by Carl Strang
One of the more significant but less publicized aspects of evolution is the importance of polyploidy. This is the rare form of mutation in which an organism’s genome is doubled. In other words, where once there was one set of genes now there are two. The descending members of that species, and the species that subsequently evolve from them, carry that doubled set. My literature review notes from the past year include two studies of this, one from plants and one from animals.
Jiao, Yuannian, et al. Ancestral polyploidy in seed plants and angiosperms. Nature, 2011; DOI: 10.1038/nature09916
Their study of ancient plant lineages points to two polyploidy events that took place earlier than previously known ones, at 320 and at 192-210 million years ago. The earliest previously established such event was at 125-150 million years ago. With one of the two sets of genes able to carry on with their usual functions, the spares could mutate with much less potential impact on the organism. This expanded potential for evolutionary creativity made possible the huge diversity of plants we see today.
Arnegard, Matthew E., Derrick J. Zwickl, Ying Lu, and Harold H. Zakon. 2010. Old gene duplication facilitates origin and diversification of an innovative communication system—twice. doi: 10.1073/pnas.1011803107
Polyploidy events apparently happened less often, or at least are less studied, in animals. These researchers looked at two large groups of electric fishes in Africa and South America. Electric fishes have evolved in large muddy rivers, where visibility is very limited and there is an advantage to sensing their surroundings by measuring disturbances in electric fields. This study used evo-devo methodology, examining the genetic control of embryological development in an evolutionary context. They found that an ancient episode of duplication released a copy of a “voltage-gated sodium channel gene” for modification into a potential use in electric communication. In other words, the fishes went beyond simply sensing their surroundings and evolved the capability of using their electric field generators and sensors to communicate with one another. The two groups of fishes on the now separate continents shared the polyploidy duplication, and so had the same starting point. A subsequent sequence of events provided similar early modifications in the two groups, but then they followed different mutational pathways to reach essentially the same endpoint independently from one another. Today the fishes on the two continents have similar physiology and similar associated physical structures in their bodies, but that similarity is the result of convergence. The researchers estimate that the common endpoint was not reached until 100 million years after the genome duplication.