Fourth International Bemisia Workshop International Whitefly Genomics Workshop
Positive evidence for interbreeding and differential gene flow between three well characterized biotypes of the Bemisia Tabaci complex (Gennadius) (Hemiptera: Aleyrodidae) excludes geographic and host barriers as isolating factors
1 Department of Entomology and
2 Department of Plant Sciences, The University of Arizona, Tucson, AZ USA. Correspondence: jbrown@ag.arizona.edu
Reciprocal crosses were carried out with single pairs or groups of males and females for three well-characterized biological types (biotypes) of the whitefly Bemisia tabaci. Two polyphagous biotypes, the polyphagous Arizona A biotype (AzA) originating in the Western U.S.A., the polyphagous B biotype (AzB) from the Eastern Hemisphere, and the monophagous Jatropha (Jat) biotype from Puerto Rico, were used in crossing experiments (27°C, 80% humidity, 12:12 hr photoperiod). Reciprocal and homologous crosses were established between single pairs or groups of 20 virgin females and males. All crosses employed cotton as the oviposition host except for Jat female x AzA or AZB male crosses, for which Jatropha gossypifolia (Euphorbiaceae) was the host. Whitefly pairs and groups were caged together on the respective oviposition host for 7 and 10 days, respectively. Genetic identification of parents and selected F1 and F2 offspring was carried out using the maternally inherited mitochondria cytochrome oxidase I sequence. All homologous crosses yielded female offspring. Female offspring were produced in all but two reciprocal crosses. These exceptions were AzB♀ x AzA♂, or AzB♀x Jat♂, for which female offspring were produced in a single direction. Because this distinctive pattern could be attributable to endosymbiont-induced incompatibility, whitefly colonies were screened for bacterial presence using PCR and 16S rRNA primers revealing that the AZA biotype harbored Cardinium, and that a Wolbachia spp. was associated with Jat. That Wolbachia and Cardinium are implicated in cytoplasmic incompatibility (CI) and/or other types of reproductive incompatibility makes this finding intriguing, and so their discovery has been the subject of a follow-on study. These results provide the first evidence for gene flow between monophagous and polyphagous New World biotypes (A, Jat, respectively), and between an Old World (B) and two New World (A, Jat) biotypes, indicating that geographic and host barriers do not mandate reproductive isolation. These results challenge the proposed classification of the B biotype as a species unique from all other B. tabaci. Based on the biological species concept, reproductively compatible organisms that produce viable offspring would be considered the same species. Several lines of evidence, including the results presented here, suggest that B. tabaci constitutes a single group of related variants, of which some are reproductively compatible while others are not. Also, certain variants are recognizable by definitive sets of (adaptive) behaviors, and are referred to as biotypes, a designation that has contributed to taxonomic confusion. Even so, results indicate that gene flow occurs between three of these B. tabaci variants, in this instance ‘biotypes’, which have experienced prolonged physical separations imposed by geography and/or host plant. It is not known whether the A, B, Jat, or other biotypes and haplotypes have hybridized in nature and/or if so, whether they are naturalized. Hybridization may more commonly be employed as a means of diversification in this species than previously thought.