Abstract
ᅟSymmetry is an eye-catching feature of animal body plans, yet its causes are not well enough understood. The evolution of animal form is mainly due to changes in gene regulatory networks (GRNs). Based on theoretical considerations regarding fundamental GRN properties, it has recently been proposed that the animal genome, on large time scales, should be regarded as a system which can construct both the main symmetries – radial and bilateral – simultaneously; and that the expression of any of these depends on functional constraints. Current theories explain biological symmetry as a pattern mostly determined by phylogenetic constraints, and more by chance than by necessity. In contrast to this conception, I suggest that physical effects, which in many cases act as proximate, direct, tissue-shaping factors during ontogenesis, are also the ultimate causes – i.e. the indirect factors which provide a selective advantage – of animal symmetry, from organs to body plan level patterns. In this respect, animal symmetry is a necessary product of evolution. This proposition offers a parsimonious view of symmetry as a basic feature of the animal body plan, suggesting that molecules and physical forces act in a beautiful harmony to create symmetrical structures, but that the concert itself is directed by the latter.ReviewersThis article was reviewed by Eugene Koonin, Zoltán Varga and Michaël Manuel.
Highlights
Symmetry is a frequent pattern in nature, often perceived as a source of beauty, and is a salient property of animal body plans
The idea that symmetry is mainly shaped by physical forces, has deep roots in time; with the advent of modern molecular biology, the molecular approach has taken the leading role in science
Any idea in which symmetry is mainly a genetic and developmental “burden” about which we do not really know why it changes in certain instances and why it remains the same for hundreds of millions of years, and which fails to explain why bilaterality is associated with a free-moving lifestyle in certain cases and why it is not in others, remains, in my opinion, unsatisfactory
Summary
Row 33: “The concept of the body plan can be defined as an ontogenetic pattern-organising algorithm, thanks to which the body develops in a specific order.”. The more deeply I considered my answer to this criticism as regards the comparison between the different models for embryonic conservation (and mathematical approaches), the more clearly I had to realise that the argumentation on early embryonic processes will not provide sufficient support for the main line of the reasoning of the paper, because the question of the diversity of early embryonic developmental strategies to adapt to a wide range of niches does not, in principle, either bolster the flexible use of symmetries in the animal body, nor contradict it – the argument will still remain necessarily weak.
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