Turn it over and turn it over

Genetic reversal allows one plant species to grow in two completely different forms. Research results on the mimulus plant

By: Noam Levitan and Yonat Ashhar Galileo

In the west of North America grows yellow mimulus (Mimulus guttatus), a plant from the family of sedums known in English as the monkey flower because of the scattered dots on its face. On the cool, humid, fog-shrouded coasts of the west, the yellow mimulus grows as a perennial plant, while inland, where the Mediterranean climate is warm, it grows as an annual plant that wilts and dies with the coming of summer. The most noticeable difference between these two ecotypes, in addition to the lifespan, is the flowering date.

The annual type blooms early and quickly, producing seeds before being damaged and dying in the hot, dry summer. On the other hand, the perennial type, which does not suffer from a lack of water, benefits from favorable growing conditions that allow it to invest in growth and leaf growth before its late flowering. Since the flowering of the two types of mimulus occurs at different times, they have almost no opportunity to pollinate and fertilize each other. This reproductive isolation may lead over time to the splitting of the two types into separate biological species of Mimulus.
genetic hybridization

In a study recently published in the online journal PLoS Biology, David Lowry (Lowry) from Duke University reveals the genetic component that causes the differences between the two types of flowers. Lowry, interested in the formation of biological species, focused on the yellow mimulus to discover how each type adapted to the environment in which it grew. Since Lori did not know which genes are involved in the adaptation of mimulus plants to their environment, he interbred between the different types in an attempt to find the genes that turn yellow mimulus from a perennial to an annual and vice versa.

During the hybridization and mapping of regions in the genome of the hybridized plants, he noticed a difference in the arrangement of a DNA segment in one of the plants' chromosomes - the segment that is arranged in one way in the perennial plants is arranged in the opposite way in the annual plants. This situation is called inversion. An inversion is a mutation in which a segment of the chromosome is placed in the opposite way. This situation occurs when breaks in two sites in the chromosome cause a section to break off, which is inserted back into the chromosome in the same place but in the wrong direction, after a 180 degree reversal.

The genetic information in the chromosome does not change following an inversion, only its position and order. Since Alfred Sturtevant discovered inversions in 1921, many inversions have been found that separate closely related species, for example about 1,500 inversions separate humans from chimpanzees, in addition to other genetic differences.
An inversion is necessarily a passing one?

As mentioned, Lowry noticed a reversal during the hybridization of the different types. He discovered the inversion because he noticed that in hybrids the part of the opposite chromosome does not participate in the process of gene predation that occurs during the creation of the gametes - the process of crossing over. In this process, a part of one chromosome is replaced by a part of another chromosome, when both the chromosomes and the replaced parts are homologous to each other, that is, they are very similar to each other.

In the case of an inversion, no replacement occurs because a part of one chromosome faces the opposite direction from the corresponding part of the other chromosome. Therefore, in hybrids of the different types each segment faces the opposite direction and crossing over does not occur, while in unmixed types the direction of the segment on each chromosome is the same and crossing over can occur. In any case, even in the hybrids (most of whom are fertile), most of the chromosome is not inverted and therefore a crossover takes place.

Since the large reverse section, which includes about 360 genes, passes as one piece without transition, it can actually be regarded as a kind of super gene that allows the mimulus plants to survive in different growing environments. Lowry found further evidence that the super garden is related to survival when he noticed that in inland areas, where the climate is Mediterranean and most mimulus plants are annuals, you can find perennial mimulus plants in places where there is no shortage of water, such as on the banks of streams and near springs. Examining these perennials revealed that they are identical to the plants that grow along the coast and that they have the same inversion that characterizes the perennials that grow there.
How does the inversion affect nature?

To test whether the distinguishing features between the perennials and the annuals really stem from the inversion, Laurie bred many plants until he was able to obtain perennials whose inverted segment was replaced by the annual segment, and vice versa. Such perennials flowered much earlier than their normal brethren while annuals with the reverse segment of the perennial flowered later than usual.

To test the effect of the reversal in nature, Lori planted the plants in experimental fields in the different habitats - on the coast and inland. Although the plants did not do as well in the field as their native brethren, for example, a common annual plant in the dry interior did better than any other plant. But, in any case plants with the annual inversion did better than plants with the perennial inversion in the dry interior. Opposite results were obtained on the coast, where plants with the perennial inversion did better. From this it is clear that the inversion is a major factor, although not the only one, that affects the adaptation of the yellow mimulus to the different growing conditions.

Lori believes that in the yellow Mimulus different forms (alleles) of genes have evolved that help survive in the dry continental inland environment or in the coastal environment. When the inversion occurred, it included these alleles and thus prevented them from crossing over with the corresponding group of alleles in the plants of the second type, and allowed them to pass as one group responsible for adapting the plants to their environment. Other possibilities, but less likely, are that the inversion occurred first and only after that additional mutations accumulated that created different alleles suitable for the different environments, or that the inversion affected a gene that sits on one of the breakpoints where the inversion occurred and this gene is responsible for the differences between the types of plants.

Reversal in nature: two forms for one plant

The results of Lowry's research prove for the first time that reversals do affect adaptation to the environment. In addition to this, the inversion in the yellow Mimulus actually causes a reproductive separation between the different ecological types of the plants by affecting the time of their flowering and since each of the types fails to survive in the environment where the other type grows. The perennial plants die of desiccation in the inland before they can even bloom, and the annuals grow slowly and bloom too early on the coast and lose out in competition for resources with all the other plants growing there. This separation, which is maintained since the inverted segment does not cross over and therefore prevents the exchange of beneficial genes between the different types, may eventually lead to the development of two separate biological species of yellow mimulus plants and further inversions.

The full article was published in Galileo magazine, November 2010