Limb Regeneration in Lady Beetles: Product of Selection or Developmental Byproduct?
By John P. Roche, Ph.D.
Some animal species have the ability to regenerate limbs. This amazing capability is seen in several groups of insects and even in some vertebrates, such as salamanders. In insects, does the degree of limb regeneration vary in different species with different ecologies and evolutionary relatedness? And why is limb regeneration maintained as a trait in some insect species? These are intriguing questions that were examined in lady beetles by J. P. Michaud, Ph.D., entomology professor at Kansas State University, and colleagues around the world in a new study published in August in the Annals of the Entomological Society of America.
Limb regeneration has long been studied in insects with incomplete metamorphosis that lack a pupal stage, such as true bugs (Hemiptera), grasshoppers and katydids (Orthoptera), cockroaches and termites (Blattodea), and stick insects (Phasmatodea). Recently, limb regeneration is also being studied in insect groups with complete metamorphosis that do have a pupal stage, such as butterflies and moths (Lepidoptera) and beetles (Coleoptera). Michaud and his colleagues studied lady beetles (order Coleoptera; family Coccinellidae).
Most species of lady beetles are predatory. Some species specialize on particular types of insect prey, such as mites or scale insects, while other lady beetles have broad diets that include many types of prey. As predators, many lady beetles are beneficial because they consume crop pests such as aphids.
In their study, Michaud and colleagues raised lady beetles for analysis in colonies in the U.S., Egypt, India, Brazil, China, Iran, and the Czech Republic. They looked at a wide range of lady beetle species with varying ecologies and evolutionary relatedness. In each location, they separated lady beetles into treatment groups and control groups. In treatment groups, the investigators removed the right forelimb of fourth instar larvae. In control groups, they did not remove the right forelimb. They measured the duration of the pupal stage, the mass of adults, and the number of larvae that survived to become adults.
The investigators created a Limb Regeneration Index (LRI) that recorded whether a limb was regenerated, whether it was regenerated fully or partly, and whether there were costs to the individual, such as a lengthened pupal stage or reduced adult body mass. The range of the index was zero to one, with one indicating the highest amount of regeneration. They also calculated the degree of evolutionary relatedness among all the species studied.
How Prevalent is Limb Regeneration in Lady Beetles?
The average LRI value found in Michaud’s study was 0.598. In two species, the sevenspotted lady beetle (Coccinella septempunctata) and the multicolored Asian lady beetle (Harmonia axyridis) limb regeneration was observed in almost all individuals (100 percent and 97.9 percent, respectively). “Virtually every species we tested had some capacity to regenerate, and most had very good capacity,” says Michaud.
There was a trend of greater differences in the LRI between species with greater evolutionary distances. The investigators also found that the most commonly observed cost of limb regeneration was a delay in pupal development time; this pupal development delay was significant in nine of the 14 species studied.
Ecologically, Michaud et al. found that invasive species with wide geographic ranges scored high in the Limb Regeneration Index. Two populations of the highly invasive multicolored Asian lady beetle were examined and the investigators found that the LRI for the invasive population in North America was 20 percent higher than in the indigenous population in China. Similarly, when the team examined two populations of the lady beetle Hippodamia variegata in the Czech Republic and in Egypt, the two populations showed pronounced differences in limb regeneration.
Ecology therefore seems to be a key factor in limb regeneration. “Invasive populations of the Asian multicolored lady beetle are now biologically different from ancestral, indigenous populations in Asia, which contain genotypes that never became invasive. Invasion is associated with aggressive traits, robust competitiveness, and ecological dominance, all characteristics shared by the best regenerators. The fact that these two populations differed as they did support the apparent correlation between general ecological success and a high LRI,” Michaud explains.
This study found that limb regeneration was common in the lady beetle groups investigated. Why is this trait maintained in these beetles? The investigators examined two potential explanations. One is that limb regeneration is directly favored by natural selection, and is thus maintained in the population because it provides an advantage in survival and reproductive success. The second explanation relates to pleiotropy, in which single genes often code for multiple traits: there is a possibility that limb regeneration itself does not offer a selective advantage, but that other traits arising from the genes coding for limb regeneration do offer a selective advantage.
Which Hypothesis is Supported by the Research So Far?
“I believe that limb regeneration has been conserved as a consequence of pleiotropic effects of the genes mediating pupal development, rather than any direct selection on the trait itself,” Michaud says. That is, there is support for the hypothesis that other traits arising from the genes coding for limb regeneration do offer a selective advantage.
“Many of the genes enabling limb regeneration are probably important developmental genes that are activated during pupation,” Michaud says. When regeneration is required, these developmental genes are likely upregulated, leading to cascading effects on the expression of other genes, as well as other phenotypic traits that could be subject to selection.
There is evidence from some insect species, such as walking sticks, that limb regeneration provides a direct selective advantage because these insects often damage limbs when escaping from predators during their nymphal stages. But limb damage is not often seen in lady beetles—when attacked by predators, they either survive or are eaten—making direct selection on limb regeneration unlikely.
When asked about ideas for future research, Michaud says, “There are many species in other groups that would be interesting to test. We lacked lady beetle species in the Scymninae, for example, and we didn’t test scale-feeding species in the Chilocorinae or fungi-feeding species in the Psylloborini.”
Limb regeneration was found in this study to be more common in invasive, ecologically competitive species. Why this is remains unclear, but in keeping with the pleiotropy hypothesis, it is possible that traits arising from genes expressed via the limb regeneration pathway offer advantages in competition. This topic offers intriguing possibilities for future research and might offer insights that could inform our understanding of invasive lady beetle species and their ecological roles as biological control agents of crop pests.
Annals of the Entomological Society of America
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com/.