Cheater gene explains multicellular evolution

The first lifeforms that appeared on earth were composed of single-celled organisms, and after millions of years, they evolved into multicellular entities. Cells cooperate to form organs, and systems of organs combine to form beings, from trees to whales. Though many aspects of organismal evolution are well-understood, scientists have struggled to discover the exact process by which single cells developed into multicellular organisms. Recently, a research team from New Zealand Institute and the Max Planck Institute have found a possible theory for the development of multicellular life.

Under normal physiological conditions, cells develop mutations through their life cycle. Most mutations are eliminated from the population through the process of natural selection. The remaining mutations might impart advantages to the mutated cells that help them compete with the rest of the population. Through successive generations, mutations that favor cooperation between cells might develop, forming the first multicellular life.

The researchers at the New Zealand Institute postulated a potential problem encountered by the evolution of multicellular organisms involving a category of cells called cheaters. Cheaters within a population do not develop mutations that favor cooperation. Instead, they take advantage of features formed by multicellular structures. The role of cheaters can best be studied through Pseudomonas fluorescens, a type of single-celled bacteria.

The bacteria in the experiment normally live independently of each other. However, when researchers breed the bacteria in test tubes, some bacteria developed mutations that produce glue. The glue helps bacteria to stick to the top of the tube where there is more oxygen, providing the bacteria with more nutrients to survive. Over generations, a mat made of bacterial glue was developed and maintained by mutated bacteria. Researchers speculate that multicellular life might have originated from processes similar to those of the bacterial glue.

During the analysis of the glue, scientists found that within the mat, some bacteria in the multicellular colony do not produce glue. These cells took advantage of the mat, staying close to the oxygen-rich surface without contributing to the formation and maintenance of the mat. These cheaters are detrimental to the development of cellular cooperation. In colonies with a high amount of cheaters, the cheaters use all the oxygen at the surface and strain the resources of the glue-producing bacteria. The mat then falls apart due to the lack of glue produced by the mutated cells.

The scientists question the role of cheaters in the role of evolution, since the existence of cheaters might hinder the development of unicellular organisms into multicellular organisms. To further understand the effect of cheaters on the mats, researchers developed two types of mats. One mat consists of both cheater and glue-producing bacteria, whereas the other only contains glue-producing bacteria. The two mats were then placed in the same container to test their fitness. The mat with both cells, surprisingly, outperformed the mat with only glue-producing bacteria.

The cheaters within the population were found to have the function of germ cells. These bacteria allowed natural selection to occur at the mat level, producing cells with traits that are even better at maintaining the mat. As a result, individual cells are less able to survive individually, but the mat as a whole increases its fitness. The researchers speculate that these cheaters may be the ancestors of germ cells such as sperm and ova.