Image of a pear psylla in its winter form, which is darker in colour and has a thicker outer cuticle layer. Image credit: Songdou Zhang (CC BY 4.0)
A bug known as pear psylla is a common pest on pear trees in China and other East Asian countries. It feeds off the sap of young leaves and shoots, causing damage to the trees and decreasing the amount of fruit they produce. To survive all-year round, pear psylla changes its body between distinct summer- and winter-forms, a phenomenon known as seasonal polyphenism. In winter, cooler temperatures trigger young pear psylla to darken and thicken the protective cuticle layer coating their bodies. However, the mechanisms behind this seasonal transformation are not fully understood.
One possible regulator of this process is the hormone Bursicon which is known to control cuticle development in juvenile insects. The hormone has two subunits that join to form dimers, which then activate specific receptors that initiate signaling pathways within the insect’s body. Here, Zhang et al. used molecular biology and genetic techniques to study the role of Bursicon dimers in seasonal polyphenism in pear psylla.
Bursicon can assemble either as a homodimer (made up of two identical subunits), or a heterodimer (made up two different subunits). Zhang et al. found that low temperatures triggered the formation of both homodimers and heterodimers of Bursicon. However, only the heterodimers activated a receptor, called CcBurs-R, which enabled the pear psylla to transition into their winter-form. The team also identified a small molecule called a micro RNA that regulates this switch by decreasing the production of the CcBurs-R receptor.
The findings by Zhang et al. advance our understanding of how seasonal polyphenism operates in pear psylla. Many other insects display seasonal polyphenism, and further research could reveal whether Bursicon plays a similar regulatory role across different species.
