Molecular evolution of HoxA13 and the multiple origins of limbless morphologies in amphibians and reptiles

Developmental processes and their results, morphological characters, are inherited through transmission of genes regulating development. While there is ample evidence that cis-regulatory elements tend to be modular, with sequence segments dedicated to different roles, the situation for proteins is less clear, being particularly complex for transcription factors with multiple functions. Some motifs mediating protein-protein interactions may be exclusive to particular developmental roles, but it is also possible that motifs are mostly shared among different processes. Here we focus on HoxA13, a protein essential for limb development. We asked whether the HoxA13 amino acid sequence evolved similarly in three limbless clades: Gymnophiona, Amphisbaenia and Serpentes. We explored variation in ω (dN/dS) using a maximum-likelihood framework and HoxA13sequences from 47 species. Comparisons of evolutionary models provided low ω global values and no evidence that HoxA13 experienced relaxed selection in limbless clades. Branch-site models failed to detect evidence for positive selection acting on any site along branches of Amphisbaena and Gymnophiona, while three sites were identified in Serpentes. Examination of alignments did not reveal consistent sequence differences between limbed and limbless species. We conclude that HoxA13 has no modules exclusive to limb development, which may be explained by its involvement in multiple developmental processes

MiR-10 Represses HoxB1a and HoxB3a in Zebrafish

BACKGROUND: The Hox genes are involved in patterning the anterior-posterior axis. In addition to the protein coding Hox genes, the miR-10, miR-196 and miR-615 families of microRNA genes are conserved within the vertebrate Hox clusters. The members of the miR-10 family are located at positions associated with Hox-4 paralogues. No function is yet known for this microRNA family but the genomic positions of its members suggest a role in anterior-posterior patterning. METHODOLOGY/PRINCIPAL FINDINGS: Using sensor constructs, overexpression and morpholino knockdown, we show in Zebrafish that miR-10 targets HoxB1a and HoxB3a and synergizes with HoxB4 in the repression of these target genes. Overexpression of miR-10 also induces specific phenotypes related to the loss of function of these targets. HoxB1a and HoxB3a have a dominant hindbrain expression domain anterior to that of miR-10 but overlap in a weaker expression domain in the spinal cord. In this latter domain, miR-10 knockdown results in upregulation of the target genes. In the case of a HoxB3a splice variant that includes miR-10c within its primary transcript, we show that the microRNA acts in an autoregulatory fashion. CONCLUSIONS/SIGNIFICANCE: We find that miR-10 acts to repress HoxB1a and HoxB3a within the spinal cord and show that this repression works cooperatively with HoxB4. As with the previously described interactions between miR-196 and HoxA7 and Hox-8 paralogues, the target genes are located in close proximity to the microRNA. We present a model in which we postulate a link between the clustering of Hox genes and post-transcriptional gene regulation. We speculate that the high density of transcription units and enhancers within the Hox clusters places constraints on the precision of the transcriptional control that can be achieved within these clusters and requires the involvement of post-transcriptional gene silencing to define functional domains of genes appropriately

Behavior of Dynamical Systems in the Regime of Transient Chaos

The transient chaos regime in a two-dimensional system with discrete time (Eno map) is considered. It is demonstrated that a time series corresponding to this regime differs from a chaotic series constructed for close values of the control parameters by the presence of "nonregular" regions, the number of which increases with the critical parameter. A possible mechanism of this effect is discussed.Comment: 4 pages, 2 figure

Three Drosophila Hox Complex microRNAs Do Not Have Major Effects on Expression of Evolutionarily Conserved Hox Gene Targets during Embryogenesis

The discovery of microRNAs has resulted in a major expansion of the number of molecules known to be involved in gene regulation. Elucidating the functions of animal microRNAs has posed a significant challenge as their target interactions with messenger RNAs do not adhere to simple rules. Of the thousands of known animal microRNAs, relatively few microRNA:messenger RNA regulatory interactions have been biologically validated in an normal organismal context. Here we present evidence that three microRNAs from the Hox complex in Drosophila (miR-10-5p, miR-10-3p, miR-iab-4-5p) do not have significant effects during embryogenesis on the expression of Hox genes that contain high confidence microRNAs target sites in the 3′ untranslated regions of their messenger RNAs. This is significant, in that it suggests that many predicted microRNA-target interactions may not be biologically relevant, or that the outcomes of these interactions may be so subtle that mutants may only show phenotypes in specific contexts, such as in environmental stress conditions, or in combinations with other microRNA mutations

Meiosis in Mice without a Synaptonemal Complex

The synaptonemal complex (SC) promotes fusion of the homologous chromosomes (synapsis) and crossover recombination events during meiosis. The SC displays an extensive structural conservation between species; however, a few organisms lack SC and execute meiotic process in a SC-independent manner. To clarify the SC function in mammals, we have generated a mutant mouse strain (Sycp1−/−Sycp3−/−, here called SC-null) in which all known SC proteins have been displaced from meiotic chromosomes. While transmission electron microscopy failed to identify any remnants of the SC in SC-null spermatocytes, neither formation of the cohesion axes nor attachment of the chromosomes to the nuclear membrane was perturbed. Furthermore, the meiotic chromosomes in SC-null meiocytes achieved pre-synaptic pairing, underwent early homologous recombination events and sustained a residual crossover formation. In contrast, in SC-null meiocytes synapsis and MLH1-MLH3-dependent crossovers maturation were abolished, whereas the structural integrity of chromosomes was drastically impaired. The variable consequences that SC inactivation has on the meiotic process in different organisms, together with the absence of SC in some unrelated species, imply that the SC could have originated independently in different taxonomic groups

De Novo Genesis of Enhancers in Vertebrates

Whole genome duplication in teleost fish reveals that a few changes in non-regulatory genomic sequences are a source for generating new enhancers