Morphological and genetic diversity of the new invasive species Bactrocera invadens (Diptera: Tephritidae) in Africa

Abstract

Species of the genus Bactrocera in the family Tephritidae, or 'true fruit flies', are among the most important pests of fruits and vegetables. Because of the broad larval host range and cosmopolitan distribution, pest fruit flies are highly invasive, with adults often exhibiting a strong tendency for dispersal and larvae being readily transported to new areas via fruit movement. Recently, a pest species was recorded for the first time in Kenya and has subsequently been found in countries across tropical Africa. The insect was described as Bactrocera invadens, due to its rapid invasion of the African continent. Taxonomic description of the B. invadens depicted different thoracic colourations that are morphotypes of the same pest. As invasive species spread around the globe, it is becoming increasingly evident that a detailed knowledge of the biology, genetic structure and geographical variability, of a given species is a prerequisite to planning strategies for quarantine, control or eradication. In this study, the morphometry, population structure and the genetic variability of different populations of the B. invadens distributed across the actual species range of tropical Africa and a sample from the presumed aboriginal home of Sri Lanka was investigated. Morphometry using wing veins and tibia lengths were used to separate the morphotypes of the B. invadens and a comparison to other closely related Bactrocera species. Fourteen distances between 15 selected landmarks on the wing were computed to characterize the shape and size of the wings for differentiation. Bactrocera invadens from all the localities analysed could not be separated by both the Principal component and canonical variate analyses. These two parameters separated the B. invadens from B. correcta, B. cucurbitae, B. oleae and B. zonata and it clustered together with B. dorsalis and B. kandiensis. Eleven polymorphic micro satellite loci were isolated and characterized and used for population genetic studies. The polymorphism of these loci was tested in individual flies from two natural populations (Sri Lanka and Democratic Republic of Congo). Allele number per locus ranged from three to 15 and eight loci displayed a polymorphic information content greater than 0.5. These markers were used to address three major issues on 13 geographic populations of the B. invadens, namely: (i) The historical origin of the B. invadens, (ii) the origin of the invasion of the B. invadens in Africa, and (iii) the extent of the establishment of the B. invadens populations in Africa and the genetic variability of these populations. Few years after its discovery in East Africa and its identification as a member of Bactrocera dorsalis complex, B. invadens samples from all equatorial Africa showed a high level of genetic diversity associated with an evident absence of geographic structure. These features are indicative of processes of rapid population growth and expansion with possible multiple introductions. The association of these two aspects reveals that B. invadens was not an indigenous African species that remained undetected for long period of time. Instead this species is a recent and an aggressive invader. Data from the timing of historical records, that indicated this insect as a new entrant in a region, are in this case concordant with the chronology of the spread as documented in this study. DNA barcoding using the COl gene was used to investigate the identity and species integrity of the B. invadens by comparing it with other Bactrocera species such as B. correcta, B. cucurbitae, B. dorsalis, B. kandiensis, B. oleae, B. paraverbascifoliae and B. zonata.