Why cocoons are more compact in some lepidopteran species than in others

Photo description: Lepidopteran silk genes are known for their tissue-specific expression; fibroins are produced in the posterior silk gland section (PSG) and sericins in different parts of middle section (MSG).

Lepidopteran silk is a complex assembly of proteins produced by a pair of highly specialized labial glands called silk glands. Silk composition has been examined only in a handful of species. In attempt to detect all components of silk from greater wax moth Galleria mellonella, (Pyralidae) we analyzed silk-gland-specific transcriptomes prepared from three developmental periods: penultimate instar larvae, wandering stage larvae, and prepupae. Our results revealed at least 27 novel candidate silk components, which were further characterized. Eight candidates were verified by proteomic analysis or microsequencing of cocoon proteins, and several others were confirmed by similarity with known silk genes and their expression patterns. We show that most candidates encode abundant secreted proteins produced by middle silk glands including ten sericins, two seroins, one or more mucins, and several sequences without apparent similarity to known proteins. We did not detect any novel posterior silk gland-specific protein, confirming that there are only three fibroin subunits. Our data not only show that the number of sericin genes in the greater wax moth is higher than in other lepidopteran species thus far examined, but also the total content of soluble proteins in silk is twice as high in G. mellonella than in B. mori or A. yamamai. The expansion of soluble silk components in G. mellonella might contribute to the compact character of the cocoon, which provides protection against parasitoids and bees. Our data will serve as a foundation for future identification and evolutionary analysis of silk proteins in the Lepidoptera.

Kludkiewicz B., Kučerová L., Koníková T., Strnad H., Hradilová M., Žaloudíková A., Sehadová H., Koník P., Sehnal F., Žurovec M. (2019) The expansion of genes encoding soluble silk components in the greater wax moth, Galleria mellonella. Insect Biochemistry and Molecular Biology 106: 28-38.  DOI: 10.1016/j.ibmb.2018.11.003