Biology: The Dynamic Science p. 492 (2013)
By Peter J. Russell, Paul E. Hertz, Beverly McMillan

Monkey-Flower Speciation: How many genes contribute to reproductive isolation?

Reproductive isolation is the primary criterion that biologists use to distinguish species. As noted earlier, the monkey-flower species Mimulus lewisii and Mimulus cardinalis experience mechanical reproductive isolation in nature because differences in flower structure keep bumblebees or hummingbirds from carrying pollen from one species to the other (see Figure 22.7). However, the two species are easily crossed in the laboratory and produce fertile F1 hybrids. The F2 offspring have flowers with various forms intermediate between the two parental types (see bottom row of Figure), suggesting that several gene loci control the traits separating the species.

Research Question How many gene loci contribute to the differences in flower traits that foster prezygotic reproductive isolation between the two Mimulus species?


Experiment

Because little was known about the genetics of the two monkey-flower species, it was not possible to use a direct genetic analysis to identify and map the flower trait genes to the chromosomes. Instead, H. D. ("Toby") Bradshaw and other researchers at the University of Washington used an indirect molecular approach that identified DNA sequence variations (analogous to those used in DNA fingerprinting; see Chapter 18) at various loci in the genome. Just as morphological and biochemical traits vary within a population, DNA sequences vary at particular sites in the genome; and the different DNA sequence alleles can be distinguished using PCR.

The researchers used a random set of 153 DNA sequence variations. They correlated the segregation of those variations with the segregation of flower traits in 93 plants of the F2 generation. Some of the DNA sequences segregated closely with a particular flower trait. This result indicated that the particular DNA sequence variation locus was very near the gene for that flower trait on the chromosome. In other words, the flower trait locus was identified indirectly through the close linkage between the DNA variation locus and the flower trait locus. But where are the genes? To answer this question, the researchers used the DNA sequences linked to flower trait loci as probes to find the sites on the chromosomes where they originated. (The use of DNA probes to identify complementary DNA in the genome is described in Figure 18.9.) For any given DNA variation locus, once its position on the chromosomes was determined, the investigators knew that the flower trait locus correlated with it must be nearby.

Results

Their results indicate that reproductive isolation of M. lewisii and M. cardinalis results from differences in eight floral traits: the amount of (1) anthocyanin pigments and (2) carotenoid pigments in petals; (3) flower width; (4) petal width; (5) nectar volume; (6) nectar concentration; and the lengths of the stalks supporting the (7) male and (8) female reproductive parts. Although the investigators could not directly determine the number of genes controlling each trait, the characteristics of the traits, their locations at eight sites on six of the eight chromosomes, and their pattern of inheritance make it most likely that each trait is controlled by a single gene, giving a likely minimum of eight genes.

Conclusion

Mutations in as few as eight genes may have established reproductive isolation between these two species of monkey-flower. Thus, it appears that in some cases surprisingly little genetic change is required for the evolution of a new species. This research was the first in which random differences in DNA sequences were used to answer the fundamental evolutionary question of how much genetic change is needed to produce a new species.

Source: H. D. Bradshaw Jr., S. M. Wilbert, K. C. Otto, and D. W. Schemske. 1995. Genetic mapping of floral traits associated with reproductive isolation in monkey-flowers (Mimulus). Nature 376:762-765,

Mimulus Reversions (1999)