Crowded in a small room in the Earth Sciences Building basement, students, researchers, and U of T faculty recently attended a seminar titled “Local Adaptation, Sexual Selection, and Chromosome Evolution.” Sponsored by the Department of Ecology and Evolutionary Biology, the talk was given by University of Texas at Austin geneticist Dr. Mark Kirkpatrick. He came to discuss new research findings concerning the relationship between local adaptation and chromosome inversions, and sexually antagonistic selection.
During the first half of the talk, Dr. Kirkpatrick spoke about the relationship between chromosome inversions and adaptation. An inversion is a type of mutation that occurs when a chromosome breaks in two locations and the region in between rotates 180 degrees before the pieces rejoin. Lethal gene mutations can occur when an inversion-induced chromosome break occurs in the middle of a gene. However, in diploid organisms—those with two sets of chromosomes—this mutation is not harmful as long as an identical, viable gene is present on its paired, or homologous, chromosome. Inversions generally do not have deleterious effects, as they do not cause the amount of genetic material to change. In fact, if an individual from one species who has experienced an inversion mates with members of a closely related species, within a few generations a large proportion of the latter species’ population will likely possess the chromosome inversion.
The inversion will spread, due to its effect on recombination, a process that involves the exchange of genetic information between homologous chromosomes during gamete formation. Chromosome inversions suppress recombination in heterozygotes—individuals possessing two different forms, or alleles, of a given gene. This is significant, as locally adapted genes give organisms an advantage in surviving and reproducing in their local environment. If at least two of these genes are found on either side of an inversion site, their alleles will rarely undergo recombination, remaining in that particular combination for generations. The accompanying inversions then spread through the population, remaining linked to genes that provide a reproductive advantage. “Inversions represent islands in the genome of local adaptation,” explained Kirkpatrick. “They are there to protect pieces of genome that interact well with each other or with the local environment.”
The second part of the seminar focused on sex chromosome evolution through sexually antagonistic selection. Sexual selection is a type of natural selection that acts on an individual’s ability to successfully mate and produce offspring. An allele having sexually antagonistic effects provides an advantage to one sex but not the other. These types of alleles can be found on sex chromosomes as well as non-sex chromosomes, known as autosomes.
Dr. Kirkpatrick presented a scenario in which an autosome containing an allele that makes an individual male successful develops a mutation that makes it male, regardless of what sex chromosomes it possesses. If this combination gives the mutant higher fitness—an advantage in terms of reproduction or survival over males who only carry the ancestral Y chromosome—this altered chromosome can spread through the population, “hijacking” the old Y chromosome by taking its place as the new sex-determining chromosome. A similar process can occur with X chromosomes. However, establishment of the new Y chromosome does not always result in replacement of the ancestral chromosome. There are also situations in which both the new and the old Y chromosomes contribute to sex determination.
So why don’t sex chromosomes in humans and other animals undergo the same changes? One reason is that the ancestral Y chromosome in these species contains genes necessary for male fertility and viability, and it is unlikely that every one of these essential genes would undergo a translocation to another chromosome.
The evolution of new X and Y chromosomes from autosomes is just one example of sex chromosome evolution. ZW sex chromosomes are found in birds, reptiles, amphibians, and fish. Some species of fish possess Z, W, X, and Y sex chromosomes, and it is suspected that other species undergo frequent transitions between the ZW and XY chromosome systems.
These processes are part of what Dr. Kirkpatrick calls a “feedback loop” between sexual selection and local adaptation. It is common to think about the ways in which genes influence the appearance and behaviour of organisms. However, it is evident that the behaviour of individuals can also influence the evolution of the genome.
