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Parasitic wasps are highly diverse and play a major role in suppression of herbivorous pest populations, but relatively little is known of the mechanisms driving their diversity. Molecular studies indicate that cryptic species complexes resulting from adaptations to specific hosts or host-foodplants may be common. The gregarious endoparasitoid, Cotesia congregata (Braconidae), is a model system for understanding parasitic wasp biology. It is reported to attack at least 15 species of sphingid caterpillars, most of which are plant family specialists. Molecular studies have demonstrated genetic differentiation of two host-foodplant complex sources originating from Manduca sexta on tobacco (MsT) and Ceratomia catalpae on catalpa (CcC). Response to female pheromone and elements of their courtship songs differ. Wasps from both sources mated and produced F1 hybrid offspring in the laboratory; however, 90% of hybrid females resulting from one of the reciprocal crosses failed to produce offspring. I built on this previous work by evaluating an ecological barrier, the evolution of courtship songs within the genus, and patterns of hybrid sterility among four additional host-foodplant complexes, as well as differentiation of their symbiotic bracovirus. Tests of developmental tolerance to nicotine demonstrate that MsT wasps are highly adapted to hosts feeding on tobacco, whereas CcC wasps experience high mortality. Acoustic analysis of courtship songs among host-foodplant sources of C. congregata and eleven additional species of Cotesia demonstrates that songs are species specific and appear to be correlated with genetic relatedness. Cotesia congregata from all sources mated and produced F1 hybrid offspring in the laboratory; however, hybrid females resulting from specific reciprocal crosses failed to produce progeny. Dissections of hybrid females revealed that sterile wasps lacked mature ovaries and functional bracovirus, a symbiotic virus integrated into the wasp genome and necessary to suppress the host immune system. Relative in vivo expression of wasp bracovirus genes differs between MsT and CcC host-foodplant complexes. Cumulatively, these behavioral, ecological, and genetic barriers to reproduction indicate that C. congregata is diverged into two incipient species with limited gene flow, and provides insight into the role of varied reproductive barriers in speciation of parasitic wasps.

In mammals, the most frequently observed hybrid dysgenesis effects are growth disturbances and male sterility. Profound defects in placental development have been described and our work on hybrids in genus Mus has demonstrated putative hybrid dysgenesis effects that lead to defects in lipid homeostasis and maternal behavior. Interestingly, mammalian interspecies hybrids exhibit strong parent-of-origin effects in that offspring of reciprocal matings, even though genetically identical, frequently exhibit reciprocal phenotypes. Recent studies have provided strong link between epigenetic regulation and growth, behavior and placental development. Widespread disruption of genomic imprinting has been described in hybrids between closely related species of the genus Peromyscus. The studies presented in this thesis aim to investigate the effects of disrupted epigenetics states on altered growth, female infanticide and placental dysplasia observed in Mus hybrids. We showed that loss-of-imprinting (LOI) of a paternally expressed gene, Peg1, was correlated with increased body weight of F1 hybrids. Furthermore, we investigated whether LOI of Peg1 in F1 females would interfere with maternal behavior. A subset of F1 females indeed exhibited highly abnormal maternal behavior in that they rapidly attacked and killed the pups. By microarray hybridization, a large number of differentially expressed genes in the infanticidal females as compared to normally behaving females were identified. In addtion to Peg1 LOI, we studied allelic expression of numerous imprinted genes in adult Mus interspecies hybrids. In contrast to the study from Peromyscus, patterns of LOI were not consistent with a direct influence of altered expression levels of imprinted genes on growth. Finally, we investigated the allelic interaction between an X-linked locus and a paternally expressed gene, Peg3, in placental defects in Mus hybrids. This study further strengthened the notion that divergent genetic and epigenetic mechanisms may be involved in hybrid dysgenesis in diverse groups of mammals.

Different species are separated by pre-zygotic reproductive barriers which impede gene flow between them. Rarely, when pre-zygotic barriers break down, interspecific hybrids are produced that display abnormal phenotypes, collectively called hybrid dysgenesis effects. Interspecies hybrid placental dysplasia (IHPD) in the genus Mus is a very consistent X-linked hybrid dysgenesis effect. Reproductive cloning and mutation of the gene Esx1 lead to placental hyperplasias with phenotypic similarities to IHPD. Comparative gene expression analysis of these three different models of placental hyperplasia showed that different mechanisms underlie these placental hyperplasias. We also identified several genes for which roles in placentation had not been studied earlier. We screened five of these genes, Car2, Ncam1, Fbln1, Cacnb3 and Cpe for their functions in placentation. Analysis of the spatio-temporal expression patterns of these genes during mouse placental development showed that they are ectopically expressed in IHPD placentas. Placental phenotype and gene expression was then studied in mice mutant for these genes. Our results show that complicated by the expression of functional counterparts, deletion of these genes failed to produce any consistent phenotype. Incompletely penetrant phenotypes were found in Cacnb3 and Cpe mutants. The Cpe mutant placentas recapitulated some IHPD phenotypes, despite co-expression of Cpd, a functionally redundant gene. Deregulated expression of Cpe and Cpd prior to manifestation of IHPD phenotype indicated that these are causally involved in IHPD and might be speciation genes in the genus Mus. We found that AT24 placentas also exhibit deregulated expression of these genes and could be used as a model to study IHPD. We tried rescuing the AT24 placental phenotype, by decreasing the expression of the over expressed genes. Normalization of transcript levels of these genes did not rescue the AT24 phenotype, thus indicating that up-regulation of these genes is a down-stream event in the generation of IHPD.

La reproduction sexuée implique une différentiation extrême des gamètes qui s’accompagne de profonds remaniements des chromosomes parentaux. Au moment de la fécondation, ces chromosomes doivent être rendus compétents pour la formation du premier noyau zygotique. Au cours de ma thèse, j’ai étudié l’importance fonctionnelle de plusieurs voies moléculaires paternelles et maternelles participant à cette étape chez la drosophile. Le complexe HIRA est impliqué dans l’assemblage de nucléosomes dans le pronoyau mâle à la fécondation. J’ai décrit le rôle de HIRA et de son partenaire Yemanucléine-α dans cette voie. J’ai caractérisé plus finement ce complexe en étudiant son rôle somatique dans l’assemblage des nucléosomes et son implication dans la stabilité de l’hétérochromatine, améliorant notre compréhension des besoins biologiques qui conditionnent sa conservation et son évolution. Je me suis aussi intéressé à diverses situations affectant l’intégrité des chromosomes parentaux à la fécondation. (1) J’ai décrit les conséquences catastrophiques pour la méiose femelle de l’expression naturelle d’un transposon à travers l’étude d’un cas de dysgénésie hybride. (2) J’ai contribué à montrer que la protéine K81 est essentielle pour la protection des télomères dans les chromosomes paternels au cours de la spermatogénèse. (3) J’ai participé à caractériser les conséquences pour les chromosomes paternels de l’incompatibilité cytoplasmique induite par la bactérie Wolbachia. Ensemble, ces travaux soulignent les particularités des chromosomes parentaux à la fécondation et aident à cerner l’importance des voies maternelles et paternelles dans leur intégration dans le premier noyau du zygote / Sexual reproduction involves dramatic gamete differentiation and profound parental chromosomes remodelling. At fertilization, these chromosomes need to be rendered competent for the formation of the fist zygotic nucleus. I have studied the functional relevance of several paternal and maternal molecular pathways that participate during this process in Drosophila. The HIRA complex is required for nucleosome assembly in the male pronucleus at fertilization. I have further described the rôle of HIRA and its obligatory partner Yemanuclein-α during this step. I have characterized the somatic roles of this complex during nucleosome assembly and its involvment in heterochromatin stability, which gives us a better understanding of the biological needs that drive its conservation and evolution. I have also focused on several situations where parental chromosomes integrity at fertilization is compromised. (1) I have described a meiotic catastrophe associated with the natural expression of a transposon in the female germline during hybrid dysgenesis. (2) I have contributed to show that K81 is an essential protein for telomere protection in paternal chromosomes during spermiogenesis. (3) I have participated in the characterization of the chromosomal abnormalities associated with cytoplasmic incompatibility induced by Wolbachia. Together, these results underscore the specificities of parental chromosomes at fertilization and shed light into the importance of maternal and paternal pathways for their integration in the first zygotic nucleus

Comme tous les ciliés, P. tetraurelia réarrange son génome à chaque génération sexuelle pendant le développement de son macronoyau somatique ¿ partir du micronoyau germinal. Les réarrangements incluent l’excision précise de courtes séquences dérivant de transposons et appelés IES (Internal Eliminated Sequences) dont la majorité sont intragéniques. L’excision d’une fraction d’entre elles dépend de petits ARN maternels (appelés scnARN) qui sont produits à partir de tout le génome germinal pendant la méiose. Ce mécanisme pose un problème lors d’une conjugaison entre deux souches présentant des polymorphismes d’insertion : une cellule sera théoriquement incapable d’exciser une IES portée par l’allèle paternel reçu si cette IES est absente de l’allèle maternel ou si la séquence est trop divergente. Mes résultats montrent cependant que les allèles paternels divergents sont correctement excisés en utilisant les scnARN produit par la cellule paternelle. Dans le cas d’un polymorphisme absence/présence, l’IES que j’ai étudié est excisée chez 70 % des hétérozygotes F1, également via les scnARN paternels. Nous avons exploré deux hypothèses pour expliquer comment ils pouvaient agir. Il pourrait s’agir d’une programmation précoce des noyaux gamétiques ou alors d’un échange cytoplasmique des scnARN. Finalement, j’ai montré qu’un défaut de scnARN maternels n’est pas une cause possible de dysgénésie hybride. Cependant, 30 % des hétérozygotes F1 présentent une rétention variable de l’IES étudié via un mécanisme inconnu. Si cela est généralisable à toutes les IES homozygotes, alors ce mécanisme aurait un effet délétère sérieux sur les F1 et pourrait contribuer à l’isolement reproductif. / Like all ciliates, P. tetraurelia entirely rearranges its genome during development of the somatic macronucleus from the germline micronucleus, in each sexual generation. Rearrangements include the precise excision of IESs (Internal Eliminated Sequences), single-copy intervening sequences likely derived from transposon insertions. At least for a fraction of IESs, correct excision, which is required to reconstitute functional genes in the macronucleus, is thought to depend on their recognition by Piwi-bound small RNAs (called scnRNAs) produced from the maternal germline genome during meiosis. This raises a problem during conjugation between strains presenting insertion polymorphisms: a cell will be theoretically unable to excise an IES from the incoming (paternal) allele if that IES is absent from the maternal allele, or if its sequence is too divergent. Our results, however, indicate that divergent paternal alleles are correctly rearranged, using scnRNAs produced by the paternal cell. In the case of an absence/presence polymorphism, the IES we studied is excised in 70% of heterozygotes, also using paternal scnRNAs. We explored two hypotheses to explain how they can act. It could be either an early programming of the gametic nuclei or through cytoplasmic exchange of scnRNAs. My results seem to favor the latter. Overall, I showed that the lack of maternal scnRNAs is not a possible cause of hybrid dysgenesis. However, 30% of heterozygous F1 display a variable retention of the IES through an unknown mechanism. If this is true for all hemizygous IESs then it will have a strong deleterious effect on hybrid F1s and may contribute to reproductive isolation.