Natural populations of fruit flies of the Bactrocera dorsalis complex exhibit chromosomal variation based on differences in the amount and distribution of constitutive heterochromatin in the centromeric regions of the autosomes and the sex chromosomes. The chromosomal variation, coupled with differences in external morphology and host plant specific preferences, strongly suggest the existence of 5 closely related species within the B. dorsalis complex that have provisionally been designated B. dorsalis species B, C, D, and E in contrast with B. dorsalis s.s. (species A). Analysis of heterochromatin in autosomes and sex chromosomes has revealed 4 distinct groups of mitotic karyotypes. Bactrocera dorsalis is the only representative of Group I, which is characterized by the typical metacentric X chromosome and major blocks of centromeric heterochromatin in autosomes 5 and 6. Group 2 consists of species B and C, which show prominent landmarks of pericentric heterochromatin in all autosomes and in the X chromosome. Group 3 comprises species D, which is characterized by conspicuous blocks of pericentric heterochromatin in all autosomes but the long arm of the subtelocentric X chromosome is euchromatic and lacks a major portion of centromeric heterochromatin. Species E belongs to Group 4, which differs from Group 3 in having major blocks of heterochromatin at the distal portion of the X chromosome in addition to the prominent landmarks of pericentric heterochromatin in all autosomes. Chromosomal evolution among closely related species within the B. dorsalis complex clearly involves the presence or absence of constitutive heterochromatin in the centromeric regions of autosomes as well as in the X chromosome.

Analysis of mitotic karyotypes of wild specimens of larvae of the Bactmcera tau-like fruit flies (Diptera: Tephritidae) in Thailand has revealed seven distinct chromosome forms, based on the amount and distribution of heterochromatin in sex chromosomes and autosomes. Such cytological differences are perfectly correlated with morphological observations and molecular genetics data. These findings clearly suggest that B. tau is a cluster of at least seven closely related species temporarily designated as species A (= B. tau), B, C, D, E, F and G. On the basis of the gross quantity of heterochromatin accumulation in the genome, three groups of mitotic karyotypes can be recognized. Group 1 comprises species A and E. Species E specifically occurs only in fruits of Shychnos thorelii while species A attacks many kinds of host plants. Group 2 contains species B, F and G. Species B has been found only in Siphonodon celastrineur fruit, whereas species F and G attack the same host species, the medically important plant, Hydnocarpus anthelminthicus, albeit in different localities. Group 3 includes species C and D, each of which comprises larger amounts ofpericentric heterochromatin in all chromosomes than the other two groups. Hence, these two species are cytologically remote from those of groups 1 and 2. Species C and D occur allopatrically but they attack the same host plant species, Momordica c0chinchinenSi.s. Thus, genetic differentiation at the chromosomal level to specific host plant species and geographic isolation seem to play an important role in speciation of members of the B. tau complex. 0 2000 The Linnean Society of London ADDITIONAL KEY WORDS:-Bachocera tau complex -metaphase chromosome heterochromatin -karyotypic evolution.

Summary Wild samples of larvae of 6 new fruit fly species belonging to the Bactrocera dorsalis complex have been temporarily designated as species I, J, K, L, M and N, based on cytological evidence. The flies are also morphologically distinguishable as adults. The larvae of these species seem to specifically infest fruits of different host plants. Cytological examination of larvae of these new species has revealed distinct patterns of metaphase karyotypes on the basis of different amounts and distribution of constitutive heterochromatin in sex chromosomes and autosomes. Thus the mitotic karyotype of species I is somewhat similar to that of B. dorsalis and is classified into Group 1. On the other hand, species L, M and N exhibit mitotic karyotypes of Group 4 showing specific patterns of heterochromatin in the centromeric regions of the X chromosome. Species J and K show a general feature of the mitotic karyotype of Group 3, which exhibits distinctive patterns of heterochromatin in sex chromosomes. These findings support the important role of constitutive heterochromatin in karyotypic evolution of these members of the B. dorsalis complex. Such a gross difference in heterochromatin is also a useful tool in cytotaxonomic study of closely related species of the fruit flies of Thailand.

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Wolbachia are bacteria that infect the reproductive tissues of numerous arthropod species, including tephritid fruit flies. These bacteria are potentially useful for management of tephritid fruit fly pest populations and may also influence tephritid speciation. The extent to which Wolbachia is distributed among tephritid fruit flies is currently unknown. We conducted a PCR-based survey from October 1995 to June 1998 to determine the prevalence and types of Wolbachia present in tephritid fruit flies in Thailand. Infected flies emerged from twenty of 126 fruit and flower collections and comprised 13 (of 46) species, twelve of which were in the genus Bactrocera. However, only 40% of individuals from infected populations, on average, were infected. Further, for infected species collected at numerous locations, less than one-half of locations had infected flies. Finally, 80% of infected collections were taken during just 3 months (October to December 1997) throughout the 33 month long survey. The use of PCR primers revealed that members of both A and B Wolbachia groups were present. They comprised the Aus subgroup (Group A), and the Con, Dei and Pip subgroups (Group B). Wolbachia from flies of nine collections could not be typed and may represent new subgroups, although there are other possibilities. Most flies carried a single subgroup of Wolbachia. Flies of two collections, however, were doubly infected. Our results indicate that Wolbachia prevalence and type varies substantially among and within tephritid fruit fly species. It is, therefore, crucial to understand the effects of these subgroups on tephritids as well as the factors that control Wolbachia prevalence before a role in pest management or speciation can be assessed for these bacteria.