Animal Origins: Genome reveals early complexity

A microscopic spermlike organism contained many of the same tools for cell-to-cell communication found in animals today, two new reports find.

Analysis of DNA from a choanoflagellate, the closest known living nonanimal relative of animals, allows scientists to infer the genetic starter kit possessed by the first animal, the studies suggest.

“It blows open doors for future research,” says Nicole King, coauthor on both papers, published this week in the Feb. 15 Science and the Feb. 14 Nature.

Over 600 million years ago, the single-celled choanoflagellates (Latin for “bearing a collar and a whiptail”) branched off from the same ancestor that gave rise to animals. Studies of their genetic makeup allow scientists to deduce which genes evolved before or after the transition to multicellular forms.

Developmental biologist Sean Carroll of the University of Wisconsin–Madison says King’s work helps answer key questions: “How sophisticated was the genetic toolkit at the dawn of animal origins?” he says.

In the last decade, genomes from flies, mice, and humans have revealed unexpected similarity. On the other hand, the decoded genomes of single-celled organisms like yeast, paramecia, and giardia are much simpler than animal genomes. Although also single-celled, choanoflagellates have twice as many genes as yeast.

The wee creature’s DNA contains as many sequences that code for proteins per gene as animals’ genes do. And a number of those genes coded for proteins involved in vital processes in animals.

But choanoflagellates do little but swim and eat. They may not even have sex. They don’t have specialized membranes. Yet they have loads of genes used by animals to create characteristic body membranes. Although mainly solitary, they have genes for signaling other cells.

King, a biologist at the University of California, Berkeley, and collaborators from Berkeley and nine other institutions, say that the flagellates used these genes in other roles before they were “hijacked” for different functions in the lineage leading to animals. This “co-option” is analogous to using a cell phone to tell time. For example, genes encoding proteins that help cells stick together in modern animals may correspond to genes for proteins used by choanoflagellates to attach to the seafloor. Similarly, genes responsible for immune reactions in animals might have been used by choanoflagellates to help recognize the bacteria they eat.

Among the surprises in the flagellates’ genes was a complete set of phosphotyrosine (pTyr) signaling molecules, known to regulate communication inside and between animal cells. But many of the pTyr genes occurred in combinations not seen in animals. “It highlights how little we know about their biology,” says King.

Some collared flagellates form colonies. One hypothesis holds that multicellular organisms arose when single-celled organisms began living in groups to protect themselves against predators or to buffer against environmental extremes. To this end, one of the colonial species is being sequenced by the Joint Genome Institute in Berkeley to see if cell-signaling genes facilitate interaction between individuals.

From the Nature Index

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