End Of Meiosis Research Articles

NLRP4E regulates actin cap formation through SRC and CDC42 during oocyte meiosis

BackgroundMembers of the nucleotide-binding oligomerization domain, leucine rich repeat and pyrin domain containing (NLRP) family regulate various physiological and pathological processes. However, none have been shown to regulate actin cap formation or spindle translocation during the asymmetric division of oocyte meiosis I. NLRP4E has been reported as a candidate protein in female fertility, but its function is unknown.MethodsImmunofluorescence, reverse transcription polymerase chain reaction (RT-PCR), and western blotting were employed to examine the localization and expression levels of NLRP4E and related proteins in mouse oocytes. small interfering RNA (siRNA) and antibody transfection were used to knock down NLRP4E and other proteins. Immunoprecipitation (IP)-mass spectrometry was used to identify the potential proteins interacting with NLRP4E. Coimmunoprecipitation (Co-IP) was used to verify the protein interactions. Wild type (WT) or mutant NLRP4E messenger RNA (mRNA) was injected into oocytes for rescue experiments. In vitro phosphorylation was employed to examine the activation of steroid receptor coactivator (SRC) by NLRP4E.ResultsNLRP4E was more predominant within oocytes compared with other NLRP4 members. NLRP4E knockdown significantly inhibited actin cap formation and spindle translocation toward the cap region, resulting in the failure of polar body extrusion at the end of meiosis I. Mechanistically, GRIN1, and GANO1 activated NLRP4E by phosphorylation at Ser429 and Thr430; p-NLRP4E is translocated and is accumulated in the actin cap region during spindle translocation. Next, we found that p-NLRP4E directly phosphorylated SRC at Tyr418, while p-SRC negatively regulated p-CDC42-S71, an inactive form of CDC42 that promotes actin cap formation and spindle translocation in the GTP-bound form.ConclusionsNLRP4E activated by GRIN1 and GANO1 regulates actin cap formation and spindle translocation toward the cap region through upregulation of p-SRC-Tyr418 and downregulation of p-CDC42-S71 during meiosis I.

Qualitative rather than quantitative phosphoregulation shapes the end of meiosis I in budding yeast

Exit from mitosis is brought about by dramatic changes in the phosphoproteome landscape. A drop in Cyclin-dependent kinase (Cdk) activity, the master regulatory kinase, and activation of counteracting phosphatases such as Cdc14 in budding yeast, results in ordered substrate dephosphorylation, allowing entry into a new cell cycle and replication licensing. In meiosis however, two cell divisions have to be executed without intermediate DNA replication, implying that global phosphorylation and dephosphorylation have to be adapted to the challenges of meiosis. Using a global time-resolved phosphoproteomics approach in budding yeast, we compared the phosphoproteome landscape between mitotic exit and the transition from meiosis I to meiosis II. We found that unlike exit from mitosis, Cdk phosphomotifs remain mostly stably phosphorylated at the end of meiosis I, whereas a majority of Cdk-unrelated motifs are reset by dephosphorylation. However, inducing an artificial drop of Cdk at metaphase of meiosis I leads to ordered substrate dephosphorylation, comparable to mitosis, indicating that phosphoregulation of substrates at the end of meiosis I is thus mainly qualitatively rather than quantitatively ordered.

Improving Crossing Efficiency by Exploiting the Genetic Potential of Allotetraploid Cooking Bananas

Conventional breeding involves considerable genetic recombination. To maximize breeding efforts, it is necessary to increase seed yield. This study proposes to improve seed yield by using tetraploid bananas as part of a genetic improvement programme for cooking varieties. To this end, the female fertility (seeding rate and embryo sac maturity) of tetraploid banana plants was characterised. The pollen potential of tetraploids was then assessed at anthesis and at the end of meiosis, as well as during crosses with seminiferous diploids, leading to a phenotypic characterisation of the value of an offspring. This study highlighted the low seed production rate of tetraploid genitors. At best, 7% of seeds are produced per fruit, but 57% of the ovules per fruit have mature embryo sacs. The use of tetraploids as male parents, free from any source of sterility associated with the domestication process, did not generally improve the seed rate, except in the case of FHIA 21 (2930 seeds). Its pollen potential was used in a cross with Banksii 0623 (♀). This resulted in progeny with interesting vigour and phenotypic characteristics. This latter combination has the potential to transfer its “cooking” genetic structure.

Paternal imprinting in Marchantia polymorpha.

We are becoming aware of a growing number of organisms that do not express genetic information equally from both parents as a result of an epigenetic phenomenon called genomic imprinting. Recently, it was shown that the entire paternal genome is repressed during the diploid phase of the life cycle of the liverwort Marchantia polymorpha. The deposition of the repressive epigenetic mark H3K27me3 on the male pronucleus is responsible for the imprinted state, which is reset by the end of meiosis. Here, we put these recent reports in perspective of other forms of imprinting and discuss the potential mechanisms of imprinting in bryophytes and the causes of its evolution.

Induction to tetraploidy in Pacific oysters (Crassostrea gigas)

As an alternative to the use of cytochalasin B (CB), 6-dimethylamino-purine (6-DMAP) and thermal shock (heat shock by increasing the temperature from 25 to 36ºC) could be used to induce tetraploidy in Pacific oyster (Crassostrea gigas) diploids. Induction was performed by applying shocks after elimination of the first polar corpuscle at the end of meiosis I. Ploidy rates were verified using flow cytometry. Tetraploid larvae were obtained using all inductor (6-DMAP, thermal shock and CB) treatments. No difference in the efficiency of tetraploidy induction was noted among 6-DMAP, thermal shock and CB. The number of D-larvae and their yield, determined by calculating the percentage of well-formed D-larvae in relation to the total number of larvae, was similar (p > 0.05) among the evaluated induction methods. We suggest that 6-DMAP and thermal shock should be used in tetraploidy induction protocols, thereby avoiding the use of CB, which is a harmful agent for both humans and the environment.

ADAD2 regulates heterochromatin in meiotic and post-meiotic male germ cells via translation of MDC1.

ABSTRACTMale germ cells establish a unique heterochromatin domain, the XY-body, early in meiosis. How this domain is maintained through the end of meiosis and into post-meiotic germ cell differentiation is poorly understood. ADAD2 is a late meiotic male germ cell-specific RNA-binding protein, loss of which leads to post-meiotic germ cell defects. Analysis of ribosome association in Adad2 mouse mutants revealed defective translation of Mdc1, a key regulator of XY-body formation, late in meiosis. As a result, Adad2 mutants show normal establishment but failed maintenance of the XY-body. Observed XY-body defects are concurrent with abnormal autosomal heterochromatin and ultimately lead to severely perturbed post-meiotic germ cell heterochromatin and cell death. These findings highlight the requirement of ADAD2 for Mdc1 translation, the role of MDC1 in maintaining meiotic male germ cell heterochromatin and the importance of late meiotic heterochromatin for normal post-meiotic germ cell differentiation.

Nuclear variations and tapetum polyploidy related to pollen grain development in Passiflora L. (Passifloraceae).

Tapetal cells comprise an anther tissue fundamental to pollen grain development. They are associated with endoreduplication events, which culminate in polyploid and multinucleated cells, high metabolic activity, and different organelle arrangements to support all the development of the pollen grains. Passiflora species present a secretory tapetum, with diversity in the number and size of nuclei. Tapetal cells undergo numerous changes in a short period of development when compared to the plant's life span. To improve our knowledge of tapetum development, tests assessing ploidy levels, anatomy, cytochemistry, transmission electron microscopy, flow cytometry, as well as conventional and molecular cytogenetics were used in Passiflora actinia and P. elegans. The current data show striking differences in nuclear organisation during tapetal cell development, including mono to quadrinucleate cells, and ploidy levels from 2n to 32n. One of the most peculiar features was the atypical behaviour of the endoplasmic reticulum (ER), which accumulated in the cell border, similar to a 'cER', as well as large dictyosomes. This endomembrane configuration may be related to the tapetum nutritional network and secretion of compounds at the end of meiosis. Another atypical feature of the ER was the formation of an invagination to establish 'binucleated' polyploid cells. This membrane projection appears when the nuclei form two lobes, as well as when it organises a nucleoplasmic reticulum. These data demonstrate that there are important ultrastructural changes in tapetal cells, including organelle arrangements, ploidy levels, and nuclear activity, common to P. actinia and P. elegans, but different from the plant model A. thaliana.

Defects in microtubule cytoskeleton during microsporogenesis relate to pollen abortion in male-sterile pepper (Capsicum annuum L.)

Microtubule (MT) cytoskeleton of pepper male-sterile microsporocytes during meiosis was examined to characterise the defects during microsporogenesis by comparison with that of fertile ones. In sterile microsporocytes, after prophase I, abnormal arrangement of MTs occurred until the end of meiosis. At metaphase I, about 10% of microsporocytes initially formed abnormal spindles, including monopolar spindle, non-polar spindle with short kinetochore MTs, and atypical bipolar spindles with two half arrays of MTs not on the same plane. Then, the homologous chromosomes could not separate, or they could move towards the two poles with lagged chromosomes at anaphase I. At telophase I, anisotropic short MTs were formed around the two sister nuclei, or the interzonal MTs depolymerised and nuclei agglutinated. About 10% of microsporocytes degraded in advance from metaphase I to telophase I. About 90% of microsporocytes proceeded to meiosis II. At metaphase II, MTs constructed two spindles at a shallow angle with upper and lower assignment, or four spindles with different size. During anaphase II and telophase II, four nuclei were arranged in a non-tetrahedral distribution with unevenly interzonal MTs between two nuclei; or the cell formed more than four nuclei with different size. The cytokinesis formed the aberrant microspores with different size, irregular shape, and even different-sized nuclei. Therefore, the defects of MTs during microsporogenesis in the male-sterile pepper affected the process of meiosis and cytokinesis, leading to some microsporocytes degraded in advance and the microspores at the end of meiosis completely decayed.

Regulated Activation of the PAR Polarity Network Ensures a Timely and Specific Response to Spatial Cues

SummaryHow do cells polarize at the correct time and in response to the correct cues? In the C. elegans zygote, the timing and geometry of polarization rely on a single dominant cue—the sperm centrosome—that matures at the end of meiosis and specifies the nascent posterior. Polarization requires that the conserved PAR proteins, which specify polarity in the zygote, be poised to respond to the centrosome. Yet, how and when PAR proteins achieve this unpolarized, but responsive, state is unknown. We show that oocyte maturation initiates a fertilization-independent PAR activation program. PAR proteins are initially not competent to polarize but gradually acquire this ability following oocyte maturation. Surprisingly, this program allows symmetry breaking even in unfertilized oocytes lacking centrosomes. Thus, if PAR proteins can respond to multiple polarizing cues, how is specificity for the centrosome achieved? Specificity is enforced by Polo-like and Aurora kinases (PLK-1 and AIR-1 in C. elegans), which impose a delay in the activation of the PAR network so that it coincides with maturation of the centrosome cue. This delay suppresses polarization by non-centrosomal cues, which can otherwise trigger premature polarization and multiple or reversed polarity domains. Taken together, these findings identify a regulatory program that enforces proper polarization by synchronizing PAR network activation with cell cycle progression, thereby ensuring that PAR proteins respond specifically to the correct cue. Temporal control of polarity network activity is likely to be a common strategy to ensure robust, dynamic, and specific polarization in response to developmentally deployed cues.

Meiotic MCM Proteins Promote and Inhibit Crossovers During Meiotic Recombination.

Crossover formation as a result of meiotic recombination is vital for the proper segregation of homologous chromosomes at the end of meiosis I. In many organisms, crossovers are generated through two crossover pathways: Class I and Class II. To ensure accurate crossover formation, meiosis-specific protein complexes regulate the degree to which each pathway is used. One such complex is the mei-mini-chromosome maintenance (MCM) complex, which contains MCM and MCM-like proteins REC (ortholog of Mcm8), MEI-217, and MEI-218. The mei-MCM complex genetically promotes Class I crossovers and inhibits Class II crossovers in Drosophila, but it is unclear how individual mei-MCM proteins contribute to crossover regulation. In this study, we perform genetic analyses to understand how specific regions and motifs of mei-MCM proteins contribute to Class I and II crossover formation, and distribution. Our analyses show that the long, disordered N-terminus of MEI-218 is dispensable for crossover formation, and that mutations that disrupt REC’s Walker A and B motifs differentially affect Class I and Class II crossover formation. In rec Walker A mutants, Class I crossovers exhibit no change but Class II crossovers are increased. However, in rec Walker B mutants, Class I crossovers are severely impaired and Class II crossovers are increased. These results suggest that REC may form multiple complexes that exhibit differential REC-dependent ATP-binding and -hydrolyzing requirements. These results provide genetic insight into the mechanisms through which mei-MCM proteins promote Class I crossovers and inhibit Class II crossovers.

Analysis of the meiotic transcriptome reveals the genes related to the regulation of pollen abortion in cytoplasmic male-sterile pepper (Capsicum annuum L.)

CMS, which refers to the inability to generate functional pollen grains while still producing a normal gynoecium, has been widely used for pepper hybrid seed production. Pepper line 8214A is an excellent CMS line exhibiting 100% male sterility and superior economic characteristics. A TUNEL assay revealed the nuclear DNA is damaged in 8214A PMCs during meiosis. TEM images indicated that the 8214A PMCs exhibited asynchronous meiosis after prophase I, and some PMCs degraded prematurely with morphological features typical of PCD. Additionally, at the end of meiosis, the 8214A PMCs formed abnormal non-tetrahedral tetrads that degraded in situ. To identify the genes involved in the pollen abortion of line 8214A, the transcriptional profiles of the 8214A and the 8214B anthers (i.e., from the fertile maintainer line) during meiosis were analyzed using an RNA-seq approach. A total of 1355 genes were determined to be differentially expressed, including 424 and 931 up- and down- regulated genes, respectively, in the 8214A anthers during meiosis relative to the expression levels in the 8214B. The expression levels of ubiquitin ligase and cell cycle-related genes were apparently down-regulated, while the expression of methyltransferase genes was up-regulated in the 8214A anthers during meiosis, which likely contributed to the PCD of these PMCs during meiosis. Thus, our results may be useful for revealing the molecular mechanism regulating the pollen abortion of CMS pepper.

Bloom syndrome helicase in meiosis: Pro-crossover functions of an anti-crossover protein.

The functions of the Bloom syndrome helicase (BLM) and its orthologs are well characterized in mitotic DNA damage repair, but their roles within the context of meiotic recombination are less clear. In meiotic recombination, multiple repair pathways are used to repair meiotic DSBs, and current studies suggest that BLM may regulate the use of these pathways. Based on literature from Saccharomyces cerevisiae, Arabidopsis thaliana, Mus musculus, Drosophila melanogaster, and Caenorhabditis elegans, we present a unified model for a critical meiotic role of BLM and its orthologs. In this model, BLM and its orthologs utilize helicase activity to regulate the use of various pathways in meiotic recombination by continuously disassembling recombination intermediates. This unwinding activity provides the meiotic program with a steady pool of early recombination substrates, increasing the probability for a DSB to be processed by the appropriate pathway. As a result of BLM activity, crossovers are properly placed throughout the genome, promoting proper chromosomal disjunction at the end of meiosis. This unified model can be used to further refine the complex role of BLM and its orthologs in meiotic recombination.

Activated proline-rich tyrosine kinase 2 regulates meiotic spindle assembly in the mouse oocyte.

Proline-rich tyrosine kinase 2 (PYK2), a member of the protein tyrosine kinase family, plays an important role in various cellular processes. PYK2 can be phosphorylated on tyrosine 402 by diverse stimuli at the cell surface, and recent studies have shown that this activated form of PYK2 is enriched in oocytes and required for fertilization. However, the subcellular localization and functions of activated PYK2 in oocytes remain elusive. In this study, we demonstrate that the localization of p-PYK2 undergoes dynamic changes during in vitro maturation of mouse oocytes. The signal of p-PYK2 is initially dispersed in the cytoplasm, but begins to decorate organized microtubules after the germinal vesicle breakdown and localizes to spindle poles at metaphase. Our data further show that p-PYK2 colocalizes with γ-tubulin from the germinal vesicle stage through the end of meiosis in mouse oocytes. Nocodazole treatment and washout experiments confirm that p-PYK2 associates with the oocyte spindle and spindle poles. Moreover, pharmacological inhibition of PYK2 activity dramatically alters the morphology of the bipolar spindle and prevents oocyte maturation. Together, these data suggest that activated PYK2 may function as a component of the microtubule organizing center to regulate spindle assembly during the meiotic process of mouse oocytes.

Observation of Extensive Chromosome Axis Remodeling during the "Diffuse-Phase" of Meiosis in Large Genome Cereals.

The production of balanced fertile haploid gametes requires the faithful separation of paired (synapsed) chromosomes toward the end of meiotic prophase I (desynapsis). This involves the timely dissolution of the synaptonemal complex during the pachytene-diplotene transition, a stage traditionally referred to as the “diffuse stage.” In species with large genomes such as, barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) we know most about the early stages of meiotic prophase I. There, synapsis initiates at the telomeric ends of chromosomes and progresses toward the centromeric regions through the ordered assembly of the synaptonemal complex (SC). Synapsis is impacted by recombination (crossing over, CO) which locally modifies the extent of chromatin compaction and extension. CO is uneven along the chromosomes, occurring mainly toward the telomeric regions resulting in a highly skewed distribution of recombination events. However, we know very little about the process of desynapsis which occurs during the “diffuse stage,” where the synapsed and recombined chromosomes faithfully desynapse and separate into daughter cells. Here, using 3D-SIM super-resolution immuno-cytology combined with the use of antibodies directed against two crucial SC proteins, ASY1 and ZYP1, we followed the whole of meiosis I (i.e., both synapsis and desynapsis) in both barley and wheat. We showed that synapsis forms a characteristic tri-partite SC structure in zygotene (more clearly seen in barley). Toward the end of meiosis I, as the SC starts to disassemble, we show that extensive chromosome axis remodeling results in the formation of characteristic “tinsel-like” structures in both wheat and barley. By using a mutant (des10) that is severely compromised in polymerization of ZYP1during synapsis, we show that tinsel structure formation during SC dissolution is not dependant on full synapsis and may relate instead to changes in expansion stress. Our observations highlight a potentially new role for ASYNAPSIS1 (ASY1) in desynapsis, in addition to chromosome synapsis and cohesion.

Accurate Chromosome Segregation at First Meiotic Division Requires AGO4, a Protein Involved in RNA-Dependent DNA Methylation in Arabidopsis thaliana.

The RNA-directed DNA methylation (RdDM) pathway is important for the transcriptional repression of transposable elements and for heterochromatin formation. Small RNAs are key players in this process by regulating both DNA and histone methylation. Taking into account that methylation underlies gene silencing and that there are genes with meiosis-specific expression profiles, we have wondered whether genes involved in RdDM could play a role during this specialized cell division. To address this issue, we have characterized meiosis progression in pollen mother cells from Arabidopsis thaliana mutant plants defective for several proteins related to RdDM. The most relevant results were obtained for ago4-1 In this mutant, meiocytes display a slight reduction in chiasma frequency, alterations in chromatin conformation around centromeric regions, lagging chromosomes at anaphase I, and defects in spindle organization. These abnormalities lead to the formation of polyads instead of tetrads at the end of meiosis, and might be responsible for the fertility defects observed in this mutant. Findings reported here highlight an involvement of AGO4 during meiosis by ensuring accurate chromosome segregation at anaphase I.

Primary rectal mature teratoma: a case report

This report describes a rare case of primary rectal mature teratoma in a 56-year-old woman. She was referred to the outpatient clinic with a large pedunculated rectal mass, which was found during a regular health check-up. Polypectomy was performed and microscopic findings showed various structures derived from all three germ cell layers. Epidermis, hair follicles, sebaceous glands, eccrine glands, and apocrine sweat glands, with some scattered melanophages and lymphocytes were present as ectodermal derivatives. Smooth muscle fibers, blood vessels, and fibrous and adipose tissues were found as mesodermal derivatives. In addition, thyroid follicles, mucinous glands, and bronchial respiratory epithelium with peribronchial glands were detected as endodermal derivatives. She is healthy and has shown no evidence of recurrence or distant metastasis for 25 months post-surgical resection. Primary rectal teratomas are generally benign and primarily affect women. Therefore, minimally invasive surgical procedures, such as endoscopic polypectomy for a pedunculated polyp and segmentectomy for a larger mass, are satisfactory in most cases. Induction of primary rectal teratomas has been suggested to occur mainly by errors in a single germ cell after the end of meiosis I; in addition, it has also been suggested that the difference in gender incidence may be associated with differences in sex chromosomes between males and females rather than with anatomical proximity between ovary and rectum.

Embracing Diversity: Uncovering the Mechanism Underlying Interhomologous Recombination Bias during Meiosis

sis. These mutants, hop2-2 and hop2-3 ,h ave short siliques and a reduced number of seeds but otherwise appear normal. On closer inspection, they exhibit strong abnormalities in meiotic progression. During male meiosis, normal chromosome pairing does not occur in prophase, and very few bivalents (but numerous univalents) are observed among hop2-2 metaphase cells (see figure). Segregation at meiosis II yields unbalanced pools of chromosomes at the end of meiosis; thus, four haploid daughter cells are not always produced, leading to abnormal pollen and, ultimately, fewer seeds. It turns out that in these mutants, RAD51 and DMC1 are loaded onto the nucleoprotein filament as usual but only the intersister DNA repair pathway is activated, making hop2-2 and hop2-3 excellent tools for the study of interhomologous recombination bias. The hop2-2 phenotype is correlated with a decrease in HOP2/MND1 complex abundance, while the hop2-3 mutant contains truncated HOP2 protein, which binds to MND1 in an inefficient manner compared with wild-type HOP2. Therefore, both mutants have less available functional HOP2/MND1 complex than the wild type, which makes it more difficult for DMC1 to enable repair via the homologous chromosome. By contrast, genetic analysis demonstrated that HOP2/MND1 is dispensable for RAD51-mediated intersister DNA repair. In addition, mutant analysis revealed that in the presence of DMC1 and in the absence of HOP2, RAD51-mediated repair is suppressed, suggesting that DMC1 is a negative regulator of RAD51 during meiosis. This negative regulation appears to be transient and is important for the establishment of DMC1-mediated, interhomolog connections during meiotic DNA repair. Thus, while RAD51 andDMC1bothformanucleoproteinfilament

Induced desynaptic variation in poppy (Papaver somniferum L.)

Cytological investigation of EMS (ethyl methane sulphonate) treated population demonstrated enhanced univalent frequency per cell with unequal separation at Anaphase I. In contrast to controlled plants, medium strong type desynaptic plants were obtained from 0.6 % EMS treated set, revealing high frequency of univalents at Metaphase I, along with bivalents, which were loosely paired. The univalents remained unpaired till the end of meiosis, leading to formation of micronuclei and abnormal tetrads. These plants had high pollen inviability and sterile seeds. It might be possible that EMS had acted on some genes responsible for chiasma formation, resulting in early chiasma dissociation, which suggests that EMS can act as a potential tool in the development of male sterile lines. The study demonstrated the feasibility of chemical mutagenesis in mutation breeding programme on poppy (Papaver somniferum L.).

Organelle aggregations during microsporogenesis with simultaneous cytokinesis in species from the family Malvaceae (Gossypium arboreum, Alcea rosea, Lavatera thuringiaca) / Przemieszczanie się organelli komórkowych podczas mikrosporogenezy z cytokinezą równoczesną u gatunków z rodziny Malvaceae (Gossypium arboreum, Alcea rosea, Lavatera

During meiosis in microsporogenesis in spermatophytes and in sporogenesis in bryophytes and ferns, regular rearrangement of cytoplasmic organelles (chondriokinesis) occurs. In the present paper, the course of chondriokinesis has been analysed in several representatives of the family Malvaceae (Gossypium arboreum, Alcea rosea and Lavatera thuringiaca). It was revealed that cell organelles in all the species analysed aggregated around the nucleus at the end of prophase I, and next they surrounded two nuclei in telophase I. This position persisted not only until the end of meiosis, but also in post-meiotic cells after formation of microspore sporoderm on mononuclear pollen grains. Currently, this type of cell organelle aggregation has not been reported from other plant groups, and it seems to be characteristic of the representatives of the family Malvaceae only. The role of this type of cell organelle rearrangement still requires elucidation. The current theories are concerned with dividing cells, whereas the cell organelle aggregation described in the present work was observed in post-meiotic cells as well, i.e. in mononuclear pollen grains.

The cilium like region of theDrosophilaspermatocyte: an oncoming flagellum?

Primary cilia and flagella are distinct structures with different functions in eukaryotic cells. Despite the fact that they share similar basic organization and architecture, a direct developmental continuity among them has not been reported until now. The primary cilium is a dynamic structure that typically assembles and disassembles during mitotic cell cycles, whereas the sperm axoneme is nucleated by the centriole inherited by the differentiating spermatid at the end of meiosis. Fruit flies display a remarkable exception to this general rule. Drosophila spermatocytes have an unusual axoneme-based structure reminiscent of primary cilia (the cilium-like region, or CLR). This structure persists through the meiotic divisions when it is internalized with the centriole to organize the centrosome and is finally inherited by young spermatids. Examination of elongating spermatids by transmission electron microscopy (EM) and cold regrowth experiments suggests that the motile axoneme derives directly from the elongation and remodelling of the immotile CLR. Both the CLR and elongating spermatid flagella have incomplete C-tubules that form longitudinal sheets associated with the B-tubule wall, unlike axonemes of other organisms in which C-tubules stop growing at the transition between the basal body and the axonemal doublets. Moreover, both the CLR and spermatid flagella lack a structured transition zone, a characteristic feature of ciliated cells. Uncoordinated (unc) mutants that lack C-remnants have short centrioles, suggesting that the C-sheets play a role in the elongation of the centriole after it docks to the cell membrane. The structural similarities between CLR and sperm axoneme suggest that the CLR can be considered as the basal region of the future axoneme and could represent the start point for its elongation.