Evol Ecol Res 16: 195-202 (2014)     Full PDF if your library subscribes.

Certainty versus stochasticity: cell replication biases DNA movement from endosymbionts and organelles into nuclei

Peter Nonacs and Sarah J. Tolley

Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, California, USA

Correspondence: P. Nonacs, Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, CA 90095, USA.
e-mail: pnonacs@biology.ucla.edu


Background: Endosymbiotic bacteria such as Wolbachia spend their entire life histories within other organisms’ cells. This close proximity of endosymbiont and host genomes allows for transfers of DNA between them. Such events are observed to be strongly biased, however, with overall DNA migration from cytoplasmic elements to host nuclei.

Question: Are DNA transfers from cytoplasmic to nuclear genomes more likely to be retained than those in the opposite direction based on how mitotic and meiotic cell division disperses nuclear and cytoplasmic DNA to daughter cells?

Mathematical model: Simulations track the survival of individual DNA intergenomic transfers in populations across 100 non-overlapping generations. Reproduction is separately modelled as either asexual in a haploid species or sexual in a diploid species.

Key assumptions: Transfers can either have no effect or increase chances of host reproduction by up to 20%. The distribution of genomes into offspring is stochastic (i.e. a given modified genome is as likely to be transmitted as an unmodified one).

Conclusions: Even when DNA transfers are equally bidirectional, transfers into host nuclei are retained more often than ones into cytoplasmic genomes. Consequently, biased migration has potential consequences for life-history evolution, whereby genes that exchange locations also switch ‘sides’ for intergenomic conflict. Thus, biased migration of genes is a long-term evolutionary process favouring host interests over that of their endosymbionts and organelles.

Keywords: endosymbiont, horizontal gene transfer, intergenomic conflict, mitochondria, Wolbachia.

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