Being a placozoan might not be so bad after all — a new study has concluded that these microscopic organisms can have sex.
Ana Signorovitch GRD ’07, a graduate student in the Department of Ecology and Evolutionary Biology, has recently provided conclusive evidence that placozoans, the simplest, multi-cellular, free-living animals, once thought only to be capable of asexual reproduction, are able to have sex.
Leo Buss, director of undergraduate studies of ecology and evolutionary biology and curator of invertebrates at the Peabody Museum, co-authored Signorovitch’s paper, which was published in the early edition of the Proceedings of the National Academy of Sciences, the Academy’s journal. Buss said Signorovitch’s findings are important because the majority of major animal groups’ lifecycles have been worked out for the past century, and she has solved one of the relatively few remaining mysteries.
“Ana has basically solved a classic problem in invertebrate zoology,” Buss said.
The round, soft-bodied placozoan measures approximately .5 millimeters across and resembles a large amoeba. They also only have four distinct cell types, as opposed to the hundreds that a more biologically sophisticated species, such as adult humans, have. Until now, it was unknown whether placozoans were capable of sexual reproduction.
While it may seem that determining whether a species has sex is relatively simple, Signorovitch said, this is not the case.
“Some animals may reproduce only once a year, or laboratory conditions may not be conducive to sex,” she said.
The failure to observe species sexually reproducing in a laboratory is never conclusive evidence that they are not capable of having sex, Signorovitch said.
Since they were discovered a little over a century ago, placozoans have only been observed reproducing asexually through binary fission — the process of a larger animal splitting into two smaller but genetically identical organisms.
German biologist Karl Grell observed placozoan eggs 30 years ago, but because placozoan sperm or embryos were not observed, the placozoan capability to have sex could not be definitively determined. The question of whether placozoans could reproduce sexually has been a mystery ever since.
Through DNA analysis — not direct observation — Signorovitch has been able to solve the mystery. Looking at seven different genes in 10 individual placozoans, Signorovitch and her team identified genetic patterns that are characteristic of species that reproduce sexually.
The process of sexual reproduction involves genetic recombination, which, over millions of years, will cause the genome of a sexually reproducing species to homogenize.
This discovery has implications in both the specialized field of placozoan biology as well as the broader scientific community.
Signorovitch said knowing that placozoans reproduce sexually will be helpful in mapping out the genome of the species.
“In the end, if you cannot get an animal to reproduce sexually, you cannot even do classical genetics with them,” she said.
Mapping out the genome of the placozoan species has the potential to profoundly affect evolutionary biology in general. Because they are the simplest free-living animals, placozoans are considered one of the most ancient lineages in the evolutionary tree.
Knowing what genes placozoans have in common with more derived species also will be useful, Signorovitch said. It will provide insight into the different functions these same genes perform in other species.
Even scientists who do not specialize in placozoan biology said they appreciate the ramifications of this research.
“Although simpler organisms are not the same as complex ones, they still function in the same basic way,” biology major Kevin Davis ’08 said. “Understanding the simple basis of these functions is the first step to understanding these same functions at a higher level, and this discovery could prove to be a big step in that direction.”