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PubMed2001

Relationship of hepatocyte ploidy levels with body size and growth rate in mammals.

To elucidate possible causes of the elevation of genome number in somatic cells, hepatocyte ploidy levels were measured cytofluorimetrically and related to the organismal parameters (body size, postnatal growth rate, and postnatal development type) in 53 mammalian species. Metabolic scope (ratio of maximal metabolic rate to basal metabolic rate) was also included in 23 species. Body masses ranged 10(5) times, and growth rate more than 30 times. Postnatal growth rate was found to have the strongest effect on the hepatocyte ploidy. At a fixed body mass the growth rate closely correlates (partial correlation analysis) with the cell ploidy level (r = 0.85, P < 10(-6)), whereas at a fixed growth rate body mass correlates poorly with ploidy level (r = -0.38, P < 0.01). The mature young (precocial mammals) of the species have, on average, a higher cell ploidy level than the immature-born (altricial) animals. However, the relationship between precocity of young and cell ploidy levels disappears when the influences of growth rate and body mass are removed. Interspecies variability of the hepatocyte ploidy levels may be explained by different levels of competition between the processes of proliferation and differentiation in cells. In turn, the animal differences in the levels of this competition are due to differences in growth rate. A high negative correlation between the hepatocyte ploidy level and the metabolic scope indicates a low safety margin of organs with a high number of polyploid cells. This fact allows us to challenge a common opinion that increasing ploidy enhances the functional capability of cells or is necessary for cell differentiation. Somatic polyploidy can be considered a "cheap" solution of growth problems that appear when an organ is working at the limit of its capabilities.

Animals↗
PubMed2002

Molecular evolution of ribosomal intergenic spacers in Odontophrynus americanus 2n and 4n (Amphibia: Anura).

Ribosomal intergenic spacers (IGSs) of Odontophrynus americanus 2n and 4n were cloned, restriction mapped, and partially sequenced. Three distinct regions, namely alpha, beta, and delta, were identified in the IGSs. The alpha and beta regions flanked the 28S and 18S rRNA genes, respectively, conserving an identical restriction pattern at each ploidy level. The delta region, located between alpha and beta, was highly variable in size and restriction pattern, enclosing different BamHI subrepeats (B-SR), 87- to 530-bp-long. Sequence analysis showed that B-SRs were composed mainly of different arrangements of similar blocks of sequences. Another family of repetitive sequences was found in the delta region, clustered inside large BamHI fragments. These subrepeats are 189-bp-long and, although very similar in diploid and tetraploid IGSs, show a pattern of concerted evolution. A hypothetical functional role for the 189-bp repeats is discussed in view of their predicted secondary structure and presence of potential E2 binding sites inside diploid subrepeats. Although the same structural elements were present both in diploid and tetraploid IGSs, the higher level of repeatability of tetraploid IGSs suggests that common ancestor sequences have undergone several rounds of amplification after O. americanus polyploidy.

Animals↗
PubMed2005

Evolution of Buchloë dactyloides based on cloning and sequencing of matK, rbcL, and cob genes from plastid and mitochondrial genomes.

Buffalograss (Buchloë dactyloides (Nutt.) Englem), a C4 turfgrass species, is native to the Great Plains region of North America. The evolutionary implications of buffalograss are unclear. Sequencing of rbcL and matK genes from plastid and the cob gene from mitochondrial genomes was examined to elucidate buffalo grass evolution. This study is the first to report sequencing of these genes from organelle genomes in the genus Buchloë. Comparisons of sequence data from the mitochondrial and plastid genome revealed that all genotypes contained the same cytoplasmic origin. There were some rearrangements detected in mitochondrial genome. The buffalograss genome appears to have evolved through the rearrangements of convergent subgenomic domains. Combined analyses of plastid genes suggest that the evolutionary process in Buchloë accessions studied was monophyletic rather than polyphyletic. However, since plastid and mitochondrial genomes are generally uniparentally inherited, the evolutionary history of these genomes may not reflect the evolutionary history of the organism, especially in a species in which out-crossing is common. The sequence information obtained from this study can be used as a genome-specific marker for investigation of the buffalograss polyploidy complex and testing of the mode of plastid and mitochondrial transmission in genus Buchloë.

Chloroplasts↗
PubMed1992

Incidence of multinucleated and polyploid aortic smooth muscle cells cultured from different age groups of spontaneously hypertensive rats.

Cell size and incidence of multinucleated, polyploid cells in cultured aortic smooth muscle cells from different age groups of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) were compared. Smooth muscle cells from SHR were generally larger than those from WKY, and the percentage of multinucleated smooth muscle cells was always higher in SHR than WKY in the three age groups of rats studied (3-4, 10-12, and 28-30 weeks). In smooth muscle cells from the 3- to 4-week group, there was a positive correlation between cell diameter and the percentage of multinucleated smooth muscle cells. Microdensitometric measurements also showed that the incidence of polyploid smooth muscle cells was always higher in SHR than WKY in the three age groups. There was a positive correlation between DNA density and nuclear area measurements in all the age groups of SHR and WKY. We conclude that cultured aortic smooth muscle cells from different age groups of SHR and WKY contained heterogeneous populations of cells and that, under our culture conditions, the polyploidy of the smooth muscle cells found in vivo was maintained in the SHR and WKY.

Aging↗
PubMed2000

Replication banding patterns of the diploid-tetraploid treefrogs Hyla chrysoscelis and H. versicolor.

Populations of the diploid-tetraploid treefrogs Hyla chrysoscelis and H. versicolor can be defined by the polymorphic positions of the nucleolar organizing regions (NORs) on their chromosomes. Evidence from NOR positions and interstitial telomere sequence data shows that gene flow between H. chrysoscelis populations appears to be restricted, with contact occurring only in narrow "hybrid" zones. Hyla versicolor appears to have had multiple origins from H. chrysoscelis populations, and this, too, is reflected in the NOR positions. We used replication banding to determine if genetic isolation of H. chrysoscelis populations was accompanied by karyotype evolution in the populations or in contact zones. We also sought to detect karyotype alteration or replication differences associated with polyploidy in H. versicolor. Homologous chromosome pairs of all H. chrysoscelis studied displayed no differences in replication banding patterns, nor did they differ from those of H. versicolor. Although NOR positions differed between the populations studied, no disturbance of the replication banding patterns was found, indicating that structural rearrangements were not involved in creating the multiple NOR positions seen in populations of H. versicolor and H. chrysoscelis.

Animals↗
PubMed2000

Induction of polyploidization by jaspamide in HL-60 cells.

Jaspamide, a natural peptide isolated from the marine sponge Hemiastrella minor, was used in the study of polyploidy in HL-60 cells. Jaspamide at 5 x 10(-8) M concentration exhibited antiproliferative activity and an increased CD4 and CD14 surface expression. After 2 days of cultivation, 56.3% of the exposed cells became multinuclear compared with 2.4% in controls. Moreover, the size and the number of nuclei of the cells increased in a time-dependent manner. An increased number of metaphase chromosomes was observed by microscopical enumeration after colcemid treatment for 60 min. The analysis of the DNA content of these cells, measured by propidium iodide staining, revealed a significant increase in the cells percentage with increased DNA content. Taken together, these findings indicate that the jaspamide treatment induces polyploidization in the HL-60 cell line.

Antineoplastic Agents↗
PubMed2005

Identification of individual chromosomes and parental genomes in Brassica juncea using GISH and FISH.

The three diploid (B. nigra, B. oleracea, B. campestris) and three allotetraploid (B. carinata, B. juncea, B. napus) species of Brassica, known as the "U-triangle" are one of the best model systems for the study of polyploidy. Numerous molecular investigations have provided a wealth of new insights into the polyploid origin and changes during the evolution of Brassica, but there are still many controversial aspects of their relationship and evolution. Interpretation of genome changes during evolution requires individual chromosome identification within the genome and clear distinction of genomes within the allotetraploid. The aim of this study was to identify individual chromosomes of B. juncea (genome AABB; 2n = 4x = 36) and to determine their genomic origin. Fluorescence in situ hybridization with 5S and 45S rDNA probes enabled discrimination of a substantial number of chromosomes, providing chromosomal landmarks for 20 out of 36 chromosomes of B. juncea. Additionally, along with double target genomic in situ hybridization, it allowed assignment of all chromosomes to either the A or B genomes.

Brassica↗
PubMed2004

Centrosome hyperamplification and chromosomal damage after exposure to radiation.

OBJECTIVE: In order to elucidate the effects of radiation on centrosome hyperamplification (CH), we examined the centrosome duplication cycle in KK47 bladder cancer cells following irradiation. METHODS: KK47 cells were irradiated with various doses of radiation and were examined for CH immunostaining for gamma-tubulin. RESULTS: Nearly all control cells contained one or two centrosomes, and mitotic cells displayed typical bipolar spindles. The centrosome replication cycle is well regulated in KK47. Twenty-four hours after 5-Gy irradiation, approximately 80% of irradiated cells were arrested in G2 phase, and at 48 h after irradiation, 56.9% of cells contained more than two centrosomes. Laser scanning cytometry performed 48 h after irradiation showed the following two pathways: (1) unequal distribution of chromosomes to daughter cells, or (2) failure to undergo cytokinesis, resulting in polyploidy. With mitotic collection, M-phase cells with CH could be divided into G1 cells with micronuclei and polyploidal cells. Fluorescence in situ hybridization analysis showed clear signs of chromosomal instability (CIN) at 48 h after irradiation. The present study had two major findings: (1) continual duplication of centrosomes occurred in the cell cycle-arrested cells upon irradiation, leading to centrosome amplification; (2) cytokinesis failure was due to aberrant mitotic spindle formation caused by the presence of amplified centrosomes. Abnormal mitosis with amplified centrosomes was detected in the accumulating G2/M population after irradiation, showing that this amplification of centrosomes was not caused by failure to undergo cytokinesis, but rather that abnormal mitosis resulting from amplification of centrosomes leads to cytokinesis block. CONCLUSION: These results suggest that CH is a critical event leading to CIN following exposure to radiation.

Blotting, Western↗
PubMed1975

Incidence and origin of heteroploidy, especially haploidy, in chick embryos from intraline and interline matings.

Preparations for chromosomal analysis were made from 2107 chick embryos at 16 hours of incubation. The embryos resulted from intraline and reciprocal interline matings of two genetically different stocks (AG and D6). The two stocks had been previously characterized as producers of high (AG) and low (D6) frequencies of chromosomally aberrant embryos. The overall frequency of aberrant embryos was 4.0 plus or minus 0.42%. The types and frequencies of abnormalities were: haploidy and haploid-euploid mosaics, 57%; polyploidy and polyploid-diploid mosaics, 19%; aneuploidy, 17%; aneuploid-diploid mosaics, 5%; and structural aberrations, 2%. Although there were no significant differences among the four types of matings in the overall frequency of heteroploid embryos (P greater than 0.1), a significant difference (P smaller than 0.01) in the frequency of haploid and haploid-euploid mosaic embryos was found. The difference was entirely attributable to the line of dam; D6 dams had 1.2% and AG dams had 2.7% haploid and haploid-euploid mosaic embryos. The difference between lines of sires was not significant. In addition, there was evidence of nonrandom distribution, among dams of both lines, of haploid and haploid-euploid mosaic embryos. It was concluded that the genotype of dam is an important influencing factor in the production of haploid cell lines in embryos. The superfluous genome in triploid embryos is usually maternal in origin, resulting from retention of the second polar body in the ovum. Sex-chromosome aneuploidy in chick embryos apparently derives from nondisjunction at meiosis I of oogenesis. The sex proportion of 2023 chromosomally sexed, diploid embryos was 50.2 plus or minus 1.1% male. No significant heterogeneity was observed among the types of matings. Sex proportion was not significantly influenced by any of a number of nongenetic variables.

Aneuploidy↗
PubMed1977

Specific chromosome changes associated with rabbit cell lines cultured in vitro.

Thirty-three rabbit cell lines were established from various fetal tissues of the inbred strain III of the New Zealand rabbit (Oryctolagus cuniculus). None of these lines exhibited senescence during a growth period of more than 2 years. Karyologic studies of most cell lines at 10 to 20 cell-passage intervals revealed that the karyotype stability of the rabbit cells in vitro was correlated with the organs from which the cell lines were derived. Thus, lines derived from cornea, spleen, and kidney tissues usually contained high frequencies of polyploidy in their early passages, whereas most of those derived from lung and skin were found to retain the normal diploid karyotype for much longer periods of time. One line derived from fetal lung tissue, designated Lung 16, remained diploid up to 100 passages. In late passages of the majority of all the lines studied, the cells became pseudodiploid, hyperdiploid, or polyploid. Among the pseudodiploid and the hyperdiploid cell lines, the chromosomal changes followed three basic patterns: (1) a gain of one or more telocentric chromosomes; (2) a loss of one telocentric chromosome plus a metacentric marker chromosome (M); or (3) a gain of a long telocentric marker chromosome with or without changes in the number of telocentric D chromosomes. By the G-banding technique, the telocentric chromosome involved in these three patterns was identified as the D-group chromosome 18 and the M marker chromosome as an isochromosome of 18. These results suggest that chromosomal rearrangement in rabbit cells involving trisomy of 18 may be responsible for the longevity of these cell lines cultured in vitro.

Animals↗
PubMed1993

Alzheimer's disease and ageing: a chromosomal approach.

Alzheimer's disease (AD) is the most common type of dementia in elderly people. Interrelations between AD and senescence have been the subject of many studies. Some researchers have suggested that chromosomal alterations may be involved in the etiology or pathogenesis of AD. We present cytogenetic findings in patients with Alzheimer's disease, normal elderly controls and young controls. Aneuploidy, premature centromere division, polyploidy and C-anaphase, were analysed and the results suggest that the cytogenetic alterations observed are inherent to the cellular ageing process and not specifically related to Alzheimer's disease.

Adolescent↗
PubMed2002

Megakaryocyte polyploidization is associated with a functional gene amplification.

It is believed that polyploidy induces an orchestrated increase in gene expression. To know whether all alleles remain functional during megakaryocyte polyploidization, we used a well-established fluorescence in situ hybridization technique which allows one to simultaneously detect pre-mRNAs and assess ploidy level in a single cell. All alleles of GPIIb, GPIIIa, VWF, beta-actin, hsp70, c-mpl, Fli-1, and FOG-1 genes are transcriptionally active in megakaryocytes from 4N to 32N. All X chromosomes in male cells are transcriptionally active but only half of them are transcriptionally active in female megakaryocytes, as revealed by the transcriptional activity of the GATA-1 gene. Nuclear untranslated XIST RNA accumulates on the inactivated X chromosomes, indicating that they are subjected to a normal inactivation process. Altogether, our results demonstrate that megakaryocyte polyploidization results in a functional gene amplification whose likely function is an increase in protein synthesis parallel with cell enlargement.

Alleles↗
PubMed2003

Survivin regulates hematopoietic progenitor cell proliferation through p21WAF1/Cip1-dependent and -independent pathways.

The cyclin-dependent kinase inhibitor p21WAF1/Cip1 and Survivin enhance granulocyte macrophage colony-forming unit (CFU-GM) cell cycle and proliferation and have been implicated as antiapoptotic proteins. We investigated the relationships between p21 and Survivin in primary CFU-GM and c-kit+, lineage-negative (Lin-) cells and demonstrate p21-dependent and -independent pathways whereby Survivin regulates progenitor cell proliferation. Ectopic Survivin enhanced p21+/+ CFU-GM formation and expansion of c-kit+, Lin- cells, whereas p21 gene loss abrogated these effects, indicating a p21 requirement. A dominant-negative form of Survivin and p21 gene deletion accelerated the loss of CFU-GM upon growth factor deprivation, and wild-type Survivin overexpression inhibited apoptosis of p21+/+ CFU-GM and c-kit+, Lin- cells but not p21-/- cells, suggesting that both Survivin and p21 block apoptosis of progenitors and that Survivin-mediated antiapoptosis requires p21. In contrast to the p21-dependent antiapoptotic effects, Survivin increased the proportion of CFU-GM in S-phase in both p21+/+ and p21-/- cells. Furthermore, modulating Survivin expression increased polyploidy in c-kit+, Lin- cells, which was accentuated by p21 deficiency. These results suggest that the Survivin-p21 axis plays an important role in the proliferation of normal hematopoietic cells and that Survivin regulates apoptosis through a p21 WAF1/Cip1-dependent pathway but may control S-phase entry independent of p21.

Animals↗
PubMed2004

Aberrant quantity and localization of Aurora-B/AIM-1 and survivin during megakaryocyte polyploidization and the consequences of Aurora-B/AIM-1-deregulated expression.

Megakaryocytes skip late anaphase and cytokinesis during endomitosis. We found normal expression and localization of a fundamental regulator of mitosis, Aurora-B/AIM-1, during prophase in polyploidizing mouse bone marrow megakaryocytes. At late anaphase, however, Aurora-B/AIM-1 is absent or mislocalized. Megakaryocytes treated with a proteasome inhibitor display Aurora-B/AIM-1 properly expressed and localized to the midzone, suggesting that protein degradation contributes to this atypical appearance. In contrast, survivin, an Aurora-B/AIM-1 coregulator of mitosis, is not detected at any stage of the endomitotic cell cycle, and in most megakaryocytes proteasome inhibition does not rescue this phenotype. To further explore the importance of reduced Aurora-B/AIM-1 for polyploidization, it was overexpressed in megakaryocytes of transgenic mice. The phenotype includes increased transgenic mRNA, but not protein, in polyploidy megakaryocytes, further suggesting that Aurora-B/AIM-1 is regulated at the protein level. Aurora-B/AIM-1 protein is, however, elevated in diploid transgenic megakaryocytes. Transgenic mice also exhibit enhanced numbers of megakaryocytes with increased proliferative potential, and some mice exhibit mild decreases in ploidy level. Hence, the molecular programming involved in endomitosis is characterized by the mislocalization or absence of at least 2 critical mitotic regulators, Aurora-B/AIM-1 and survivin. Future studies will examine the impact of survivin restoration on mouse megakaryocyte polyploidization.

Anaphase↗
PubMed2005

Induction of polyploidization in leukemic cell lines and primary bone marrow by Src kinase inhibitor SU6656.

Megakaryocytes (MKs) undergo successive rounds of endomitosis during differentiation, resulting in polyploidy (typically, 16-64N). Previous studies have demonstrated that this occurs through an interruption of normal cell cycle progression during anaphase. However, the molecular mechanism(s) controlling this unique process is undefined. In the present report, we examine the effect of an Src kinase inhibitor, SU6656, on thrombopoietin (TPO)-induced growth and differentiation. Remarkably, when SU6656 (2.5 microM) was added to a megakaryocytic cell line, UT-7/TPO, the cells ceased cell division but continued to accumulate DNA by endomitosis. During this interval, CD41 and CD61 expression on the cell surface increased. Similar effects on polyploidization and MK differentiation were seen with expanded primary MKs, bone marrow from 2 patients with myelodysplastic syndrome, and other cell lines with MK potential. Our data suggest that SU6656 might be useful as a differentiation-inducing agent for MKs and is an important tool for understanding the molecular basis of MK endomitosis.

Bone Marrow Cells↗
PubMed2006

Visualization of C. elegans transgenic arrays by GFP.

BACKGROUND: Targeting the green fluorescent protein (GFP) via the E. coli lac repressor (LacI) to a specific DNA sequence, the lac operator (lacO), allows visualization of chromosomes in yeast and mammalian cells. In principle this method of visualization could be used for genetic mosaic analysis, which requires cell-autonomous markers that can be scored easily and at single cell resolution. The C. elegans lin-3 gene encodes an epidermal growth factor family (EGF) growth factor. lin-3 is expressed in the gonadal anchor cell and acts through LET-23 (transmembrane protein tyrosine kinase and ortholog of EGF receptor) to signal the vulval precursor cells to generate vulval tissue. lin-3 is expressed in the vulval cells later, and recent evidence raises the possibility that lin-3 acts in the vulval cells as a relay signal during vulval induction. It is thus of interest to test the site of action of lin-3 by mosaic analysis. RESULTS: We visualized transgenes in living C. elegans by targeting the green fluorescent protein (GFP) via the E. coli lac repressor (LacI) to a specific 256 sequence repeat of the lac operator (lacO) incorporated into transgenes. We engineered animals to express a nuclear-localized GFP-LacI fusion protein. C. elegans cells having a lacO transgene result in nuclear-localized bright spots (i.e., GFP-LacI bound to lacO). Cells with diffuse nuclear fluorescence correspond to unbound nuclear localized GFP-LacI. We detected chromosomes in living animals by chromosomally integrating the array of the lacO repeat sequence and visualizing the integrated transgene with GFP-LacI. This detection system can be applied to determine polyploidy as well as investigating chromosome segregation. To assess the GFP-LacI*lacO system as a marker for mosaic analysis, we conducted genetic mosaic analysis of the epidermal growth factor lin-3, expressed in the anchor cell. We establish that lin-3 acts in the anchor cell to induce vulva development, demonstrating this method's utility in detecting the presence of a transgene. CONCLUSION: The GFP-LacI*lacO transgene detection system works in C. elegans for visualization of chromosomes and extrachromosomal transgenes. It can be used as a marker for genetic mosaic analysis. The lacO repeat sequence as an extrachromosomal array becomes a valuable technique allowing rapid, accurate determination of spontaneous loss of the array, thereby allowing high-resolution mosaic analysis. The lin-3 gene is required in the anchor cell to induce the epidermal vulval precursors cells to undergo vulval development.

Animals↗
PubMed2026

Generational variation and stabilization in resynthesized allotetraploid Brassica juncea derived from diploid progenitors B. rapa and B. nigra.

BACKGROUND: Polyploidy is a major driver of plant evolution and crop improvement, generating novel variation in morphology, physiology, and agronomic traits. Brassica juncea (AABB, 2n&#x2009;=&#x2009;36), a natural allotetraploid derived from B. rapa (AA) and B. nigra (BB), is an important oilseed and vegetable crop; however, its narrow genetic base limits further breeding gains. Resynthesized B. juncea (RBJ), developed from known progenitors, provides a tractable system to investigate polyploid stabilization, trait diversification, and generational variation. This study evaluated RBJ across nine generations (F1-S8) to elucidate generational variation in morphological, molecular, cytological, and oil content traits during progressive stabilization. RESULTS: Substantial variation was observed for key yield-related traits, including siliqua length, seeds per siliqua, and thousand-seed weight. High estimates of heritability, genotypic variance, and genetic advance indicated their potential utility in selection based improvement. Comparative analyses revealed a clear generational progression, characterized by relatively enhanced performance in early generations, increased recombination-driven variability in intermediate generations, and the partial stabilization of several traits in later generations. Generation mean analysis suggested the involvement of additive, dominance, and epistatic gene effects in trait inheritance. Molecular analysis using SSR markers confirmed the amphidiploid origin and genomic integrity of RBJ generations. Cytological assessments, pollen viability assays, and flow cytometric analysis collectively demonstrated stable chromosome numbers, improved fertility, and maintenance of ploidy stability across successive generations. CONCLUSIONS: The study provides valuable insights into the generational variation and stabilization of morphological, molecular, and oil content traits in resynthesized B. juncea. The findings suggest that variability arising from polyploidization and interspecific hybridization undergoes gradual reorganization across successive generations, leading to increased trait stabilization and more consistent expression of selected agronomic characteristics. Collectively, these results contribute to the understanding of early stabilization processes in RBJ, highlighting resynthesized polyploids as useful systems for studying variation and stabilization in allopolyploid crops.

Mustard Plant↗
PubMed2001

Notch-Delta signaling induces a transition from mitotic cell cycle to endocycle in Drosophila follicle cells.

In many developmental processes, polyploid cells are generated by a variation of the normal cell cycle called the endocycle in which cells increase their genomic content without dividing. How the transition from the normal mitotic cycle to endocycle is regulated is poorly understood. We show that the transition from mitotic cycle to endocycle in the Drosophila follicle cell epithelium is regulated by the Notch pathway. Loss of Notch function in follicle cells or its ligand Delta function in the underlying germline disrupts the normal transition of the follicle cells from mitotic cycle to endocycle, mitotic cycling continues, leading to overproliferation of these cells. The regulation is at the transcriptional level, as Su(H), a downstream transcription factor in the pathway, is also required cell autonomously in follicle cells for proper transitioning to the endocycle. One target of Notch and Su(H) is likely to be the G2/M cell cycle regulator String, a phosphatase that activates Cdc2 by dephosphorylation. String is normally repressed in the follicle cells just before the endocycle transition, but is expressed when Notch is inactivated. Analysis of the activity of String enhancer elements in follicle cells reveals the presence of an element that promotes expression of String until just before the onset of polyploidy in wild-type follicle cells but well beyond this stage in Notch mutant follicle cells. This suggests that it may be the target of the endocycle promoting activity of the Notch pathway. A second element that is insensitive to Notch regulation promotes String expression earlier in follicle cell development, which explains why Notch, while active at both stages, represses String only at the mitotic cycle-endocycle transition.

Animals↗