Evidence for Gene Flow between Two Sympatric Mealybug Species (Insecta; Coccoidea; Pseudococcidae)

Abstract

Occurrence of inter-species hybrids in natural populations might be evidence of gene flow between species. In the present study we found evidence of gene flow between two sympatric, genetically related scale insect species – the citrus mealybug Planococcus citri (Risso) and the vine mealybug Planococcus ficus (Signoret). These species can be distinguished by morphological, behavioral, and molecular traits. We employed the sex pheromones of the two respective species to study their different patterns of male attraction. We also used nuclear ITS2 (internal transcribed spacer 2) and mitochondrial COI (Cytochrome c oxidase sub unit 1) DNA sequences to characterize populations of the two species, in order to demonstrate the outcome of a possible gene flow between feral populations of the two species. Our results showed attraction to P. ficus pheromones of all tested populations of P. citri males but not vice versa. Furthermore, ITS2 sequences revealed the presence of ‘hybrid females’ among P. citri populations but not among those of P. ficus. ‘hybrid females’ from P. citri populations identified as P. citri females according to COI sequences. We offer two hypotheses for these results. 1) The occurrence of phenotypic and genotypic traits of P. ficus in P. citri populations may be attributed to both ancient and contemporary gene flow between their populations; and 2) we cannot rule out that an ancient sympatric speciation by which P. ficus emerged from P. citri might have led to the present situation of shared traits between these species. In light of these findings we also discuss the origin of the studied species and the importance of the pherotype phenomenon as a tool with which to study genetic relationships between congener scale insects.

Paternal inheritance in mealybugs (Hemiptera: Coccoidea: Pseudococcidae)

“…High mortality among hybrid male crawlers was also documented in that study (Rotundo and Tremblay 1982). In a previous study, we used pherotype compositions and ITS2 gene sequences to demonstrate the occurrence of gene flow between P. citri and P. ficus among wild populations (Kol-Maimon et al 2014), and the results suggested a directional gene flow from P. ficus to P. citri but not vice versa. We further showed that genetic features of P. ficus were present among P. citri populations but not vice versa.…”

“…Chromatograms showing double peaks among representative type H sequences likewise served as an indication for heterozygosity of those sequences ( Fig. 3; Kol-Maimon et al 2014). By means of the BioPerl software, the probability for each nucleotide to occupy each position in the amplified ITS2 segment (see Tables S1 and S2 for the analysis) was calculated, and the expected proportion-8 %-of single-nucleotide polymorphisms (SNPs) that differentiate P. citri from P. ficus was obtained.…”

“…We further showed that genetic features of P. ficus were present among P. citri populations but not vice versa. Males attracted to LI were also found in P. citri populations from the East Mediterranean but not among those from the West Mediterranean (Kol-Maimon et al 2014).…”

“…ITS2 genotyping was monitored only for the cross between P. ficus and P. citri because no consistent differences between the two subpopulations of P. ficus were found. Segments of ITS2 gene were amplified, purified, and sequenced as recently described by Kol-Maimon et al (2014). The CLUSTALW software (SFI, Dublin, Ireland) was used for multiple sequence alignment with GenBank reference sequences and for specimen genotyping.…”

“…Genetic linkage was demonstrated between the respective pherotypes and development times among P. ficus populations (Kol-Maimon et al 2010). Sequencing of the nuclear internal transcribed spacer 2 (ITS2) of suspected hybrid specimens is frequently used to demonstrate hybridization and gene flow between various organisms, such as plants (Moody and Les 2002), insects (Stevens et al 2002;Kol-Maimon et al 2014), nematodes (La Rosa et al 2003), and sea stars (Wakabayashi et al 2008). Additionally, sequencing segments of the ITS2 gene is often used to distinguish between P. citri and P. ficus (Malausa et al 2010).…”