Takao K. Watanabe

Hybrid lethal systems in the Drosophila melanogaster species complex

Lethal phases of the hybrids betweenDrosophila melanogaster and its sibling species,D. simulans are classified into three types: (1) embryonic lethality in hybrids carryingD. simulans cytoplasm andD. melanogaster X chromosome, (2) larval lethality in hybrids not carryingD. simulans X, and (3) temperature-sensitive pupal lethality in hybrids carryingD. simulans X. The same lethal phases are also observed when either of the two other sibling species,D. mauritiana orD. sechellia, is employed for hybridization withD. melanogaster. Here, we describe genetic analyses of each hybrid lethality, and demonstrate that these three types of lethality are independent phenomena. We then propose two models to interpret the mechanisms of each hybrid lethality. The first model is a modification of the conventional X/autosome imbalance hypothesis assuming a lethal gene and a suppressor gene are involved in the larval lethality, while the second model is for embryonic lethality assuming an interaction between a maternal-effect lethal gene and a suppressor gene.

Evolutionary change of the chromosomal polymorphism in drosophila melanogaster populations

Drosophila melanogaster is a chromosomally polymorphic species, a fact that has been confirmed in various natural populations by many workers (Dubinin et al., 1937; Warters, 1944; Ives, 1947; Carson, 1965; Yang and Kojima, 1972; Mukai and Yamaguchi, 1974; Ashburner and Lemeunier, 1976; Stalker, 1976; Choi, 1977; Mettleretal., 1977; Inoue and Watanabe, 1979; Paik, 1979; Zacharopoulou and Pelecanos, 1980; Knibb et al., 1981; Knibb, 1982). Chromosome variations in nature are usually restricted to paracentric inversions on the four arms of the two major autosomes. Other variations such as X-chromosome and pericentric inversions, deficiencies, duplications, translocations and transpositions rarely are found in nature. Inoue and Watanabe (1979) classified the inversions into four groups, using a classification partly modified from that of Mettler et al. (1977): (1) Common cosmopolitan inversions which are the most frequent world-wide inversions on each arm of the major autosomes. (2) Rare cosmopolitan inversions which are distributed world wide but in lower frequency than the common cosmopolitan inversions. Some populations often lack one or two of these inversions. (3) Recurrent endemic inversions which show temporal polymorphisms in some populations. (4) Unique endemic inversions which are observed in a single individual or its brood from a single population and never found in other populations. Designations of the common cosmopolitan inversions are In(2L)t, In(2R)NS, In (3L)P and In(3R)P. The rare cosmopolitan inversions are labelled In(2L)A, In(3L)M, In(3R)C, In(3R)Mo and In(3R)K. In Japan two inversions, In(2L) W and In(3L) Y, are categorized as recurrent endemic inversions. The unique endemic inversions are often reported as rare inversions, having differing break points which probably correspond to recent mutations. Mettler et al. (1977), in their studies of eastern United States populations, demonstrated highly significant negative correlations between the frequencies of the common cosmopolitan inversions and latitude. Stalker (1980) obtained similar results in addition to documenting seasonal changes of some inversions in midwestern and eastern U.S. populations. Recently, Knibb et al. (1981) reported a north-south cline from Australian populations with inversion frequencies increasing toward the equator. These studies suggest that the standard arrangementbearing chromosomes were more adaptive in cooler environments than were the inversion-containing chromosomes. Inoue and Watanabe (1979) compared the inversion frequencies of several Japanese populations showing a north-south dline in inversion In(3R)P. Clines were not always confirmed in other inversions which was attributed to the small number of populations examined. Inoue and Watanabe (1979) also compared several natural populations from the viewpoint of frequency order of the four common cosmopolitan inversions: In order of decreasing frequency the relation 2Lt > 2RNS > 3RP > 3LP was observed in the middle part of Japan in the past (1960s) and still (1970s) in the northern part of Japan. However, the present populations in the middle part of Japan showed the relation of 3RP > 2Lt _ 2RNS > 3LP. The order change in the inversion frequencies occurred

Structural and genetic studies of the proliferation disrupter genes of Drosophila simulans and D. melanogaster

To examine whether structural and functional differences exist in the proliferation disrupter (prod) genes between Drosophila simulans and D. melanogaster, we analyzed and compared both genes. The exon–intron structure of the genes was found to be the same – three exons were interrupted by two introns, although a previous report suggested that only one intron existed in D. melanogaster. The prod genes of D. simulans and D. melanogaster both turn out to encode 346 amino acids, not 301 as previously reported for D. melanogaster. The numbers of nucleotide substitutions in the prod genes was 0.0747u2009±u2009 per synonymous site and 0.0116u2009±u20090.0039 per replacement site, both comparable to those previously known for homologous genes between D. simulans and D. melanogaster. Genetic analysis demonstrated that D. simulans PROD can compensate for a deficiency of D. melanogaster PROD in hybrids. The PRODs of D. simulans and D. melanogaster presumably share the same function and a conserved working mechanism. The prod gene showed no significant interaction with the lethality of the male hybrid between these species.

CHROMOSOMAL POLYMORPHISM IN ISOFEMALE LINES AND CAGE POPULATIONS OF DROSOPHILA MELANOGASTER

Drosophila melanogaster populations in nature usually carry inversion polymorphisms. When they were transferred to and maintained in the laboratory as large cage populations, frequencies of polymorphic inversions were drastically decreased and finally eliminated. This “cage effect” was observed irrespective of the geographical origin of the population or the initial frequency of each inversion. The decrease and elimination of inversions in the cage was not overcome by changing conditions such as medium, temperature, or the number of isofemale lines (40‐600) introduced. On the other hand, in the sets of isofemale lines derived from the same geographical origins as the cage populations, each of which was maintained as a small vial population, the inversion frequencies, though decreased from the initial frequencies, were kept at significantly high levels. The cage populations initiated with one or two isofemale lines also maintained the inversion polymorphisms that were as high as vial populations.