Tremblaya is one of a growing family of extremely small endosymbiotic bacteria, discovered within the last seven years, that have challenged scientists’ assumptions about the minimal blueprint of life. “It somehow puts a limit to evolution; how much can you evolve towards efficiency and still be intact?” asked Moselio Schaechter, an emeritus professor of microbiology at Tufts University School of Medicine in Boston, Mass.
For most of the last 40 years, scientists thought the smallest genomes belonged to bacteria of the Mycoplasma genus. Mycoplasma genitalium, which lives in the human genital tract with just 482 protein-coding genes (compared to about 20,000 in the human genome), became the second bacterial genome ever sequenced, in 1995, and remained the smallest known to scientists for about a decade. “The insect endosymbionts blew the doors off that number,” said McCutcheon. (M. genitaliumis still considered to have the smallest genome of a free-living organism — unlike Tremblaya, it can be grown in the lab.)
Many scientists are interested in studying these small-genome organisms for practical reasons. Researchers at the J. Craig Venter Institute, for example, are developing a stripped-down bacterium that can be used as a chassis for biological machines designed to make fuel, medicines or other useful chemicals.
Nature’s most streamlined life forms also provide a lesson in thrift and cooperation. “An endosymbiont like Tremblaya is an illustration of how clever organisms can get,” said Schaechter. “You can see evolution in front of you.”
The collection of bacteria with tiny genomes is surprisingly diverse, having emerged from an array of bacterial ancestries, and having retained and shed a variety of genes. Thanks to the protected environment of the host cell, these organisms tend to evolve rapidly, with the smallest mutating the fastest. Tremblaya and its counterparts have shed many of the genes involved in DNA repair, further accelerating their rates of evolution. They have also lost genes required to make the protective membranes that enclose them and instead are thought to rely on membrane components from the host cell. The genes these organisms retain tend to be involved in producing nutrients for the host, as well as carrying out so-called information repair, which includes DNA replication and the translation of genes into proteins. (Beneficial endosymbionts, such as Tremblaya, are fairly common in invertebrates, but are rare in humans and other vertebrates.)
One of the most intriguing reasons for studying endosymbionts like Tremblayais to learn about the evolution of mitochondria and chloroplasts, membrane-bound structures within cells that produce energy. Their emergence more than one billion years ago was a foundational event in the development of eukaryotes, which include plants, animals, protists and fungi.
Scientists proposed the idea that these organelles evolved from bacteria as early as the late 1800s, though the theory didn’t become popular until the 1970s. Two key events enabled organelles to develop: The precursor bacteria transferred many of their genes to the host’s genome, and they developed a method of transporting the proteins produced by these and other genes back inside their own membranes. Human mitochondria, for example, have just 13 genes that code for proteins of their own but employ thousands of proteins in their quest to make energy for the cell.