Content area
Full text
ABSTRACT
Sexual reproduction and recombination are important for maintaining a stable copy number of transposable elements (TEs). In sexual populations, elements can be contained by purifying selection against host carriers with higher element copy numbers; however, in the absence of sex and recombination, asexual populations could be driven to extinction by an unchecked proliferation of TEs. Here we provide a theoretical framework for analyzing TE dynamics under asexual reproduction. Analytic results show that, in an infinite asexual population, an equilibrium in copy number is achieved if no element excision is possible, but that all TEs are eliminated if there is some excision. In a finite population, computer simulations demonstrate that small populations are driven to extinction by a Muller's ratchet-like process of element accumulation, but that large populations can be cured of vertically transmitted TEs, even with excision rates well below transposition rates. These results may have important consequences for newly arisen asexual lineages and may account for the lack of deleterious retrotransposons in the putatively ancient asexual bdelloid rotifers.
(ProQuest-CSA LLC: ... denotes formulae omitted.)
TRANSPOSABLE elements (TEs) are mobile DNA sequences that are abundant in the genomes of nearly all living organisms, including bacteria, protists, fungi, plants, and animals (CRAIG et al. 2002). Although there are cases in which mobile genetic elements may be coopted to serve host regulatory or structural functions (KIDWELL and LISCH 2001), like most other mutator mechanisms, TEs are known to reduce the fitness of their host organism.However, unlike other classes of mutation, TEs are capable of autonomous self-replication. When elements transpose, they replicate faster than their host genome, with rates of transposition above rates of spontaneous deletion (CHARLESWORTH and LANGLEY 1989; NUZHDIN and MACKAY 1995; MASIDE et al. 2000). This permits TE persistence despite their deleterious effects, as postulated by the "selfish DNA" hypothesis (DOOLITTLE and SAPIENZA 1980; Orgel and Crick 1980).
Experimental and theoretical studies suggest that the ability of TEs to propagate can be balanced by natural selection against individuals with high element copy number (CHARLESWORTH and CHARLESWORTH 1983; KAPLAN and BROOKFIELD 1983). Three main sources of deleterious effects on fitness of segregating TEs have been postulated (see review by NUZHDIN 1999): insertions disrupting gene function (FINNEGAN 1992), chromosomal rearrangements generated by ectopic exchange (MONTGOMERY...