The reprogramming of X-chromosome inactivation during the acquisition of pluripotency in vivo and in vitro is accompanied by the repression of Xist, the trigger of X-inactivation, and the upregulation of its antisense counterpart Tsix. We have shown that key factors supporting pluripotency-Nanog, Oct4 and Sox2-bind within Xist intron 1 in undifferentiated embryonic stem cells (ESC) to repress Xist transcription. However, the relationship between transcription factors of the pluripotency network and Tsix regulation has remained unclear. Here we show that Tsix upregulation in embryonic stem cells depends on the recruitment of the pluripotent marker Rex1, and of the reprogramming-associated factors Klf4 and c-Myc, by the DXPas34 minisatellite associated with the Tsix promoter. Upon deletion of DXPas34, binding of the three factors is abrogated and the transcriptional machinery is no longer efficiently recruited to the Tsix promoter. Additional analyses including knockdown experiments further demonstrate that Rex1 is critically important for efficient transcription elongation of Tsix. Hence, distinct embryonic-stem-cell-specific complexes couple X-inactivation reprogramming and pluripotency, with Nanog, Oct4 and Sox2 repressing Xist to facilitate the reactivation of the inactive X, and Klf4, c-Myc and Rex1 activating Tsix to remodel Xist chromatin and ensure random X-inactivation upon differentiation. The holistic pattern of Xist/Tsix regulation by pluripotent factors that we have identified suggests a general direct governance of complex epigenetic processes by the machinery dedicated to pluripotency.

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The ability of mothers to transfer antibodies (Abs) to their young and the temporal persistence of maternal Abs in offspring constitute important life-history traits that can impact the evolution of host-parasite interactions. Here, we examined the effects of food availability and parental immunization on the transfer and persistence of maternal antibodies in nestling pigeons (Columba livia). This species can transmit maternal Abs to offspring before hatching through the egg yolk and potentially after hatching through crop milk. However, the role of this postnatal substance in immunity remains elusive. We used a full cross-fostering design to disentangle the effects of food limitation and parental immunization both before and after hatching on the levels and persistence of maternal Abs in chicks. Parents were immunized via injection with keyhole limpet hemocyanin antigens. Using an immunoassay that specifically detected the IgY antibodies that are known to be transmitted via the yolk, we found that the levels of anti-KLH Abs in newly hatched chicks were positively correlated with the levels of anti-KLH Abs in the blood of their biological mothers. However, this correlation was not present between chicks and their foster parents, suggesting limited IgY transfer via crop milk to the chick’s bloodstream. Interestingly, biological mothers subjected to food limitation during egg laying transferred significantly fewer specific maternal Abs, which suggests that the transfer of antibodies might be costly for them. In addition, the persistence of maternal Abs in a chick’s bloodstream was not affected by food limitation or the foster parents’ anti-KLH Ab levels; it was only affected by the initial level of maternal anti-KLH Abs that were present in newly hatched chicks. These results suggest that the maternal transfer of Abs could be costly but that their persistence in an offspring’s bloodstream may not necessarily be affected by environmental conditions.

Urban areas encompass both favorable and stressful conditions linked with human activities and pollution. Pollutants remain of major ecological importance for synanthropic organisms living in the city. Plumage of urban birds harbour trace metals, which can result from external deposition or from internal accumulation. External and internal plumage concentrations likely differ between specific trace metals, and may further differ between males and females because of potential sex-linked differential urban use, physiology or behaviour. Here, we measured the concentrations in four trace metals (cadmium, copper, lead and zinc) in both unwashed and washed feathers of 49 male and 38 female feral pigeons (Columba livia) from Parisian agglomeration. We found that these concentrations indeed differed between unwashed and washed feathers, between males and females, and for some metals depended on the interaction between these factors. We discuss these results in the light of physiological and behavioural differences between males and females and of spatial repartition of the four trace metals in the city.