Characterization of dark and pale forms of frankliniella schultzei (trybom) – an ecological, biological, morphological and molecular approach

Abstract

Frankliniella schultzei Trybom is an important pest of ornamental and vegetable crops worldwide and a vector of Tospoviruses, which are plant viruses belonging to the family Bunyaviridae. It occurs as two color forms (dark and pale) which are morphologically very similar hence has been considered as one species. However, differences in vector competency and geographic spread has led to debate on whether they are one or separate species. Moreover, evidence is lacking on whether these two forms can interbreed to qualify as one species. Hence to ascertain the taxonomic status of the dark and pale forms of Frankliniella schultzei laboratory reared dark and pale forms maintained at icipe were characterized using ecological, biological, morphological and molecular approaches. Differences in ecological preferences among the colour forms were determined by analyzing the host plants and ecological characteristics of the collection sites. Biological characterization was undertaken by assessing the parthenogenetic reproduction strategies of the two color forms and assess their potential to interbreed between colour forms. The two colour forms were also differentiated through morphometrics of character states. Molecular characterisation was undertaken through Restriction fragment length polymorphism (RFLP) analysis and further sequencing of ITS 2 sequences. The results indicated that ecologically, the dark forms were observed predominantly in the western regions of Kenya, while the pale forms predominated the coastal and eastern regions of Kenya. The two colour forms differed in their mode of parthenogenetic reproduction with the pale forms being theletokous while the dark forms being arrhenotokus. The dark male pale female cross consistently resulted only in pale females indicating parthenogenesis and absence of interbreeding between the two colour forms. Pale males were absent in the colony and further field samples of F. schultzei males in different host plants indicated only presence of dark males in the environment. The two colour forms also differed in their morphological features such as length of third ocellar setae, setae on tergite 10 and distance of the third ocellar setae from tangent. The two colour forms exhibited significant molecular differences, by having consistent sequence differences resulting in their clading separately on a phylogenetic tree, with a sequence divergence of up to 23% between the two colour forms. The RFLP banding profile of the two colour forms of F.schultzei showed the two forms having different banding patterns. Considering the distinct differences between the dark and pale colour forms of Frankliniella schultzei for ecological preference, parthenogenetic reproduction, in ability to interbreed, differences in morphological and molecular characteristics, the dark and pale colour forms of F. schultzei needs to be considered as two separate species.