Heterochronic Mutants Research Articles

The F-box protein FBXL-5 governs vitellogenesis and lipid homeostasis in C. elegans.

The molecular mechanisms that govern the metabolic commitment to reproduction, which often occurs at the expense of somatic reserves, remain poorly understood. We identified the Caenorhabditis elegans F-box protein FBXL-5 as a negative regulator of maternal provisioning of vitellogenin lipoproteins, which mediate the transfer of intestinal lipids to the germline. Mutations in fbxl-5 partially suppress the vitellogenesis defects observed in the heterochronic mutants lin-4 and lin-29, both of which ectopically express fbxl-5 at the adult developmental stage. FBXL-5 functions in the intestine to negatively regulate expression of the vitellogenin genes; and consistently, intestine-specific over-expression of FBXL-5 is sufficient to inhibit vitellogenesis, restrict lipid accumulation, and shorten lifespan. Our epistasis analyses suggest that fbxl-5 functions in concert with cul-6, a cullin gene, and the Skp1-related gene skr-3 to regulate vitellogenesis. Additionally, fbxl-5 acts genetically upstream of rict-1, which encodes the core mTORC2 protein Rictor, to govern vitellogenesis. Together, our results reveal an unexpected role for a SCF ubiquitin-ligase complex in controlling intestinal lipid homeostasis by engaging mTORC2 signaling.

Abstract IA17: The stem cell factor LIN28 in pediatric cancer

Abstract LIN28, a developmental regulator originally identified as a heterochronic mutant in C. elegans, functions as an RNA binding protein that regulates protein expression through effects on mRNA translation and through control of processing of the let-7 family of tumor suppressor microRNAs. Lin28 has two paralogs, Lin28A and Lin28B, and studies have revealed a role for these factors in a wide array of biological phenotypes including organismal growth and metabolism, sexual maturation, and oncogenesis. LIN28/let-7 act antagonistically to regulate the balance of self-renewal and differentiation in embryonic stem cells, and LIN28 is one of the factors shown to reprogram somatic cells to pluripotency. In pediatric cancers, high level expression of LIN28 is a consistent marker of germ cell tumor, frequently observed in neuroblastoma and hepatoblastoma, and activated by chromosomal translocation in Wilms tumor. Thus, the stem cell factor LIN28 is commonly dysregulated in pediatric tumors and may represent a novel therapeutic target. Citation Format: George Q. Daley. The stem cell factor LIN28 in pediatric cancer. [abstract]. In: Proceedings of the AACR Special Conference on Chromatin and Epigenetics in Cancer; Jun 19-22, 2013; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2013;73(13 Suppl):Abstract nr IA17.

Heterochronic genes in plant evolution and development

Evolution of morphology includes evolutionary shifts of developmental processes in space or in time. Heterochronic evolution is defined as a temporal shift. The concept of heterochrony has been very rewarding to investigators of both animal and plant developmental evolution, because it has strong explanatory power when trying to understand morphological diversity. While for animals, extensive literature on heterochrony developed along with the field of evolution of development, in plants the concept has been applied less often and is less elaborately developed. Yet novel genetic findings highlight heterochrony as a developmental and evolutionary process in plants. Similar to what has been found for the worm Caenorhabditis, a heterochronic gene pathway controlling developmental timing has been elucidated in flowering plants. Two antagonistic microRNA’s miR156 and miR172 target two gene families of transcription factors, SQUAMOSA PROMOTOR BINDING PROTEIN-LIKE and APETALA2-like, respectively. Here, we propose that this finding now allows the molecular investigation of cases of heterochronic evolution in plants. We illustrate this point by examining microRNA expression patterns in the Antirrhinum majus incomposita and choripetala heterochronic mutants. Some of the more beautiful putative cases of heterochronic evolution can be found outside flowering plants, but little is known about the extent of conservation of this flowering plant pathway in other land plants. We show that the expression of an APETALA2-like gene decreases with age in a fern species. This contributes to the idea that ferns share some heterochronic gene functions with flowering plants.

Wnt signaling controls temporal identities of seam cells in Caenorhabditis elegans

The Wnt signaling pathway regulates multiple aspects of the development of stem cell-like epithelial seam cells in Caenorhabditis elegans, including cell fate specification and symmetric/asymmetric division. In this study, we demonstrate that lit-1, encoding the Nemo-like kinase in the Wnt/β-catenin asymmetry pathway, plays a role in specifying temporal identities of seam cells. Loss of function of lit-1 suppresses defects in retarded heterochronic mutants and enhances defects in precocious heterochronic mutants. Overexpressing lit-1 causes heterochronic defects opposite to those in lit-1(lf) mutants. LIT-1 exhibits a periodic expression pattern in seam cells within each larval stage. The kinase activity of LIT-1 is essential for its role in the heterochronic pathway. lit-1 specifies the temporal fate of seam cells likely by modulating miRNA-mediated silencing of target heterochronic genes. We further show that loss of function of other components of Wnt signaling, including mom-4, wrm-1, apr-1, and pop-1, also causes heterochronic defects in sensitized genetic backgrounds. Our study reveals a novel function of Wnt signaling in controlling the timing of seam cell development in C. elegans.

Upregulation of the let-7 microRNA with precocious development in lin-12/Notch hypermorphic Caenorhabditis elegans mutants

The lin-12/Notch signaling pathway is conserved from worms to humans and is a master regulator of metazoan development. Here, we demonstrate that lin-12/Notch gain-of-function (gf) animals display precocious alae at the L4 larval stage with a significant increase in let-7 expression levels. Furthermore, lin-12(gf) animals display a precocious and higher level of let-7 gfp transgene expression in seam cells at L3 stage. Interestingly, lin-12(gf) mutant rescued the lethal phenotype of let-7 mutants similar to other known heterochronic mutants. We propose that lin-12/Notch signaling pathway functions in late developmental timing, upstream of or in parallel to the let-7 heterochronic pathway. Importantly, the human microRNA let-7a was also upregulated in various human cell lines in response to Notch1 activation, suggesting an evolutionarily conserved cross-talk between let-7 and the canonical lin-12/Notch signaling pathway.

The rice heterochronic gene SUPERNUMERARY BRACT regulates the transition from spikelet meristem to floral meristem

Regulating the transition of meristem identity is a critical step in reproductive development. After the shoot apical meristem (SAM) acquires inflorescence meristem identity, it goes through a sequential transition to second- and higher-order meristems that can eventually give rise to floral organs. Despite ample information on the molecular mechanisms that control the transition from SAM to inflorescence meristems, little is known about the mechanism for inflorescence development, especially in monocots. Here, we report the identification of the SUPERNUMERARY BRACT (SNB) gene controlling the transition from spikelet meristem to floral meristem and the floral organ development. This gene encodes a putative transcription factor carrying two AP2 domains. The SNB:GFP fusion protein is localized to the nucleus. SNB is expressed in all the examined tissues, but most strongly in the newly emerging spikelet meristems. In SNB knockout plants, the transition from spikelet meristems to floral meristems is delayed, resulting in the production of multiple rudimentary glumes in an alternative phyllotaxy. The development of additional bracts interferes with subsequent floral architecture. In some spikelets, the empty glumes and lodicules are transformed into lemma/palea-like organs. Occasionally, the number of stamens and carpels is altered and an ectopic floret occurs in the axil of the rachilla. These phenotypes suggest that snb is a heterochronic mutant, affecting the phase transition of spikelet meristems, the pattern formation of floral organs and spikelet meristem determinancy.

PLASTOCHRON1, a timekeeper of leaf initiation in rice, encodes cytochrome P450.

During postembryonic development of higher plants, the shoot apical meristem produces lateral organs in a regular spacing (phyllotaxy) and a regular timing (plastochron). Molecular analysis of mutants associated with phyllotaxy and plastochron would greatly increase understanding of the developmental mechanism of plant architecture because phyllotaxy and plastochron are fundamental regulators of plant architecture. pla1 of rice is not only a plastochron mutant showing rapid leaf initiation without affecting phyllotaxy, but also a heterochronic mutant showing ectopic shoot formation in the reproductive phase. Thus, pla1 provides a tool for analyzing the molecular basis of temporal regulation in leaf development. In this work, we isolated the PLA1 gene by map-based cloning. The identified PLA1 gene encodes a cytochrome P450, CYP78A11, which potentially catalyzes substances controlling plant development. PLA1 is expressed in developing leaf primordia, bracts of the panicle, and elongating internodes, but not in the shoot apical meristem. The expression pattern and mutant phenotype suggest that the PLA1 gene acting in developing leaf primordia affects the timing of successive leaf initiation and the termination of vegetative growth.

A rice heterochronic mutant, mori1, is defective in the juvenile-adult phase change.

We have identified five recessive allelic mutations, mori1-1 to mori1-5, which drastically modify the shoot architecture of rice. The most remarkable feature of mori1 plants is a rapid production of small leaves and short branches. The mori1 plants are about 5 cm in height even 7 months after sowing. No reproductive growth was attained in mori1 plants even if inductive short-day treatment was applied. Leaves of mori1 at any position were very small and the size and shape were comparable to those of the wild-type 2nd leaf. The stem of mori1 7 months after sowing did not differentiate node and internode and had randomly oriented vascular bundles, which were characteristic of the basal part of the wild-type stem where 2nd and 3rd leaves were inserted. These structural characteristics indicate that mori1 maintains the 2nd-leaf stage (juvenile phase) of the wild type. The short plastochron and high cell division activity in the shoot apical meristem further confirmed the juvenility of mori1, corresponding to the 2nd-leaf-differentiation stage in the wild-type embryo. Furthermore, the apparent photosynthetic rate in mori1 leaves was low as in the wild-type 2nd leaf. Thus, mori1 is a heterochronic mutation that suppresses the induction of adult phase and the termination of the juvenile phase. Therefore, MORI1 plays an important role in the juvenile-adult phase change. The importance of heterochronic mutations in modifying shoot architecture is discussed.

Regulation and function of the Arabidopsis ABA-insensitive4 gene in seed and abscisic acid response signaling networks.

We have characterized developmental, environmental, and genetic regulation of abscisic acid-insensitive (ABI)4 gene expression in Arabidopsis. Although expressed most strongly in seeds, ABI4 transcripts are also present at low levels in vegetative tissue; vegetative expression is not induced by abscisic acid (ABA) or stress treatments. Comparison of transcript levels in mature seeds of ABA-insensitive, ABA-hypersensitive, ABA-deficient, or heterochronic mutants indicates that ABI4 expression is altered in only two of the backgrounds, the ABA-insensitive mutants abi1-1 and abi3-1. To determine whether ABI4 is necessary and/or sufficient for ABA response, we assayed the effects of loss of ABI4 function and ectopic ABI4 expression on growth and gene expression. We examined genetic interactions among three ABA response loci, ABI3, ABI4, and ABI5, by comparing phenotypes of mutants, ectopic expression lines, mutants carrying an ectopically expressed transgene, and the corresponding wild-type lines. Our results indicate some cross-regulation of expression among ABI3, ABI4, and ABI5 and suggest that they function in a combinatorial network, rather than a regulatory hierarchy, controlling seed development and ABA response.

Timekeeping in worms

During the past few years, circadian clock genes have been discovered in various model systems, from Drosophila and Neurospora to cyanobacteria. Many clock components share conserved features and encode transcription factors of the PAS domain family, which function within transcriptional autoregulatory feedback loops. Surprisingly, given the wealth of information concerning its genetics and development, one animal model, Caenorhabditis elegans, has been conspicuously absent from these studies because a circadian rhythm, as yet, had not been demonstrated in it. In this paper1xSimilarity of the C. elegans developmental timing protein LIN-42 to circadian rhythm proteins. Jeon, M. et al. Science. 1999; 286: 1141–1146Crossref | PubMed | Scopus (110)See all References1, Jeon et al. show that worms do have clock gene homologues. By studying heterochronic mutants in which the timing and sequence of various developmental events is abnormal, they found that one gene, lin-42, which affects hypodermal differentiation, encodes a PAS domain protein that closely resembles the Drosophila clock gene per product, period. However, unlike the circadian rhythm of per expression in Drosophila, lin-42 oscillates with a 6-hour rhythm that correlates with the molting cycle. Also unlike per, lin-42 expression continues to oscillate in lin-42 mutant worms – thus, the protein is not required to generate the transcript rhythm. Drosophila period heterodimerizes with another clock component, timeless (encoded by tim), to translocate into the nucleus. Expression of a C. elegans tim homologue, which Jeon et al. identified by computer analysis of the complete genomic sequence, does not oscillate, unlike Drosophila tim expression. Furthermore, its expression pattern does not overlap with that of lin-42. Nevertheless, by linking a clock gene homologue with a developmental timing function, this work opens up the exciting possibility that different biological timing systems might have a similar molecular organization and might even share common components.

Lax Midrib1‐O, A systemic, heterochronic mutant of maize

Lxm1-O, a dominant EMS (ethyl methanesulfonate) induced mutation in maize (Zea mays, Poaceae), was originally reported to affect the blade/sheath boundary over the midrib region of the leaf. Here we present a more extensive analysis of the Lxm phenotype in nine different inbred lines. Lxm leaves are longer and narrower, and can initiate ectopic leaves. Additionally, Lxm1-O affects all plant organs observed. Compared to wild-type siblings, Lxm plants have fewer nodes, basal displacement of reproductive structures, and advance more quickly to the reproductive phase. We address questions as to whether Lxm1-O abbreviates a specific developmental phase, using hair, wax, and ear node data. We found that each phase was affected, although to varying degrees, depending on the inbred line. We interpret Lxm1-O to be a heterochronic mutation, causing the developmental acceleration of each phase of the shoot. Lxm1-O is novel, since other systemic heterochronic maize mutants prolong the juvenile phase, thereby extending shoot development. We discuss the importance of heterochronic mutations in the context of morphological evolution.

The generation and genetic analysis of suppressors of lethal mutations in the Caenorhabditis elegans rol-3(V) gene.

The Caenorhabditis elegans rol-3(e754) mutation is a member of a general class of mutations affecting gross morphology, presumably through disruption of the nematode cuticle. Adult worms homozygous for rol-3(e754) exhibit rotation about their long axis associated with a left-hand twisted cuticle, musculature, gut and ventral nerve cord. Our laboratory previously isolated 12 recessive lethal alleles of rol-3. All these lethal alleles cause an arrest in development at either early or mid-larval stages, suggesting that the rol-3 gene product performs an essential developmental function. Furthermore, through the use of the heterochronic mutants lin-14 and lin-29, we have established that the expression of rol-3(e754)'s adult specific visible function is not dependent on the presence of an adult cuticle. In an attempt to understand rol-3's developmental role we sought to identify other genes whose products interact with that of rol-3. Toward this end, we generated eight EMS induced and two gamma irradiation-induced recessive suppressors of the temperature sensitive (ts) mid-larval lethal phenotype of rol-3(s1040ts). These suppressors define two complementation groups srl-1 II and srl-2 III; and, while they suppress the rol-3(s1040) lethality, they do not suppress the adult specific visible rolling phenotype. Furthermore, there is a complex genetic interaction between srl-2 and srl-1 such that srl-2(s2506) fails to complement all srl alleles tested. These results suggest that srl-1 and srl-2 may share a common function and, thus, possibly constitute members of the same gene family. Mutations in both srl-1 and srl-2 produce no obvious hermaphrodite phenotypes in the absence of rol-3(s1040ts); however, males homozygous for either srl-1 or srl-2 display aberrant tail morphology. We present evidence suggesting that the members of srl-2 are not allele specific with respect to their suppression of rol-3 lethality, and that rol-3 may act in some way to influence proper posterior morphogenesis. Finally, based on our genetic analysis of rol-3 and the srl mutations, we present a model whereby the wild-type products of the srl loci act in a concerted manner to negatively regulate the rol-3 gene.

Flower development in the organ number mutant clavata1‐1 of Arabidopsis thaliana (Brassicaceae)

Flowers of the organ number (meristic) mutant clavata1‐1 of Arabidopsis thaliana (Brassicaceae) were studied to examine timing and patterns of floral organogenesis as compared to the wild type. All clavata1‐1 flowers examined had four‐ instead of two‐loculed gynoecia; half showed increased numbers of stamens; and 10% formed increased numbers of sepals. An inflorescence plastochron index was used to establish the timing of developmental events during flower organogenesis. clavata1‐1 flowers initiate faster but grow more slowly than in the wild type. The stages of sepal and stamen initiation were prolonged compared to those of the wild type. Although gynoecial initiation was not prolonged, the preceding stage was and it was characterized by a proliferation of meristematic cells above the initiating stamens. The clavata1‐1 flower apex did not become wider than that of the wild type until after the establishment of the gynoecium. We propose that clavata1‐1 is a heterochronic mutant, where flower organ number increases are due partly to prolongation of organ initiation stages.

Flower Development in the Organ Number Mutant Clavata1-1 of Arabidopsis thaliana (Brassicaceae)

Flowers of the organ number (meristic) mutant clavata1-1 of Arabidopsis thaliana (Brassicaceae) were studied to examine timing and patterns of floral organogenesis as compared to the wild type. All clavata1-1 flowers examined had four- instead of two-loculed gynoecia; half showed increased numbers of stamens; and 10% formed increased numbers of sepals. An inflorescence plastochron index was used to establish the timing of developmental events during flower organogenesis. clavata1-1 flowers initiate faster but grow more slowly than in the wild type. The stages of sepal and stamen initiation were prolonged compared to those of the wild type. Although gynoecial initiation was not prolonged, the preceding stage was and it was characterized by a proliferation of meristematic cells above the initiating stamens. The clavata1-1 flower apex did not become wider than that of the wild type until after the establishment of the gynoecium. We propose that clavata1-1 is a heterochronic mutant, where flower organ number increases are due partly to prolongation of organ initiation stages.

Mutation of protein kinase A causes heterochronic development of Dictyostelium.

In heterochronic mutants the relative timing of developmental events is altered compared with the wild type. This generally results in a disordered embryo, though heterochronic mutations may also be an important source of evolutionary variation. In the rapidly developing (rde) mutants of Dictyostelium, stalk and spore cells differentiate before morphogenesis is complete. We have traced the lesion in one class of these mutants to the regulatory subunit of cyclic AMP-dependent protein kinase (pk-A). Inactivation of this protein results in the unrestrained activity of the catalytic subunit, so prematurely triggering terminal cell differentiation.

Early-Flowering Mutants of Arabidopsis thaliana

The transition from vegetative to reproductive growth represents a major change in angiosperm development. During the floral transition, shoot meristem activity progresses through the overlapping developmental phases: vegetative→inflorescence→floral. The transition to flowering is regulated by graft-transmissible substances, and the arrival of these substances at the shoot apex is correlated with the establishment of the inflorescence meristem. To identify the genes involved in regulating the floral transition, we have screened for mutants of Arabidopsis thaliana that display altered timing of flowering. We expect that such heterochronic mutants are defective either in the production or transport of floral signals, or in the response of the shoot meristem to these signals. We have isolated four early-flowering mutants: terminal flower 1 (tfl1) and early-flowering (elf) 1, 2 and 3. Three phenotypic classes are represented by these mutations: (I) early-flowering, photoperiod-sensitive, and displaying abnormal inflorescence development (tfl1); (II) early-flowering photoperiod-sensitive (elf1 and elf2); and (III) early-flowering, photoperiod-insensitive (elf3). This paper describes the initial phenotypic and genetic analysis of the mutants, and discusses how the genes represented by these mutations may fit into the signal/response pathway that leads to reproductive development.

Altered expression of an L1-specific, O-linked cuticle surface glycoprotein in mutants of the nematode Caenorhabditis elegans.

Mouse mAb M38 was used in indirect immunofluorescence experiments to detect a stage-specific antigen on the surface of the first larval stage (L1) of the free-living nematode Caenorhabditis elegans, and to detect alterations in the apparent expression of this antigen in two distinct classes of C. elegans mutants. In previously described srf-2 and srf-3 mutants (Politz S. M., M. T. Philipp, M. Estevez, P.J. O'Brien, and K. J. Chin. 1990. Proc. Natl. Acad. Sci. USA. 87:2901-2905), the antigen is not detected on the surface of any stage. Conversely, in srf-(yj43) and other similar mutants, the antigen is expressed on the surface of the first through the fourth (L4) larval stages. To understand the molecular basis of these alterations, the antigen was characterized in gel immunoblotting experiments. After SDS-PAGE separation and transfer to nitrocellulose, M38 detected a protein antigen in extracts of wild-type L1 populations. The antigen was sensitive to digestion by Pronase and O-glycanase (endo-alpha-N-acetylgalactosaminidase), suggesting that it is an O-linked glycoprotein. This antigen was not detected in corresponding extracts of wild-type L4s or srf-2 or srf-3 L1s, but was detected in extracts of srf-(yj43) L4s. The antigen-defective phenotype of srf-3 was epistatic to the heterochronic mutant phenotype of srf-(yj43) in immunofluorescence tests of the srf-3 srf-(yj43) double mutant, suggesting that srf-(yj43) causes incorrect regulation of a pathway of antigen formation that requires wild-type srf-3 activity.

Hairy-Sheath Frayed # 1-0: A Systemic, Heterochronic Mutant of Maize that Specifies Slow Developmental Stage Transitions

Hairy sheath frayed-1 (Hsf1-O), an ethyl methanesolfonate-induced dominant mutant in maize, was first described as an excessively hairy plant with pronglike projections of sheath that extended from the margin of the leaf blade. We describe several other phenotypes and show that Hsf1-O affects virtually every segment of the plant. We have identified genetic backgrounds that suppress some of the Hsf1-O phenotypes. The morphological descriptions reveal a prolongation of cell differentiation in certain tissues and organs. In general, transformations are from more adult to more juvenile, not older to younger. On the assumption that Hsf1-O acts with the same developmental logic in both the apical meristem and leaf primordia, the leaf sheath is a juvenile blade, not a young blade. Genetic background effects show that the expressivity of the phenotype is influenced by the plant's time to anthesis. The suppression and temporal shift in Hsf1-O expression patterns in early flowering lines is due to the inherent accelerated maturation of cells in these lines. In early flowering lines, transformations are suppressed and shifted toward the less mature (younger) tissues, organs, and segments of the plant. We conclude that Hsf1-O is a heterochronic mutation that prolongs the time to maturity (delays terminal differentiation), and that a primordial cell's developmental maturity rather than the cell's position can dictate that cell's fate. Hsf1-O disconnects the rate of developmental stage transition from the overall flowering-time rate.

The Prophyllum of Grasses

Previous articleNext article FreeBriefer ArticlesThe Prophyllum of GrassesG. N. CollinsG. N. Collins Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by Volume 78, Number 3Nov., 1924 Article DOIhttps://doi.org/10.1086/333403 Views: 23Total views on this site Citations: 3Citations are reported from Crossref Journal History This article was published in the Botanical Gazette (1876-1991), which is continued by International Journal of Plant Sciences (1992-present). PDF download Crossref reports the following articles citing this article:Ralph Bertrand-Garcia, Michael Freeling HAIRY-SHEATH FRAYED #1-O: A SYSTEMIC, HETEROCHRONIC MUTANT OF MAIZE THAT SPECIFIES SLOW DEVELOPMENTAL STAGE TRANSITIONS, American Journal of Botany 78, no.66 (Jun 1991): 747–765.https://doi.org/10.1002/j.1537-2197.1991.tb14477.xH. Weston Blaser STUDIES IN THE MORPHOLOGY OF THE CYPERACEAE. II. THE PROPHYLL, American Journal of Botany 31, no.11 (Jan 1944): 53–64.https://doi.org/10.1002/j.1537-2197.1944.tb08001.xADRIANCE S. FOSTER SALIENT FEATURES OF THE PROBLEM OF BUD-SCALE MORPHOLOGY, Biological Reviews 3, no.22 (Mar 1928): 123–164.https://doi.org/10.1111/j.1469-185X.1928.tb00853.x