A novel defence strategy displayed in response to marine viruses by some of the most abundant unicellular organisms found in our oceans has recently been demonstrated by researchers in the Laboratoire Adaptation et diversité en milieu marin (CNRS, UPMC) working in collaboration with other European scientists.
These results enable a clearer understanding of the origin of, and reasons for, sexual reproduction in eukaryotes (1).
The researchers studied the impact of marine viruses on Emiliania huxleyi, one of the most abundant unicellular eukaryotes in oceans that significantly influences the carbon cycle and climates. In their diploid form, i.e. when they contain a pair of chromosomes (2N), Emiliania huxleyi produce mineral scales and form gigantic populations that are visible from space. But when attacked by marine viruses, they transform into haploid cells which only contain a single chromosome (N). These new, non-calcifying, highly motile cells are totally invisible to viruses (and undetectable on satellite photos) so that the species can live in peace to await safer times.
These scientists have called this the "Cheshire Cat" strategy, in homage to Lewis Carroll's novel " Alice in Wonderland". In this book, the crafty and philosophical Cheshire Cat escapes being beheaded on the order of the Red Queen by rendering his body transparent. In the same way, by changing their form during the haploid phase, eukaryotes can evade biotic pressure and reinvent themselves within their own species.
Our ancestors, unicellular eukaryotes, appeared in oceans some one billion years ago and "invented" sexuality. These species are characterized by a life cycle where haploid individuals (carrying a single copy of the genome, like gametes(2)) unify to form diploid individuals that will subsequently generate haploid cells once again. During this eukaryote "double life", humans and other multicellular eukaryotes whose haploid gametes remain imprisoned within a diploid body, tend to be the exception. Originally, and in most eukaryotes, haploid cells multiply in their environment to form independent populations. Sexuality has allowed eukaryotes to evade constant attacks by viruses so that they could evolve towards more complex, high-performance organisms, the ecological importance of which is still markedly underestimated.
1) Cells where genetic material is preserved within a nucleus
2) Reproductive cells
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