Sex versus survival: A tradeoff at geographical range limits
Northern plants must 'use it or lose it,' says Queen's study
(Kingston, ON) – A new, Queen's-led study shows that plants growing in harsh northern climates are losing the ability to reproduce sexually, an evolutionary phenomenon similar to the loss of sight in cave-dwelling fish.
"Our genetic analysis shows that northern plant populations acquire mutations that disable sex itself, a trait central to the biology of almost all higher organisms," says Queen's biologist Christopher Eckert, co-author of the study and an expert in reproductive evolution.
These findings are provocative because they point to the possibility of rapid reproductive evolution in other species at the northern fringes of their range, Dr. Eckert explains. "This is significant because almost all of the designated species at risk in Canada consist of populations at their northern range limit."
"Rapid reproductive evolution at the range limit will clearly affect decisions about the management of these marginal populations," he continues. "A shift in how plants reproduce will also greatly affect whether or not they will be able to move with changing climates, especially rapid global warming caused by humans."
Focusing on Decodon verticillatus, a dominant shrub in wetlands throughout eastern North America, a series of studies led by Dr. Eckert show that populations switch from being sexual to totally asexual across the northern limit of the species' geographical range. This switch leads to northern populations becoming "enormous, genetically homogeneous superclones."
By comparing reproduction in natural populations versus a benign greenhouse environment, the research team learned that the reproductive switch is due to genetic factors causing sexual sterility.
These sterility mutations can spread in northern populations because the harsher environment makes sex relatively unsuccessful compared to asexual clonal reproduction (where plants make genetically identical offspring by vegetative budding). This is akin to the evolutionary loss of eyes in cave organisms where a lack of light makes visual stimuli useless.
Published in the journal Proceedings of the Royal Society, the paper is co-authored by Queen's student Kathryn Neville and Queen's graduate Marcel Dorken (now at Oxford University).
Evidence gathered under the controlled environmental conditions of the Queen's Phytotron shows that the genes that disable sex in northern populations of Decodon actually improve other aspects of plant function such as survival. Hence, plants growing in cold climates appear to have made an evolutionary "tradeoff" between sexual reproduction and enhanced survival, says Dr. Eckert. The doubled-edged nature of these sterility mutations can cause them to spread quickly in northern populations where sex is not very useful. This shows how complex traits such as sexual reproduction can quickly degenerate and even disappear when they are no longer useful.
Evolutionary biologists have long viewed these so-called vestigial traits – which appear to have degenerated under conditions where they no longer enhance reproductive fitness – as the flip-side of Darwin's mechanism of evolution by natural selection.
"No other theory can explain why organisms have collected these degraded vestigial traits," says Dr. Eckert, noting that in humans an example of a vestigial, or lost trait, is our tailbone. "If our data are borne out by other genetic studies, it means that these complex traits can be eroded very quickly."
Source: Eurekalert & others
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