Experimental taphonomy of giant sulphur bacteria: implications for the interpretation of the embryo-like Ediacaran Doushantuo fossils.

The Ediacaran Doushantuo biota has yielded fossils interpreted as eukaryotic organisms, either animal embryos or eukaryotes basal or distantly related to Metazoa. However, the fossils have been interpreted alternatively as giant sulphur bacteria similar to the extant Thiomargarita. To test this hypothesis, living and decayed Thiomargarita were compared with Doushantuo fossils and experimental taphonomic pathways were compared with modern embryos. In the fossils, as in eukaryotic cells, subcellular structures are distributed throughout cell volume; in Thiomargarita, a central vacuole encompasses approximately 98 per cent cell volume. Key features of the fossils, including putative lipid vesicles and nuclei, complex envelope ornament, and ornate outer vesicles are incompatible with living and decay morphologies observed in Thiomargarita. Microbial taphonomy of Thiomargarita also differed from that of embryos. Embryo tissues can be consumed and replaced by bacteria, forming a replica composed of a three-dimensional biofilm, a stable fabric for potential fossilization. Vacuolated Thiomargarita cells collapse easily and do not provide an internal substrate for bacteria. The findings do not support the hypothesis that giant sulphur bacteria are an appropriate interpretative model for the embryo-like Doushantuo fossils. However, sulphur bacteria may have mediated fossil mineralization and may provide a potential bacterial analogue for other macroscopic Precambrian remains.

A light and electron microscopical study of the spermathecae and ventral receptacle of Anastrepha suspensa (Diptera: Tephritidae) and implications in female influence of sperm storage.

Female insects with multiple sperm storage organs may potentially influence patterns of paternity by differential storage of sperm from competing males. The Caribbean Fruit Fly, Anastrepha suspensa, stores sperm differentially with respect to its three spermathecae. To understand the mechanisms and processes responsible for patterns of sperm storage and use in A. suspensa, details of the fine structure of female sperm storage organs were resolved by UV-light microscopy, confocal microscopy, tissue sectioning, and scanning and transmission electron microscopy. Structures not previously described for this species include a ventral receptacle for sperm storage and osmoregulation, a conical-shaped valve at the junction between the spermathecal capsules and their ducts, laminar and granular secretions, secretions from the signum, hemocytes surrounding the spermathecae, and spermathecae with sclerotized, hollow projections that terminate in single glandular cells. The independent organization of sperm storage organs, spermathecal ducts, associated musculature, gland cells, and innervation offer possible mechanisms by which sperm movement may be influenced by females. The implications of these structures for insemination and fertilization events are discussed.

Tubulin sorting during dimerization in vivo.

We demonstrate sorting of beta-tubulins during dimerization in the Drosophila male germ line. Different beta-tubulin isoforms exhibit distinct affinities for alpha-tubulin during dimerization. Our data suggest that differences in dimerization properties are important in determining isoform-specific microtubule functions. The differential use of beta-tubulin during dimerization reveals structural parameters of the tubulin heterodimer not discernible in the resolved three-dimensional structure. We show that the variable beta-tubulin carboxyl terminus, a surface feature in the heterodimer and in microtubules, and which is disordered in the crystallographic structure, is of key importance in forming a stable alpha-beta heterodimer. If the availability of alpha-tubulin is limiting, alpha-beta dimers preferentially incorporate intact beta-tubulins rather than a beta-tubulin missing the carboxyl terminus (beta 2 Delta C). When alpha-tubulin is not limiting, beta 2 Delta C forms stable alpha-beta heterodimers. Once dimers are formed, no further sorting occurs during microtubule assembly: alpha-beta 2 Delta C dimers are incorporated into axonemes in proportion to their contribution to the total dimer pool. Co-incorporation of beta 2 Delta C and wild-type beta 2-tubulin results in nonmotile axonemes because of a disruption of the periodicity of nontubulin axonemal elements. Our data show that the beta-tubulin carboxyl terminus has two distinct roles: 1) forming the alpha-beta heterodimer, important for all microtubules and 2) providing contacts for nontubulin components required for specific microtubule structures, such as axonemes.

Axoneme-specific beta-tubulin specialization: a conserved C-terminal motif specifies the central pair.

Axonemes are ancient organelles that mediate motility of cilia and flagella in animals, plants, and protists. The long evolutionary conservation of axoneme architecture, a cylinder of nine doublet microtubules surrounding a central pair of singlet microtubules, suggests all motile axonemes may share common assembly mechanisms. Consistent with this, alpha- and beta-tubulins utilized in motile axonemes fall among the most conserved tubulin sequences [1, 2], and the beta-tubulins contain a sequence motif at the same position in the carboxyl terminus [3]. Axoneme doublet microtubules are initiated from the corresponding triplet microtubules of the basal body [4], but the large macromolecular "central apparatus" that includes the central pair microtubules and associated structures [5] is a specialization unique to motile axonemes. In Drosophila spermatogenesis, basal bodies and axonemes utilize the same alpha-tubulin but different beta-tubulins [6--13]. beta 1 is utilized for the centriole/basal body, and beta 2 is utilized for the motile sperm tail axoneme. beta 2 contains the motile axoneme-specific sequence motif, but beta 1 does not [3]. Here, we show that the "axoneme motif" specifies the central pair. beta 1 can provide partial function for axoneme assembly but cannot make the central microtubules [14]. Introducing the axoneme motif into the beta 1 carboxyl terminus, a two amino acid change, conferred upon beta 1 the ability to assemble 9 + 2 axonemes. This finding explains the conservation of the axoneme-specific sequence motif through 1.5 billion years of evolution.

Embryonic expression of the divergent Drosophila beta3-tubulin isoform is required for larval behavior.

We have sought to define the developmental and cellular roles played by differential expression of distinct beta-tubulins. Drosophila beta3-tubulin (beta3) is a structurally divergent isoform transiently expressed during midembryogenesis. Severe beta3 mutations cause larval lethality resulting from failed gut function and consequent starvation. However, mutant larvae also display behavioral abnormalities consistent with defective sensory perception. We identified embryonic beta3 expression in several previously undefined sites, including different types of sensory organs. We conclude that abnormalities in foraging behavior and photoresponsiveness exhibited by prelethal mutant larvae reflect defective beta3 function in the embryo during development of chordotonal and other mechanosensory organs and of Bolwig's organ and nerve. We show that microtubule organization in the cap cells of chordotonal organs is altered in mutant larvae. Thus transient zygotic beta3 expression has permanent consequences for the architecture of the cap cell microtubule cytoskeleton in the larval sensilla, even when beta3 is no longer present. Our data provide a link between the microtubule cytoskeleton in embryogenesis and the behavioral phenotype manifested as defective proprioreception at the larval stage.

Conserved axoneme symmetry altered by a component beta-tubulin.

Ninefold microtubule symmetry of the eukaryotic basal body and motile axoneme has been long established [1-3]. In Drosophila, these organelles contain distinct but similar beta-tubulin isoforms [4-10]: basal bodies contain only beta1-tubulin, and only beta2-tubulin is used for assembly of sperm axonemes. A single alpha-tubulin functions throughout spermatogenesis [11,12]. Thus, differences in organelle assembly reside in beta-tubulin. We tested the ability of beta1 to function in axonemes and found that beta1 alone could not generate axonemes. Small sequence differences between the two isoforms therefore mediate large differences in assembly capacity, even though these two related organelles have a common evolutionarily ancient architecture. In males with equal beta1 and beta2, beta1 was co-incorporated at equimolar ratio into functional sperm axonemes. When beta1 exceeded beta2, however, axonemes with 10 doublets were produced, an alteration unprecedented in natural phylogeny. Addition of the tenth doublet occurred by a novel mechanism, bypassing the basal body. It has been assumed that the instructions for axoneme morphogenesis reside primarily in the basal body, which normally serves as the axonemal template. Our data reveal that beta-tubulin requirements for basal bodies and axonemes are distinct, and that key information for axoneme architecture resides in the axonemal beta-tubulin.

A transient specialization of the microtubule cytoskeleton is required for differentiation of the Drosophila visual system.

Drosophila beta3-tubulin is an essential isoform expressed during differentiation of many cell types in embryos and pupae. We report here that during pupal development transient beta3 expression demarcates a unique subset of neurons in the developing adult visual system. beta3 is coassembled into microtubules with beta1, the sole beta-tubulin isoform in the permanent microtubule cytoskeleton of the adult eye and brain. Examination of beta3 mutant phenotypes showed that beta3 is required for axonal patterning and connectivity and for spatial positioning within the optic lobe. Comparison of the phenotypes of beta3 mutations with those that result from disruption of the Hedgehog signaling pathway shows that beta3 functions early in the establishment of the adult visual system. Our data support the hypothesis that beta3 confers specialized properties on the microtubules into which it is incorporated. Thus a transient specialization of the microtubule cytoskeleton during differentiation of a specific subset of the neurons has permanent consequences for later cell function.

Clonal tests of conventional kinesin function during cell proliferation and differentiation.

Null mutations in the Drosophila Kinesin heavy chain gene (Khc), which are lethal during the second larval instar, have shown that conventional kinesin is critical for fast axonal transport in neurons, but its functions elsewhere are uncertain. To test other tissues, single imaginal cells in young larvae were rendered null for Khc by mitotic recombination. Surprisingly, the null cells produced large clones of adult tissue. The rates of cell proliferation were not reduced, indicating that conventional kinesin is not essential for cell growth or division. This suggests that in undifferentiated cells vesicle transport from the Golgi to either the endoplasmic reticulum or the plasma membrane can proceed at normal rates without conventional kinesin. In adult eye clones produced by null founder cells, there were some defects in differentiation that caused mild ultrastructural changes, but they were not consistent with serious problems in the positioning or transport of endoplasmic reticulum, mitochondria, or vesicles. In contrast, defective cuticle deposition by highly elongated Khc null bristle shafts suggests that conventional kinesin is critical for proper secretory vesicle transport in some cell types, particularly ones that must build and maintain long cytoplasmic extensions. The ubiquity and evolutionary conservation of kinesin heavy chain argue for functions in all cells. We suggest interphase organelle movements away from the cell center are driven by multilayered transport mechanisms; that is, individual organelles can use kinesin-related proteins and myosins, as well as conventional kinesin, to move toward the cell periphery. In this case, other motors can compensate for the loss of conventional kinesin except in cells that have extremely long transport tracks.

A novel ontogenetic pathway in hybrid embryos between species with different modes of development.

To investigate the bases for evolutionary changes in developmental mode, we fertilized eggs of a direct-developing sea urchin, Heliocidaris erythrogramma, with sperm from a closely related species, H. tuberculata, that undergoes indirect development via a feeding larva. The resulting hybrids completed development to form juvenile adult sea urchins. Hybrids exhibited restoration of feeding larval structures and paternal gene expression that have been lost in the evolution of the direct-developing maternal species. However, the developmental outcome of the hybrids was not a simple reversion to the paternal pluteus larval form. An unexpected result was that the ontogeny of the hybrids was distinct from either parental species. Early hybrid larvae exhibited a novel morphology similar to that of the dipleurula-type larva typical of other classes of echinoderms and considered to represent the ancestral echinoderm larval form. In the hybrid developmental program, therefore, both recent and ancient ancestral features were restored. That is, the hybrids exhibited features of the pluteus larval form that is present in both the paternal species and in the immediate common ancestor of the two species, but they also exhibited general developmental features of very distantly related echinoderms. Thus in the hybrids, the interaction of two genomes that normally encode two disparate developmental modes produces a novel but harmonious ontongeny.

Drosophila centrosomin protein is required for male meiosis and assembly of the flagellar axoneme.

Centrosomes and microtubules play crucial roles during cell division and differentiation. Spermatogenesis is a useful system for studying centrosomal function since it involves both mitosis and meiosis, and also transformation of the centriole into the sperm basal body. Centrosomin is a protein localized to the mitotic centrosomes in Drosophila melanogaster. We have found a novel isoform of centrosomin expressed during spermatogenesis. Additionally, an anticentrosomin antibody labels both the mitotic and meiotic centrosomes as well as the basal body. Mutational analysis shows that centrosomin is required for spindle organization during meiosis and for organization of the sperm axoneme. These results suggest that centrosomin is a necessary component of the meiotic centrosomes and the spermatid basal body.

Structurally similar Drosophila alpha-tubulins are functionally distinct in vivo.

We used transgenic analysis in Drosophila to compare the ability of two structurally similar alpha-tubulin isoforms to support microtubule assembly in vivo. Our data revealed that even closely related alpha-tubulin isoforms have different functional capacities. Thus, in multicellular organisms, even small changes in tubulin structure may have important consequences for regulation of the microtubule cytoskeleton. In spermatogenesis, all microtubule functions in the postmitotic male germ cells are carried out by a single tubulin heterodimer composed of the major Drosophila alpha-84B tubulin isoform and the testis-specific beta 2-tubulin isoform. We tested the ability of the developmentally regulated alpha 85E-tubulin isoform to replace alpha 84B in spermatogenesis. Even though it is 98% similar in sequence, alpha 85E is not functionally equivalent to alpha 84B. alpha 85E can support some functional microtubules in the male germ cells, but alpha 85E causes dominant male sterility if it makes up more than one-half of the total alpha-tubulin pool in the spermatids. alpha 85E does not disrupt meiotic spindle or cytoplasmic microtubules but causes defects in morphogenesis of the two classes of singlet microtubules in the sperm tail axoneme, the central pair and the accessory microtubules. Axonemal defects caused by alpha 85E are precisely reciprocal to dominant defects in doublet microtubules we observed in a previous study of ectopic germ-line expression of the developmentally regulated beta 3-tubulin isoform. These data demonstrate that the doublet and singlet axoneme microtubules have different requirements for alpha- and beta-tubulin structure. In their normal sites of expression, alpha 85E and beta 3 are coexpressed during differentiation of several somatic cell types, suggesting that alpha 85E and beta 3 might form a specialized heterodimer. Our tests of different alpha-beta pairs in spermatogenesis did not support this model. We conclude that if alpha 85E and beta 3 have specialized properties required for their normal functions, they act independently to modulate the properties of microtubules into which they are incorporated.

Microtubule architecture specified by a beta-tubulin isoform.

In Drosophila melanogaster, a testis-specific beta-tubulin (beta2) is required for spermatogenesis. A sequence motif was identified in carboxyl termini of axonemal beta-tubulins in diverse taxa. As a test of whether orthologous beta-tubulins from different species are functionally equivalent, the moth Heliothis virescens beta2 homolog was expressed in Drosophila testes. When coexpressed with beta2, the moth isoform imposed the 16-protofilament structure characteristic of that found in the moth on the corresponding subset of Drosophila microtubules, which normally contain only 13-protofilament microtubules. Thus, the architecture of the microtubule cytoskeleton can be directed by a component beta-tubulin.

Genetic analysis of the Drosophila beta3-tubulin gene demonstrates that the microtubule cytoskeleton in the cells of the visceral mesoderm is required for morphogenesis of the midgut endoderm.

We have investigated the cellular basis for lethality of mutant alleles of the Drosophila melanogaster beta3-tubulin gene, betaTub60D. Lethal beta3 mutations can be grouped into two classes: the most severe mutations (Class I alleles) cause death during the first larval instar, while weaker alleles (Class II) cause death in later larval stages or in early pupal development. Since beta3 is not expressed during larval development, lethality of the Class I mutations must reflect essential functions of beta3 in embryogenesis. Beta3-tubulin is zygotically expressed during midembryogenesis in the developing mesoderm, and the major site of beta3 accumulation is in the developing muscles during myogenesis. We show that the embryonic pattern of beta3 expression, including accumulation in the developing musculature, is conserved in other Drosophila species. However, we found that loss of beta3 function does not cause discernible defects in either the ultrastructure or function of the larval muscle. Thus beta3-tubulin is dispensable in its highest site of accumulation. Rather, the essential site of function of beta3 in embryos is in cells of the visceral mesoderm. Lethality of Class I alleles is caused by defects in midgut morphogenesis and failure of gut function. Although the folding pattern is irregular and the gut is smaller than normal, a complete folded gut forms in mutant larvae, and the visceral muscle functions normally to move food through the gut. However, mutant larvae cannot absorb nutrients across the gut wall. Thus loss of beta3 function in the mesoderm results in defects in the underlying endodermally derived layer of the gut. Our data provide an assay for cellular interactions between mesoderm and endodermal tissues and reveal a role for the microtubule cytoskeleton of the visceral mesodermal cells in differentiation of the endodermal cell layer of the larval gut.

Pathway analysis of radiation-sensitive meiotic mutants of Coprinus cinereus.

We have isolated 37 radiation-sensitive mutants of the basidiomycete Coprinus cinereus. Each mutation is recessive, and the collection defines at least ten complementation groups for survival of gamma irradiation. Four complementation groups define the genes rad3, rad9, rad11 and rad12, which are required both for survival of gamma irradiation and for meiosis. Mutants in each of these four groups fail to complete meiosis and produce mushrooms with greatly reduced numbers of viable spores. Propidium iodide staining of meiotic nuclei showed a characteristic terminal appearance for each mutant: few cells of any of the meiotic mutants progress beyond prophase I, and both condensation and fragmentation or dispersal of meiotic chromatin are frequently observed. Scanning electron micrographs showed that the meiotic mutants make varying numbers (0-6) of basidiospore initials and that few of these initials develop into mature spores. When initials are present they are always symmetrically arrayed on the basidium, regardless of initial number. In quantitative measurements of gamma ray sensitivity, double mutants of every tested combination of rad3, rad9, rad11 and rad12 consistently showed the same gamma ray sensitivity as the more sensitive single mutant parent of the cross. Therefore, these four genes are in the same pathway for the repair of gamma radiation damage, and this pathway also represents one or more functions essential for meiosis.

Relationships between complex Delta expression and the specification of retinal cell fates during Drosophila eye development.

Analysis of retinal development in Delta (Dl) temperature-sensitive mutants reveals requirements for Delta function in the specification of all retinal cells, including photoreceptors, cone cells, pigment cells and cells that make up interommatidial bristles. In situ hybridization and immunohistochemistry indicate that Delta is expressed dynamically during the specification of different cell types. Comparisons of Delta expression patterns with developmental defects in Dl mutants implies that Delta functions in a cell-nonautonomous manner in the specification of photoreceptors. Delta protein resides predominantly in subcellular vesicles located primarily at the apical ends of developing retinal cells. Localization of Delta protein in Dl and shibire tsl mutants implies that Delta is targeted to the cell surface, but is efficiently removed via endocytosis, resulting in vesicular accumulation.

Structural analysis of mutations in the Drosophila beta 2-tubulin isoform reveals regions in the beta-tubulin molecular required for general and for tissue-specific microtubule functions.

We have determined the lesions in a number of mutant alleles of beta Tub85D, the gene that encodes the testis-specific beta 2-tubulin isoform in Drosophila melanogaster. Mutations responsible for different classes of functional phenotypes are distributed throughout the beta 2-tubulin molecule. There is a telling correlation between the degree of phylogenetic conservation of the altered residues and the number of different microtubule categories disrupted by the lesions. The majority of lesions occur at positions that are evolutionarily highly conserved in all beta-tubulins; these lesions disrupt general functions common to multiple classes of microtubules. However, a single allele B2t6 contains an amino acid substitution within an internal cluster of variable amino acids that has been identified as an isotype-defining domain in vertebrate beta-tubulins. Correspondingly, B2t6 disrupts only a subset of microtubule functions, resulting in misspecification of the morphology of the doublet microtubules of the sperm tail axoneme. We previously demonstrated that beta 3, a developmentally regulated Drosophila beta-tubulin isoform, confers the same restricted morphological phenotype in a dominant way when it is coexpressed in the testis with wild-type beta 2-tubulin. We show here by complementation analysis that beta 3 and the B2t6 product disrupt a common aspect of microtubule assembly. We therefore conclude that the amino acid sequence of the beta 2-tubulin internal variable region is required for generation of correct axoneme morphology but not for general microtubule functions. As we have previously reported, the beta 2-tubulin carboxy terminal isotype-defining domain is required for suprastructural organization of the axoneme. We demonstrate here that the beta 2 variant lacking the carboxy terminus and the B2t6 variant complement each other for mild-to-moderate meiotic defects but do not complement for proper axonemal morphology. Our results are consistent with the hypothesis drawn from comparisons of vertebrate beta-tubulins that the two isotype-defining domains interact in a three-dimensional structure in wild-type beta-tubulins. We propose that the integrity of this structure in the Drosophila testis beta 2-tubulin isoform is required for proper axoneme assembly but not necessarily for general microtubule functions. On the basis of our observations we present a model for regulation of axoneme microtubule morphology as a function of tubulin assembly kinetics.

Regulation of beta-tubulin function and expression in Drosophila spermatogenesis.

In this study we examined two aspects of beta-tubulin function in Drosophila spermatogenesis: 1) beta-tubulin structural requirements for assembly of different categories of microtubules and 2) regulatory requirements for production of the correct tubulin protein level. In normal Drosophila spermatogenesis, the testis-specific beta 2-tubulin isoform supports multiple microtubule functions. Our previous work showed that another Drosophila isoform, beta 3, cannot support spermatogenesis, whereas a carboxyl-truncated form of beta 2, beta 2 delta C, can at least to some extent provide all of beta 2's normal functions, save one: beta 2 delta C cannot support organization of axonemal microtubules into the supramolecular architecture of the axoneme. Here, to test whether beta 2 carboxyl sequences can rescue the functional failure of the beta 3 isoform in spermatogenesis, we constructed a gene encoding a chimeric protein, beta 3 beta 2C, in which beta 3 sequences in the carboxyl region are replaced with those of beta 2. Unlike either beta 3 or beta 2 delta C, beta 3 beta 2C can provide partial function for both assembly of axonemal microtubules and their organization into the supramolecular architecture of the axoneme. In particular, the beta 2 carboxyl sequences mediate morphogenesis of the axoneme doublet tubule complex, including accessory microtubule assembly and attachment of spokes and linkers. However, our data also reveal aspects of beta 2-specific function that require structural features other than the primary sequence of the isotype-defining variable regions, the C terminus and the internal variable region. Tests of fecundity in males that coexpress beta 2 and the chimeric beta 3 beta 2C protein showed that in Drosophila there are differential requirements for sperm motility in the male and in the female reproductive tract. Since some aspects of microtubule function in spermatogenesis are sensitive to the tubulin pool size, we examined the mechanisms for control of tubulin protein levels in the male germ cells. We found that both beta 2-tubulin mRNA accumulation and protein synthesis are dependent on gene dose, and that the level of expression is regulated by 3' noncoding sequences in the beta 2 gene. Our data show that the regulatory mechanisms that control tubulin pool levels in the Drosophila male germ line differ from those observed in cultured animal somatic cells. Finally, expression of transgenic constructs is consistent with early cessation of X chromosome expression in Drosophila spermatogenesis.

Tissue-specific microtubule functions in Drosophila spermatogenesis require the beta 2-tubulin isotype-specific carboxy terminus.

beta-Tubulins are encoded by members of multigene families and are generally highly conserved at the sequence level. The carboxyl terminal 15 amino acids are markedly more diverged than the rest of the sequence and constitute an "isotype defining region," which is conserved in corresponding beta-tubulin isoforms in different vertebrate species. It is thought that the carboxy terminus of beta-tubulin may not be required for assembly of microtubules per se, but it may be necessary for conferring properties on beta-tubulins required for isotype-specific functions. We have determined the extent to which a beta-tubulin isoform that lacks its carboxy terminus can assemble into functional suprastructures by generating two early-stop-codon variants of the gene for the testis-specific beta-tubulin (beta 2) in Drosophila melanogaster. We have also sequenced the null allele of this gene and discovered that it also contains an early-stop codon. By examining the products of these genes and the phenotypes they confer, we have determined that the beta-tubulin variants with large truncations (171 or 50 amino acids) do not accumulate to detectable levels and provide no beta-tubulin function. However, a small truncation missing only the terminal 15 amino acids is capable of being assembled into ultrastructurally normal looking microtubules in vivo, even though the truncated protein is less stable than wildtype beta 2. The functional failings of this truncated beta-tubulin are manifested in defective microtubule-based spermatogenic suprastructures, rather than at the level of assembly of individual microtubules. The most remarkable defect conferred by the truncated beta 2 is the failure of axonemes to assemble with proper organization, even though microtubules with presumptive axoneme identity are clearly present. We therefore demonstrate that the carboxy terminus of beta 2-tubulin is indeed required for organization of microtubule suprastructures in spermatogenesis. This observation supports the hypothesis that the variable carboxy terminus mediates isotype-specific microtubule-dependent functions.

In situ localization of storage protein mRNAs in developing meristems of Brassica napus embryos.

Probes derived from cDNA clones of napin and cruciferin, the major storage proteins of Brassica napus, and in situ hybridization techniques were used to examine changes in the spatial and temporal distribution of storage protein messages during the course of embryogeny, with a special emphasis on the developing apical meristems. Napin mRNAs begin to accumulate in the cortex of the axis during late heart stage, in the outer faces of the cotyledons during torpedo stage and in the inner faces of the cotyledons during cotyledon stage. Cruciferin mRNAs accumulate in a similar pattern but approximately 5 days later. Cells in the apical regions where root and shoot meristems develop do not accumulate storage protein messages during early stages of embryogeny. In the upper axis, the boundary between these apical cells and immediately adjacent cells that accumulate napin and cruciferin mRNAs is particularly distinct. Our analysis indicates that this boundary is not related to differences in tissue or cell type, but appears instead to be coincident with the site of a particular set of early cell divisions. A major change in the mRNA accumulation patterns occurs halfway through embryogeny, as the embryos enter maturation stage and start drying down. Final maturation of the shoot apical meristem is associated with the development of leaf primordia and the accumulation of napin mRNAs in the meristem, associated leaf primordia and vascular tissue. Cruciferin mRNAs accumulate only in certain zones of the shoot apical meristem and on the flanks of leaf primordia. Neither type of mRNA accumulates in the root apical meristem at any stage.

A genetic and developmental analysis of mutations in labial, a gene necessary for proper head formation in Drosophila melanogaster.

We have undertaken a developmental genetic analysis of labial (lab), the most proximal gene in the Antennapedia complex (ANT-C) of Drosophila melanogaster. The terminal phenotype of mutant embryos was examined in cuticle preparations, in thin sections, and by scanning electron microscopy. These preparations revealed a failure of head involution and the loss or disruption of several head structures, including the salivary glands and the H-piece and ventral arm of the cephalopharyngeal apparatus. Although these structures are presumed to derive from the gnathocephalic segments, we argue that the observed defects are likely to be a secondary consequence of a failure of head involution. A function for lab in the development of the adult head was inferred from the phenotype of animals bearing hypomorphic alleles and from clones of lab- tissue generated by mitotic recombination. Two aspects of the mutant phenotype were manifested. Ventrally, a deletion and/or disruption of tissue occurred in the maxillary palp and vibrissae regions. Dorsally, the posterior head appeared to be transformed to a thoracic-like identity. Mutations in lab, like those in the Deformed and proboscipedia loci of the ANT-C, reveal a homoeotic phenotype only in the adult stage of the life cycle.