Horizontal transfer (HT) is the transmission of genetic material by means other than reproduction, possibly between species which are genetically distant. In multicellular eukaryotes the impact of HTs on their evolution is poorly understood, as well as the mechanisms and factors which are involved. In this thesis I focus on animals, with an emphasis on insects, to bring some insights on some of the mechanisms and factors that have been proposed. For mechanisms, I investigated whether viruses could act as vectors of HT, i.e. transport genetic material from one species to another. For this, I investigated two kinds of viruses: a free virus and polydnavirus. We showed that an infection by a baculovirus, a free virus, moderately impacts the activity of transposable elements (TE) of the host, which might increase the chance of a transposition in the virus during infections. We also showed that a TE that was inserted in the virus was expressed, which means that it might be able to transpose again in the genome of another species during a second infection. Regarding polydnaviruses, they are very particular viruses which are found in the genome of some parasitoid wasps that domesticated them. These parasitoids inject polydnaviruses in their host at the same time as their eggs. We showed that these polydnaviruses were able to integrate massively in several tissues of the hosts. Although the transmission to the next generation of surviving hosts seems quite limited in the system we investigated in detail (Cotesia typhae [wasp] - Sesamia nonagrioides [lepidoptera]), we found many traces of polydnavirus integrations in the genomes of many lepidopteran species, the main hosts of parasitoid wasps. Finally, we investigated four possible factors that might promote HTs, although the results are still preliminary: the geographical proximity, the phylogenetic proximity, the aquatic habitat and the mode of fertilization.