Constraints on the evolution of asexual reproduction

A combinational theory for maintenance of sex

Sexual reproduction implies high costs, but it is difficult to give evidence for evolutionary advantages that would explain the predominance of meiotic sex in eukaryotes. A combinational theory discussing evolution, maintenance and loss of sex may resolve the problem. The main function of sex is the restoration of DNA and consequently a higher quality of offspring. Recombination at meiosis evolved, perhaps, as a repair mechanism of DNA strand damages. This mechanism is most efficient for DNA restoration in multicellular eukaryotes, because the initial cell starts with a re-optimized genome, which is passed to all the daughter cells. Meiosis acts also as creator of variation in haploid stages, in which selection can purge most efficiently deleterious mutations. A prolonged diploid phase buffers the effects of deleterious recessive alleles as well as epigenetic defects and is thus optimal for prolonged growth periods. For complex multicellular organisms, the main advantage of sexuality is thus the alternation of diploid and haploid stages, combining advantages of both. A loss of sex is constrained by several, partly group-specific, developmental features. Hybridization may trigger shifts from sexual to asexual reproduction, but crossing barriers of the parental sexual species limit this process. For the concerted break-up of meiosis-outcrossing cycles plus silencing of secondary features, various group-specific changes in the regulatory system may be required. An establishment of asexuals requires special functional modifications and environmental opportunities. Costs for maintenance of meiotic sex are consequently lower than a shift to asexual reproduction.

Oogenesis: making the mos of meiosis

Meiosis is an ancient type of cell division whose advent allowed the evolution of sexual reproduction. The evolutionary history of the specialization that allowed gamete production to emerge from a simple reduction division has been unclear. New data now suggest that the molecular mechanisms involved in animal oocyte specialization may have origins that predate the emergence of bilaterian animals.

Sexual and asexual (apomictic) seed development in flowering plants: molecular, morphological and evolutionary relationships

Reproduction in the flowering plants (angiosperms) is a dynamic process that relies upon the formation of inflorescences, flowers and eventually seed. Most angiosperms reproduce sexually by generating gametes via meiosis that fuse during fertilisation to initiate embryo and seed development, thereby perpetuating the processes of adaptation and evolution. Despite this, sex is not a ubiquitous reproductive strategy. Some angiosperms have evolved an alternate form of reproduction termed apomixis, which avoids meiosis during gamete formation and leads to the production of embryos without paternal contribution. Therefore, apomixis results in the production of clonal progeny through seed. The molecular nature and evolutionary origin of apomixis remain unclear, but recent studies suggest that apomixis evolved from the same molecular framework supporting sex. In this review, we consider physical and molecular relationships between the two pathways, with a particular focus on the initial stages of female reproduction where apomixis deviates from the sexual pathway. We also consider theories that explain the origin of apomictic processes from sexual progenitors. Detailed characterisation of the relationship between sex and apomixis in an evolutionary and developmental sense is an important step towards understanding how apomixis might be successfully integrated into agriculturally important, but currently sexual crops.