Genome scientists discover that evolution sometimes 'reinvents the wheel'

If a particular biological innovation is good enough to evolve once, it may sometimes be good enough to evolve multiple times independently in different species. This is one of the discoveries from a study of genome evolution led by Jay F. Storz, a biological scientist at the University of Nebraska-Lincoln, and published this week in the online edition of the Proceedings of the National Academy of Sciences.

In the study by Storz and UNL colleagues, the biological innovation in question is the ability to synthesize functionally distinct forms of the oxygen-carrying hemoglobin protein that are specialized for different stages of development.

The oxygen transport needs of the developing embryo are quite distinct from those of an adult with fully developed lungs and a fully developed circulatory system. So it is easy to imagine why it might be advantageous to make use of specialized types of hemoglobin that are adapted to the special physiological conditions during different stages of development, said Storz, an assistant professor of biological sciences.

For example, during human pregnancy, oxygen transfer from the maternal bloodstream to the fetal bloodstream is made possible because the developing fetus produces a specialized type of hemoglobin that is especially good at grabbing oxygen. Humans are able to synthesize certain specialized types of hemoglobin during the earliest stages of embryonic development, other types during later stages of fetal development, and still other types after birth.

"Given that all living mammals possess a developmentally regulated system of hemoglobin synthesis, it seems reasonable to assume that it represents a unique and ancient innovation that traces back to the common ancestor of all mammals," Storz said. But, the surprising finding in the study by Storz and colleagues is that the same system of developmentally regulated hemoglobin synthesis has been invented twice independently in different branches of the mammalian family tree.

Embryo-specific and adult-specific hemoglobins evolved independently in the ancestors of monotremes (a group of Australian egg-laying mammals, including the duck-billed platypus and the spiny anteater) and once in the ancestors of therian mammals (a group that includes marsupials, such as kangaroos and opossums, as well as placental mammals, such as humans, dogs, cattle and bats). In the monotreme and therian mammal lineages, a physiological division of labor between embryonic and adult hemoglobins was made possible by gene duplication.

"In each case, a single-copy hemoglobin gene with a generalized adult function became duplicated -- such that two functionally redundant copies of the same gene came to lie side-by-side on the same chromosome," Storz said. With the passage of time, each of the two initially identical gene copies changed through mutation. Natural selection gradually adapted one copy to a uniquely 'adult' respiratory function while simultaneously adapting the other copy to a uniquely 'embryonic' respiratory function.

"It is especially remarkable that natural selection fashioned the same physiological division of labor between the products of the two independent gene duplication events. It thus appears that sometimes evolution does need to 'reinvent the wheel,'" he said.

Storz and his collaborators -- postdoctoral researchers Juan Opazo and Federico Hoffmann -- were able to decipher the evolutionary history of the hemoglobin genes by analyzing genome sequences from a diverse array of mammals and other vertebrates. The recently completed platypus genome sequence proved to be a valuable source of evidence for the researchers' detective work.

"Proponents of 'intelligent design' often argue that many features of living organisms are so complex that they could not possibly have been built up from scratch by a blind, non-deliberate process such as natural selection," Storz said. "However, studies of genome evolution such as these demonstrate that natural selection is capable of fashioning exquisitely intricate and complex physiological systems, and that such systems may often be cobbled together from the products of duplicated genes that are co-opted for new roles."