Metaphase karyotypes of fruit flies of Thailand. V. Cytotaxonomy of ten additional new species of the Bactrocera dorsalis complex

Abstract

Analysis of mitotic chromosomes, with special emphasis on the amount and distribution of constitutive heterochromatin, from larval samples of Thai populations of flies in the Bactrocera dorsalis complex has revealed 10 distinctive forms of metaphase karyotypes. This cytological evidence coupled with morphological observations of adults and host relationships with different host plant species have suggested the existence of 10 new cytotaxonomic species provisionally named species Q-Z belonging to the B. dorsalis complex. Metaphase karyotypes of Thai fruit flies were previously classified into 4 groups based on the quantitative differences in pericentric heterochromatin. Metaphase karyotypes of species Q, R, W, X, Y and Z are characterized by the presence of prominent blocks of pericentric heterochromatin in the X chromosomes and autosomes similar to the mitotic patterns found in B. kanchanaburi and B. raiensis. Hence, they are placed in Group 2 of this classification. Species U and V exhibit chromosome X patterns similar to that of B. carambolae and other related species. They are thus assigned to Group 4. In contrast, species S shows a minimum amount of pericentric heterochromatin in all autosomes, although the X chromosome follows the pattern of species Y. Interestingly, species T exhibits a unique pattern of euchromatin and heterochromatin in the X chromosome and conspicuous pericentric heterochromatin in all autosomes. Thus, species S and T are assigned to 2 new categories. Heterochromatin accumulations in the genome, as demonstrated in this study, appear to have played a significant role in chromosomal evolution. The different degree of variation in pericentric heterochromatin in mitotic chromosomes is, therefore, useful for cytotaxonomically distinguishing members of some cryptic species complexes of the dipteran insects.