Scientists Discover the Secret to Divergent Animal Life Cycles in Unlocking the Evolutionary Mystery

Scientists Discover the Secret to Divergent Animal Life Cycles in Unlocking the Evolutionary Mystery ...

Researchers sequenced the genome of a worm. Credit: Queen Mary University of London

Scientists establish a connection between the time it takes to activate dozens of genes in the embryo and how animal life cycles evolved.

Biologists have been perplexed by the varying life cycles of different animal species for over a century. Some species, including humans and most vertebrates, develop directly into a fully formed—but smaller—adult form, while others undergo a fascinating metamorphic process that involves the emergence of intermediate forms known as larvae, which eventually evolve into their adult counterparts.

Despite this, the knowledge scientists had about why larvae exist and how they came to be was inadequate. More importantly, large-scale comparative studies addressing this issue had previously not employed modern methods based on sequencing the genetic information of an animal—the genome—and discovering how the organism utilizes this information while growing.

Researchers at Queen Mary University of London (QMUL) discover for the first time the mechanism that might explain why embryos develop either a larva or a miniature version of the adult.

The timing of activation of essential genes involved in embryogenesis, the transformation of a fertilized egg into an organism, is correlated with the presence or absence of a larval stage and whether the larva feeds from their surroundings or relies on the mother's nourishment in the egg.

"It's striking to see how evolution shaped the way animal embryos "tell the time" to activate important groups of genes earlier or later in development. Suppose a larval stage is no longer necessary for your survival. In that case, it might be advantageous to, for example, activate the genes to form the trunk earlier and develop straight into an adult."

In a large-scale research involving over 600 datasets of more than 60 species separated by more than 500 million years of evolution, Dr Ferdinand Marlétaz, a main collaborator of the research from University College London.

"While the techniques have been around for some years, no team had ever used them for this purpose." The datasets we generated and the methodologies we developed will be invaluable to other researchers.

"Developmental biology primarily concentrates on mice, flies, and other well-known species we know as model organisms," says Dr. Chema Martn-Durán, the senior author in this research. "Our study demonstrates that understanding how animal development works and how it evolved is crucial."

The genes involved in forming the trunk—the body region that follows the head and runs until the tail—are of great importance. Some animals may have developed larvae with little trunk as early as in the ancestor of all animals with head and tail. In many animal groups, like us and most vertebrates, direct development and forming a small adult would have evolved later as genes to form the trunk get activated earlier in embryogenesis.

"We are hopeful that other scientists in the field will continue to research the fascinating topic of animal life cycles and provide further evidence for the hypothesis we proposed," said Dr. Andreas Hejnol, professor at the Friedrich-Schiller-University Jena, Germany.

Francisco M. Martn-Zamora, Yan Liang, Kero Guynes, Rory D. Donnellan, Yongkai Tan, Nicholas M. Luscombe, Andreas Hejnol, and José M. Martn-Durán, 25 January 2023, Nature. DOI: 10.1038/s41586-022-05636-7 The present study is led by QMUL researchers; other collaborators include researchers from the Okinawa Institute for Science and Technology

The European Research Council (ERC) Horizon 2020 grant financed the research, as well as the Biotechnology and Biological Sciences Research Council (BBSRC), the Royal Society, and the Japan Society for the Promotion of Science.

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