Genome expansion in early eukaryotes drove the transition from lateral gene transfer to meiotic sex

Prokaryotes acquire genes from the environment via lateral gene transfer (LGT). Recombination of environmental DNA can prevent the accumulation of deleterious mutations, but LGT was abandoned by the first eukaryotes in favour of sexual reproduction. Here we develop a theoretical model of a haploid population undergoing LGT which includes two new parameters, genome size and recombination length, neglected by previous theoretical models. The greater complexity of eukaryotes is linked with larger genomes and we demonstrate that the benefit of LGT declines rapidly with genome size. The degeneration of larger genomes can only be resisted by increases in recombination length, to the same order as genome size – as occurs in meiosis. Our results can explain the strong selective pressure towards the evolution of sexual cell fusion and reciprocal recombination during early eukaryotic evolution – the origin of meiotic sex.

Genome expansion in early eukaryotes drove the transition from lateral gene transfer to meiotic sex

ABSTRACTProkaryotes generally reproduce clonally but can also acquire new genetic material via lateral gene transfer (LGT). Like sex, LGT can prevent the accumulation of deleterious mutations predicted by Muller’s ratchet for asexual populations. This similarity between sex and LGT raises the question why did eukaryotes abandon LGT in favor of sexual reproduction? Understanding the limitations of LGT provides insight into this evolutionary transition. We model the evolution of a haploid population undergoing LGT at a rate λ and subjected to a mutation rate μ. We take into account recombination length, L, and genome size, g, neglected by previous theoretical models. We confirm that LGT counters Muller’s ratchet by reducing the rate of fixation of deleterious mutations in small genomes. We then demonstrate that this beneficial effect declines rapidly with genome size. Populations with larger genomes are subjected to a faster rate of fixation of deleterious mutations and become more vulnerable to stochastic frequency fluctuations. Muller’s ratchet therefore generates a strong constraint on genome size. Importantly, we show that the degeneration of larger genomes can be resisted by increases in the recombination length, the average number of contiguous genes drawn from the environment for LGT. Large increases in genome size, as in early eukaryotes, are only possible as L reaches the same order of magnitude as g. This requirement for recombination across the whole genome can explain the strong selective pressure towards the evolution of sexual cell fusion and reciprocal recombination during early eukaryotic evolution – the origin of meiotic sex.

Extended Defects Evolution in Pre-Amorphised Silicon after Millisecond Flash Anneals

In this paper, we investigate the evolution of extended defects during a millisecond Flash anneal after a preamorphising implant. The experimental results, supported by predictive simulations, indicate that during the ultra-fast temperature ramp-up and rump-down occurring in a millisecond Flash anneal, the basic mechanisms that control the growth and evolution of extended defects are not modified with respect to the relatively slower annealing processes, such as “soak” and “spike” Rapid Thermal Annealing. In addition, we have observed a decrease in the number of trapped interstitials in the End-Of-Range (EOR) defects when decreasing the Ge+ amorphisation energy from 30 keV down to 2 keV. This result is ascribed to two concomitant phenomena: (i) the increase of the initial number of interstitials, Ni, created by the amorphisation step, when the implant energy is decreased and (ii) the efficient interstitial annihilation at the silicon surface, whose recombination length, Lsurf, is in the nanometer range even at the very high temperatures employed in millisecond Flash anneals.

Recombination Rate Predicts Inversion Size in Diptera

Most species of the Drosophila genus and other Diptera are polymorphic for paracentric inversions. A common observation is that successful inversions are of intermediate size. We test here the hypothesis that the selected property is the recombination length of inversions, not their physical length. If so, physical length of successful inversions should be negatively correlated with recombination rate across species. This prediction was tested by a comprehensive statistical analysis of inversion size and recombination map length in 12 Diptera species for which appropriate data are available. We found that (1) there is a wide variation in recombination map length among species; (2) physical length of successful inversions varies greatly among species and is inversely correlated with the species recombination map length; and (3) neither the among-species variation in inversion length nor the correlation are observed in unsuccessful inversions. The clear differences between successful and unsuccessful inversions point to natural selection as the most likely explanation for our results. Presumably the selective advantage of an inversion increases with its length, but so does its detrimental effect on fertility due to double crossovers. Our analysis provides the strongest and most extensive evidence in favor of the notion that the adaptive value of inversions stems from their effect on recombination.

Asymptotically matched sheath and presheath in a dense plasma

Asymptotically matched solutions for electron and ion density, electron and ion velocity, and electric potential are obtained in the boundary region of a dense low-temperature plasma adjacent to perfectly absorbing walls – walls that absorb, without reflection, incident electrons and ions. Leading-order composite solutions, valid throughout the boundary region, are constructed from solutions in three subdomains distinguished by different physical length scales: the geometric length, the ion mean free path and the Debye length. The composite solutions are used to assess the impact of electron–ion recombination in the ionization nonequilibrium region on sheath and presheath profiles, and on quantities evaluated at the wall. While, at leading order, the velocity profiles throughout the boundary region are not influenced by recombination, the density and potential profiles are significantly altered when recombination is included. These results show that the region of rapid change in these profiles lies closer to the wall when recombination is explicitly included in the model. The influence of recombination on the presheath potential, and consequently the wall potential, is found to scale as the natural logarithm of the recombination length. The broadening of the density profile results in a larger flux of ions accelerating through the sheath and impacting on the wall. The influence of recombination on the ion power flux to the wall is found to scale with the inverse recombination length. This scaling influences the prediction of surface erosion rates in technological applications that utilize these plasmas.