Identifying the molecular mechanisms behind reproductive isolation between closely related yeast species provides avaluable understanding of their evolution. Sequence divergence and chromosomal rearrangements are the main post-zygotic barriers behind reproductive isolation within Saccharomyces "sensu strico' species, where hybrids are readily formed but sterile upon meiosis. Saccharomyces paradoxus and Saccharomyces cariocanus have an almost identical genome in terms of sequence, and therefore provide good model systems to explore the impact of karyotypic rearrangements on reproductive isolation. According to the biological species concept they are considered two different species despite having low sequence divergence. Since the karyotypic analysis revealed that the genomic differences are restricted to four chromosomal translocations, we hypothesized that such rearrangements may be the cause of low spore viability between them. To test this expectation, we engineered two chromosomal translocations in S. paradoxus YPS138, via Cre-loxP mediated recombination event, to render those parts of genome collinear to S.cariocanus UFRJ50816. Our analysis revealed that hybrids between S. cariocanus and engineered S. paradoxus harbouring two translocations showed a significant increase in spore viability (12.7%) compared to control hybrids harbouring five translocations (3.4%) (P=0.0031and P=0.0125, respectively, Two-sample t-test). Consequently, fitness in meiosis was improved four fold by undoing two translocations. Given this result, the prediction for spore viability in complete collinear crossing would be around 50.8 %, which is still far from the value of ca. 100%, which would be expected for strains with very low sequence divergence and belonging to the same species. This indicates that other factors may contribute to meiotic fitness in these hybrids. Further investigation was carried to determine the genome structures by using the PacBio sequencing approach. Our DNA sequencing data revealed other, previously undetected, rearrangements in S. cariocanus strain: one new reciprocal translocation between chromosomes XIII and XIV and 11 inversions distributed in 6 chromosomes. The variations in meiotic viability observed in the engineered hybrids could be because of these 5 chromosomal translocations. Further experiments were also carried out to evaluate the impact of translocations on mitotic fitness and gene expression; we observed a significant drop in the mitotic fitness of engineered translocant strains under different nutritional and temperature stresses. These changes were also accompanied with alteration in genes expression throughout the genome. Our RNA- seq data revealed that many genes were up- or down- regulated because of the translocation. Several genes with altered expression in translocant strains are correlated with morphology changes when they are up- or down- regulated. Therefore, the cell morphology was evaluated under light microscopy and different abnormal cells were detected compared to the wild type. Irregular cell morphology included elongated and clumped cells. Overall, these data confirmed that chromosomal translocations were the cause of reproductive isolation between S. paradoxus and S. cariocanus and play an important role in altering the phenotype and gene expression.