The Drosophila Eip78C gene is not vital but has a role in regulating chromosome puffs.

We have generated a number of chromosomal aberrations that disrupt the early-late ecdysone-induced 78C puff gene (Eip78C, ecdysone-induced protein, FlyBase name for the E78 gene of Stone and Thummel 1993), which encodes the two members of the nuclear hormone receptor superfamily Eip78C-A and Eip78C-B. The aberrations include deletions of the ligand-binding/dimerization domain of both, inversions that split Eip78C-A but retain residual Eip78C-B expression, and a small deletion specific for Eip78C-B. We find that wild-type Eip78C functions are completely dispensable for normal development under laboratory conditions. However, we show that Eip78C-B is required for the maximal puffing activity of a subset of late puffs (63E and 82F) since these puffs are reduced in size in Eip78C-B mutant backgrounds. Paradoxically the same late puffs are reduced, as well as at least one other, when the Eip78C-B cDNA is overexpressed from a heat shock promoter. These data indicate either that Eip78C function is redundant or that it plays a subtle modulating role in the regulation of chromosome puffing.

Egalitarian and the choice of cell fates in Drosophila melanogaster oogenesis.

Oogenesis in Drosophila females begins with the formation of a cyst of 16 interconnected sister cells, one of which eventually becomes the definitive oocyte. The other 15 become nurse cells, performing nutritive roles for the oocyte. There are four alternative developmental pathways in each cyst: winning pro-oocyte, losing pro-oocyte and cells with three ring canals, all of which enter meiosis, and the twelve obligate pro-nurse cells, which do not. In flies homozygous for the female-sterile mutation egalitarian (egl) all 16 cells follow the same intermediate pathway. All nuclei enter meiosis (shown by their attainment of synaptonemal complex of at least mid-zygotene levels and by their attainment of pachytene numbers and locations of recombination nodules), then all exit it and become morphologically indistinguishable from nurse cells in wild-type flies. The wild-type allele of egl therefore performs two active but opposite roles in cell fate choice. Early on it is necessary for inhibition of meiotic entry in the 'obligate' pro-nurse cells; later it is necessary for meiotic maintenance in the 'winning' pro-oocyte. One can account for both roles from a single function by invoking gradients through the early cyst; the egl+ function appears to be required for the normal function of these gradients.

Stereochemical studies on the formation of melanin by monophenol monooxygenase.

The oxidation of tyrosine by monophenol monooxygenase (tyrosinase: EC 1.10.3.1) to melanin has been studied by a combination of ultraviolet, circular dichroism, and nuclear magnetic resonance techniques. It is demonstrated that the chiral intermediate (dopachrome) is generated stereoselectively in this enzymic reaction.

Are there morphologically abnormal early recombination nodules in the Drosophila melanogaster meiotic mutant mei-218?

Early recombination nodules have been suggested to perform a role in meiotic gene conversion recombination events. The meiotic recombination-defective mutant mei-218 greatly reduces the frequency of meiotic crossover (reciprocal) recombination events and reduces the number of late recombination nodules to the same extent. However, it does not reduce the frequency of simple gene conversion events, although they are abnormal in having shorter coconversion tracts than controls. The original cytological study yielded somewhat fewer early nodules in mei-218 than in controls, although very abnormal ones might have been missed. The present study failed to identify a mei-218 specific abnormal category. However, because recombination nodules are at present recognizable only by their morphology, a definitive answer to this question must await a specific probe for recombination nodules. Moreover, the possibility remains that early nodules in mei-218 are more ephemeral than are early nodules in wild type.

The effect of sequence homozygosity on the frequency of X-chromosomal exchange in Drosophila melanogaster females.

The repair of mismatched heteroduplex DNA has been implicated in the normal resolution of meiotic exchange events. Although sequence microheterogeneity over defined intervals of homologous chromosomes has been correlated with local effects on recombination, this correlation has not previously been extended to effects on chromosomal levels of exchange. In order to determine the role of microheterogeneity in normal exchange between homologs, a system was devised for monitoring exchange between isogenic X chromosomes. Lack of microheterogeneity did not significantly alter the frequency of exchange along the isogenic X chromosomes relative to controls or to previously reported values. There were, however, characteristic levels of exchange intrinsic to the cloned X chromosomes in each of the lines tested.

Recombination nodules and the mechanism of crossing-over in Drosophila.

Meiotic recombination involves a complex sequence of regulated enzymatic and structural functions. These functions and their possible interrelations are discussed using evidence mainly from studies on Drosophila.

Mismatch repair, gene conversion, and crossing-over in two recombination-defective mutants of Drosophila melanogaster.

Recombination-defective mutants at two loci that are known to decrease drastically the frequency of meiotic crossing-over do not decrease the frequency of gene conversion at the rosy locus. mei-9 mutant alleles produce frequent postmeiotic segregants manifested as mosaic progeny whereas controls and mei-218 mutants produce none. It is concluded that (i) recombination in Drosophila involves a biparental DNA intermediate and (ii) correction of heteroduplex DNA or recognition of biparental DNA or both is necessary, but not sufficient, for this intermediate to result in crossing-over of flanking markers. It is therefore likely, at least in Drosophila, that the isomerization step in Meselson-Radding type molecular models of recombination is under genetic control.

On the Control of the Distribution of Meiotic Exchange in DROSOPHILA MELANOGASTER.

The effects of eight recombination-defective meiotic mutants on crossing over within the X heterochromatin were examined. Since none permit substantial frequencies of exchange within heterochromatin although six lessen or abolish constraints on the location of exchanges within euchromatin, the systems that prohibit exchange within heterochromatin and that govern where exchanges will occur in euchromatin are under separate genetic control.-A minor component of the effects of mei-218 is the production of nonhomologous exchanges; of mei-9 is the recovery of deleted chromatids; and of mei-41 is the recovery of deleted chromatids and/or a low frequency of heterochromatic exchanges.

EM autoradiographic evidence that DNA synthesis occurs at recombination nodules during meiosis in Drosophila melanogaster females.

Serial section electron microscopic autoradiography was used to examine the relationship between recombination nodules and 3H-thymidine incorporation during pachytene in Drosophila melanogaster females. For both ellipsoidal and spherical recombination nodules, the number of nodules that are associated with an autoradiographic grain is higher than that expected by chance; this observation is consistent with the hypotheses that recombination involves DNA synthesis and that recombination nodules are the sites of meiotic recombination. Moreover, general DNA replication (S-phase) and synapsis (synaptonemal complex formation) were found to be temporally distinct events, contrary to previous reports; Drosophila females therefore are not exceptional in this regard.

Effects of recombination-deficient and repair-deficient loci on meiotic and mitotic chromosome behavior in Drosophila melanogaster.

The results of recent genetic and cytological studies on recombination-defective and repair-defective mutants of Drosophila melanogaster are summarized. These studies show that there is substantial overlap between the functions used in various aspects of DNA metabolism in Drosophila. Most loci first identified by either recombination-defective or mutagen-sensitive mutants have been shown also to function in nonmutagenized mitotic cells where their action is necessary to maintain the integrity of the genome: mutants at particular loci produce elevated frequencies of chromosome breakage, mitotic exchange, mutation, and/or chromosome loss. Genetic studies of meiotic recombination show that many of the loci identified by recombination-defective mutants restrict where along the chromosome arms exchange may occur. Recent EM studies suggest that the products of at least some of these loci are components of recombination nodules. Region-specific control of DNA metabolism is also indicated by the finding of nonrandom patterns of chromosome breakage in some mutagen-sensitive mutants. Recombination-defective mutants at two loci have been studied for their effects on sister chromatid exchanges (SCEs) and x-ray induced aberrations. Mutants at both loci are defective in steps necessary for the production of symmetrical chromatid interchanges but have little effect on SCEs.

Synaptonemal complex and recombination nodules in wild-type Drosophila melanogaster females.

Electron microscope serial section reconstruction analysis of all zygotene-pachytene nuclei of meiotic cells from three wild-type germaria (a subunit of the ovary containing the early meiotic stages arrayed in temporal developmental sequence) of Drosophila melanogaster females corroborates and extends earlier observations (Carpenter 1975a) on the nature and sequence of ultrastructural events occurring during the time of meiotic recombination. Emphasis has been placed on (1) the time of appearance and disappearance of the synaptonemal complex (SC) and the changes in its dimensions that accompany a cell's progression through pachytene, and (2) the appearance, disappearance, number and chromosomal locations of recombination nodules (Carpenter 1975b). For both the SC and the recombination nodule the availability of several developmental series has provided an estimate of the biological variability in the properties of these recombination-associated structures. The much more extensive data presented here substantiate the earlier hypothesis that recombination nodules occur at sites where reciprocal meiotic recombination will occur, has occurred, or is occurring. A second morphological type of recombination nodule is reported; it is suggested that the presence of the latter type of nodule may correlate with sites of gene conversion. The hypothesis that there may be two types of meiotic recombination processes is discussed.

Recombination nodules and synaptonemal complex in recombination-defective females of Drosophila melanogaster.

The cytological effects of mutant alleles of the mei-9, mei-218, and mei-41 loci during prophase I have been examined by electron microscopy. None of these mutants affect synaptonemal complex structure, continuity, or temporal behavior. Both the precondition-defective mutants mei-218 and mei-41 affect both number and morphology of spherical recombination nodules and apparently affect at least the numbers of ellipsoidal recombination nodules, whereas in the exchange-defective mutant mei-9 the numbers and morphologies of both ellipsoidal and spherical recombination nodules are normal. The parallel effects of mei-218 and mei-41 on meiotic recombination and on recombination nodules indicate that spherical recombination nodules at least mark the site of exchange events; the effects of these mutants on nodule morphology suggest that the nodule performs an active role in the recombination process. The nodule phenotype of mei-9 indicates that spherical nodules are present, and presumably functioning, well before the concluding stages of the recombination event. The parallel effects of all 3 mutants on ellipsoidal and spherical nodules indicate that these are indeed related structures but does not ellucidate the nature of the relationship. It is suggested that all aspects of meiotic recombination are under the aegis of recombination nodules.

The Utilization during Mitotic Cell Division of Loci Controlling Meiotic Recombination and Disjunction in DROSOPHILA MELANOGASTER.

To inquire whether the loci identified by recombination-defective and disjunction-defective meiotic mutants in Drosophila are also utilized during mitotic cell division, the effects of 18 meiotic mutants (representing 13 loci) on mitotic chromosome stability have been examined genetically. To do this, meiotic-mutant-bearing flies heterozygous for recessive somatic cell markers were examined for the frequencies and types of spontaneous clones expressing the cell markers. In such flies, marked clones can arise via mitotic recombination, mutation, chromosome breakage, nondisjunction or chromosome loss, and clones from these different origins can be distinguished. In addition, meiotic mutants at nine loci have been examined for their effects on sensitivity to killing by UV and X rays.-Mutants at six of the seven recombination-defective loci examined (mei-9, mei-41, c(3)G, mei-W68, mei-S282, mei-352, mei-218) cause mitotic chromosome instability in both sexes, whereas mutants at one locus (mei-218) do not affect mitotic chromosome stability. Thus many of the loci utilized during meiotic recombination also function in the chromosomal economy of mitotic cells.-The chromosome instability produced by mei-41 alleles is the consequence of chromosome breakage, that of mei-9 alleles is primarily due to chromosome breakage and, to a lesser extent, to an elevated frequency of mitotic recombination, whereas no predominant mechanism responsible for the instability caused by c(3)G alleles is discernible. Since these three loci are defective in their responses to mutagen damage, their effects on chromosome stability in nonmutagenized cells are interpreted as resulting from an inability to repair spontaneous lesions. Both mei-W68 and mei-S282 increase mitotic recombination (and in mei-W68, to a lesser extent, chromosome loss) in the abdomen but not the wing. In the abdomen, the primary effect on chromosome stability occurs during the larval period when the abdominal histoblasts are in a nondividing (G2) state.-Mitotic recombination is at or above control levels in the presence of each of the recombination-defective meiotic mutants examined, suggesting that meiotic and mitotic recombination are under separate genetic control in Drosophila.-Of the six mutants examined that are defective in processes required for regular meiotic chromosome segregation, four (l(1)TW-6(cs), ca(nd), mei-S332, ord) affect mitotic chromosome behavior. At semi-restrictive temperatures, the cold sensitive lethal l(1)TW-6(cs) causes very frequent somatic spots, a substantial proportion of which are attributable to nondisjunction or loss. Thus, this locus specifies a function essential for chromosome segregation at mitosis as well as at the first meiotic division in females. The patterns of mitotic effects caused by ca(nd), mei-S332, and ord suggest that they may be leaky alleles at essential loci that specify functions common to meiosis and mitosis. Mutants at the two remaining loci (nod, pal) do not affect mitotic chromosome stability.

Genetic controls of meiotic recombination and somatic DNA metabolism in Drosophila melanogaster.

Recombination-defective meiotic mutants and mutagen-sensitive mutants of D. melanogaster have been examined for their effects on meiotic chromosome behavior, sensitivity to killing by mutagens, somatic chromosome integrity, and DNA repair processes. Several loci have been identified that specify functions that are necessary for both meiotic recombination and DNA repair processes, whereas mutants at combination and DNA repair processes, whereas mutants at other loci appear to be defective in only one pathway of DNA processing.

Electron microscopy of meiosis in Drosophila melanogaster females: II. The recombination nodule--a recombination-associated structure at pachytene?

The recombination nodule is a transient structure present during pachytene in intimate association with the synaptonemal complex. The total numbers of these structures per nucleus and their locations along the bivalents correspond to the total numbers and distributions of genetic exchanges. It is suggested that this structure may be involved in the recombination process.