Newswise — STRATFORD, NJ – Scientists from the UMDNJ-School of Osteopathic Medicine and the UMDNJ-Graduate School of Biomedical Sciences have shown that the origin of some traits during evolution is shaped by developmental biology – changes that occur as animals develop from embryos into adults – rather than driven by natural selection.
In previously published research, the UMDNJ scientists showed that changes in just two genes in the roundworm species Caenorhabditis remanei cause females to become self-fertile hermaphrodites. In the current paper, they show that these self-fertile hermaphrodites produce dramatically smaller sperm than males. This size difference was found in the laboratory, where natural selection could not have been a factor.
“Several related species of worms produce self-fertile hermaphrodites in the wild,” said Ronald Ellis, PhD, the study’s corresponding author and a professor of molecular biology at the UMDNJ-School of Osteopathic Medicine. “These worms also make smaller sperm than males, and our work in the lab shows that this difference was not caused by natural selection.”
Darwin showed that natural selection – over many generations – can produce new traits. As these new traits accumulate, the animals slowly change, eventually resulting in new species. Many traits show a variety of subtle differences within a population of animals, which gives natural selection something to work on. For example, some mice have lighter fur and others have darker fur.
Ellis explained, “Our results teach us something fundamental about evolution. We showed that some traits can only take one form. When we make artificial hermaphrodites, we don’t get some with larger sperm than males, some with smaller sperm and some with sperm of the same size. Instead, they all make smaller sperm. This size difference was not caused by natural selection.”
Further genetic tests showed that hermaphrodites make small sperm because their bodies lack features that help males make large sperm. “This is a case where the laws of developmental biology influenced the course of evolution,” Ellis explained. “But it works for these species, since hermaphrodites can make lots of small sperm quickly, and use them to fertilize their own eggs. So once hermaphrodites first evolved, natural selection kept them around.”
The research, which Ellis co-authored with Christopher Baldi and Jeffrey Viviano of the UMDNJ-Graduate School of Biomedical Sciences, appears online in Current Biology. Their work is supported by grants from the National Science Foundation and the National Institutes of Health.
The University of Medicine and Dentistry of New Jersey (UMDNJ) is the nation's largest free-standing public health sciences university with more than 6,000 students on five campuses attending the state's three medical schools, its only dental school, a graduate school of biomedical sciences, a school of health related professions, a school of nursing and New Jersey’s only school of public health. UMDNJ operates University Hospital, a Level I Trauma Center in Newark, and University Behavioral HealthCare, which provides a continuum of healthcare services with multiple locations throughout the state.
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