Recent breeding experiments have revealed that bright pink, not green, could be the most genetically dominant colouring of the North American oblong-winged katydid.
For almost as long as we've known that oblong-winged katydids (Amblycorypha oblongifolia) come in an array of colours, we’ve known that green is by far the most common, while the pink, yellow and orange colourings are far more rare, based on their appearance in the wild. But over the past five years, researchers at the Audubon Butterfly Garden and Insectarium in New Orleans have been breeding pink katydids, and the results are challenging everything we thought we knew about the genetics of katydid colouring.
First described in 1874, pink katydids have inspired more than a century of discussion over the hows and whys of this incredible hue. At the turn of the 20th century, Harvard entomologist, Hubbard Scudder, suggested that the pink colouring could be seasonal, so the green insects would change their colours with the autumn leaves as the temperature drops. But having found bright pink katydid nymphs in the prairies of Wisconsin and Illinois during July in 1907, American entomologist and myrmecologist, William Morton Wheeler, rejected this theory, suggesting instead that the condition was genetic.
For the first time, pink katydids were recognised as genetic 'mutants' in the scientific literature, and Wheeler compared the condition to albinism. "They have, in fact, every appearance of belonging to a category of colour forms similar to that of the albino mammals and birds and certain kinds of white-flowering plants," he observed in The American Naturalist that year.
Fast-forward to present-day thought on pink katydids and it looks like Wheeler nailed it. Right now it's widely accepted that pink, yellow and orange katydids occur as a result of erythrism – a genetic mutation controlled by recessive genes that causes an absence of a normal pigment, such as green, and/or an excessive production of another pigment, such as red or pink. It made sense, seeing how rare pink, yellow and orange katydids are in the wild. "It apparently is similar to the situation in albino animals where the condition is due to recessive genes," said Professor of Entomology Tom Turpin, from Indiana's Purdue University in 2009. "When the correct combination of recessive genes is present, the condition is expressed as an albino animal. The same apparently is true of pink katydids, although very little scientific data exist. In the past, people have tried to produce pink katydids in captive breeding experiments, but with limited success."
In 1916, Chicago-based entomologist, Joseph Hancock, managed to breed pink katydids for the first time using a pink female and a green male. It took almost a century for Hancock's success to be repeated, this time by the team at the Audubon Butterfly Garden and Insectarium. Led by entomologist and Director of Animal and Visitor Programs, Jayme Necaise, the team received a donation of eight pink male and female oblong-winged katydids in 2008, scooped up in several locations in southern Louisiana. Wild yellow and orange katydids were integrated into the breeding program last year, and a ‘rainbow cage’ was set up to allow all colours, or morphs, to mate at will. The newly captive katydids enjoyed a buffet of romaine lettuce, oats, bran, dog food, Cheerios, sliced orange, apples and grapes. They reached sexual maturity at around 60 days of age and had a lifespan of between four to six months. Their eggs can take up to 13 months to hatch, and the males always hatch first.
The first thing the team noticed when their rainbow katydids started breeding was that not all colour morphs developed in the same way:
"All pink individuals hatched out of the eggs pink body coloured with pink eyes. Throughout the molting process, they retained this pink eye colour and pink body colouration. However, all yellow and orange-coloured individuals hatched out of the eggs with green body colour and green eyes.
The body colour changed from green to yellow or orange over successive molts, resulting in a yellow or orange colouration as an adult. These individuals retained the green eye colour throughout the molting process and throughout their entire adult lifespan. On one occasion, an individual from the ‘rainbow cage’ hatched green and changed to yellow through molting. At the final molt it was yellow. The next day it was orange with green eyes and retained the orange colouration throughout the remainder of its adult lifespan."
The different colour morphs are controlled by alleles – different forms of the same gene that sit in the same location on a pair of chromosomes – and the dominant allele has the ability to mask the recessive allele. So through their breeding experiments, Necaise’s team tested the hypotheses that the allele that controls the green morph is the dominant allele, and those that control both the pink and yellow/orange morphs are recessive. According to the laws of Mendelian inheritance, if the parents contained one of each type of allele (heterozygous), this would produce offspring at a ratio of 3:1 dominant to recessive. Katydids that ended up as pink or yellow/orange would have to possess two recessive alleles (homogenous recessive), because if they had a green allele, it would mask the expression of this recessive trait.
With this in mind, the first thing the team did was breed two wild-caught pink females with a wild-caught pink male. They produced a clutch of 35 eggs, and it was predicted that 100% of the offspring would be pink, as there could not possibly be any green alleles to pass on. But what they got was 31 pink (89%) and four green (11%) – a 3:1 ratio in favour of the pink morph, which meant that the pink could not be dependent on the parents being homogenous recessive. In fact, in order to yield these results, each parent would have had to have a pink and a green allele, with pink being the dominant allele over the green.
Next they bred green males with pink females; pink females with green males; green males with green females from pink parents; and wild-caught green males and females. Finally, they matched up lab-reared pink males and females and wild-caught yellow males with yellow females. The results of all of the six crosses are below:
The results suggest that not only is green the recessive trait, but judging but the ratio of offspring, it looks like the extremely rare yellow/orange morph is also dominant to green, just as the pink morph is. “When we factor in the results of the green to green crosses, both wild-caught and those from pink parents, the results further suggest that green is the recessive allele," Necaise and the Audubon Insectarium's senior entomologist, Tabitha Holloway, report in a paper to be published in the upcoming 2012 Proceedings for the Invertebrates in Education and Conservation Conference. "In neither of those crosses did any pink emerge. The only way this would be possible is if the parents were homozygous recessive. In that scenario, the offspring would be 100% homozygous recessive and green, which matches our results. This strongly suggests that the predominant genotype in nature is homozygous recessive green."
So assuming that green is the recessive trait of oblong-winged katydids, are how is it that North America is littered when greens, not the candy-coloured pinks or yellows or oranges? The team suggest that it could be a case of directional selection, whereby one allele, regardless of being dominant or recessive, has greater fitness than another, causing the gene pool to change over time until eventually a species arrives a situation called fixation, where only one of the alleles remains. Combined with their leaf-like body shape, the green katydids are particularly well camouflaged in their environment, while the bright yellow/orange morph is less so. The pink katydids are even worse off in the wild, because unlike the yellows and oranges that start off green when they are young and vulnerable, the pinks are pink and super-conspicuous from the moment they’ve hatched. “The small percentage of pink individuals in wild populations that do achieve sexual maturity and do breed with green [will] produce approximately 50% pink and 50% green. This could explain why the pink colouration is so rare in nature, but not completely eradicated,” Necaise and Holloway suggest.
The importance of camouflage has seen a similar case of directional selection in insects – the famous case of the peppered moth (Biston betularia). A light body colour was the dominant trait in English peppered moths because they could camouflage against the similarly coloured trees and lichens that were abundant in their habitat. But widespread pollution during the Industrial Revolution in the mid-18th century rendered it more beneficial to have the recessive trait of a dark, soot-coloured body to hide from predators, so the percentage of dark peppered moths increased from 0.001% to around 90% of the population in England.
The team's breeding experiments also hint at co-dominance between the pink and yellow/orange morphs, as individuals in the rainbow cage started appearing with yellow bodies and pink legs, or mixed colours such as taupe, grey and peach. "By breeding pink, yellow and green, we selected for the extremes," report Necaise and Holloway. "Based on the results of the rainbow cage, there appear to be more subtleties in colour variability than demonstrated in nature and in our cross-breeding experiments. Further research and experimentation should provide new possible explanations for the colour variability within this species of katydid."
"We would love to delve more deeply into the extremely interesting genetics of these beautiful animals, adds Necaise. "Unfortunately, we do not have a full bore genetic lab at our disposal, so we are limited to crossbreeding experiments. At this time, we are continuing to breed these katydids in our exhibit at the Audubon Butterfly Garden and Insectarium without restrictions. To date, we continue to see very strange mixing of colours that demonstrate signs of incomplete and co-dominance. In fact, the other day we had an adult katydid that had a yellow body, green eyes and pink legs on exhibit."
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