Genome size reversely correlates with host plant range in Helicoverpa species

Authors: ZHANG, SHEN - Chinese Academy Of Agricultural Sciences; GU, SHAOHUA - Chinese Academy Of Agricultural Sciences; Ni, Xinzhi; LI, XIANCHUN - University Of Arizona

Submitted to: Frontiers in Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2019
Publication Date: 1/30/2019
Citation: Zhang, S., Gu, S., Ni, X., Li, X. 2019. Genome size reversely correlates with host plant range in Helicoverpa species. Frontiers in Physiology. 10:Article 29.
DOI: https://doi.org/10.3389/fphys.2019.00029

Interpretive Summary: Speciation often leads to expansion or contraction in genome size and host range. Host range may positively or negatively correlate with genome size in pathogenic bacteria and fungi, depending on their host expansion strategy—host adaptation by evolving additional host use related metabolic pathways and digestion/detoxification genes or host modification via pathogen secretions. To test if this holds true for herbivorous insects whose genomes have a greater percentage of repetitive transposable elements (i.e. jumping genes) than those of pathogenic bacteria and fungi, four closely-related important agricultural pests differed in host range were examined for a potential correlation between their genome sizes and host ranges. The four important agricultural pests used in this study were the corn earworm (also known as the cotton bollworm), the old-world bollworm, the oriental tobacco budworm, and the tobacco budworm. The findings of the present study show a trend of genome contraction along with expansion of host range in this group of insect pests. And species of this group appear to depend more on host modification than host adaptation to become a generalist.

Technical Abstract: In organisms with very low percentages of transposable elements (TEs), genome size may positively or negatively correlate with host range, depending on whether host adaptation or host modification is the main route to host generalism. To test if this holds true for insect herbivores with greater percentages of TEs, we conducted flow cytometry to measure the endopolyploidy levels and C-values of the host modification (salivary gland and mandibular gland in head), host adaptation (midgut), and host use-independent tissues (male gonad, hemolymph, and Malpighian tubules) of the generalist Helicoverpa armigera and the head of its older specialist sister H. assulta. Larval salivary gland displayed a consecutive chain of endopolyploidy particles from 8Cx to higher than 32Cx and larval head and midgut had endopolyploidy nuclei clusters of 16Cx and 32Cx, whereas larval male gonad, hemolymph, and Malpighian tubules possessed no endopolyploidy nuclei of higher than 8Cx. The estimated genome size of the Solanaceae plant specialist H. assulta is 430 Mb, significantly larger than that of its older generalist sister Heliothis virescens (408 Mb) and those of its two generalist descendants H. armigera (394 Mb) and H. zea (363 Mb). These data not only reveal a negative correlation between host plant range and genome size in this terminal lineage, but also imply that Helicoverpa species appear to depend more on host modification than on host adaptation to achieve polyphagy.