What They Did
The researchers examined the DNA of 11 species of fig wasps, 6 fig pollinators and 5 non-pollinators. The pollinating species enter the synocium (the structure that completely encloses the flowers and, later, the tiny multiple fruits), while the non-pollinating species pierce the synocium with their ovipositors to lay eggs. The researchers particularly looked at the transposable elements, segments of DNA that can move from one area of the genome to another. They found that the non-pollinating wasps had significantly more transposable content in their genomes and that the individual transposable elements of the pollinating wasps were shorter.
They compared the genetics of the different species to develop a phylogeny, which shows that the pollinating wasps form a clade that diverged from the non-pollinating wasps. Based on the differences among transposable elements and the background mutation rate, they determined that the transposable elements in the pollinating wasps were 10 to 30 million years old, while the those of the non-pollinating wasps were less than 5 million years old.
In addition, the pollinator wasp species have smaller effective population sizes: a less varied population has a smaller effective population size than a more varied population with the same number of individuals. The researchers suggest that since the pollinating wasps spend most of their lives in the stable, enclosed environment of the fig synocium, the selection pressures tend towards conservation of the well-adapted phenotype. The non-pollinating wasps, in contrast, encounter more varied environments, such that more genetic variability is likely to be adaptive.
Further Exploration
Transposable elements (also called transposons) are fascinating and difficult to understand. There are two major types: one type (Type I) gets copied and pasted elsewhere in the genome, but the original copy remains. The other type (Type II) gets cut and pasted, moving from one location to another.
In Type I, the transposon DNA is transcribed into RNA and then reverse transcribed back into DNA. Repeating sequences of bases determine where the new DNA can be integrated into the genome with the help of enzymes. The Type II transposons, meanwhile, are not copied into RNA. Instead, both strands of DNA are enzymatically cut and held together, then moved elsewhere in the genome (see https://www.integra-biosciences.com/united-states/en/blog/article/transposons-jumping-genes-revolutionizing-genetics).
Of course, no discussion of transposons is complete without a mention of Barbara McClintock. By observing the variegated color patterns of corn, she realized that something other than mutation or Mendelian genetics must be responsible for the changes from one generation to the next. Her ideas were dismissed for decades, but she won the Nobel Prize for her work in 1983. (see https://www.nobelprize.org/stories/women-who-changed-science/barbara-mcclintock/). If a particular transposon is near the gene that codes for pigment in a corn cell, it stops pigmentation from occurring, but if it’s farther away, it has no effect. Since the location of the transposon can vary among cells within a kernel, the kernel ends up with a mottled pattern (see https://www.waynesword.net/transpos.htm). It would be interesting to learn how transposons may have evolved in the first place, but that’s a rabbit hole for another day!
Image credit: Alandmanson
https://commons.wikimedia.org/wiki/File:Philotrypesis_2019_06_29_4560.jpg
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