You Say Tomato, I Say Tomahto: Colorado State University Scientist Discovers Related Tomato Species Reorder Their Chromosomes

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The chromosomes of a garden tomato and its closest relatives do not share the same organization and in fact differ by major rearrangements – an important discovery by a Colorado State University biologist that reverses 50 years of existing research and helps define differences between species.

The research by Lorinda Anderson, assistant professor of biology, is featured in the current issue of the journal Cytogenetic and Genome Research.

Anderson studies meiosis – a special process that is fundamental to sexual reproduction. In humans and tomatoes (and most other large organisms), each cell has two sets of chromosomes – one from the mother and one from the father. In humans, 23 chromosomes equals one set. During meiosis, cells in reproductive tissues divide to produce egg and sperm cells that each have only one set of chromosomes.

“Meiosis has been compared to a dance because each chromosome from the father has to find its partner from the mother and ‘hold hands’ before they separate again into egg or sperm cells,” Anderson said. “If the chromosomes don’t complete the dance of meiosis correctly and the number of chromosomes in an egg or sperm cell is not exactly equal to one set of chromosomes, then genetic disorders, like Down syndrome in humans, happen.”

Anderson’s research specifically examines the pairing and “hand-holding” aspects of meiosis. During hand-holding, parts of the parental chromosomes recombine and trade large segments of genes. The dance and recombination of meiosis is responsible for genetic variations that make each of us unique.

In her journal paper, Anderson took advantage of the pairing process of meiosis to look at the organization of chromosomes from the tomato compared with related tomato species. Because the chromosomes find their partners and line up closely together during meiosis, any differences in structure are easy to detect. Contrary to 50 years of previously reported scientific research, Anderson and her team found several large differences in the chromosomes from different tomato species. Some of the differences, like an inversion where part of a chromosome is flipped with respect to its partner, can lead to sterility.

How they made the discovery: Scientists took pollen-producing cells from plants – prior to the production of flowers – and removed the cell walls. They added a detergent solution to the cells that caused them to burst and reveal their chromosome structure.

“We’re a long way from understanding what leads to the pairing and recombination of chromosomes,” Anderson said. “Once we understand how that is controlled, we might eventually say, ‘I want to have this piece of DNA from that species moved into this one.’ Potentially, you could cross two species to move specific genes, like disease resistance, from a wild species into a commercial variety using natural recombination processes.”

The tomato and its relatives were thought to be unusual because differences between species were due to changes in genes, not in chromosomes. With the tomato study, “we have shown that chromosome rearrangements must be taken into consideration when evaluating evolutionary patterns in tomato as well as in other organisms.”

Anderson’s co-authors on the research are Stephen Stack and Patricia Bedinger, also biology professors at Colorado State; Lauren Larsen, an undergraduate biology student; and Paul Covey, a recent biology alumnus who now works for the U.S. Department of Agriculture.

The research was supported by the National Science Foundation.