Summary
Highlights
Animals exhibit incredible diversity in form and function, from flying and swimming creatures to complex organisms like nudibranchs and jellyfish. All animals are composed of millions of cells working in concert. Nicole King's lab focuses on understanding the origin of multicellularity – the transition from single-celled to complex multi-celled life. Darwin's 'endless forms most beautiful' refers to this vast diversity, and current research combines embryology, evolutionary biology, and genetics to understand the molecular basis of animal evolution.
Key questions include how genome evolution contributed to animal origins, how genes for animal development functioned before animals evolved, the origin of specialized cell types like neurons and epithelial cells, and the influence of bacteria on animal origins. The evolutionary implications of multicellularity form a crucial underlying theme. The fossil record provides some insights into ancient animal groups but doesn't explain how multicellularity or cellular interactions first arose.
To understand animal origins, comparing living organisms is essential. Choanoflagellates, an unusual group of organisms, offer special insight. They are the closest living relatives of animals, sharing unique cell biological features. Genomic analyses confirm their close relationship, and studying them helps reveal the genome and biology of the first animals that lived almost a billion years ago. Multicellularity evolved independently in various lineages, including animals, plants, fungi, and slime molds, making direct comparisons difficult due to differences in cell biology and unique genes for cell-cell interactions within each lineage.
Choanoflagellates are single-celled microbial eukaryotes, roughly the size of yeast, characterized by a collar and a flagellum. This flagellum creates fluid currents, pushing water and allowing them to swim, but more importantly, it pulls bacteria towards the collar for feeding. Historically, scientists like Ernst Haeckel and William Saville-Kent noted the striking resemblance between choanoflagellates and choanocytes, the collar cells found in sponges. This similarity led them to hypothesize a close evolutionary link, which is now supported by modern phylogenetic and genomic evidence.
Mapping the distribution of collar cells onto a phylogenetic tree suggests that choanocytes were present in the last common ancestor of choanoflagellates and animals. By comparing choanoflagellate and animal biology, specific inferences can be made about the first animals. It's hypothesized that the first animal had a simple epithelium, adhering cells, some capable of differentiating into collar cells for bacterivorous feeding, and was capable of programmed cell death (apoptosis) and cell differentiation within the soma. It also likely underwent gametogenesis, producing differentiated eggs and sperm, which fused to form a zygote that developed into the adult form.
Choanoflagellates, long overlooked in molecular biology, are now being studied genomically. Sequencing the genomes of various choanoflagellates and comparing them to animal genomes reveals the genomic landscape of animal origins. Many genes crucial for animal multicellularity and development—including those for cell adhesion, signaling, gene regulation, and extracellular matrix interactions—are found in choanoflagellates. This suggests these genes existed before the evolution of animal multicellularity and might have served different functions in unicellular ancestors.
Despite sharing many 'animal' genes, choanoflagellates are not animals. Genomic comparisons highlight specific genetic innovations that characterize animals. Some genes, such as certain developmental signaling genes, classical cadherins (essential for epithelial cell interaction), Hox genes (developmental patterning), and specialized extracellular matrix components like Type IV collagens, have so far only been found in animals. This ongoing research aims to understand how these genes functioned in ancestral organisms and what precise genomic changes contributed to the emergence of animals.
The study of choanoflagellates has illuminated the cell biology and genome of animal progenitors, indicating that the first animals likely ate bacteria and possessed collar cells. A remarkable number of genes essential for multicellularity in animals demonstrably evolved before multicellularity itself. Future research will explore the functions of these 'pre-animal' genes. The speaker concludes by thanking her lab, collaborators, and funding institutions, and looks forward to discussing the regulation of multicellularity in Part II of her talk.