National Geographic

Transgender Algae Show How Males and Females Came to Be

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 photo of a Volvox Colony of green algae.
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.

A composite photo of four different kinds of algae, some of which have genderswapped.
Various dishes of V. carteri. Males are pictured upper left, females are upper right, pseudo females are lower left, and pseudo males are lower right. Photograph by Sa Geng and James Umen

“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.

Follow Carrie Arnold on Twitter and Google+.


  1. Phillip Lake
    July 15, 1:30 am

    Obviously the only way you are allowed to comment on the great National Geo. site is if you are a scientist or a Rhodes Scholar. All other comments are simply ignored.

  2. Frank B
    July 11, 10:34 am

    If a hermaphrodite is considered a transgender, there should be no problem in calling these algae transgendered. Transgender is not limited to one idea, oddly enough, Cynthia.

  3. Lisa Ellis
    Los Angeles
    July 10, 11:23 am

    You also imply that Chlamydomonas reinhardtii is a four-celled colonial organism. It is not. It is a unicellular alga.

    • Carrie Arnold
      July 10, 6:53 pm

      Thank you for the correction. I have updated the post.

  4. Cynthia
    July 9, 10:57 pm

    Please, revise this. You’ve used the term transgender incorrectly, and it is not only offensive but misinforming. The equation of gender and sex within this article embodies a critical misunderstanding of both concepts that is common within our society. The misconception is, unfortunately, no less common within the scientific community, and this article is a prime example of that. We strive to learn more, and these and false ideas are weighing us down. If we are to understand both sex and gender, we must not continue to approach the concepts with this incorrect view.

  5. annonimase
    July 9, 2:15 pm

    its safe to be trasgender

  6. Nathan
    The Seattle area.
    July 9, 12:33 pm

    Just the origin of the sexes by evolution would have taken a lot longer than 4 to 5 billion years.

  7. Joe M
    Houston, TX
    July 9, 11:53 am

    I must protest the usage of the term “transgender”. Gender is a mental state, not a physical condition. These are “transsexual” algae.

  8. Kat
    July 9, 11:44 am

    Your article in National Geographic has a huge error, unless you can show algae have culture. Gender is a cultural class, sex is a biological class. When you “go to bed” with someone, do you have sex with them, or do you have gender with them? Most transgendered people do not have sex change surgery, but all transsexuals desire the operation. If you randomly kidnap someone and perform the operation on them, they most likely will continue to perform their original gender, no one viewing them with their clothes on will have a clue you messed with their sex.
    I read the article in, and even though they say they decoupled gender behavior from the physically expressed sex, the altered algae behaved according to the expressed *sex*; the sperm-carrying algae mated with females, the egg-carrying algae mated with males. What’s new about that? And show me how that has anything to do with gender, which is a cultural performance.

    • Carrie Arnold
      July 9, 8:29 pm

      Thank you for your comment. The Plos Biology paper used “sex” and “gender” somewhat interchangeably in the article, as did the researcher in my interview with him and the accompanying press release. I apologize for not being more careful about those terms.

  9. Gerry
    July 9, 11:29 am

    Good story, but I have a nitpick. The story speaks of the genus Volvox, then introduces Chlamydomonas as though it were a member of that genus, though that’s clearly not the case: Chlamydomonas is a genus name. In fact, the two organisms aren’t even in the same family.

    • Carrie Arnold
      July 9, 8:29 pm

      Thank you for your comments. I have updated the article to reflect your correction.