Research highlight: Dr. Aleksander Popadić

Small animals, big questions
With hundreds of thousands of different species, insects are by far the most diverse group of animals on the planet. Beyond their stunning number, insects offer something much more. Over the past few years, a WSU research group has used them to provide a window into the amazing variation that exists between species and even between insects of the same species, and also to shed light on evolution itself.
“Look at an insect leg. All insect legsare made of the same six segments, including the femur, tibia, and tarsus –names they share with human legs –but the long and very muscular jumping leg of a grasshopper is very different from its other legs,” says Aleksandar Popadić, associate professor in the WSU Department of Biological Sciences. “The difference lies in the size and in the level of development of these six parts, and my lab is looking to nature to try to understand the origin of these differences.”
His research group has made a number of discoveries, which have been reported in the pages of prestigious scientific journals. Back in 2004, the cover of Proceedings of the National Academy of Sciences (PNAS) heralded his group’s work showing that a gene called Ultrabithorax (Ubx) controls the size and robustness of a leg part. The paper revealed that when the gene is expressed (turned on) in the femur or tibia, those leg segments become enlarged, and the higher the gene’s expression, the larger the increase in segment’s size. “What this did was identify the master gene involved in the evolution of leg morphology, and we have continued our studies from there,” Popadić explains.
Recently, his research group found that the same Ubx master gene can also affect the creation of a worker honey bee’s pollen basket, a cavity on the leg that these insects use to store and transport pollen to the hive (Biology Letters 2014). They found that Ubx turns off the development of bristles that normally appear on insect legs, and that naked bit of cuticle allows the development of the pollen basket. “Without creating a pollen basket, you couldn’t have the social behavior we see in the colony, because the worker bees couldn’t carry pollen back and feed the queen and other bees,” Popadić says, noting that this illustrates the cascade effect such structural modifications can have.
Outside of leg structures, the research group is also investigating coloration, especially the striking pattern seen on large milkweed bugs (Oncopeltus fasciatus). These orange-red insects are adorned with a large black diamond fore and aft, and a black bar in between. The group’s findings indicate that a set of five genes are involved in the patterning of the body, with some genes working in concert in areas where red-orange in predominant, and others cooperating in mainly black areas. The study suggests that various combinations of these genes may similarly be responsible for color patterns in all insects.
Another especially captivating area of research is the lab’s study of the evolution of insect wings, a question that has intrigued biologists for years. When organisms evolve, the do so slowly: The organism develops an initial structure that confers some advantage, and over vast time scales, that structure becomes more and more pronounced as long as it continues to provide the organism with a greater advantage. With a grasshopper, for instance, slightly larger hind legs might help it escape predators more readily, and over time, the legs have evolved to become increasingly oversized. The puzzle with wing development is an initial wing bud doesn’t afford any advantage, Popadić explains.“If the wing isn’t already fully developed so it can move and allow flight, it’s really not good for the organism. So how does a structure that doesn’t yet have a function start to evolve?”
To figure out the age-old enigma, Popadić and his research group took a closer look at the pair of little pads that extend from each of the three segments of thorax (T1-T3) in fossils of longextinct insects. Only the pads on the second segment and third segments (T2 and T3) eventually evolved into wings. “There’s a mystery as to why the T1 pads never became fully developed wings and the T2 and T3 pads did, and we are now trying to do some molecular work and testing hypotheses in a very rigorous way to find out,” he says.
In their recent work published in PNAS (2015), he and his group showed how another homeotic gene, known as Scr, as well as additional related genes act to suppress T1 wing development. In addition, they found that the transition from extended pads, or flaps, into wings on T2 and T3 required cells originating on the underside of the flap to migrate up and fuse with cells originating on the topside of the flap, which ultimately resulted in large wings with all of the mechanical components necessary for flight.
While they haven’t solved the riddle yet, Popadić is happy with the progress so far, and looks forward to employing advances in genomic and other technologies to gain an even clearer picture of wing evolution.
All of this work is on the basic-science front —providing a fundamental understanding of biology and development — but in theory, it could have an applied side, he contends.“Once we understand how to make a naked cuticle, for instance, we could possibly make even bigger pollen baskets to the benefit of the hive. Or maybe we could change one of the wing or leg genes to cause a defect in a pest species, which might mean that we could stop trying to kill them with chemicals, and instead use an alternative, environmentally-safe transgenic approach to affect the species.”
With the fast-paced advances in science today, he believes the doors will open on many research possibilities within the next 10 years. “It’s just going to be a matter of which areas we find important enough that we want to put our time and money into them, and which areas can give us a reasonable result."


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Research highlight: Dr. Aleksander Popadić 11/28/2018
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