Abstract
BackgroundQuest for understanding the nature of mechanisms governing the life span of clonal organisms lasts for several decades. Phylogenetic evidence for recent origins of most clones is usually interpreted as proof that clones suffer from gradual age-dependent fitness decay (e.g. Muller's ratchet). However, we have shown that a neutral drift can also qualitatively explain the observed distribution of clonal ages. This finding was followed by several attempts to distinguish the effects of neutral and non-neutral processes. Most recently, Neiman et al. 2009 (Ann N Y Acad Sci.:1168:185-200.) reviewed the distribution of asexual lineage ages estimated from a diverse array of taxa and concluded that neutral processes alone may not explain the observed data. Moreover, the authors inferred that similar types of mechanisms determine maximum asexual lineage ages in all asexual taxa. In this paper we review recent methods for distinguishing the effects of neutral and non-neutral processes and point at methodological problems related with them.Results and DiscussionWe found that contemporary analyses based on phylogenetic data are inadequate to provide any clear-cut answer about the nature and generality of processes affecting evolution of clones. As an alternative approach, we demonstrate that sequence variability in asexual populations is suitable to detect age-dependent selection against clonal lineages. We found that asexual taxa with relatively old clonal lineages are characterised by progressively stronger deviations from neutrality.ConclusionsOur results demonstrate that some type of age-dependent selection against clones is generally operational in asexual animals, which cover a wide taxonomic range spanning from flatworms to vertebrates. However, we also found a notable difference between the data distribution predicted by available models of sequence evolution and those observed in empirical data. These findings point at the possibility that processes affecting clonal evolution differ from those described in recent studies, suggesting that theoretical models of asexual populations must evolve to address this problem in detail.ReviewersThis article was reviewed by Isa Schön (nominated by John Logsdon), Arcady Mushegian and Timothy G. Barraclough (nominated by Laurence Hurst).
Highlights
Quest for understanding the nature of mechanisms governing the life span of clonal organisms lasts for several decades
We found a notable difference between the data distribution predicted by available models of sequence evolution and those observed in empirical data
These findings point at the possibility that processes affecting clonal evolution differ from those described in recent studies, suggesting that theoretical models of asexual populations must evolve to address this problem in detail
Summary
Quest for understanding the nature of mechanisms governing the life span of clonal organisms lasts for several decades. Such results are interpreted as evidence that increased accumulation of deleterious mutations is a prominent force that determines the ages of clones We see this interpretation as problematic because strictly speaking, such studies document the proximate mechanisms underlying clonal decay but do not test whether selection really favours young clones and sexuals at the expense of ancient clones. Such studies do not evaluate the generality of such processes because numerous studies failed to detect higher mutation or parasite loads in asexuals or prove comparable evolutionary plasticity to related sexual populations [[5,6,7,8], rev. Guex et al [5] discuss putative parasexual processes that explain the similar performances of young and ancient asexual genomes, such findings may suggest that at least in some cases, the ages of natural clonal genomes are not determined by clonal decay
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