One of the few things older than the battle of the sexes is the origin of the sexes. How sexes evolved in the first place has been a lasting mystery in biology. Thanks to some transgender algae, scientists may have cracked this evolutionary whodunit.
A simple trick of genetic engineering forced female Volvox carteri algae to produce sperm and males to produce eggs. The process revealed that the evolution of males and females was much more straightforward than anyone thought.A colony of Volvox, magnified 50 times. Photograph by E.R. Degginger, Alamy
“Much to our surprise, one single gene had evolved the capacity to affect the male/female difference,” said the study’s lead author James Umen, a biologist at the Danforth Plant Science Center in St. Louis. (See “How a Tiny Critter Has Seven (Yes, Seven) Sexes.”)
Most of the many forms of life we see every day contain millions of cells. Whether they take the form of chirping songbirds or a resolute oak tree, they have many different types of cells and at least two different sexes. Life didn’t start out this way, however. For most of Earth’s history, life consisted of single-celled organisms. Only relatively recently did life grow from one cell into many.
The switch to multicellularity happened not once but approximately a dozen times, says Umen. Each time it occurred, the new multicellular life forms had to navigate a variety of new complexities, including reproduction. (Get a genetics overview.)
Evolving hand in hand with multicellularity was sexual reproduction. Single-celled organisms generally reproduce by simply dividing in two. Multicellular organisms can’t do that. Instead, many larger life-forms reproduce using sex. The transition between simple cell division and more complex sexual reproduction was traditionally thought to have been a complicated affair.
Divide and Conquer
To study this transition, Umen turned to Volvox algae, a genus that contains a wide variety of algae, from simple unicellular organisms to large, complex multicellular life. V. carteri lives in colonies of 2,000 cells, whereas another species from a different family, such as Chlamydomonas reinhardtii, are unicellular. Their diversity in size is also matched by a diversity in reproductive methods.
“It’s almost as if these algae were tailor-made to help us understand important evolutionary transitions,” Umen said.
Previous work had closely investigated the genetics behind reproduction in Chlamydomonas, which has two basic mating types, known simply as + and -. To reproduce sexually, a + Chlamydomonas must find and fuse with a – Chlamydomonas. Previous work had shown that mating types were controlled by a gene known as MID. If MID was present, Chlamydomonas was a – mating type. If not, then it was a +.
V. carteri‘s mating system was a little more complicated. Instead of simple + and -, it had sperm-producing males and egg-producing females. It also had a functioning copy of the MID gene, leading Umen to wonder whether that gene might be involved in the development of males and females in V. carteri. (See “How to Survive 50 Million Years Without Sex.”)
To test this idea, Umen and colleagues inserted a copy of MID in females and switched it off in males. The females with the activated MID gene produced viable sperm and were able to successfully mate with control females that produced eggs as usual. Something similar happened in the males, which produced eggs instead of sperm.
Their work was published July 8 in PLOS Biology. “It was surprising that all of this could be controlled by one gene,” said Matthew Herron, a postdoc at the University of Montana and an expert in Volvox algae. “Their work was exceptionally thorough.”
Now that scientists have more information on how the sexes evolved in this one group of organisms, Umen and Herron hope that it can be used to better understand the evolution of sexes across all of life.