University of Montana Researcher Lila Fishman Reveals Chromosomes at War

In a simple world, natural selection would efficiently remove detrimental traits from animal or plant populations. If inherited DNA causes sterility, for example, evolution by natural selection should quickly remove that trait.

The world is a complex place, however, and most populations include stubborn detrimental traits that won’t disappear. How does this happen? New research at The University of Montana may have solved part of the puzzle.

Lila Fishman is a plant evolutionary geneticist in UM’s Division of Biological Sciences http://dbs.umt.edu/ . While studying monkeyflowers – plants with small yellow blossoms found across much of the West – she uncovered a case in which the detrimental trait of pollen sterility persisted in a population.

Under normal circumstances, genes that cause pollen sterility would fail to transmit to the next generation and be eliminated. However, Fishman discovered that while the sterility hurts the plant, it is caused by a chromosome that selfishly helps itself.

Fishman’s work on this topic was published in the Dec. 5 issue of the prestigious journal Science. Her co-author is Arpiar Saunders, who did post-baccalaureate work at UM. The work was funded by the National Science Foundation and enabled by a Joint Genome Institute effort to sequence the genome of the yellow monkeyflower.

“We tend to think about natural selection among individuals shaping traits to fit the environment,” Fishman said, “but natural selection can also occur at the gene or chromosome level. Genes that can out-compete other genes will spread, sometimes to the detriment of the individual. The basic biology of most plants and animals actually sets the stage for this kind of genetic conflict.”

In plants, as in humans, each individual carries two copies of every chromosome (which contain the genes) – one from mom, one from dad. During the first step of sexual reproduction, called meiosis, these paired chromosomes divide to provide a single set of chromosomes for each egg or sperm.

Generally, it is a random coin-flip that determines which of the two copies of each chromosome gets transmitted to the offspring. The newly discovered monkeyflower chromosome, however, somehow outraces its partner during meiosis in females and thus manages to get into more than its fair share of the next generation’s seeds.

“If a variant chromosome can bias its own transmission during meiosis, it can become common, even if it has negative effects on other traits,” Fishman aid. “So, the sterile pollen is collateral damage from a hidden war between chromosomes.”

Fishman showed that nonrandom chromosomal transmission likely results from competition between structures called centromeres, which are regions of chromosomes that mediate their division during meiosis. While other researchers have shown that genes often behave selfishly, this research offers some of the first evidence to pinpoint selfish centromeres.

“Our work shows for the first time that something associated with centromeres can strongly bias chromosomal segregation and also has fitness costs in natural populations,” Fishman said. “This process could be important for bringing deleterious traits into any population, including human populations.”

Contact: Lila Fishman, assistant professor, UM Division of Biological Sciences, 406-243-5122, [email protected]

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