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Hybrid dysgenesis in Paramecium aurelia species

Irina Nekrasova
Alexey Potekhin - Saint Petersburg State University, Russia
Eric Meyer - Institut de Biologie de l`Ecole Normale Superieure, Paris, France
Saturday, April 4, 2015 to Sunday, May 31, 2015

Nuclear dualism provides several exciting possibilities to enhance speciation in ciliates. The unique way of transgenerational maintenance of somatic genome has a huge potential for creating reproductive barriers within and between species. The aim of the STSM was to reveal possible mechanisms of initial speciation in Paramecium. In particular, we tested the hypothesis that “polymorphic IESs” (i.e. IESs absent from genome of one partner and present in genome of the other partner) due to the genome scanning mechanism are epigenetically incompatible and may lead to hybrid dysgenesis in Paramecium, causing lethality in F2 of interstrain crosses.

We performed the large scale cross between two strains of P.tetraurelia – 32 and 51. 72 parental couples were obtained, their F1, and, in number of cases, F2 progeny was analyzed. Genomic DNA of mixtures of F1 hybrid clones (10 F1 clones from each side of the cross) was prepared for genome sequencing. Two polymorphic IESs over 100 bp were often retained in MAC genomes of F1 strains; however, they were retained not only in MAC genome of the exconjugant cell deriving from IES-naïve parent, but also in MAC genome of F1 cells originating from that parent which had this IES in its MIC. Though frequency of IES retention in F1’s deriving from IES-naïve parent was higher (22%) than in F1’s deriving from IES-possessing parent (13%), still it is evident that the model of genome scanning is not sufficient to explain the observed phenomenon, and that hypothesis on hybrid dysgenesis in Paramecium may not hold. About 80% of F2 clones obtained died for unclear reasons. We prepared total genomic DNA of 13 F2 clones from “side 51” and 29 F2 clones from “side 32” for further sequencing, which will allow to explain lethality in F2 and also will be used for mapping of the MIC genome of strain 51 of P.tetraurelia.

We performed also crosses between two phylogenetically distant strains of another species of the P.aurelia complex – P.sexaurelia. Genomic DNA of mixtures of F1 hybrid strains and of F2 surviving strains from both parental sides was prepared for further sequencing. All F1 hybrid clones were perfectly viable, and in F2 lethality was about 40% at one side of the cross and about 25% at another side, confirming data of rather high F2 lethality in interstrain crosses obtained in P.tetraurelia.

Also we obtained interspecific F1 hybrids and checked their ability to survive and to produce viable F2 progeny. We performed crosses between P.octaurelia and P.tetraurelia using two combinations of strains: we used P.tetraurelia 51 and 32 strains, and strain 138 of P.octaurelia. In cross 51 x 138 F1 viability was close to zero: of 30 obtained clones only 3 survived; all of them were very weak, and it was impossible to obtain F2 passing these F1 hybrids through autogamy. In cross 32 x 138 three of 24 hybrid strains survived, and two of these were able to produce F2 progeny. Lethality in F2 progeny in one case appeared to be about 40%, in the other – only 14%. At the moment we are checking the faithfullness of F1 and F2 hybrids by sequencing mating type genes – mtA, mtB, and mtC; if F1 clones appear to be heterozygous for these genes, and F2 clones appear to be true recombinants by them, it will be possible to conclude that reproductive isolaion between P.tetraurelia and P.octaurelia is incomplete.

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