Insights & Perspectives Sex is not a solution for reproduction: The libertine bubble theory Thierry Lode ´ Ã Introduction Although sexual species appeared about 1.2 billion years ago [1], asexual ‘species’, such as parthenogenetic populations, have been clearly shown to produce more progeny. Because sexual reproduction limits the overall number of offspring, sex is known to have an evolutionary price to pay, the ‘twofold cost’ of sex [2]. While an asexual parthenogenetic population doubles with each generation, a sexual population has to bear the cost of males. Ultimately, with female-only offspring, the asexual lineage can grow exponen- tially through the generations, showing greater fitness. Thus, the maintenance of sexual reproduction continues to be an essential evolutionary issue [3]. Consequently, numerous competing theories have attempted to identify the advantages of sexual reproduction. DNA repair hypothesis Meiosis was first considered basically to be a ‘DNA repair process’ [4]. In fact, in its primitive forms, sex could favour the survival of organisms whereby each strand of the original double-stranded DNA molecule served as a template for the replication of a new, complementary DNA molecule, allowing damaged DNA to be repaired. In the ‘repair and com- plementation hypothesis’ [5, 6], genetic recombination is regarded as a response to the ‘noise’ that occurs when genetic information is transmitted. One chromo- some can duplicate information from another, and use it to recover lost genetic information. Nonetheless, this mechanism requires that recombination leads to a repair process that should be faster than the rate of natural damage. Recombination repair systems exist in both prokaryotes and eukaryotes, but meiosis is limited to eukaryotes, suggesting that the DNA repair hypoth- esis is not sufficient to explain the advantage of sexual reproduction. Likewise, the ability of meiosis to faith- fully transfer information from one generation to the next is affected both by low numbers of sexual partners and by a relatively high rate of unfavourable mutations. The addition of bad transcripts tends to weaken organisms, and non-neutral mutations can reduce fitness. Based on a ‘Muller’s ratchet-like’ mechanism, Kondrashov [7] in his mutation deter- ministic hypothesis alleged that sexual reproduction is a process to remove del- eterious recessive mutations. As a result, by generating numerous new genotypes with a lower genetic load, sexual reproduction could be a signifi- cant advantage when there is a high rate of deleterious mutations. Nonetheless, the movement of Muller’s ratchet can be associated with a substantial reduction in genetic diversity that is below the classical neutral expectation [8] and the mechanism is often considered as functioning too slowly to provide short-term benefits [9]. That said, some computer models [10] have shown the impact of Muller’s ratchet to be much greater than expected, even in large populations. With recombination, the selective advantage of beneficial mutations is dis- connected from neighbouring detrimen- tal ones, thereby allowing additive effects of beneficial mutations to coalesce for their next generations. In summary, these hypotheses have emphasised that the recombination process is very conservative, preventing gross detrimental changes in phenotype in the offspring. Promoting variations By contrast, another research framework emphasised the importance of generated variation to explain why sex is so wide- spread in nature. Sexual reproduction is said to provide a substantial advantage because it produces genetic variations through allelic recombination, whereas reproduction of parthenogenetic species is expected to result in similar genomes among descendants. Keywords: .asexual species; libertine bubble theory; meiosis; parthenogenesis; sex; sexual reproduction DOI 10.1002/bies.201000125 UMR CNRS 6552 ETHOS, Universite ´ de Rennes 1, Rennes, France *Corresponding author: Thierry Lode ´ E-mail: thierry.lode@univ-rennes1.fr Bioessays 33: 419–422,ß 2011 WILEY Periodicals, Inc. www.bioessays-journal.com 419 Ideas & Speculations