Genome Size Reversely Correlates With Host Plant Range in Helicoverpa Species.

Shen Zhang,Xinzhi Ni,Shaohua Gu,Xianchun Li

doi:10.3389/fphys.2019.00029

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.

Highlights

Speciation often leads to expansion or contraction in genome size and host range (Moran, 2002; Parkhill et al, 2003; Cho et al, 2007; Thomson et al, 2008; Merhej et al, 2013; Langridge et al, 2015; Skoracka et al, 2018)
A phylogenetic comparative analysis of 191 human pathogenic bacteria species (121 zoonotic species, 70 azoonotic species) shows that the host range of these pathogens is positively correlated with their secretome size but negatively correlated with their genome size, which is consistent with the prediction of the host modification strategy (McNally et al, 2014)
In order to verify if the positive correlation between genome size and host plant range revealed from the 21 distantly related lepidopteran herbivores by Calatayud et al (2016) holds true among the three closely related Helicoverpa species, six larval tissues of H. armigera were used to determine a suitable tissue for estimation of their genome size by flow cytometry

Summary

INTRODUCTION

Speciation often leads to expansion or contraction in genome size and host range (Moran, 2002; Parkhill et al, 2003; Cho et al, 2007; Thomson et al, 2008; Merhej et al, 2013; Langridge et al, 2015; Skoracka et al, 2018). If the expansion of host use related genes and non-coding regulatory sequences are too small to dictate the evolution of genome size relative to that of TEs, there will be lack of correlation between genome size and host plant range This is likely to happen when species that belong to different higher taxonomic levels such as different families or even genera are compared. The contribution of host use related genes and regulatory sequences may be large enough to govern the evolution of genome size among closely related species or intraspecific populations that are similar in all life-history traits but differ mainly in host plant range In the latter case, a positive or negative correlation may be found between genome size and host range. The resultant tree agrees with their phylogenetic relationship reported previously (Mallet et al, 1993; Mitter et al, 1993; Fang et al, 1997; Behere et al, 2007; Cho et al, 2008; Pearce et al, 2017)

RESULTS

Findings

DISCUSSION