Here we see a happy, typical family of sea monkeys. Note the red bow and plump lips that indicate the female of the species, and the tall body and protective stance of the male. I assume that the father’s well-placed tail blocks some other clues to his identity. The parallels between the sea monkeys and the human family (see inset) are uncanny and surely nothing more than a coincidence.
The real life of sea monkeys (brine shrimp, or Artemia) is a pretty far cry from Ozzie and Harriet. Sea monkeys don’t live in families, for one thing. And in a lot of populations, the females have no need for males. Their eggs can develop into healthy embryos–and, eventually, adults–without the need of sperm. You can take that picture of sea monkeys and wipe Dad out.
From an evolutionary perspective, this father-free way of life has a lot going for it. Let’s say you’ve got a sexual pair of male and female shrimp in one tank, and two asexual females in the other. Let them breed for a while. Sexual species typically produce a roughly even ratio of sons and daughters. So only half of the sexual population can produce eggs, while every individual in the asexual one can. It won’t be long before the asexual population is far bigger than the sexual one. Out in the wild, this proliferation should mean that the genes for male-free reproduction should quickly dominate populations. Down with sex, in other words.
But this has only occurred in only about one in every ten thousand species of animals. Sex must have a powerful advantage that overcomes its disadvantage–what the late biologist John Maynard Smith dubbed the two-fold cost of sex.
Scientists have given this question a lot of thought, and they’ve come up with some possible answers that they’ve been testing in recent years. Maybe sex lets adaptations evolve faster, because mothers and fathers can combine genes into new combinations. Defenses against ever-evolving parasites might be especially important. There may be different explanations for different cases. Very often, when an asexual lineage emerges, it gains an extra set of chromosomes. That’s a lot of extra DNA to build when a cell divides–which requires a lot of phosphorus and other ingredients. Perhaps that’s a cost too great to balance the advantage of giving up fathers.
Or perhaps the rarity of asexual animals is the result of evolution playing out not in short-term competitions, but over vast stretches of time. Populations of sexual animals may be less prone to going extinct because they can adapt to more niches.
To better understand the evolution of sex, a number of biologists are looking to the exceptions to the rule. If the advantages of sex overwhelm its costs for 9,999 species out of every 10,000, then why is the opposite true in the remaining one? One lineage of microscopic animals called bdelloid rotifers has been asexual for 80 million years. Cornell scientists have suggested that they have remained asexual because they’ve found a way to resist parasites that’s as good as sex–by drying up and blowing away from their pathogens.
But there are other puzzles to the evolution of sex. And one involves sea monkeys. In a paper appearing in the Journal of Evolutionary Biology, Marta Maccari of the University of Hull and her colleagues describe a massive survey of brine shrimp from across Europe and Asia. They reared cysts from dozens of populations and closely examined the offspring over the course of two generations. The females in these populations can reproduce on their own. And yet in most of the populations they studied, they discovered a few males.
The males were exceedingly rare–around one in thousand in many cases, and around one in a hundred in a few. And yet they were healthy and fertile. The males couldn’t mate with females of their own population, but they readily had sex with other species. What’s more, their hybrid offspring were healthy and fertile, too.
If asexual animal species are rare, species with asexual females and rare males are even more rare. Only a few other examples have turned up, such as certain populations of snails in New Zealand. Maccari and her colleagues don’t think there’s a clear answer to why these rare males exist. But there are a few plausible possibilities.
Maybe it’s just a fluke. It’s possible, for example, that as eggs develop, a few accidentally lose a chromosome, altering their sex. Sons, in other words, are birth defects.
It’s also possible that some of the asexual brine shrimp have mutations that lead sometimes to males, and they pass their mutated genes down to their offspring. In her study on New Zealand snails last year, Maurine Neiman of the University of Iowa and her colleagues found, surprisingly, that producing a few sons that can’t mate with any females of your species doesn’t put asexual animals at a major disadvantage.
On the other hand, maybe rare males are a side-effect of brine shrimp biology. One way for females to reproduce is to combine two eggs, joining together their chromosomes into a full set. This process can produce lots of different combinations of the shrimp’s DNA, and that variation may help them adapt to the changing environment. Sometimes, though, those combinations may produce a fertile son.
The most interesting possibility Maccari and her colleagues raise is that the rare males are a way for the genes for asexuality to spread themselves. The males can’t mate with their own species, but they can interbreed with others. They may then introduce genes for asexual reproduction into the species, causing them to turn male-free. For brine shrimp, in other words, fathers may be a way of getting rid of fathers. I have no idea how you’d paint that on a box of sea monkeys, but I’d be curious to see someone try.