Here, we discuss the roles that microRNAs play in providing canalization to animal development, citing recent theoretical and experimental. Abstract: Animal development is an extremely robust process resulting in stereotyped outcomes. Canalization is a design principle wherein developmental . Canalization refers to the process by which phenotypes are stabilized within . Many miRNAs play a role in critical steps of animal development (Carrington and .

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On the surface, evolutionary changes and phenotypic stability may seem like devvelopment concepts, but in the language of genetics they are really two sides of the same issue. Thanks to the direct action on the transcripts, miRNAs can tune the expression quickly, but the extent of tuning is often modest.

From This Paper Figures, tables, and topics from this paper. Analysis of chromosome substitutions.

Evolution under canalization and the dual roles of microRNAs—A hypothesis

The expression-tuning motifs include: Second, the nearly neutral model makes other predictions. The original nongenetic version of the Darwinian theory encountered many difficulties. For example, if a shock absorber works well in an incoherent FFL of Figure 1Done might expect the expression of miRNAs themselves to vary more canslization the targets. We analyzed the eevelopment conservation of the target sites between human and mouse.

These authors found that, on average, the number of predicted targets is several times larger in human than canalisation Drosophila ; hence, broad-scale nonconservation seems possible. One more canalizatikn in the tuning mode is shown in Figure 1C. The difference is that living organisms carry imcrornas the tasks under extremely variable environments, whereas no computers are made to withstand even moderate fluctuations in voltage input.

Any model on the function and evolution of miRNAs has to micronas for these paradoxical properties. The fluctuation in miR is thus dampened and its effect on the Nodal pathway, which controls mesendoderm formation, would be robust.


Figure 4A and B show the conservation of the first 8 bp referred to as the core and all 18 bp, respectively. In the literature, the proposed functions of miRNAs may be broadly classified into two categories. In a control experiment for the experiments of Figure 2we used nonnative miRNAs as the transgene.

The analysis was not able to incorporate divergence in miRNA expression as such information was generally unavailable. Although miR Myb interaction should drvelopment embedded in a more complex network, it appears that this repression dominates other interactions in the particular cellular context.

The miRs cluster have been evolving since their formation 40—50 Mya and are still evolving adaptively in the recent past Lu et al.

There is little doubt that individual loci might play a canalizing role. Under the buffering hypothesis, miRNAs stabilize whole-genome expression against input noises. The second bin with one change in the core is canailzation the same between the two panels of Figure 4A.

In the canalization metaphor, some gene actions vy responsible for moving the species to the right valley, while others play the role of making its development canalized.

Since all miRs should be absent, the lethality of Dicer-1 deletion is hardly unexpected. One of the major discoveries in the last decade is the existence of microRNAs miRNA or miRwhich constitute a class of post-transcriptional regulators of gene expression Lee et al.

This abstract may be abridged. However, in the absence of the heat shock protein HSP90, many abnormalities were observed.

Correlation between the expression ratios in miRNAs caanlization their targets. B Changes in the expression of the predicted targets between miRs transgenic lines and the wild-type line.

Another example of simple repression that has to be viewed in the context of a larger motif is the miR-9a senseless repression in Drosophila. Like many other miRNA examples, the deletion of miR does not produce an obvious phenotype.

An additional point concerns the evolution between the two functions of miRNAs tuning and buffering. We will return to this hypothesis later. Canalizafion contrast, the ratios are 1.


When there are no input noises, the buffering function might not be easily noticeable.

It is customary to explain the dispensability in terms of functional redundancy, but it is nevertheless difficult to reconcile the extreme conservation with redundancy.

New miRNAs, in the process of becoming integrated into the genome, may have to micrkrnas the disruption to the transcriptome while providing some benefits. Furthermore, the sequences of their targets appear to be weakly constrained.

Canalization of development by microRNAs

Articles by Tang, T. In this model, evolution can proceed if the deleterious effects of the new mutations including new genes are sufficiently small.

Recent evidence has increasingly hinted that microRNAs may be involved in canalizing gene expression. Filtering transcriptional noise during development: Given that the tuning functions attributed to miRNAs often underlie important phenotypes, one might expect strong conservation in the interaction between miRNAs and their targets. Most of these studies were designed to study the interactions between miRNAs and their targets. One also notices in Figure 3 that a sizable fraction of miRNAs vary mmicrornas expression by more than twofold between the two species of Drosophila.

EBSCOhost | | Canalization of development by microRNAs.

This bristle number does not vary within species for example, always four in Drosophila melanogaster but has diverged among Drosophila species, making it a diagnostic trait in species identification. The gene, senselessactivates proneural gene expression, which plays a key role in the development of sense organ precursors SOPs Li et al.

Figure 1 highlights only the simpler network motifs. Given that HSP90 is a chaperon protein that helps the folding of many other proteins, it can conceivably influence the phenotypic consequences of those gene products.

Although the results in Table 1 do not lend themselves to a quantitative estimate of target-site conservation, they do suggest, qualitatively, that the miR—target coupling is labile during evolution.

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