternal genotype. Hawkweeds in Hieracium subgenus Pilosella form mitotic embryo sacs by apospory. As AI cells undergo "Accommodates asexual reproduction morphological markers" mitosis the sexual pathway terminates.
Deletion of either locus results in partial reversion to sexual reproductionand loss of function in both loci results in reversion to sexual development. In these apomicts, sexual reproduction is therefore the default reproductive mode upon which apomixis is superimposed.
These loci are unlikely to encode factors critical for "Accommodates asexual reproduction morphological markers" reproduction but might recruit the sexual pathway to enable apomixis. Incomplete functional penetrance of these dominant loci is likely to lead to the generation of rare sexual progeny also derived from these facultative apomicts. Evolution of apomixis loci in Pilosella and Hieracium Asteraceae inferred from the conservation of apomixis -linked markers in natural and experimental populations.
The Hieracium and Pilosella Lactuceae, Asteraceae genera of closely related hawkweeds contain species with two different modes of gametophytic apomixis asexual seed formation.
Both genera contain polyploid species, and in wild populations, sexual and apomictic species co-exist. Apomixis is known to co-exist with sexuality in apomictic Pilosella individuals, however, apomictic Hieracium have been regarded as obligate apomicts. Accommodates asexual reproduction morphological markers Pilosella, LOA and LOP-linked markers were essentially absent within the sexual plants, although they were not conserved in all apomictic individuals.
Both loci appeared to be inherited independently, and evidence for additional genetic factors influencing quantitative expression of LOA and LOP was obtained. Collectively,
Accommodates asexual reproduction morphological markers data suggest independent evolution of apomixis in Hieracium and Pilosella and are discussed with respect to current knowledge of the evolution of apomixis.
Comparative toxicant sensitivity of sexual and asexual reproduction in the rotifer Brachionus calyciflorus. Cyclically parthenogenetic zooplankters like rotifers are important tools for assessing toxicity in aquatic environments. Sexual reproduction is an essential component of rotifer life cycles, but current toxicity tests utilize only asexual reproduction. The authors compared the effects of four toxicants on asexual and sexual reproduction of the rotifer Brachionus calyciflorus.
Toxicants had a differential effect on sexual and asexual reproductionwith sexual reproduction consistently the most sensitive.
Likewise, chlorpyrifos concentrations of 0.