The Origin of Sex

July 22, 2014 |

Volvox gender quartet 2Algae researchers think they may have discovered a trigger that establishes gender, and sexual reproduction.

We humans experience asexual cellular division, or mitosis, though we rarely write songs or poetry in celebration of it. Our earliest life experience reflects that process, as two cells become four, and four become eight.

As multicellular organisms, however — and at the point of conception — our story is dominated by the phenomenon of sex. We are products of it, obsess about it, worry about it, classify it, celebrate it, demonize it, and struggle mightily to understand it.

It probably should come as no surprise that a classification of living organisms as broad and ancient as algae should feature asexual cellular division, and sexual reproduction, just as we do. Well, not exactly as we do, but you get the idea.

marvin-gayeYes, there are a tremendous range of algae — so vast a grouping that, genetically, we humans are more closely related to fungus than some algae are related to each other. Within that diverse array — for some species, there are boy algae and girl algae. Occasionally, as Marvin Gaye encouraged us to do, they get it on.

The presence of sex in algae has led a number of researchers to suspect over the years that sex may well have originated here. Thereupon passed along to (or independently developed by) plants and animals — and ultimately, to a boy and a girl perhaps now longingly staring at each other across the front seat of a late-model Chevy.

And there are pretty good reasons for chasing down this question — reasons that have little to do with summer romance and everything to do with taking advantage of (and perhaps some day expanding) the evolutionary benefits of sex.

The evolution of the gametes

You see, there are some evolutionary reasons, for example, that eggs are really huge as cells go, and contain tremendous resources available to the little zygote after conception. That sperm are so very tiny and really don’t transport all that much in the way of useful information except the vital transfer of the second half of a zygote’s complement of chromosomes.

"When Mommy Algae and Daddy Algae love each other very , very much....". Here: algae sperm

“When Mommy Alga and Daddy Alga love each other very, very much….”. Here: algae sperm

Think of it this way. Let’s say that you are going on a cross-country driving trip — and you decide to look for a driving partner, and you find 50 “potentials” via Craig’s List. From a logistical point of view, it’s more efficient if you have the car already at hand, instead of requiring that each potential partner show up with half a car and you bring half a car.

Leaving aside the welding problem, those 50 partners will show up with, in total, 50 half-cars, and 49 of them will be discarded. It takes 51 half-cars to make a complete car this way. Versus just the one car the other way.

The strategy is so compelling, in terms of maximizing genetic diversity but minimizing the amount of physical resource employed, that it is quite likely that sexual reproduction and the development of big eggs and tiny sperm may have evolved repeatedly and independently over time. But maybe not.

About mating types and sexes

It’s been a puzzle. One reason? Not too many algae have distinct sexes. The distant unicellular relatives of plants, animals and other multicellular species on the whole=have mating types–a system in which gametes of one mating type can only fuse with those with a different mating type, but the cells of each mating type are indistinguishable from each other in size and morphology. It’s a world of “plus” and “minus” instead of “girl” and “boy”. Far less fascinating, and you almost never see it in multicellular organisms.

For those of you looking for a leg up in “The Battle of the Sexes”, girls equate to “plus” and boys equate to “minus”. Possibly accounting for toilet-seat learning deficit disorder. You never know.

We may never be sure where sex came from, because one thing is for sure and that is that sex is really, really ancient. And it’s hard enough to reassemble decent football stats from the 1800s much less the origin of sex some hundreds of millions of years ago.

But the origin of sex is a topic well worth chasing, even if there’s no particular reason that Cinemax After Dark might take an interest in it, and that has a lot to do with hybridization. Whether you are breeding dogs or plants, everyone knows that cross-breeding is about the oldest and most reliable way of producing advantaged characteristics. The emergence of double-cross hybrids in food plants is primarily responsible for the fact that we can feed a global population of more than 7 billion people.

Male algae. Note, persistent trait: have not evolved reflexive toilet-seat-down skills.

Male alga cell.

And if genetic modification gives you the willies — cross-breeding is just about the best way available of conferring new advantages on a genome, without transporting DNA across inter-special boundaries that were set up by Nature for reasons that Nature alone knows.

So, it’s big news in the world of sex — and plant breeding — that a team of researchers led by James Umen at the Donald Danforth Plant Center and the Enterprise Institute for Renewable Fuels have discovered what they believe is a genetic trigger that links asexually reproducing algae to their more sophisticated, Noel Coward-esque algae cousins who have evolved sexual reproduction strategies.

Evolving sexes from mating types

Unlike the case in plants and animals whose unicellular ancestors are very distantly related, male and female sexes in Volvox evolved relatively recently from mating types in an ancestor that was similar to Chlamydomonas. Based on a previous study published in Science, Umen and co-workers, postdoctoral fellows Sa Geng and Peter DeHoff, had identified a gene in Volvox males called MID whose counterpart in Chlamydomonas was known to control differentiation of its two mating types called “plus” and “minus”.

A discovery in the multicellular green alga, Volvox carteri, has revealed the origin of male and female sexes, showing how they evolved from a more primitive mating system in a unicellular relative.  The Danforth team identified the master regulatory gene for sex determination in Volvox and found that it has acquired new functions compared to a related gene in its close relative, the unicellular alga Chlamydomonas reinhardtii which does not have dimorphic sexes.

Their findings, published on July 8, 2014 in PLOS Biology, also provide a possible blueprint for how sexes in other multicellular organisms like plants and animals may have originated.

Unlike the case in plants and animals whose unicellular ancestors are very distantly related, male and female sexes in Volvox evolved relatively recently from mating types in an ancestor that was similar to Chlamydomonas. Based on a previous study published in Science, Umen and co-workers, postdoctoral fellows Sa Geng and Peter DeHoff, had identified a gene in Volvox males called MID whose counterpart in Chlamydomonas was known to control differentiation of its two mating types called “plus” and “minus”.

Gender bending and algae

By forcing chromosomally female Volvox to express MID, the team led by Umen was able to convert what would have been egg cells into packets of functional sperm cells.  Conversely, by using a method of gene inactivation called RNA interference (RNAi), the Danforth scientists were able to block MID expression in chromosomal males causing them to develop with functional eggs in place of their sperm packets.  The team was even able to use their gender-swapped strains to carry out successful matings between pairs of chromosomally male or chromosomally female Volvox.

The gender-bending algae Volvex, subject of the study

The gender-bending algae Volvex, subject of the study

Importantly, even though the MID genes from the two species of algae are related, the Chlamydomonas MID gene was unable to substitute for Volvox MID.   The finding of a master regulatory gene for sexes and mating types in this group of green algae shows that these two forms of reproduction share a common genetic origin, and hint that a similar evolutionary scenario may underlie the origin of sexes in animals, plants and other multicellular lineages.

The Bottom Line

OK, you may well be wondering — what the heck has any of this to do with biofuels, in a practical sense?

The mechanism, illustrated

The mechanism, illustrated

“Just as the case for crop plants, breeding will be an important tool for making improved algal strains that can serve as biofuel feed stocks or other purposes.  However, sexual reproduction in most algal species is poorly understood. The identification of a conserved regulatory gene that controls sex and mating in the algae may lead to clues about how sex is controlled in other related groups of algae that are used for biotechnological applications,” Umen said.

To be able to understand, control or otherwise improve mating and sex — well, that has huge implications for breeding of organisms that we use as feedstocks or for microbial-scale biobased processing. Enhancing or expanding the means available for conferring, suppressing or transferring traits — it isn’t hard to see potential value there.

Down the line? Would it be possible to evolve certain algae in a directed manner, from  depending on mating types to having sex-differentiation and ultimately establishing advantaged opportunities for breeding, cross-breeding and hybridization? In short, importing the idea of crop improvement into algae?

Well, that remains to be seen. But the idea that hybridization might come one day to a broader class of algae — and that, in the nearer term, we will understand more and more about the origin of sex and the origin of the sexes — that’s compelling on a lot of levels.

Random House may not take the view that the story of girl algae and boy algae will add to the canon of romance novels — after all, there are no steamy romps in sight based on the science revealed so far. But who knows? Someone once told me that peaches have feelings, probably a frutarian friend of mine.

Perhaps one day we’ll discover that algae are lusty, promiscuous organisms, as hormonally addled as teenagers. After all, we got our feelings about sex from somewhere. Hollywood might never be the same again.

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