Fungi with a primitive sexuality may shed light on the origins of maleness.
A closer look at fungal sex. I'm Bob Hirshon and this is Science Update.
To understand how sex evolved, researchers need to look back at the most primitive kinds of sexuality. That's why Duke University microbiologist Joseph Heitman is studying an ancient fungus, which comes in two varieties that aren't technically male and female, but still play distinctive roles in reproduction. Heitman and his colleagues have identified a gene that codes for a kind of protein called a transcription factor, which solely determines the fungus' sexual type.
The homologous protein that's involved in sex determination in humans, called SRY, is a member of the same family of transcription factors.
And SRY is a key product of the human Y chromosome. So tracking this protein from the primordial soup to people may explain how sex as we know it came to be. I'm Bob Hirshon for AAAS, the science society.
Making Sense of the Research
The simplest, most primitive one-celled organisms are asexual—meaning that they are neither male nor female, and reproduce from a single parent, often simply by dividing in two. Asexual reproduction is actually quicker and more energy efficient than sexual reproduction. However, most multicellular organisms rely on sexual reproduction, in which two separate parents contribute different reproductive cells called gametes (such as a sperm and an egg), which fuse together to create a new organism. Many scientists believe that sex evolved because it allows a species' genes to mix and recombine more freely, which creates higher levels of evolutionary fitness.
It's not known how exactly life got from point A, asexual reproduction, to point B, sexual reproduction. The key may lie in the origin of maleness: the sex that contributes gametes without giving birth. (Asexual parents are really more like females, since the new organism comes directly out of them.) In humans and other mammals, maleness is determined by the Y chromosome. In the womb, the Y chromosome triggers surges of testosterone and other events that make an embryo male rather than female. The Y chromosome continues to regulate male sex characteristics later in life. Understanding how this Y chromosome came to be can help scientists trace the evolution of sex itself.
In this case, the scientists studied an infectious fungus, Cryptococcus neoformans, that falls in between classical definitions of asexual and sexual. They don't have complete sex chromosomes like humans, but they do have two different kinds of DNA sequences, called “mating-type loci,” which govern reproduction. Depending on the type of DNA it has, a single fungus may be either “sex-minus” or “sex-plus.” Although the two mating types look alike, it takes one sex-minus and one sex-plus fungus to reproduce.
In their research, Heitman's team isolated a gene that codes for a protein called a transcription factor. This transcription factor determines whether the organism will function as a sex-minus or sex-plus. Furthermore, they found that this transcription factor corresponds to a transcription factor called SRY, a key determinant of maleness in humans and mammals. Since SRY is encoded by the Y chromosome, it appears that this fungus may contain a primitive blueprint for more advanced forms of sexuality. That could help scientists trace the history of sex, and better understand its genetic and evolutionary purpose today.
Now try and answer these questions:
- What is the difference between asexual and sexual reproduction?
- Why is maleness important in understanding how sex evolved?
- How is this fungus like and unlike a typical sexual organism?
- The resemblance between this species' mating-type DNA and the human Y chromosome is an example of homology. In evolutionary biology, homology refers to similar features that are found in different kinds of organisms. For example, human feet are homologous to primate feet, dog feet, and even foot-like extensions of one-celled organisms. What other examples of homology can you name?
You may want to check out the February 15, 2008, AAAS Annual Meeting Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: how sand spiders use elaborate camouflage, a new interactive map that tracks threats to the oceans, and four healthful habits of long-lived people.
The National Geographic News articles Sex Speeds Up Evolution and Study Links Origin of Sexual Reproduction With High Mutation Rates report on a possible evolutionary explanation for sex.
Also from National Geographic News, the article The End of Males? describes a recent experiment in which baby mice were created without sperm.
The Meaning of Sex: Genes and Gender, from the Howard Hughes Medical Institute, features lectures, interactives, and information related to determining the sex of a person.