Escalation by duplication: Milkweed bug trumps Monarch butterfly

Area of Science:

• * Ecology and Evolutionary Biology
• * Biochemistry and Molecular Biology

Background:

• * Monarch butterflies sequester toxic cardiac glycosides from milkweed plants for defense against predators.
• * These toxins inhibit the sodium-potassium ATPase (Na,K-ATPase) enzyme, crucial for cellular function.
• * Previous research identified adaptations in the alpha-subunit of Na,K-ATPase conferring resistance in insects.

Purpose of the Study:

• * To investigate the role of both alpha and beta subunits of Na,K-ATPase in conferring resistance to cardiac glycosides in milkweed bugs.
• * To explore how gene duplication and diversification of Na,K-ATPase subunits contribute to toxin resistance and physiological function.

Main Methods:

• * Expression of all nine combinations of three alpha- and three beta-subunits of milkweed bug Na,K-ATPase.
• * Testing the inhibitory effects of a milkweed plant-derived cardenolide on these Na,K-ATPase variants.
• * Analysis of gene duplication and subfunctionalization patterns in insect lineages.

Main Results:

• * Different combinations of alpha and beta subunits exhibit varying levels of resistance to cardenolides.
• * Gene duplication events have occurred multiple times in insect lineages feeding on cardiac glycoside-producing plants.
• * Subfunctionalization of Na,K-ATPase genes allows for a balance between toxin resistance and enzyme activity.

Conclusions:

• * Diversification of Na,K-ATPase subunits is a key evolutionary strategy for insects to cope with host plant toxins.
• * Milkweed bugs balance the benefits of chemical defense against predators with the physiological costs of toxin resistance.
• * Understanding these adaptations provides insights into the co-evolutionary dynamics between plants and insects.