Comprehensive expression analysis for the core cell cycle regulators in the chicken embryo reveals novel tissue-specific synexpression groups and similarities and differences with expression in mouse, frog and zebrafish.

The core cell cycle machinery is conserved from yeast to humans, and hence it is assumed that all vertebrates share the same set of players. Yet during vertebrate evolution, the genome was duplicated twice, followed by a further genome duplication in teleost fish. Thereafter, distinct genes were retained in different vertebrate lineages; some individual gene duplications also occurred. To which extent these diversifying tendencies were compensated by retaining the same expression patterns across homologous genes is not known. This study for the first time undertook a comprehensive expression analysis for the core cell cycle regulators in the chicken, focusing in on early neurula and pharyngula stages of development, with the latter representing the vertebrate phylotypic stage. We also compared our data with published data for the mouse, Xenopus and zebrafish, the other established vertebrate models. Our work shows that, while many genes are expressed widely, some are upregulated or specifically expressed in defined tissues of the chicken embryo, forming novel synexpression groups with markers for distinct developmental pathways. Moreover, we found that in the neural tube and in the somite, mRNAs of some of the genes investigated accumulate in a specific subcellular localisation, pointing at a novel link between the site of mRNA translation, cell cycle control and interkinetic nuclear movements. Finally, we show that expression patterns of orthologous genes may differ in the four vertebrate models. Thus, for any study investigating cell proliferation, cell differentiation, tissue regeneration, stem cell behaviour and cancer/cancer therapy, it has to be carefully examined which of the observed effects are due to the specific model organism used, and which can be generalised.

Time course and side-by-side analysis of mesodermal, pre-myogenic, myogenic and differentiated cell markers in the chicken model for skeletal muscle formation.

The chicken is a well-established model for amniote (including human) skeletal muscle formation because the developmental anatomy of chicken skeletal muscle matches that of mammals. The accessibility of the chicken in the egg as well as the sequencing of its genome and novel molecular techniques have raised the profile of this model. Over the years, a number of regulatory and marker genes have been identified that are suited to monitor the progress of skeletal myogenesis both in wildtype and in experimental embryos. However, in the various studies, differing markers at different stages of development have been used. Moreover, contradictory results on the hierarchy of regulatory factors are now emerging, and clearly, factors need to be able to cooperate. Thus, a reference paper describing in detail and side-by-side the time course of marker gene expression during avian myogenesis is needed. We comparatively analysed onset and expression patterns of the key markers for the chicken immature paraxial mesoderm, for muscle-competent cells, for cells committed to myogenesis and for cells entering terminal differentiation. We performed this analysis from stages when the first paraxial mesoderm is being laid down to the stage when mesoderm formation comes to a conclusion. Our data show that, although the sequence of marker gene expression is the same at the various stages of development, the timing of the expression onset is quite different. Moreover, marker gene expression in myogenic cells being deployed from the dorsomedial and ventrolateral lips of the dermomyotome is different from those being deployed from the rostrocaudal lips, suggesting different molecular programs. Furthermore, expression of Myosin Heavy Chain genes is overlapping but different along the length of a myotube. Finally, Mef2c is the most likely partner of Mrf proteins, and, in contrast to the mouse and more alike frog and zebrafish fish, chicken Mrf4 is co-expressed with MyoG as cells enter terminal differentiation.