For more than a hundred years, biologists have wondered why animals display different types life cycle. Humans and most other vertebrate animals develop directly into fully developed, albeit smaller, versions of adults. In contrast, many other species give rise to highly diverse intermediate forms, known as larvae, which then undergo metamorphosis into adults.
They describe for the first time the mechanisms that could explain how embryos give rise to different life cycles
But until now, researchers’ understanding of the origin and whereabouts of the larvae is still lacking. Now, a large-scale comparative study of Queen Mary University London (QMUL), led by two Spanish researchers and published in Magazine Natural, rely on techniques based on sequences of animal genetic information – the genome – to work out how organisms use this information as they develop and grow. This is how scientists arrived at a mechanism that might explain how embryos form larvae or adults.
In the work, the moment of activation and the temporal sequence of the essential genes involved in embryogenesis –the transformation of the fertilized egg in fully formed organisms–, which is related to the presence or absence of a larval stage. In addition, this important moment is also related to how the larvae are fed, either by obtaining food from their environment or based on the nutrients stored in the ovules by their parents.
Yan Liang and Francisco Martín-Zamora, the first authors of the study. / Francisco Martin-Zamora
Francisco Martín Zamora explained to SINC this way: “There are a range of genes that are already known to be important for the embryonic development or embryogenesis of animals. What we found is that temporary order of some of these genes is sufficient to produce large changes in embryo developmentand thus give rise to a distinct life cycle”.
Zamora, pre-doctoral researcher at QMUL and co-first author of the paper, explains graphically that, during this stage of development, an organism can become a “larvae — as occurs in many marine invertebrates such as molluscs or annelids — or skip that stage” to instantly become a “little adult”.
The temporal sequence of some of these genes is sufficient to produce larvae or, directly, small adults.
Francisco Martin-Zamora
In his words, “it’s impressive to see how evolution shaped animal embryos look time and time for activate important groups of genes sooner or later, in the process of development”. Zamora proposes to imagine that the larval stage is no longer essential for the survival of a species, hence the fact that “genes are activated to form stems early and embryos form directly into adults.” conversely,” could be “evolutionary advantage.”
This would explain why many mammals and vertebrates lack a larval stage, which may have been lost during evolution. “In our work, we highlight how one of the worm species we used (Dimorphilus gyrociliatus), which had a direct mode of development (forming adults directly), lost its larvae during annelid evolution, from an ancestor that had that stage,” the researchers added.
Nutrition during embryo development
The way the larva eats to grow also influences embryogenesis: “The larva that gains nutrients from the environment They generally show faster development, because they need to start eating as soon as possible, explained the researcher.
For their part, the animals have what it takes inside the ovule during this stage, “they don’t have that stress” and, instead, “they activate, very early on, the genes associated with the autophagy pathway, which is the cellular process that allows them to utilize that food”; In the case of larvae that obtain food from their environment, “these genes (for autophagy) are not activated until later,” according to Martín Zamora.
Laboratory data and computational analysis
This new study uses a cutting-edge approach to decode the genetic information of organisms, their activities and how they are regulated, in three species of invertebrate marine worms of the annelid family. Then, this information is combined with public data sets from other species, in large-scale studies that include up to 600 data sets from more than 60 species separated by over 500 million years of evolution.
“Only by combining laboratory-generated experimental data and systematic computational analysis were we able to uncover this new, unknown biology,” said Ferdinand Marletaz, principal study collaborator, from University College London.
“Even though the technique has been available for several years, no team has used it for this purpose. The data we generate and the methodology we develop will no doubt be a very useful resource for other researchers”, highlighted Yan Liang, QMUL postdoctoral researcher and co-author of this paper.
Previously neglected species
Chema Martín Durán, principal investigator of the work, says that “developmental biology has focused largely on mice, flies and other well-established species that are known to us as model organisms. Our study shows that the interesting biology of ‘non-model’ species, which is often overlooked, is critical to understanding how animal development works and has evolved.”
Up to 600 datasets of more than 60 species separated by more than 500 million years of evolution were analyzed
The genes involved in stem formation, the region of the body that follows the head and reaches the tail, is especially important. Some species form larvae which are almost trunkless and are known as ‘head larvae’. head larvae), and that they could have been present at ancestor of all animals with head and tail.
The immediate development and formation of small adults after embryogenesis will develop later in many animal groups (eg We and most vertebrates), because the gene for proboscis formation is activated earlier in embryogenesis, and larval characteristics are progressively lost.
“We hope that other researchers in the field will continue to study the exciting topic of the evolution of animal life cycles, and they will provide more information about how genetic mechanism and the molecules behind it influence how animals develop and evolve,” explains Andreas Hejnol, professor at Friedrich-Schiller-University Jena and team collaborator.
Although led by researchers at QMUL, this work is a multidisciplinary collaboration of more than a dozen scientists, with collaborators from University College London, Imperial College London, and the National Museum of Wales, in great Britain; Okinawa Institute of Science and Technology, at Japan; Friedrich-Schiller University Jena, at German; and the University of Bergen, in Norwegian.
Reference
Martín Zamora, FM et al., “Functional genomics of annelids unraveling the origins of the bilaterian life cycle”. Natural (2023).
“Entrepreneur. Internet fanatic. Certified zombie scholar. Friendly troublemaker. Bacon expert.”
