Does germ plasm accelerate evolution?

Scientists at The University of Nottingham have published research in the leading academic journal Science that challenges a long held belief about the way certain species of vertebrates evolved.

Dr Matt Loose and Dr Andrew Johnson who are experts in genetics and cell development in the School of Life Sciences carried out the research, funded by the Medical Research Council (MRC). It suggests that genes evolve more rapidly in species containing germ plasm. The results came about as they put to the test a novel theory that early developmental events dramatically alter the vertebrate body plan and the way evolution proceeds.

The original theory was proposed by Dr Johnson over 10 years ago. His view is that the relationship between the germ line (hereditary germ cells that create sperm and eggs) and the soma (cells which form the body of an organism) also impacts on species diversity. He argues that once a species evolves a substance called germ plasm, germ cells are independent of other cells, so constraints on somatic development are liberated and this enhances a species ability to evolve. As a result, he says, vertebrates such as frogs, fruit flies and birds, which look unlike their ancestors, came about, and remarkably they evolved much faster than their ancestors did.

Taking a new look at embryology

Early in animal development a subset of cells in an embryo becomes committed to produce Primordial Germ Cells (PGCs). PGCs develop into sperm or eggs. The remaining cells develop as the soma, a term that describes all of the tissues that make up an individuals body. The germ line is an immortal cell lineage, passed on from generation to generation, and charged solely with producing a new embryo at fertilisation. The soma, on the other hand, dies off in every generation. In the most classic biological sense its job is to interpret the effects of natural selection in order to determine genetic fitness, leading to so called survival of the fittest.

There are two known ways to make a PGC. There is preformation - material in the egg called germ plasm is inherited directly by a few cells in the embryo, and germ plasm instructs these cells to become PGCs. Alternatively, PGCs can develop without germ plasm. In this case PGCs can be induced to form by signals secreted from other cells, which are part of the soma. This is called epigenesis.

Dr Johnson said: Biologists are accustomed to thinking about how adult somatic traits impact natural selection and the evolution of new species. What is much less clear is the role of embryological mechanisms in evolution, and how they might contribute to species diversity.

A new computer programme tested their hypothesis

In this latest publication Dr Loose put this theory to the test. He said: To investigate the effect germ plasm has on sequence changes we had to look at as many sequences as possible nearly 12 million sequences were analysed and processed from 165 different species including mammals, reptiles, amphibians and fish.

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Does germ plasm accelerate evolution?