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.

Figure 1. ITS2 sequences amplified from three types of females.

A- Individuals with more than 98% identity with P. citri GenBank references, considered as P. citri. B- Individuals with more than 98% identity with P. ficus GenBank references, considered as P. ficus. H- Individuals with less than 92% identity with P. citri and P. ficus GenBank references, confirmed as hybrids of the two species by cloning sequencing. Black arrows mark double-peak signals indicating the existence of heterozygosity in this region. ITS2 GenBank references: P. ficus: GU134677, JQ085574, HQ852471; P. citri: JF714195. COI GenBank references: P. ficus: JN120845, EU250573, DQ238220; P. citri: AB439517, AF483204.

Figure 2. Pherotype frequency distribution among males whose mothers (gravid females) were sampled from various locations (see Table 1 ).

The males were divided into four groups: (1) F- males – attracted to one or both P. ficus pheromones; (2) C- males – attracted only to P. citri pheromone; (3) FC- males – attracted to P. citri pheromone and also to one or both of P. ficus pheromones; (4) N- males – not attracted to any of the tested pheromones. Unless otherwise specified, the sampled hosts originated from Israel. The numbers preceding the host names are the serial numbers shown in Table 2; the numbers in parentheses are the numbers of tested males per tested population. The species of the populations were determined by COI sequencing and comparison with gene bank references (P. ficus: JN120845, EU250573, DQ238220; P. citri: AB439517, AF483204).