Evidence for redundancy but not trans factor-cis element coevolution in the regulation of Drosophila Yp genes.

In Drosophila melanogaster and the endemic Hawaiian species D. grimshawi three Yolk protein (Yp) genes are expressed in a similar sex- and tissue-specific pattern. In contrast, DNA sequence comparisons of promoter/enhancer regions show low levels of similarity. We tested the functional significance of these observations by transforming D. melanogaster with the genomic region that includes the divergently transcribed D. grimshawi DgYp1 and DgYp2 genes; we found that the introduced genes were expressed in female fat body and in ovaries but not in males. Moreover, we found D. grimshawi proteins in the hemolymph and accumulating in ovaries. Using reporter constructs we showed that the intergenic region from D. grimshawi was sufficient to drive accurate expression, but some low level of ectopic expression was seen in males. Transforming D. melanogaster with constructs bearing deletions within the D. grimshawi intergenic region revealed only subtle effects in the overall level of expression, suggesting a high level of redundancy. Testing mutants in the sex-specific regulator doublesex revealed that it is capable of repressing the DgYp genes in males. Together, these data show that D. melanogaster trans-acting factors can regulate the in vivo pattern of DgYp expression and support the notion of a redundant and complex system of cis-acting elements.

Phylogeny of the island populations of the Hawaiian Drosophila grimshawi complex: evidence from combined data.

The picture-winged species Drosophila grimshawi is unique among Hawaiian Drosophila in its wide geographic range, having populations on several islands of the Hawaiian archipelago. This distribution contrasts with the pattern of single-island endemism observed in most of the picture-winged group; significantly, it does not concur with predictions of the founder theory, where speciation is the typical outcome of founder events involving colonization of a new island. To examine this anomalous situation, we have taken a phylogenetic approach in an attempt to resolve the relationships among taxa and decipher the most probable colonization scenario. We have obtained both morphological and molecular data for all the D. grimshawi populations as well as the closely related species D. pullipes, and two outgroup species, using scanning electron microscopy to score ultrastructural features of the chorion or eggshell, and PCR amplification and nucleotide sequencing to acquire sequence data on Yp1, one of the three Yolk protein genes. In addition, we have used available data on Yolk Protein electrophoretic pattern and jousting, oviposition, and mating behavioral characters. Analyses of these data sets, either individually or in combination, indicate that there are two separate and ecologically distinct clades within this species complex. One clade includes the Kauai and Oahu populations of grimshawi, as well as the closely related species D. pullipes from Hawaii, all of which are classified as ecological specialists with respect to their oviposition and breeding substrate. The other clade includes all the ecologically generalist grimshawi populations of the Maui Nui island complex. The phylogenetic results do not concur with the previously proposed hypothesis that D. pullipes originated from a founder derived from the Maui Nui complex and further suggest that these taxa are in need of taxonomic revision.

Phylogenetic analysis of DNA length mutations in a repetitive region of the Hawaiian Drosophila yolk protein gene Yp2.

Nucleotide sequence analysis has demonstrated that interspecific size variation in the YP2 yolk protein among Hawaiian Drosophila is due to in-frame insertions and deletions in two repetitive segments of the coding region of the Yp2 gene. Sequence comparisons of the complex repetitive region close to the 5' end of this gene across 34 endemic Hawaiian taxa revealed five length morphs, spanning a length difference of 21 nucleotides (nt). A phylogenetic character reconstruction of the length mutations on an independently derived molecular phylogeny showed clade-specific length variants arising from six ancient events: two identical insertions of 6 nt, and four deletions, one of 6 nt, one of 12 nt, and two identical but independent deletions of 15 nt. These mutations can be attributed to replication slippage with nontandem trinucleotide repeats playing a major role in the slipped-strand mispairing. Geographic analysis suggests that the 15 nt deletion which distinguishes the planitibia subgroup from the cyrtoloma subgroup occurred on Oahu about 3 million years ago. The homoplasies observed caution against relying too heavily on nucleotide insertions/deletions for phylogenetic inference. In contrast to the extensive repeat polymorphisms within other Drosophila and the human species, the more complex 5' Yp2 repetitive region analyzed here appears to lack polymorphism among Hawaiian Drosophila, perhaps due to founder effects, low population sizes, and hitchhiking effects of selection on the immediately adjacent 5' region.

Pattern of ecological shifts in the diversification of Hawaiian Drosophila inferred from a molecular phylogeny.

BACKGROUND: The endemic Hawaiian drosophilids, a unique group that are remarkable for their diversity and rapid proliferation, provide a model for analysis of the process of insular speciation. Founder events and accompanying random drift, together with shifts in sexual selection, appear to explain the dramatic divergence in male morphology and mating behaviour among these flies, but these forces do not account for their spectacular ecological diversification into a wide array of breeding niches. Although recognized as contributing to the success of this group, the precise role of adaptive shifts has not been well defined. RESULTS: To delineate the pattern of ecological diversification in the evolution of Hawaiian Drosophila, we generated a molecular phylogeny, using nucleotide sequences from the yolk protein gene Yp1, of 42 endemic Hawaiian and 5 continental species. By mapping ecological characters onto this phylogeny, we demonstrate that monophagy is the primitive condition, and that decaying leaves were the initial substrate for oviposition and larval development. Shifts to decaying stems, bark and tree fluxes followed in more derived species. By plotting female reproductive strategies, as reflected in ovarian developmental type, on the molecular tree, we also demonstrate a phylogenetic trend toward increasing fecundity. We find some statistical support for correlations between ecological shifts and shifts in female reproductive strategies. CONCLUSIONS: Because of the short branches at the base of the phylogram, which lead to ecologically diverse lineages, we conclude that much of the adaptive radiation into alternate breeding substrates occurred rapidly, early in the group's evolution in Hawaii. Furthermore, we conclude that this ecological divergence and the correlated changes in ovarian patterns that adapt species to their ecological habitats were contributing factors in the major phyletic branching within the Hawaiian drosophilid fauna.

Vitellogenin protein diversity in the Hawaiian Drosophila.

Egg and female hemolymph proteins were resolved via SDS-polyacrylamide gel electrophoresis in a diverse array of 33 endemic Hawaiian drosophilids, encompassing 17 picture-winged species, 3 of the antopocerus species group, 9 fungus feeders, 1 species from each of the modified mouthparts, crassifemur and ciliated tarsus groups, and 1 Scaptomyza species. Molecular weights of the two (10 species) or three vitellogenin bands (22 species) were highly variable, spanning a 7-kD range. The largest vitellogenin, V1, was the most variable, showing a change of some 10% in its mean size of 47.6 kD. The smallest V3 vitellogenin, mean size 44.1 kD, was evolutionarily the most conservative in size. The species Drosophila hawaiiensis was found to be polymorphic for two/three vitellogenin bands and, also, polymorphic with respect to the size of the V1 protein. No inter- or intrapopulation variability in vitellogenin size was detected in 10 other species examined. The major features of vitellogenin protein evolution in the Hawaiian Drosophila are change in molecular weight and regulatory differences that result in quantitative differences between species in patterns of vitellogenin protein production.

The temporal pattern of vitellogenin synthesis in Drosophila grimshawi.

The temporal pattern of protein production and, in particular, vitellogenin protein synthesis during the sexual maturation of Drosophila grimshawi females has been studied in vivo by briefly feeding the flies with 35S-methionine and 3H-amino acids. The overall level of incorporation was very low in young flies; it then progressively increased to reach a maximum with the onset of sexual maturity at 13-15 days. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analyses revealed three classes of proteins: those synthesized throughout the age spectrum, which constitute the majority of protein species; proteins synthesized primarily or only in young flies; and proteins synthesized only by the older flies. In this Drosophila species, the three vitellogenins (V1, V2, and V3) appeared to be synthesized in a two-phase pattern. In the first phase, small quantities of V1 and V2 were detected immunologically in the fat body and hemolymph of newly emerged and 1 day-old flies. These proteins did not accumulate in the hemolymph or the ovaries, apparently being unstable proteins. The second phase commenced in early vitellogenesis (7-9 days of age) with synthesis in the fat body of small quantities of V1 and V2, followed by V3 proteins. These proteins were secreted and accumulated in the hemolymph and 24 h later were found in the ovaries. Their quantities increased rapidly and a steady state of synthesis, release into the hemolymph, and uptake by the ovaries was reached by days 13-15. We have estimated that during the steady state of vitellogenin synthesis, a fly can synthesize in 24 h at least 152 micrograms of vitellogenins, which is more than 2% of its body weight, at an average rate of about 6.3 micrograms vitellogenins/h. About 2 micrograms of this are synthesized in the fat body, and about 4 micrograms in the ovaries. These findings are discussed in terms of their physiological implications and contrasted with the available data on Drosophila melanogaster.

DNA length variants contiguous to the 3' end of a vitellogenin gene in Drosophila grimshawi laboratory stocks from different Hawaiian Islands.

Two V3 vitellogenin clones isolated from genomic libraries of Drosophila grimshawi (G1, Auwahi, Maui) were found to differ in length. Structural comparison of the two clones established that the length difference could be attributed to two insertions/deletions of about 200 bp each, both within the 3' flanking sequences of the gene. The two length variants appeared to be polymorphic in the G1 laboratory strain, as demonstrated by analysis of genomic DNA isolated from single flies. The deleted variant sequence was traced by further analysis to two other D. grimshawi strains (PK9 and S10G1) which originated from the island of Molokai. The existence of this morph in the Maui strain appears to have resulted from a laboratory stock contamination at the Drosophila Stock Center. In the course of a few generations of culture of this G1 strain at New York University, the deleted morph increased its frequency surprisingly rapidly, almost replacing the original morph, while at the Bowling Green Stock Center, the original morph still predominates. These frequency changes are most likely consequences of genetic drift due to bottlenecks in the maintenance and propagation of this stock.

Comparative biochemical and immunological analysis of the three vitellogenins from Drosophila grimshawi.

1. The three female-specific vitellogenin proteins, namely V1, V2 and V3, have been isolated and characterized from Drosophila grimshawi. Their mol. wt, as determined by SDS-polyacrylamide gel electrophoresis are 46,000, 45,000 and 43,000 which are in agreement with those determined by Ferguson plot analysis. 2. All three vitellogenins appear to be monomers in the ovarian extracts and they have very similar biochemical and immunological properties. 3. Ion-exchange chromatography, double immunodiffusion tests and partial digestion with Staphylococcus aureus V8 protease indicated more physicochemical and structural similarities between the V1 and the V2 polypeptides. 4. The distribution pattern of the proteolytic polypeptides resulting from limited chymotrypsin digestion suggested partial homology in the primary structure of the three vitellogenin proteins.

Differential and temporal expression of the vitellogenin genes in Drosophila grimshawi.

The three vitellogenin transcripts from Drosophila grimshawi have an average size of 1,600 nucleotides as determined using denaturing electrophoretic conditions. Southern analysis showed that the large quantity of vitellogenin mRNAs in adult female fat body cells is not a reflection of specific gene amplification. The quantitative differences in mRNA accumulation between fat body and follicle cells, which are in concert with the onset of their translation, indicate that vitellogenin synthesis entails the regulated expression of individual genes. The expression of the vitellogenin genes during follicle development is stage specific: V1 and V2 expression starts at late stage 7, while V3 is delayed by one stage. Maximum transcription of all three genes occurs at stage 10 whereas at stage 12 none of the transcripts is present. These results suggest that, either there is more than one regulatory signal, or there is one to which each gene reacts differently. Surprisingly, in male fat body cells a V2 transcript has been detected which is also present in the poly(A)+RNA fraction: the function and the purpose of this particular vitellogenin mRNA in male fat body cells are unknown. Neither of the other two vitellogenin transcripts have been detected in male fat body cells.

Isolation and structural analysis of Drosophila grimshawi vitellogenin genes.

We isolated recombinant genomic DNA clones containing sequences coding for the female-specific vitellogenin proteins of Drosophila grimshawi. By screening with cDNA vitellogenin clones derived from female fat body mRNA we were able to isolate all three genes, namely V1, V2 and V3. The identity of these genes was established first by cell-free translation of the hybrid-selected mRNA followed by protease digestion of the in vitro translation products and second by hybridization of the three genes to electrophoretically separated mRNAs. The transcriptional orientation of these genes was determined. The V1 and V2 genes have opposite orientations with their 5'-ends 1.75 kb apart. S1 analysis demonstrated that the V1 gene has three exons of 310, 400 and 980 bp in length and two introns of about 120 bp. The V2 gene has two exons of 300 and 1260 bp in length and an intron 100 bp long. The V3 gene has three exons of 250, 375 and 820 bp in length and two introns of about 120 bp. The homology, in both sequence and structure, of the vitellogenin genes indicates that they have arisen by duplication events from an ancestral gene. Moreover, the similarity of the V1 and V2 gene positions within the genome of the two distant species D. melanogaster and D. grimshawi suggests a functional coupling of these two genes during vitellogenin gene expression.

Noncoordinate synthesis of the vitellogenin proteins in tissues of Drosophila grimshawi.

In analyzing the in vitro pattern of protein synthesis by the fat body and ovaries of the Hawaiian species Drosophila grimshawi, we have found that the ovaries synthesize much more protein than the female fat body and that the majority of the synthesized proteins are retained by the ovarian tissues. In contrast, the fat body secrets most of the proteins into the culture medium. Vitellogenins are the major class of proteins synthesized and released into the medium by both tissues. The synthesis of the three vitellogenin proteins (V1, V2, V3) is noncoordinate in the two tissues. Ovaries synthesize much more of the V2 protein, less V1 and very little V3, whereas fat body synthesizes more V1 protein with lesser quantities of the other two. The follicle cells were identified as the site of ovarian vitellogenin synthesis in D. grimshawi, confirming the findings in D. melanogaster. In D. grimshawi, the three vitellogenins are synthesized by the follicle cells in a noncoordinate and developmentally regulated manner. V2 and V1 are the predominant proteins at the onset of vitellogenesis (S8-9); their production peaks together with that of V3 a few hours later (S10) and then decreases to quantities equal to that of V3 during early choriogenesis (S11). During active choriogenesis (S12), V2 and V1 cease to be synthesized, but V3 synthesis continues. The vitellogenins synthesized by the follicles in vitro are released into the medium and not incorporated into the oocyte.

Spermatogenesis in cultured testes of the cynthia silkworm: effects of ecdysone and of prothoracic glands.

In vitro spermatogenesis takes place when intact testes are cultured in blood plasma containing ecdysone or certain other steroids possessing ecdysone activity. The ecdysone requirement can be satisfied by culturing the testes in the presence of living, active prothoracic glands. The most likely explanation of these results is that the prothoracic glands constitute the principal source of ecdysone.