Barbara McClintock first hypothesized that interspecific hybridization could provide a “genomic shock” that leads to the mobilization of transposable elements. This hypothesis is based on the idea that regulation of transposable element movement is potentially disrupted in hybrids. However, the handful of studies testing this hypothesis have yielded mixed results. Here, we set out to identify if hybridization can increase transposition rate and facilitate colonization of transposable elements in Saccharomyces cerevisiae x Saccharomyces uvarum interspecific yeast hybrids. S. cerevisiae have a small number of active long terminal repeat (LTR) retrotransposons (Ty elements), while their distant relative S. uvarum have lost the Ty elements active in S. cerevisiae. While the regulation system of Ty elements is known in S. cerevisiae, it is unclear how Ty elements are regulated in other Saccharomyces species, and what mechanisms contributed to the loss of most classes of Ty elements in S. uvarum. Therefore, we first assessed whether transposable elements could insert in the S. uvarum sub-genome of a S. cerevisiae x S. uvarum hybrid. We induced transposition to occur in these hybrids and developed a sequencing technique to show that Ty elements insert readily and non-randomly in the S. uvarum genome. We then used an in vivo reporter construct to directly measure transposition rate in hybrids, demonstrating that hybridization itself does not alter rate of mobilization. However, we surprisingly show that species-specific mitochondrial inheritance can change transposition rate by an order of magnitude. Overall, our results provide evidence that hybridization can potentially facilitate the introduction of transposable elements across species boundaries and alter transposition via mitochondrial transmission, but that this does not lead to unrestrained proliferation of transposable elements suggested by the genomic shock theory.

Embryos devoid of autonomic innervation suffer sudden cardiac death. However, whether autonomic neurons have a role in heart development is poorly understood. To investigate if sympathetic neurons impact cardiomyocyte maturation, we co-cultured phenotypically immature cardiomyocytes derived from human induced pluripotent stem cells with mouse sympathetic ganglion neurons. We found that 1) multiple cardiac structure and ion channel genes related to cardiomyocyte maturation were up-regulated when co-cultured with sympathetic neurons; 2) sarcomere organization and connexin-43 gap junctions increased; 3) calcium imaging showed greater transient amplitudes. However, sarcomere spacing, relaxation time, and level of sarcoplasmic reticulum calcium did not show matured phenotypes. We further found that addition of endothelial and epicardial support cells did not enhance maturation to a greater extent beyond sympathetic neurons, while administration of isoproterenol alone was insufficient to induce changes in gene expression. These results demonstrate that sympathetic neurons have a significant and complex role in regulating cardiomyocyte development.

Interspecific hybridization can introduce genetic variation that aids in adaptation to new or changing environments. Here, we investigate how hybrid adaptation to temperature and nutrient limitation may alter parental genome representation over time. We evolved Saccharomyces cerevisiae x Saccharomyces uvarum hybrids in nutrient-limited continuous culture at 15°C for 200 generations. In comparison to previous evolution experiments at 30°C, we identified a number of responses only observed in the colder temperature regime, including the loss of the S. cerevisiae allele in favor of the cryotolerant S. uvarum allele for several portions of the hybrid genome. In particular, we discovered a genotype by environment interaction in the form of a loss of heterozygosity event on chromosome XIII; which species’ haplotype is lost or maintained is dependent on the parental species’ temperature preference and the temperature at which the hybrid was evolved. We show that a large contribution to this directionality is due to a temperature dependent fitness benefit at a single locus, the high affinity phosphate transporter gene PHO84. This work helps shape our understanding of what forces impact genome evolution after hybridization, and how environmental conditions may promote or disfavor the persistence of hybrids over time.

38Barbara McClintock first hypothesized that interspecific hybridization could provide a "genomic shock" 39 that leads to the mobilization of transposable elements. This hypothesis is based on the idea that 40 regulation of transposable element movement is potentially disrupted in hybrids. However, the handful of 41 studies testing this hypothesis have yielded mixed results. Here, we set out to identify if hybridization can 42 increase transposition rate and facilitate colonization of transposable elements in Saccharomyces 43 cerevisiae x Saccharomyces uvarum interspecific yeast hybrids. S. cerevisiae have a small number of 44 active long terminal repeat (LTR) retrotransposons (Ty elements), while their distant relative S. uvarum 45 have lost the Ty elements active in S. cerevisiae. While the regulation system of Ty elements is known in 46 S. cerevisiae, it is unclear how Ty elements are regulated in other Saccharomyces species, and what 47 mechanisms contributed to the loss of most classes of Ty elements in S. uvarum. Therefore, we first 48 assessed whether transposable elements could insert in the S. uvarum sub-genome of a S. cerevisiae x S. 49uvarum hybrid. We induced transposition to occur in these hybrids and developed a sequencing technique 50 to show that Ty elements insert readily and non-randomly in the S. uvarum genome. We then used an in 51 vivo reporter construct to directly measure transposition rate in hybrids, demonstrating that hybridization 52 itself does not alter rate of mobilization. However, we surprisingly show that species-specific 53 mitochondrial inheritance can change transposition rate by an order of magnitude. Overall, our results 54 provide evidence that hybridization can facilitate the introduction of transposable elements across species 55 boundaries and alter transposition via mitochondrial transmission, but that this does not lead to 56 unrestrained proliferation of transposable elements suggested by the genomic shock theory. 57 58 59 60 61