Preliminary results of molecular polymorphism in field populations of Thrips tabaci Lindeman (Thysanoptera:Thripidae), occurring on onion crops in South Italy

Thrips are important direct pest as well as vectors of virus infesting vegetable crops. Thrips tabaci is one of the main insect pests of onion and also a vector of two important plant infecting tospoviruses, Tomato spotted wilt virus (TSWV) and Iris yellow spot virus (IYSV). The latter is a widespread disease infecting onion, leek, iris and wild Allium species. This virus has been recorded from North Italy since 2008 and more recently in the South (Calabria Region). In the last two years, surveys of T. tabaci on onion crops have been carried out on in the southern Region, in order to investigate the genetic variation in the field. Here, the preliminary results of the study are reported. The study was conducted using mitochondrial COI sequences, with a focus on investigating a potential relationship between genetic structure of populations of T. tabaci and the presence of IYSV. Results are based on the molecular analysis of 40 specimens collected from cultivated (Allium cepa) and wild plant hosts (Rosa spp.). The DNA sequences have been obtained by a sequencing reaction with 20 ng of DNA and generated through an Applied Biosystems automated sequencer 3130xl. The COI sequences were aligned and manually edited by the software Sequencher v 4.1 of Gene Codes Corporation. A phylogenetic analysis has been performed by MEGA version 6. The COI specific primer pairs amplified a PCR product of 634 bp long fragment for 38 individuals of T. tabaci. Pair – wise alignment showed that there were variations in 30 nucleotides, amounting to 4.7% differences between the individuals. These variations defined 8 haplotypes. All variation was in form of silent single base-pair substitutions, except for position 1, 93 and 106. More precisely, in position 1 the adenosine is replaced by thymine whereas thymine is replaced by cytosine in the two positions 93 and 106. The exchanged based have been produced replacements in three amino-acids, from isoleucine to phenylanine in the first position and from arginine to cysteine in the last two. Range and mean of intraspecific genetic distances were computed, through the Poisson correction model, of all COI variants belonging to T. tabaci examined individuals. Intraspecific genetic distance ranged from 0.00% to 0.04% (mean 0.02%). Haplotypes 1 and 4 were the most frequent: in particular, we found haplotype 1 either in rose and onion. In contrast, haplotype 4 occurred in onion crop where the virus presence was confirmed.