Abstract
The unusual occurrence and developmental diversity of asexual eukaryotes remain a puzzle. De novo formation of a functioning asexual genome requires a unique assembly of sets of genes or gene states to disrupt cellular mechanisms of meiosis and gametogenesis, and to affect discrete components of sexuality and produce clonal or hemiclonal offspring. We highlight two usually overlooked but essential conditions to understand the molecular nature of clonal organisms, that is, a nonrecombinant genomic assemblage retaining modifiers of the sexual program, and a complementation between altered reproductive components. These subtle conditions are the basis for physiologically viable and genetically balanced transitions between generations. Genomic and developmental evidence from asexual animals and plants indicates the lack of complementation of molecular changes in the sexual reproductive program is likely the main cause of asexuals' rarity, and can provide an explanatory frame for the developmental diversity and lability of developmental patterns in some asexuals as well as for the discordant time to extinction estimations.
Highlights
Sexuality is ubiquitous to most eukaryotes, but approximately one in 10,000 species is asexual
As opposed to apomixis, meiosis proceeds normally and an unreduced egg is formed either by endomitosis, fusion of two nuclei resulting from the same meiotic division, or fusion of cleavage division nuclei, which develops parthenogenetically into a new individual
The new lineage will likely exhibit low coordination of gene expression and altered reproductive developments causing low reproductive efficiency. Such patterns have been observed in different recently established organisms exhibiting asexual reproduction or tendencies to asexuality.[32,53]Later, occasional recombination and sex in these individuals may play a pivotal role for fine-tuning the efficiency of any reproductive mode and contributing to alleviate a reversal to the sexual program or to establish a persistent asexual lineage.[46]In nature, different animal and plant groups display a variable incidence of alternative reproductive modes and patterns of clonal diversity.[24,41,42]
Summary
Sexuality is ubiquitous to most eukaryotes, but approximately one in 10,000 species is asexual. As opposed to apomixis, meiosis proceeds normally and an unreduced egg is formed either by endomitosis, fusion of two nuclei resulting from the same meiotic division, or fusion of cleavage division nuclei, which develops parthenogenetically into a new individual This exclude cases of nonparthenogenetic automixis in ferns and bryophytes.[37]automixis creates endogamous recombinant offspring without male contribution and leads to homozygosity.[ 35,36,38,39 ]. Paternal chromosomes are eliminated during female meiosis and reduced gametes carry only maternal chromosomes Such gametes require fertilization by a male gamete to develop a new individual. Pseudogamous apomixis: or pseudogamy, form of asexuality that requires pollination and fertilization of the central cell in the female gametophyte to complete the formation of the endosperm and trigger parthenogenetic development of the embryo.
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