The Green Colonies
In freshwater ponds and puddles around the world, a remarkable evolutionary transition plays out in miniature. Here lives Volvox carteri, a green alga that forms a hollow, spherical colony of thousands of cells. These colonies, visible as tiny green specks about 0.5 mm in diameter, represent one of the simplest examples of a multicellular organism with specialized cells. A mature colony contains two distinct cell types. Approximately 2000 to 6000 small, biflagellate (two-tailed) somatic cells form the outer layer. These cells are responsible for motility, using their flagella in unison to propel the sphere towards light for photosynthesis.
Embedded within the gelatinous sphere are around 16 much larger reproductive cells, called gonidia. Unlike the somatic cells, the gonidia cannot swim; their sole function is reproduction. This division of labor is a basic feature of multicellular life. The swimming somatic cells are mortal and die when the colony reproduces. The gonidia, however, are potentially immortal, as they pass their genes to the next generation. This separation of sterile "body" cells from reproductive "germ" cells mirrors the organization of more complex organisms like plants and animals.
An Evolutionary Toolkit
The jump from a single-celled existence to a coordinated, multicellular life happened multiple times in Earth's history. The volvocine algae, the group that includes Volvox, show this process because their unicellular and colonial relatives still exist today. By comparing the genome of Volvox carteri with that of its single-celled cousin, Chlamydomonas reinhardtii, scientists can pinpoint the genetic changes that enabled multicellularity. This evolutionary leap is estimated to have occurred relatively recently, around 200 million years ago.
Surprisingly, the transition did not require a massive invention of new genes. The genomes of Volvox and Chlamydomonas are very similar in size and contain almost the same number of protein-coding genes—about 14,500. The larger size of the Volvox genome is mainly due to an increase in repetitive DNA sequences. Instead of creating new parts, evolution repurposed the existing genetic toolkit. A significant genetic change involved the regA gene. This gene is a master switch, suppressing cell growth and reproduction in the somatic cells, locking them into their swimming function. This prevents every cell from trying to reproduce, enforcing the division of labor that allows the colony to function as a single, coordinated organism.