Evolution of Insect Legs: From Millipedes to Beetles

Beetle Larva and Insect Leg Evolution

• A beetle larva with many legs highlights the evolutionary steps from multilimbed centipedes to six-legged insects.

Genetic Two-Step in Beetle Evolution

• Biologist Randy L. Bennett and colleagues discovered that suppressing two genes in red flour beetle larvae (Tribolium castaneum) can cause extra legs to grow.
• Suppressing these genes effectively reverses evolution, reverting insects to an earlier form with more legs.

Early Insect Development

• Early insects had many identical segments, each with legs.
• Over time, segments evolved separate identities, and abdominal segments lost limbs, leaving only six thorax legs.

Role of Ultrabithorax (Ubx) and abdominal-A (abd-A) Genes

• In fruit flies, Ubx and abd-A control the number of legs and wings.
• These genes work together to keep fruit fly abdomens limbless.
• Researchers investigated if Ubx and abd-A gave beetles leg-free abdomens.

RNA Interference (RNAi) Experiments

• Beetle eggs were injected with double-stranded RNA copies of beetle versions of Ubx and abd-A to shut down specific genes.
• RNAi was originally developed to make mutant worms and has become a popular method to inactivate genes in various animals.

Experimental Results

• When abd-A was knocked out alone, beetle larvae grew nublike appendages called pleuropodia but didn’t develop legs.
• Knocking out Ubx alone caused “something on the first abdominal segment that looks like it’s trying to form a leg.”
• Removing both genes resulted in “legs everywhere,” and the larvae died before maturing.

Significance of Experiments

• These experiments give clues to the molecular process that led to the six-legged body plan.
• Nipam Patel notes the experiments validate hints found earlier and demonstrate them experimentally.

Gene Function in Different Species

• In fruit flies, Ubx and abd-A turn off the leg-development program.
• In crustaceans and centipedes, proteins encoded by these genes are found even in segments with legs.
• Patel suggests these proteins were present in the common ancestor of all insects but didn’t suppress legs.
• Beetles represent an intermediate stage where neither abd-A nor Ubx alone can completely suppress leg growth but each modifies limb formation.

Evolution as Rearrangement of Genetic Networks

• Evolution often rearranges genetic networks to make different developmental programs.
• New genes don't need to be invented for evolution; existing genes can be repurposed.

Caution in Interpreting RNAi Results

• Capecchi warns caution when interpreting RNAi results, as the mechanism of RNAi is not fully understood.