No abstract

In the mouse embryo, transcriptional activation begins during S/G2 phase of the first cell cycle when paternal and maternal chromatin are still in separate nuclear entities within the same cytoplasm. At this time, the male pronucleus exhibits greater transcriptional activity than the female pronucleus. Since acetylation of histones in the nucleosome octamer exerts a regulatory influence on gene expression, we investigated changes in histone acetylation during the remodeling of paternal and maternal chromatin from sperm entry through to minor genome activation and mitosis. We found (1) neither mature sperm nor metaphase II maternal chromatin stained for hyperacetylated histone H4; (2) immediately following fertilization, hyperacetylated H4 was associated with paternal but not maternal chromatin while, in parthenogenetically activated oocytes, maternal chromatin became hyperacetylated; (3) in zygotes, differential levels and patterns of hyperacetylated H4 between male and female pronuclei persisted throughout most of G1 with histone deacetylases and acetyltransferases already active at this time; (4) when transcriptional differences are observed in S/G2, male and female pronuclei have equivalent levels of H4 hyperacetylation and DNA replication was not required to attain this equivalence and (5) in contrast to the lack of H4 hyperacetylation on gametic chromatin, chromosomes at the first mitosis showed distinct banding patterns of H4 hyperacetylation. These results suggest that sperm chromatin initially out-competes maternal chromatin for the pool of hyperacetylated H4 in the oocyte, that hyperacetylated H4 participates in the process of histone-protamine exchange in the zygote, and that differences in H4 acetylation in male and female pronuclei during G1 are translated across DNA replication to transcriptional differences in S/G2. Prior to fertilization, neither paternal nor maternal chromatin show memory of H4 hyperacetylation patterns but, by the end of the first cell cycle, before major zygotic genome activation at the 2-cell stage, chromosomes already show hyperacetylated H4 banding patterns.

High mobility group 1 protein (HMG1) has traditionally been considered a structural component of chromatin, possibly similar in function to histone H1. In fact, at the onset of Xenopus and Drosophila development, HMG1 appears to substitute for histone H1: HMG1 is abundant when histone H1 is absent after the midblastula transition histone H1 largely replaces HMG1. We show that in early mouse embryos the expression patterns of HMG1 and histone H1 are not complementary. Instead, HMG1 content increases after zygotic genome activation at the same time as histone H1. HMG1 does not remain associated to mitotic chromosomes either in embryos or somatic cells. These results argue against a shared structural role for HMG1 and histone H1 in mammalian chromatin.

The structure‐function relationships of the nucleolar substructures were studied in preimplantation rabbit embryos, where nucleologenesis is extending over the first four cell cycles and may not be synchronous in each blastomere. Immunocytochemical methods using light and electron microscopy were applied for protein and RNA localization as well as nick translation and terminal deoxynucleotidyl transferase techniques for DNA detection. DNA was gradually associated with the periphery of the compact nucleolar precursor bodies (NPBs) but was never found inside NPBs at the four‐cell stage. In 16‐cell embryos, some NPBs displayed a reticulated periphery forming the branching network of the dense fibrillar component (DFC) surrounding the “residual body” (remnant of NPB) in the process of activation. At the 32‐cell stage, fully reticulated nucleoli were observed in each blastomere. DNA was then associated with the DFC of reticulated nucleoli. RNA was first detected at the 16 cell‐stage in close contact with the DFC as well as inside the “residual body” which was not immunolabeled with the DNA antibodies used. When observed by light microscopy, fibrillarin, nucleolin, and protein B23 displayed a changing distribution pattern during nucleologenesis. At early stages (up to the 16‐cell stage), small dot‐ and spot‐like structures were distributed within the whole nuclei. In 16‐cell embryos, these proteins started to accumulate in an irregular thin layer around the NPBs in the process of activation. The reorganization process described may be in relation with the redistribution of chromatin and nuclear/nucleolar matrix components during the activation of rDNA transcription localized in the NPB shell. In conclusion, nucleologenesis is only achieved at the fourth cell cycle in the cleaving rabbit embryo at the corresponding time when the first detectable nucleolus‐associated RNA is detectable. Our results show a good correlation between the establishment of structure and function. Mol. Reprod. Dev. 48:34–44, 1997. © 1997 Wiley‐Liss, Inc.

Taste perception plays an important role in an animal’s detection of nutrients, conveying key dietary information, fundamental for its growth and survival. Because alternative terrestrial ingredients are known to affect the feeding of rainbow trout (RT, Oncorhynchus mikyss), we aimed to assess the importance of taste receptors in detection. Using self-feeders, we examined the feeding behavior (30 days of a feeding trial followed by 10 days of a preference trial) of RT fed with a commercial diet (C), vegetable diets supplemented with linseed oil (V1) or algal oil (V2). During the feeding trial those fed V2 decreased their food intake. The preference trial revealed that fish preferred V2 v. C and V1 v. V2 for fish which had consumed V1 and C during their feeding trial. Mechanistically, taste receptors were mainly expressed in taste organs and regulated by diet, which indicated the function of the taste receptors. Some taste receptors for fatty acids (such as the ffar receptor) and amino acids (such as the tasr receptor) were highly expressed in the RT tongue. While ffar2a transcripts were upregulated by vegetal diets in the tongue, ffar1 and ffar4, known for important roles in mammals, were very low expressed and not found in the RT genome, respectively. Overall findings show that RT displayed the fundamental mechanisms for oro-gustatory perception of nutrients related to different diet composition.