PhD student Paris XI university – doctoral Genes school, Genomes, Cells, Nature, function and evolution of a selfish gene at Drosophila simulans
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Nature, function and evolution of a selfish gene at Drosophila simulans
The distorters of meiotic segregation are selfish genetic elements. They assure their preferential transmission by the individuals who carry them to the heterozygous state by perturbing the meiose or the maturation of gametes at the expense of their counterparts non-distorters (Burt & Trivers 2006).
The distorters of sex-ratio type, carried by the X chromosome and active to males, reduces the production of sperm cells carriers of the Y chromosome. This phenomenon is translated by means of sex ratio in favour of females in the descent. Systems of distortion of meiotic segregation sex-ratio were described at more than a dozen species of the Drosophila genre and in several other genres of Dipterans (Jaenike 2001; Burt & Trivers 2006).
Because of their advantage of transmission, distorters sex-ratios can spread very quickly in the populations. The strong bias of sex ratio, as well as diverse costs which can be associated to the distortion, create then of strong pressure of selection which favor the implementation of suppressors on the Y chromosome and the autosomes to restore a sex well-balanced ratio.
Distorters sex-ratios are thus source of genetic conflicts the evolutionary consequences of which are potentially important: they can be for example at the origin of changes of the determinism of the sex, or the emergence of genetic incompatibilities which can contribute to the reproductive isolation between divergent lineages (Phadnis and Orr 2009; Tao and Meiklejohn 2009). Their very fast evolution can have effects on other genomic regions and often leaves strong molecular signatures (for example Kingan and al 2010; Ancient country-house and al 2011).
However, because of the genetic complexity inherent to these systems and their frequent association with chromosomic rearrangements, only two distorters elements sex-ratio were this day identified at the molecular level (Tao and al 2007a, b; Phadnis and Orr 2009), among which the nature and the mode of action remains still very badly known.
Chez D. simulans, trois systèmes sex-ratio génétiquement indépendants ont été mis en évidence, les systèmes Durham, Winters, et Paris dont les deux derniers sont les mieux caractérisés au niveau moléculaire (Tao et al 2007a,b; Montchamp-Moreau et al 2006). le système Paris, étudié au LEGS par l’équipe de Catherine Montchamp-Moreau, fait l’objet de ce projet de thèse. La distorsion résulte d’une non-disjonction des chromatides soeurs du chromosome Y en anaphase II (Cazemajor et al. 2000).
At D. simulans, three systems sex-ratio genetically independent were demonstrated, the Durham, Winters and Paris systems among which both last ones are the best characterized at the molecular level (Tao and al 2007a, b; Montchamp-Moreau and al 2006). The Paris system, studied at LEGS by Catherine Montchamp-Moreau’s team, is the object of this thesis project. The distortion results of one non-disjunction of chromatides sisters of the Y chromosome in anaphase II (Cazemajor and al. 2000).
A first experience of genetic mapping using recombinants showed that two locus of the X chromosome are necessary for the distortion (Montchamp-Moreau and al. 2006). The first element is inseparable of a duplication in tandem 37 kb containing 6 genes, the second was localized in a region be candidate of 25 kb not showing rearrangement.
This thesis will have for objective to genetically identify the second element necessary for the distortion sex-ratio in the Paris system, and realize a functional validation of its implication.
Helleu, Q., P. Gérard, R. Dubruille, D. Ogereau, B. Prud’homme, B. Loppin & M. Montchamp-Moreau (2016) Rapid evolution of a Y-chromosome heterochromatin protein underlies sex chromosome meiotic drive. Proceedings of the National Academy of Sciences of the USA, 113, 4110-4115. 10.1073/pnas.1519332113.
Helleu, Q., P. R. Gerard & C. Montchamp-Moreau (2015) Sex Chromosome Drive. Cold Spring Harbor Perspectives in Biology, 7.