Disclaimer: machine translated by DeepL which may contain errors.

Evolution of organisms is limited by internal factors.

What mechanism determines how organisms evolve? Modern evolutionary theory explains the evolutionary process as follows: a mutation in DNA produces the next generation with characteristics slightly different from those of the parent generation, and from there, through natural selection and accidental factors, the characteristics of the individual that produces more offspring spread within the species. Then, can we artificially evolve winged animals such as Pegasus from horses by natural selection? Looking back over hundreds of millions of years of evolutionary history, this possibility is extremely unlikely. No mammal, reptile, bird, or amphibian with two pairs of legs has ever acquired a third pair of legs (even birds have only converted their forelegs into wings). ) Although changes in body size and coloration have often occurred, strangely enough, changes in basic anatomical features have been slow to occur, even through hundreds of millions of years of evolution. In particular, basic anatomical features (body plans) such as dorsally located neural tube, ventral digestive tract, pharynx, and neural crest cell-derived organs have not changed throughout vertebrate evolution. Despite many years of debate, the reason for this has remained unclear, and the issue has tended to be settled on the poorly supported theory that changes in body plan may not have been so advantageous to survival strategies.

On the other hand, recent research has provided a clue to this problem. The hourglass model of ^{development*1} has revealed that during the developmental process from fertilized egg to adult body, the period when basic anatomical features (body plan) are formed has not been evolutionarily diversified compared to the developmental periods preceding and following the period when body plans are formed. It may seem obvious, but the reason that body plans have not changed throughout evolution is that the developmental stages that form body plans have not changed. The next step is to elucidate the mechanism by which the stages of body plan formation have not diversified throughout evolution.

As we experimentally verified and eliminated candidate possibilities one by one, we came up with a theoretical ^hypothesis*2. The embryonic stage, in which body plans are formed, is unlikely to produce variations in the characteristics of the body plans, and therefore it may not be possible to select individuals with new characteristics and diversify them. In fact, our previous ^studies*3 have shown that the body plan formation stage is robust against mutation and environmental noise, and that the characteristics of the body plan do not change even at the gene expression level. In other words, the characteristics of the body plan formation stage are not easily changed by environmental changes or mutations. Then, what about stability in the absence of environmental fluctuations and mutations?

Figure: Robustness and stability of body planning (embryogenesis) of organisms (left panel) were found to correlate with their evolutionary diversity (right panel). In the figure, embryogenesis proceeds from bottom to top. The embryonic stage, in which the basic structure of the body is being formed, is robust to environmental variation and mutation and stable to internal noise, making it difficult for phenotypic variation to emerge. This means that it is difficult to produce individuals with new characteristics that can be selected for by natural selection, resulting in the lack of evolutionary diversity (right side of the figure).

In the present study, we tested this hypothesis by establishing conditions in which the effects of mutation and environmental variation are minimal. We used ^inbred*4 killifish, a vertebrate species with few genomic differences, and conducted a large-scale analysis of gene expression information on killifish embryos of the same sex born from the same parents and raised in the same environment. The results showed that the body plan formation stage was less differentiated than the developmental stages before and after the body plan formation stage, i.e., it was more stable. These results are consistent with the hypothesis that the body plan formation stage is conserved because it is less likely to produce variation, and indicate that diversification of organismal characteristics may be limited by internal characteristics such as stability and robustness. The viewpoint that the evolution of organisms is to some extent internalized in the organisms themselves is not integrated in modern evolutionary theory, and is an important finding that will lead to the expansion of evolutionary theory in the future.

The results of this study were published in Y. Uchida et al. BMC Biology. 82, 20 (2022).

1 A rule of thumb that states that the developmental stages in which body plans are constructed have less evolutionary diversity than the early developmental stages, which are closer to the fertilized egg, or the late developmental stages, which are closer to the adult body, when the processes from fertilized egg to adult body (developmental processes) of various animals are compared. The body plan is now known to be established in multiple animal phyla, but only in groups of animals belonging to the same phylum, and not across animal phyla.

This theory is called the fluctuation response theory, and was proposed by Professor Chikara Furusawa of the Universal Biology Institute, with whom we conducted this joint research, and his colleagues. It is experimentally supported in unicellular organisms such as Escherichia coli, but it has been unclear whether it is valid for complex multicellular organisms.

EvoDevo 9: 7, (2018) *4 A lineage in which the genetic differences between individuals are very small due to repeated inbreeding.

(Press release, April 11, 2022)

Published in Faculty of Science News, July 2022.

Communicating to Faculty Research Students at the Forefront of Research>