Development III: Polyphenisms

In today’s lecture we will examine the evolution of complex life cycles focused around the topics:

• Evolutionary significance of the pupal stage in holometabolous (endoptergote) insects.

• Caste polyphenisms in ants of the genus Pheidole.

• Larval polyphenisms in the geometrid moth Nemoria arizonaria.

• Phase polyphenisms in locusts.

• General evolution of complex life cycles.

Evolutionary Significance of the Pupal Stage

Three competing theories:

• Berlese theory—pupal stage in endopterygotes is homologous to nymphal stages in exoptergotes .

• Poyarkoff theory—pupal stage is homologous to subimago stage in mayflies. Pupal stage acts as mold for adult stage tissues.

• Hinton theory—pupal stage is homologous to the last larval instar of exoptergyotes. Larval-to-pupa molt is required for proper wing evagination. Pupa-to-adult molt is required for proper muscle attachment.

Caste Polyphenism in Pheidole Ants

Caste polyphenism in Pheidole forms in the last (3rd) larval instar and depends on JH titer in the blood, protein in the diet, and the number of soldiers in the nest.

Larval Polyphenisms in the Geometrid Moth Nemoria arizonaria • Caterpillars look the same at hatching, differences emerge in

2nd instar. • Spring brood feeds on oak catkins, a transient resource of

high nutrient value. Summer brood feeds on oak leaves, a permanent resource of low nutrient value.

• Tannins in oak leaves (or their absence in oak catkins) causes the developmental switch.

Phase Polyphenisms in Locusts

Evolution of Complex Lifecycles