Abstract
BackgroundPredicted genetic consequences of asexuality include high intraindividual genetic diversity (i.e., the Meselson effect) and accumulation of deleterious mutations (i.e., Muller’s Ratchet), among others. These consequences have been largely studied in parthenogenetic organisms, but studies on fissiparous species are scarce. Differing from parthenogens, fissiparous organisms inherit part of the soma of the progenitor, including somatic mutations. Thus, in the long term, fissiparous reproduction may also result in genetic mosaicism, besides the presence of the Meselson effect and Muller’s Ratchet. Dugesiidae planarians show outstanding regeneration capabilities, allowing them to naturally reproduce by fission, either strictly or combined with sex (facultative). Therefore, they are an ideal model to analyze the genetic footprint of fissiparous reproduction, both when it is alternated with sex and when it is the only mode of reproduction.ResultsIn the present study, we generate and analyze intraindividual cloned data of a nuclear and a mitochondrial gene of sexual, fissiparous and facultative wild populations of the species Dugesia subtentaculata. We find that most individuals, independently of their reproductive strategy, are mosaics. However, the intraindividual haplotype and nucleotide diversity of fissiparous and facultative individuals is significantly higher than in sexual individuals, with no signs of Muller’s Ratchet. Finally, we also find that this high intraindividual genetic diversity of fissiparous and facultative individuals is composed by different combinations of ancestral and derived haplotypes of the species.ConclusionsThe intraindividual analyses of genetic diversity point out that fissiparous reproduction leaves a very special genetic footprint in individuals, characterized by mosaicism combined with the Meselson effect (named in the present study as the mosaic Meselson effect). Interestingly, the different intraindividual combinations of ancestral and derivate genetic diversity indicate that haplotypes generated during periods of fissiparous reproduction can be also transmitted to the progeny through sexual events, resulting in offspring showing a wide range of genetic diversity and putatively allowing purifying selection to act at both intraindividual and individual level. Further investigations, using Dugesia planarians as model organisms, would be of great value to delve into this new model of genetic evolution by the combination of fission and sex.
Highlights
Predicted genetic consequences of asexuality include high intraindividual genetic diversity and accumulation of deleterious mutations (i.e., Muller’s Ratchet), among others
The existence of mosaicism combined with the Meselson effect remains as the most plausible explanation to interpret the intraindividual genetic pattern found in most of the fissiparous and facultative individuals analyzed in the present study
Intraindividual selection in fissiparous planarians Considering the evidence for the existence of mosaicism combined with the Meselson effect in fissiparous and facultative populations, we expected to find a high incidence of Muller’s ratchet in individuals from these populations
Summary
Predicted genetic consequences of asexuality include high intraindividual genetic diversity (i.e., the Meselson effect) and accumulation of deleterious mutations (i.e., Muller’s Ratchet), among others. It has been proposed that long-lasting asexuality can promote an increased number of slightly deleterious mutations as a consequence of relaxed selection (the physical linkage among loci hinders selection’s ability to act upon loci independently), which in the long term can cause detrimental effects on the populations (i.e., Muller’s ratchet) [13,14,15,16,17] Most of these studies have been performed in parthenogenetic asexual organisms, while clonal reproduction by some type of fissioning is rarely considered, this type of reproduction is known to exist in most phyla within metazoans [18]
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