Phenetic structure of two Bactrocera tau cryptic species (Diptera: Tephritidae) infesting Momordica cochinchinensis (Cucurbitaceae) in Thailand and Laos

Morphometric variation with respect to wing venation patterns was explored for 777 specimens of the Bactrocera tau complex collected in Thailand (nine provinces) and Laos (one locality). Cryptic species B. tau A and C were identified based on their wing shape similarity to published reference images. In Thailand, the B. tau A species was identified in four provinces and the B. tau C species in seven provinces, and both species in one locality of Laos. The objective of the study was to explain the geographic variation of size and shape in two cryptic species collected from the same host (Momordica cochinchinensis). Although collected from the same host, the two species did not show the same morphological variance: it was higher in the B. tau A species, which currently infests a wide range of different fruit species, than in the B. tau C species, which is specific to only one fruit (M. cochinchinensis). Moreover, the two species showed a different population structure. An isolation by distance model was apparent in both sexes of species C, while it was not detected in species A. Thus, the metric differences were in apparent accordance with the known behavior of these species, either as a generalist (species A) or as a specialist (species C), and for each species our data suggested different sources of shape diversity: genetic drift for species C, variety of host plants (and probably also pest–host-relationship) for species A. In addition to these distinctions, the larger species, B. tau C, showed less sexual size and shape dimorphism. The data presented here confirm the previously established wing shape differences between the two cryptic species. Character displacement has been discussed as a possible origin of this interspecific variation. The addition of previously published data on species A from other hosts allowed the testing of the character displacement hypothesis. The hypothesis was rejected for interspecific shape differences, but was maintained for size differences.