The authors have declared that no competing interests exist.
Neo-Darwinian natural selection theory indicates that sudden, drastic changes in the environment place selective pressure on genetic variants in a population. As time progresses, this pressure sculpts individuals to better fit this new environment. Waddington’s classic experiment was repeated using white-eyed (the
No matter how drastic and sudden, a change in the environment does not bring a change in genetics. Such changes add constraint, by way of increased selective pressure, on those few, rare, genetic variants in the population. It is the repeated nature of selection, with time, that changes the gene frequency in favor of the most suitable individuals. This continues to the point that the whole population comprises more of these fitter variants than the older, less fit wild stock. Waddington’s Genetic Assimilation theory
Two strains of
F1 flies were maintained in sterile disposable culture bottles filled with freshly prepared fly medium until they started pupating (approximately 120 hours AEL). Crossveinless F1 flies were maintained as the Upward Selection Line and the non-crossveinless F1 flies were maintained separately as the Downward Selection Line. Pre-pupal collection was spread over 12-15 time points in a day, for five-seven days. Simultaneously, vials containing prepupae were heat-shocked after 24 hours of incubation (at 25°C) at 40.5°C for 45 min. Following the heat-shock, the vials are left in the incubator (at 25°C) for approximately 5 days, for the flies to eclose (Supplemental data, S1). As they emerge, the flies were scored as either cve or non-crossveinless (non-cve). Unlike the classic experiment
Following the repetition of the genetic assimilation experiment
Although the frequency of the crossveinless allele increased in every generation, there was a cost that crossveinless flies paid in the form of compromised viability. Nothing of the weakened viability was ever mentioned in Waddington’s original work of 1953. But this is not the first time it has been observed. Earlier works
During the usual development of
As mentioned earlier in the section, the flies normally eclose 5 days after the start of pupariation. Except in the case of cve flies, where most of the females eclosed on the fifth day and the males waited a day or two more to emerge out of their pupal cases. Also, in a particular generation females always, and by far, outnumbered males in terms of the percentage of crossveinless (
A reason behind the apparent increase in the frequency of crossveinless, seen in the upward selection line, is that the combinations of genes influencing crossveinlessness are believed to be polygenic in nature
We know that energy is used for two vital requirements in a living system: one for maintenance and the other for reproduction
Repetition of the Waddington experiment showed that just like the original experiment, the fly population strongly to the recurrent heat-shock and selection. Although the classic experiment made the selection of crossveinless look something as straightforward as merely heat-shocking pupae at specific conditions, it turns out there is a cost associated with acquiring crossveinless. It is understood that crossveinless as a phenotype does not offer any benefit to the fly
We would like to thank GRAVIDA, New Zealand and the University of Otago, New Zealand for providing funding and research facilities that was key to the successful completion of this work.