Mother Cell Meiosis Research Articles

Calcium controls the formation of vacuoles from mitochondria to regulate microspore development in wheat.

Potassium antimonite was used to investigate the localisation of calcium in developing wheat anthers to examine the relationship between Ca2+ and pollen development. During anther development, calcium precipitate formation increased in anther wall cells prior to microspore mother cell meiosis and appeared in microspores, suggesting the presence of a calcium influx from anther wall cells into the locule. Initially, the precipitates in microspore cytoplasm primarily accumulated in the mitochondria and destroyed their inner membranes (cisterns) to become small vacuoles, which expanded and fused, ultimately becoming a large vacuole during microspore vacuolisation. After microspore division and large vacuole decomposition, many calcium precipitates again accumulated in the small vacuoles, indicating that calcium from the large vacuole moved back into the cytoplasm of bicellular pollen.

Identification and expression analysis of microRNAs during ovule development in rice (Oryza sativa) by deep sequencing.

MicroRNA (miRNA) expression profiles during rice ovule development revealed the possible miRNA-mediated regulation between ovule sporophytic tissue and female gametophyte and the involvement of miRNAs in programmed cell death. MiRNAs are 20-24-nucleotide small RNAs that play key roles in the regulation of many growth and developmental processes in plants. Rice ovule development comprises a series of biological events, which are regulated by complex molecular mechanisms. To gain insight into miRNA-mediated regulation of rice ovule development, Illumina sequencing was used to examine the expression of miRNAs from the megaspore mother cell meiosis stage to the fertilized ovule stage. Based on the sequencing data, 486 known and 204 novel miRNAs were identified during rice ovule development. Moreover, 56, 65 and 11 differentially expressed miRNAs between adjacent developmental stages were identified. By analyzing transcriptome and degradome data, we identified 41, 65 and 12 coherent target genes for the differentially expressed miRNAs in ovule development. We found that changes in the expression of plant hormone-related miRNAs may play important roles in embryo sac development, providing evidence for cross-talk communication between sporophytic tissue and the female gametophyte. Additionally, we revealed that miRNAs may be involved in programmed cell death after fertilization. Finally, we constructed miRNA-mediated regulatory networks that are active during rice ovule development.

Sugars, antioxidant enzymes and IAA mediate salicylic acid to prevent rice spikelet degeneration caused by heat stress

Salicylic acid (SA) can alleviate damage to rice plants induced by heat stress, but its role in preventing spikelet degeneration under high-temperature stress has not been documented. Rice plants pretreated with SA (0–50 mmol L−1) were subjected to heat stress at 40 °C at the pollen mother cell meiosis stage for 10 days. The results indicated that there were no significant differences in grain yields and yield components among rice plants that were exogenously sprayed with SA (0–50 mmol L−1) under natural conditions. Under heat stress, the grain yield, spikelet number per panicle and setting rate in response to SA treatments were higher than under the control (0 mmol L−1 SA or NON-SA) treatment, especially with 1 and 10 mmol L−1 SA. A higher grain yield, spikelet number per panicle and setting rate were recorded in these two SA treatments compared with the NON-SA treatment. During this process, soluble sugars, proline, phytohormones including ABA, GA3, BRs, IAA, ZR and JA, and antioxidant enzymes, such as superoxide dismutase, peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX), were induced by SA. Moreover, soluble sugars, IAA, POD and APX in the spikelets with SA treatments were not only higher than with the NON-SA treatment but the changing patterns were also similar to that of the spikelet number per panicle under natural conditions and heat stress. Therefore, our results suggest that sugars, antioxidant enzymes and IAA might mediate SA to prevent spikelet degeneration caused by heat stress.

Induction of unreduced megaspores in Eucommia ulmoides by high temperature treatment during megasporogenesis

Eucommia ulmoides Oliver is an important temperate species because it contains Eucommia rubber, iridoids, and other medicinal substances. Triploid breeding may be an effective way to improve the contents of gutta percha and other secondary metabolites in Eucommia leaves. To obtain triploid trees, 2n female gametes were induced by exposing female buds to high temperatures. The results showed that although the Eucommia female flower bud development is not synchronous, it still exhibits regularity. We estimated the period of megaspore mother cell meiosis based on the number of days since the adjacent male flower produced its pollen.Using female buds at 1–4 days after pollination, we obtained triploid seedlings after female buds were treated with different temperatures and durations to induce megaspore chromosome doubling. Among the offspring, 23 of Eucommia triploid seedlings were detected. Our findings indicate that the most suitable condition for inducing megaspore chromosome doubling occurred at 3 days after adjacent male tree produce pollen when the meiosis of the megasporocytes was between pachytene and diplotene. The best treatment of 45 °C for 4 h gave triploid induction of 5.74 %.

Pollen grain development is highly sensitive to temperature stress inVitis vinifera

Background and Aims The grapevine (Vitis vinifera L.) is often exposed to above optimal temperature in the field. Although genomic analysis has been underway for over a decade, studies on grapevine chromosome behaviour and nuclear topology are scarce. Here, we explore the influence of hot-water sanitation treatments, involving immersing dormant canes in water at 50°C for 45 min or heat exposure to 42°C for 4 h during reproductive growth, on the dynamics of somatic nuclei and meiotic chromosomes. Methods and Results Chromatin behaviour was analysed by 4′,6-diamidino-2-phenylindole hydrochloride staining, fluorescent in situ hybridisation with various probes, and immunocytochemistry with an antibody against DNA methylated at cytosine residues. Sanitation procedures had no effect on somatic or meiotic chromatin behaviour or nuclear topology. In contrast, although meiosis proceeded normally during exposure to 42°C for 4 h, this heat regime resulted in highly disrupted pollen grains likely due to cell wall fragility and in a significant reduction in pollen viability. Conclusions Grapevine pollen mother cell meiosis is described in detail for the first time. Although hot-water sanitation treatments had no effect on chromatin behaviour, the range of temperature to which grapevines are often exposed to in the field induces gametophyte malformations associated with decreased pollen viability. Significance of the Study Heat stress during reproductive development disrupts pollen grain development and can therefore have implications on yield. Such knowledge may offer a tool for identification and selection of heat-tolerant genotypes.

Cytogenetic and molecular identification of small-segment chromosome translocation lines from wheat-rye substitution lines to create wheat germplasm with beneficial traits

ABSTRACTIntergeneric crop plant hybrid lines with small-segment chromosome translocations are very useful in plant genetic research and breeding. In this study, to create small-segment chromosome translocations with beneficial agronomic characters, the progeny of wheat-rye substitution lines 5R/5A and 6R/6A were selected from generations F2 to F5 for rye-specific characteristics. A PCR primer and specific simple sequence repeat marker for rye were used in F5 populations to detect rye chromatin and to amplify a specific chromosome band in six translocation lines (06-6-5, 06-6-6, 06-6-9, 6-26-1, 7-23, and 7-33). Fragment pSc119.1 cloned from 7-33 had 99% homology with the big ear gene sequence (GenBank AF512607.1) in wheat. The six lines were further characterized via pollen mother cell meiosis analysis for genetic stability, and chromosome C-banding and genomic in situ hybridization for rye chromatin. The results show that line 7-33 was still within the 5R/5A substitution lines and possessed the big ear gene. The other lines all contained small-segment rye chromosome translocations. The results indicated that substitution line hybridization is an effective method for creating small-segment chromosome translocations with useful agronomic traits. Trials for these six wheat-rye translocation lines are justified because they possess many important stably-inherited agronomic characters, including disease resistance and improved yield.

Relationship of metabolism of reactive oxygen species with cytoplasmic male sterility in pepper ( Capsicum annuum L.)

Pepper cytoplasmic male sterility (CMS) line 9704A is one of the CMS types used for hybrid pepper ( Capsicum annuum L.) production in China. Our previous studies suggested that CMS-9704A may suffer from oxidative stress as its cyanide-resistant respiration is lower than that of the maintainer line. To elucidate the metabolic mechanism of reactive oxygen species (ROS) in the CMS-pepper anthers, the metabolism changes in the production and scavenging of ROS and gene expression related to ROS-scavenging enzymes were investigated in the anther mitochondria of CMS-9704A and maintainer-9704B. At the abortion stage (pollen mother cell meiosis stage), anthers of CMS-9704A had higher contents of O 2 −, H 2O 2 and MDA than those of the maintainer. Simultaneously, there were higher activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in scavenging ROS in the anthers of the CMS line than in the maintainer. Abundance of POD, Mn-SOD, CAT1, CAT2 and CAT3 mRNA in anthers of CMS-9704A corresponded with the activities of these three genes, except for that of Cu/Zn-SOD and Irion-SOD. Therefore, it is possible that the sterility in CMS-9704A is related to the abortion of microspores, induced by chronic oxidative stress caused by an abnormal increase in ROS, and also by membrane lipid peroxidation in the mitochondria in the early stage of pollen development.

Microtubule organization during successive microsporogenesis in Allium cepa and simultaneous cytokinesis in Nicotiana tabacum

Microtubule cytoskeleton organization during microspore mother cell (MMC) meiosis in Allium cepa L. and microsporogenesis in Nicotiana tabacum L. was examined. The MMC microtubules (MTs) were short and well dispersed in the cytoplasm of both taxa. As the MMCs of both species entered metaphase of meiosis I, the MTs constructed a spindle that facilitated the chromosomes to orient in the meridian plane. At anaphase of meiosis I, the spindle MTs differentiated into two types: one MT type became short, pulled the chromosomes toward the two poles, and was designated as centromere MTs; the second type of MT connected the two poles, and was designated as pole MTs. In A. cepa, where successive cytokinesis was observed, pole MTs assumed a tubbish shape. Some new short MTs aggregated in the meridian plane and constricted to form a phragmoplast, which developed into a cell plate, divided the cytoplasm into two parts and produced a dyad. However, in tobacco, a phragmoplast was not generated in anaphase of meiosis I and II and cytokinesis did not occur. The spindle MTs depolymerized and reorganized the radial arrangement of MTs from the nucleate surface to the periplasm during anaphase. Following telophase of meiosis II, the cytoplasm produced centripetal furrows, which met in the center of the cell and divided it into four parts, serving as a form of cytokinesis. In this process, MTs appeared to bear no relationship to cytokinesis.

Allelic interaction of F1 pollen sterility loci and abnormal chromosome behaviour caused pollen sterility in intersubspecific autotetraploid rice hybrids

The intersubspecific hybrids of autotetraploid rice has many features that increase rice yield, but lower seed set is a major hindrance in its utilization. Pollen sterility is one of the most important factors which cause intersubspecific hybrid sterility. The hybrids with greater variation in seed set were used to study how the F1 pollen sterile loci (S-a, S-b, and S-c) interact with each other and how abnormal chromosome behaviour and allelic interaction of F1 sterility loci affect pollen fertility and seed set of intersubspecific autotetraploid rice hybrids. The results showed that interaction between pollen sterility loci have significant effects on the pollen fertility of autotetraploid hybrids, and pollen fertility further decreased with an increase in the allelic interaction of F1 pollen sterility loci. Abnormal ultra-structure and microtubule distribution patterns during pollen mother cell (PMC) meiosis were found in the hybrids with low pollen fertility in interphase and leptotene, suggesting that the effect-time of pollen sterility loci interaction was very early. There were highly significant differences in the number of quadrivalents and bivalents, and in chromosome configuration among all the hybrids, and quadrivalents decreased with an increase in the seed set of autotetraploid hybrids. Many different kinds of chromosomal abnormalities, such as chromosome straggling, chromosome lagging, asynchrony of chromosome disjunction, and tri-fission were found during the various developmental stages of PMC meiosis. All these abnormalities were significantly higher in sterile hybrids than in fertile hybrids, suggesting that pollen sterility gene interactions tend to increase the chromosomal abnormalities which cause the partial abortion of male gametes and leads to the decline in the seed set of the autotetraploid rice hybrids.

Hybridization among Epimedium (Berberidaceae) species native to China

The hybridization of fourteen plant populations belonging to seven Epimedium species native to China was studied by self-cross, infra-population cross, inter-population cross within species, and interspecific cross. Self-pollination studies on nine populations indicated high incompatibility; capsule-set rates were higher than zero in only three (4.61–6.76%). Interspecific cross-pollinations demonstrated high crossabilities in most cross combination (15.38–92.44% capsule-set rate), and the F 1 seeds possessed high germination rates (>20%). The F 1 hybrids of three interspecific cross combinations were raised to maturity. The morphology, karyomorphology of somatic cells, and pollen mother cell (PMC) meiosis of these F 1 plants revealed that they were all highly fertile (>76.10% in pollen viability), that there were few structural differences in chromosomes among species, and that most PMCs had 6 bivalents at MI. Abnormal chromosomal behaviors occurred in a minority, including chromosome bridges, unequal segregation of chromosome number, lagging chromosomes, and micronuclei. A series of experimental crosses provided strong evidence for an outbreeding system and a weak internal barrier to hybridizations in the taxa studied.

Abnormal PMC microtubule distribution pattern and chromosome behavior resulted in low pollen fertility of an intersubspecific autotetraploid rice hybrid

Intersubspecific autotetraploid rice hybrids have high heterosis in both vegetative and reproductive growth, but low seed set hinders commercial utilization of autotetraploid rice. Autotetraploid rice hybrids with high and low pollen fertility were used in the present study to compare microtubule distribution patterns and chromosome behavior during pollen mother cell (PMC) meiosis, using indirect immunofluorescence laser scanning confocal microscopy. Microtubule distribution patterns of autotetraploid hybrids were similar to diploid rice, but many different kinds of abnormalities were found in the hybrid with low pollen fertility and seed set. Abnormal microtubule organization including structurally distorted microtubules at pachytene, loosely knitted perinuclear microtubules at diakinesis during prophase I, and abnormal spindles, viz. multipoles, loss of spindle pole focus, abnormal size of spindles, and so on, were found at metaphase I and metaphase II. Some cells developed into triad with no formation of tetrad. Abnormal chromosome behaviors included high percentage of multivalents, chromosome lagging, chromosome bridges, and micronuclei. All these abnormalities were found more frequently in low-fertile hybrid than in high-fertile hybrid. These results suggest that abnormal microtubule distribution pattern is an important factor which affects pollen fertility and percentage seed set in autotetraploid rice, and may also have a close relationship with chromosome behavior.

Fertility Alternation of Thermo-Photo-Sensitive Genic Male Sterile (TGMS) Wheat Line C412S and its Association with Adenine Phosphoribosyltransferase Gene Expression

C412S, a novel thermo-photo-sensitive genic male sterile (TGMS) wheat ( Triticum aestivum L.) line, is derived from a cross between TGMS line C49S and the conventional line C412. The objectives of this study were to examine the fertility alternation of C412S planted at different sowing dates and to investigate the possible function of APRT1 and APRT2 in the fertility alternation of thermo-photo-sensitive wheat. Under the 10 treatments of seeding date from 26 October to 10 December in 2005–2007 growing seasons, C412S showed fertility variations from complete sterility to high sterility, semisterility, low sterility, and normal fertility, which were achieved by adjusting the environmental temperature and photoperiod conditions during the development of male sex cells. Smear and squash microscopic technique was employed to follow the developmental progress of pollen mother cell (PMC) meiosis and microsporogenesis. Fertility-sensitive period of C412S was from PMC formation stage to mature pollen stage, especially from PMC formation to PMC meiosis stage. C412S pollen abortion, which belongs to spherical abortion type, reached peak level at later mononucleate stage of microsporogenesis. Moreover, the full length of adenine phosphoribosyltransferase gene ( APRT) was cloned from C412S and its control line C412. Sequences analysis showed that there were a few base differences in APRT1 between C412S and C412. However, the deducted amino acid sequences of C412S and C412 are identical. According to the results of semiquantitative reverse transcription PCR (RT-PCR), the transcriptional level of APRT genes in the young spikes of the sterile line C412S was lower than that of the fertile line C412 from PMC formation to mononucleate microspore stage. This indicates that the transcriptional level of APRT in young spikes may associate with fertility alternation in TGMS wheat.

Cytological Observation of Microsporogenesis in Male-Sterile Lines of Chinese Pink ( Dianthus chinensis L.)

This study addressed the differences in microsporogenesis between male sterile and fertile lines of Chinese pink. The microsporogenesis processes of male sterile and fertile lines were histologically examined in squashed pollen grains and in paraffin embedded sections. A stable male-sterile line (H-37B) was obtained following six generations of inbreeding in a self-fertile line, followed by two generations of backcrossing. In the corresponding fertile line, development of the mature pollen grains was followed through the initiation of the sporogenous cell, microsporocyte formation, and the tetrad developmental period. In the male-sterile line, abortion of the developing pollen grains was observed to take place at various stages, namely, sporogenous cell growth, mother cell meiosis, and tetrad transformation to the uninuclear state. The pollen grains of the fertile line were spheroid, turgid, and viable. By contrast, the male-sterile line produced pollen that was irregular in shape, empty, and nonviable. The abortion of the microspore in the male-sterile line appeared to relate to abnormal growth of the tapetum layer.

Analysis of Meiotic Chromosome Behaviour in Diploid Individuals of Chrysanthemum zawadskii and Related Species (Asteraceae): Evidence for Chromosome Rearrangements

Diploid individuals (2n=18) of Chrysanthemum zawadskii, C. indicum and C. boreale, two or all three of which often occur together in the wild in East Asia, have diverse metaphase karyotypes. We hypothesised that the diverse karyotypes result from chromosome rearrangements. To test this, we investigated chromosomal behaviour during pollen mother cell (PMC) meiosis and pollen fertility, using 33 individuals collected from seven populations in Korea. All individuals contained one or two multivalents (mostly quadrivalents) at late prophase I and metaphase I in some PMCs (approximately 17–37%), and at least one or two bivalents composed of homologous chromosomes of different length and centromere position in all PMCs. This meiotic chromosomal behaviour indicates that chromosome rearrangements, i.e., reciprocal translocation and unequal crossing-over, are prevalent in Chrysanthemum species. Nevertheless, because of successful chromosome segregation in anaphase I and II, they produce ample stainable pollen with different chromosome complements. Fertilization by such gametes should result in diverse somatic karyotypes. We also discuss the possibility that older or relatively recent hybridization events have led to a high rate of irregular meiotic chromosome behaviours and pollen sterility, particularly in C. indicum.

Occurrence and cytological mechanism of 2n pollen formation in Chinese Diospyros spp. (Ebenaceae) staminate germplasm

SummarySome Diospyros spp. (Ebenaceae) staminate germplasm (2n = 6x = 90) recently found in Hubei Province, China, was of potential value for breeding. This study evaluated the natural rate of occurrence and cytological mechanism responsible for the formation of 2n pollen in this staminate germplasm. The frequencies of giant pollen, dyads, and triads detected in all accessions were 0 – 2.3%, 0.8 – 2.2%, and 2.3 – 4.3%, respectively. The correlation coefficient (r = 0.98**; P < 0.01) between the frequency of giant pollen and those of dyads and triads was highly significant, which strongly suggests that the giant pollen was 2n pollen. Cytological analysis of pollen mother cell meiosis showed that the main cytological mechanisms responsible for the formation of 2n pollen were tripolar spindles and fused spindles, instead of parallel spindles and vertical spindles. In addition, an abnormal disintegration of sister chromatids, which also produced 2n pollen in the end, was occasionally found at metaphase II. Diploid (2n) pollen produced by tripolar spindles and fused spindles was equivalent to first division restitution (FDR) gametes. Therefore, this Diospyros spp. staminate germplasm may have value for sexual polyploidisation in breeding programmes of Japanese persimmon.

Breeding for Male-Sterility Line with G. barbadense Nuclear Background and Cytological Observation of Its Microsporogenesis

To study the feasibility of utilizing the heterosis of interspecific ( Gossypium barbadense × G. hirsutum) hybrid cotton based on cytoplasmic male-sterility system, a G. barbadense male-sterility line HaiA with G. harknessii cytoplasm and its maintainer HaiB were developed by replacing nuclear genome from G. hirsutum to G. barbadense in a backcross breeding program. Its male cell abortion was observed using a light microscope through a paraffin slice for H.E staining technique. The results showed that the abortion characteristics in HaiA were not significantly changed at the cytological level, although G. hirsutum nuclear genome of sterile line was replaced by G. barbadense nuclear genome, and its abortion mainly took place at the stage of the microspore mother cell meiosis. HaiA sterility was very stable because of its early abortion and no-poltens in anthers. Hybrids between HaiA and G. hirsutum restorers were high in pollen viability, boll setting rate, boll weight and lint percentage which contributed to high yield heterosis. It is suggested that HaiA is of great value in the application of cotton interspecific hybrid seed production.

Metabolism of reactive oxygen species in cotton cytoplasmic male sterility and its restoration

To elucidate reactive oxygen species (ROS) metabolism of cotton cytoplasmic male sterility and the effects of restorer gene on the metabolism of ROS, the metabolism changes in the production and scavenging of ROS and gene expression related to ROS-scavenging enzymes were investigated in the anther mitochondria of CMS line, maintainer line and hybrid F(1). During the abortion preliminary stage (sporogenous cell division stage), anthers of CMS line had a little higher superoxide (O(2)(-)) production rate and hydrogen peroxide (H(2)O(2)) and malondialdehyde (MDA) contents than those of maintainer or hybrid F(1. )Simultaneously, a little higher ROS contents might serve as a signal to increase the activity of superoxide dismutase (SOD) in anthers of CMS line to reduce the ROS damage to the anther development. But at the abortion peak (pollen mother cell meiosis stage), anthers of CMS line had extraordinarily higher ROS contents and lower ROS-scavenging enzymic activities compared with the hybrid F(1), during which the ROS contents and ROS-scavenging enzymic activities in hybrid F(1) were approximate to those of maintainer line. The expression of Mn-sod and apx mRNA in anther of CMS line was obviously inhibited when ROS produced with a great deal during anther abortion, however the gene expression in hybrid F(1) kept normal with the maintainer. Excessive accumulation of O(2)(*-) , H(2)O(2 )and MDA, significant reduction of ROS-scavenging enzymic activities and lower gene expression level of ROS-scavenging enzyme were coinstantaneous with male cells death in anthers of CMS line. But when the restorer gene was transferred into CMS line, excessive production of ROS could be eliminated in the anthers of hybrid F(1). The restorer gene likely plays an important role in keeping the dynamic balance between the production and elimination of ROS.

Metabolism of Reactive Oxygen Species in the Cytoplasmic Male-Sterile Cotton Anther

Reactive oxygen species (ROS) in plant cell, including superoxide (O 2), hydrogen peroxide (H 2O 2), and malondialdehyde (MDA), are thought to be important inducible factors of cell apoptosis if excessively accumulated in cells. To elucidate the metabolic mechanism of ROS production and scavenging in anthers of the cytoplasmic male-sterile (CMS) cotton. CMS line, maintainer, and hybrid F 1 anthers, were employed for studying the relationship between CMS and metabolism of ROS, by comparing ROS changes in the sterile and fertile anthers at different developmental stages. The results showed that during the abortion preliminary stage (sporogenous cell division stage), anthers of CMS line had higher contents of O 2, H 2O 2, and MDA than those of maintainer or hybrid F 1. Simultaneously, the higher activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) in scavenging ROS were measured in the anthers of the CMS line, indicating that an increase of ROS in anthers of abortion preliminary stage had an inducible effect on the antioxidant enzymes. But during the abortion peak of CMS anther (pollen mother cell meiosis stage), on the one hand, contents of O 2. H 2O 2, and MDA were extraordinarily high in CMS anthers, on the other hand, the activities of SOD, CAT, and POD were excessively low, which disrupted the balance between the production and elimination of ROS and led to pollen mother cells apoptosis at this stage. In the following two stages (uninucleate microspore stage and mature pollen stage), the contents of O 2 and H 2O 2 in the aborted anthers were approximated to contents in the fertile anthers of the maintainer and hybrid F 1. However, MDA contents were continuously raised and enzymic activities of SOD, CAT, and POD were consistently decreased in sterile anthers, which indicated that ROS still had harmful effects on the anthers after the apoptosis of the male cells. Excessive accumulation of O 2, H 2O 2, and MDA and significant reduction of ROS scavenging-enzyme activities were coinstantaneous with male cells apoptosis in the anthers of the cotton CMS line. But when the restorer gene was transferred into the CMS line, excessive production of ROS could be eliminated in the anthers of hybrid F 1.

CYTOGENETIC STUDY OF DIPLOID ACTINIDIA CHINENSIS - KARYOTYPE, MORPHOLOGY OF SEX CHROMOSOMES AT PRIMARY DIFFERENTIATION STAGE AND EVOLUTIONARY SIGNIFICANCE

The karyotype of diploid Actinidia chinensis is 2n = 2 = 58 = 38m + 18sm (2SAT) + 2t. The ratio of the largest chromosome to the shortest chromosome is 2.35 and the karyotype is type 2B. Although the two SAT-chromosomes have similar lengths and shapes, they are probably the chromosomes reponsible for sex determination. The SAT region of nucleolar chromosomes may be the region of sex determination. At the pachytene stage of pollen mother cell (PMC) meiosis, the two-nucleolar chromosomes of pistillate plants pair tightly, in a linear manner. In staminate plants the SAT region is about 15% of the total nucleolar chromosomal length and the nucleolar chromosomes are obviously heteromorphic and they cannot pair; for the remaining 85% of their length (i.e., non-SAT regions, chromosomes can pair tightly, making the nucleolar divalent of staminate plants display a “Y” shape. Therefore, it is assumed that sex chromosomes are probably at an early stage of differentiation and that the sex determination mechanism is probably of the XX/XY type. Because of the configuration character of PMC chromosomes during M1 (1-2 quadruples or 1-2 secondary pairings between divalents) and because the 8th and 9th pairs and 11th and 12th pairs of chromosomes respectively are similar in length, Lt/St and in morphology, it is concluded that diploid Actinidia chinensis is probably derived from a paleotetraploid. The comparison of cytological characters between Actinidia ( =29) and related genera Clematoclethra =24, Saurauia ; =15）shows that Actinidia is a relatively evolved genus. Morphological analysis of the chromosomes at the pachytene stage may be an effective approach to a study of the sex determination mechanisms of dioecious plants, which are at an early stage of differentiation.

Flower developmental stage and organ sensitivity of bell pepper (Capsicum annuum L.) to elevated temperature

AbstractHigh temperatures adversely affect crop productivity of several plant species including bell pepper (Capsicum annuum L. var. annuum). The objectives of this study were: (1) to determine whether flower ontogeny is adversely affected by high temperature during different phases of development, including pre‐ and post‐pollination events; (2) to determine the duration of high temperature exposure necessary to cause reduction in fruit set; and (3) to determine whether injury to the pistil or stamen during development is responsible for reduced fruit set during high temperature. We determined that flower buds at &lt;2·5 mm in length, corresponding to microspore mother cell meiosis to tetrad dissolution, and flowers that reached anthesis during the high temperature exposure had reduced fruit set when exposed to 33 °C for 48 or 120 h. Flower buds at &lt;2·5 mm in length also had reduced pollen viability when exposed to 33 °C for 120 h. Morphological examination demonstrated that meiocytes initiated tetrad formation, but after dissolution the microspores remained small and clumped without a thick exine. High temperature exposure at a late‐development, pre‐anthesis stage did not affect pistil or stamen viability, but high post‐pollination temperatures inhibited fruit set, suggesting that fertilization is sensitive to high temperature stress.