Lessons from the pond: Clues from green algae on the origin of males and females

Apr 15, 2010
The image shows a sexual male colony. During sexual reproduction, male Volvox carteri colonies produce sperm packets each containing 64 or 128 sperm cells. After their release individual sperm packets swim as unit until they reach a female spheroid where they dissociate prior to entry of fertilization of eggs. Credit: Courtesy of the Umen laboratory, Salk Institute for Biological Research

A multicellular green alga, Volvox carteri, may have finally unlocked the secrets behind the evolution of different sexes. A team led by researchers at the Salk Institute for Biological Studies has shown that the genetic region that determines sex in Volvox has changed dramatically relative to that of the closely related unicellular alga Chlamydomonas reinhardtii.

Their findings, which will be published in the April 16th issue of the journal Science, provide the first empirical support for a model of the evolution of two different sexes whereby expansion of a sex-determining region creates followed by genes taking on new functions related to the production of male and female reproductive cells termed gametes.

"Until now, sex-determining had generally been viewed as regions of decay, steadily losing genes that are not involved in ," explains James Umen, Ph.D., assistant professor in the Plant Molecular and Cellular Biology Laboratory at the Salk Institute, who led the team conducting the study. "Our study shows the opposite-that such regions can expand and generate new much more rapidly than the rest of the genome."

Most multicellular organisms such as plants and animals have two distinct sexes with females producing large immotile eggs and males producing small motile sperm. While unicellular organisms can also reproduce sexually, the two sexes of single-celled species are typically indistinguishable from each other and are thought to represent an ancestral or early evolutionary state. However, the large distances that separate plants or animals from their closest unicellular relatives have precluded understanding the evolutionary transition to male-female .

"In unicellular organisms like Chlamydomonas, the gametes look the same. In contrast, , including Volvox, produce eggs and sperm-they are distinctly male and female. Yet no one really has any idea how the evolution of males and females occurs or what genetic changes were required to achieve it," explains Umen.

Although the genomes of Chlamydomonas and Volvox are similar in most ways, there is one glaring exception that provided the Salk researchers with an entrée into the origin of male and female sexes-the so-called mating locus that functions in much the same way as human X and Y chromosomes to determine gender.

When Umen and his colleagues examined the mating locus genes in Chlamydomonas and Volvox they found that they shared some of the same genes, as you would expect from closely related species. However, Volvox also now possessed a surprising variety of new genes that were added to its expanded mating locus, and expression of many of these genes had come under the control of the male or female differentiation programs.

The image shows a vegetative female colony. Volvox carteri forms spherical colonies, which are composed of 2,000 to 4,000 individual cells embedded in an extracellular matrix. During non-sexual reproduction, so-called gonidia in both male and female (pictured) colonies produce juvenile colonies through repeated division. Credit: Courtesy of the Umen laboratory, Salk Institute for Biological Research

"We found that the Volvox mating locus is about five times bigger than that of Chlamydomonas," says postdoctoral researcher and co-first author Patrick Ferris, Ph.D. "We wanted to understand the evolutionary basis of this. How did it happen? And where did these new genes come from?"

To trace the origin of the added genes, the team looked to see if they could also find them in Chlamydomonas. "We found that although some of the mating locus genes in Volvox are completely new, many of them have counterparts in Chlamydomonas that are near the mating locus," explains co-first author Bradley Olson, Ph.D. "So Volvox has taken these genes that initially had nothing to do with sex, incorporated them into its mating locus, and started using some of them in its sexual reproductive cycle."

The team is now studying these new mating locus to understand their individual roles in sex determination and sexual development.

They have already identified a Volvox mating locus gene named MAT3 that appears to have evolved a new role in sexual differentiation. MAT3 is related to a human gene called the retinoblastoma tumor suppressor that controls cell division and is frequently mutated in cancer cells. In Volvox, MAT3 probably has a role in controlling cell division as it does in animals and plants, but has also acquired intriguing gender-specific differences in its sequence and expression pattern that correlate with differences in male/female reproductive development. Umen's laboratory is following up on this finding to determine the newly evolved role of MAT3 in Volvox gender specification.

"This study shows that Volvox and its relatives are a powerful model in which to study the evolution of sex," says Umen. "It provides us with a system in which we can retrace evolutionary history to ask questions about the origin of gender and other traits that are difficult to approach in groups such as plants and animals."

The team is also working with collaborators to examine the mating locus of an evolutionary intermediate between Chlamydomonas and Volvox called Gonium,which has between four and 16 cells. "Gonium allows us to look at the evolutionary steps between Chlamydomonas and Volvox to better understand how the evolutionary process happened," says Ferris.

Explore further: Researchers find animals killed by anthrax leave behind enticing grasses for herbivores, allowing disease to spread

Related Stories

Fungi can tell us about the origin of sex chromosomes

Mar 17, 2008

Fungi do not have sexes, just so-called mating types. A new study being published today in the prestigious journal PLoS shows that there are great similarities between the parts of DNA that determine the sex of plants and an ...

How Volvox got its groove

Feb 19, 2009

Some algae have been hanging together rather than going it alone much longer than previously thought, according to new research.

Small But Mighty Female Lizards Control Genetic Destiny

Apr 05, 2010

(PhysOrg.com) -- "Be faithful in small things because it is in them that your strength lies." Mother Teresa's words echo throughout the world. They ring particularly true in the biological kingdom among brown ...

Why Evolution Drives Some Cells to Altruism

Sep 18, 2006

Nature has been capitalizing on the benefits of a specialized labor force long before Henry Ford made it popular. New research suggests the same principles Ford used have driven the evolution of complex organisms.

Recommended for you

What happens when good genes get lost?

8 hours ago

Scientifically speaking, there is no bad DNA, though we like to blame it for unruly hair, klutziness or poor gardening skills. There is, however, junk DNA.

Plants prepackage beneficial microbes in their seeds

Sep 29, 2014

Plants have a symbiotic relationship with certain bacteria. These 'commensal' bacteria help the pants extract nutrients and defend against invaders – an important step in preventing pathogens from contaminating fruits and ...

User comments : 0