Recombination in the B Chromosome of Maize to Produce a-B-a Chromosomes

The translocations between the supernumerary B chromosomes and the normal A chromosomes of maize provide a valuable tool for gene localizations, dosage studies and characterization of mutants as null, leaky or gain-of-function. A procedure is described, that relies on recombination in the B chromosome, for marking each of the various B-A translocations with a single dominant marker that will allow dosage classifications of individuals at the mature kernel stage. This marker is R-scm3, which conditions anthocyanin pigment in the aleurone of the endosperm and the scutellum of the embryo. A test for recombination in the B chromosome was conducted by crossing together two translocations, that were broken on opposite sides of the B centromere, and in different A chromosome arms, namely TB-1La and TB-10L18. An example was recovered that linked genetic markers on 1L and 10L to the B centromere. Cytological examination at pachytene of meiosis confirmed the new chromosomal linkage. The use of this procedure to produce a comprehensive set of uniformly marked B-A translocations is discussed.     

Association of endosperm reduction with parental imprinting in maize

Among 38 reciprocal translocations between the maize B chromosome and the proximal region of the long arm of chromosome 10 were six interchanges associated with reduced endosperm development. These six have breakpoints that are the most proximal of the set and constitute a graded series with those broken nearer the centromere which have the most abnormal phenotypes. The group of six defines three major regions that produce the endosperm effects. The remaining 32 translocations reduce kernel size very slightly, suggesting the presence of a fourth region distal to all break-points.-The affected class of kernels lacks a paternally derived representative of that segment of 10L translocated to the B centromeric element (B(10) chromosome; 10 10 B(10)). An accompanying class of kernel in which the paternal B(10) chromosome is duplicated in the endosperm (10 10 10(B) B(10) B(10)) is normal. Kernels of the same endosperm constitution synthesized by introducing both 10 and B(10) maternally, however, are defective, resembling 10 10 10(B). Maternal B(10)'s are therefore unable to compensate for the absence of a paternal B(10). Clearly expression of the 10L genes involved supports normal endosperm growth only following pollen transmission.     

Endosperm-specific demethylation and activation of specific alleles of α-tubulin genes of Zea mays L.

We have investigated the methylation status of the α-tubulin genes, and the degree of accumulation of their mRNAs in endosperm, embryo and seedling tissues of Zea mays L. We have found that many of the α-tubulin genes are differentially demethylated in the endosperm relative to the embryo and seedling. However, only for tubα2 and tubα4 could a correlation between DNA demethylation and increased RNA accumulation be detected. By analyzing the inbred lines W64A and A69Y and their reciprocal crosses, we have also identified in the endosperm two α-tubulin genes, tubα3 and tubα4, that are differentially demethylated if transmitted by the maternal germline, but that remain hypermethylated when transmitted by the paternal germline.     

玉米籽粒性状的遗传模型研究

用１０个遗传上和籽粒形态性状上具有差异的玉米自交系,依多种可能的交配方法获得亲本Ｐ１、Ｐ２、Ｆ１（Ｐ１× Ｐ２）、Ｆ２、Ｂ１（Ｆ１×Ｐ１）、Ｂ２（Ｆ１× Ｐ２）及其相应反交ＲＦ１、ＲＦ２、ＲＢ１、ＲＢ２共１０个种子世代。种植２年。依广义遗传模型建立包括种子胚乳加性、胚乳显性、母体加性、母体显性和细胞质效应的遗传模型,运用种子数量性状的精细鉴别法［１］和混合模型分析法［２,３］,对粒长、粒宽、粒长宽比、粒厚及百粒重作了性状表达遗传机制的鉴别与探讨。单个组合的遗传模型精细测验表明,５个籽粒性状的遗传主要受母体显性和胚乳基因型（包括加性和灵性）的控制,一个组合的粒宽、粒厚和百粒重上还检测到细胞质效应。对２５对 Ｆ１正反交组合世代均值依ＭＩＮＱＵＥ法分析的结果表明,５个籽粒性状的遗传方差中,母体遗传方差占６０％以上,胚乳基因型方差低于４０％,粒长和百粒重还有细胞质效应,约占１０％～３０％。可见,籽粒性状的遗传特点是受多套遗传系统控制,其中以母体基因型的作用最大。     

Cell cycle progression during endosperm development in Zea mays depends on parental dosage effects

Interploidy crosses in flowering plants often cause seed abortion. Studies in maize have shown that failure of kernel development results from dosage effects among products of imprinted but as-yet-unknown genes in the endosperm, and that the operative stoichiometry is established for a ratio of two maternal genomes to one paternal genome. In this study, we used flow cytometry to monitor cell cycle activities in developing endosperms obtained after reciprocal crosses between diploid and tetraploid maize individuals. Our data show that dosage effects alter critical events involved in the establishment of endoreduplication during maize endosperm development. Particularly, maternal genomic excess (4x x 2x crosses) forces endosperm cells to enter early into endoreduplication while paternal genomic excess (2x x 4x crosses) prevents its establishment. Our results also suggest that altering mechanisms depend on two different sets of cell cycle regulatory genes--one imprinted through the female that is required for mitotic arrest, and another responsible for re-entry into S phase that is imprinted through the male. Further, molecular and physiological analyses should provide insights into the interaction of parental imprinting action and cell cycle regulation during endosperm development.     

Sugar Uptake and Metabolism in the Developing Endosperm of Tassel-seed Tunicate (Ts-5 Tu) Maize

Factors regulating assimilate transport into developing maize (Zea mays L.) kernels have been difficult to determine because of the structural complexity of basal kernel tissues and the damage that results from tissue dissection. The sensitivity of maize kernels to experimental manipulation is such that substantial maternal tissue is required to support kernel growth in vitro. Consequently, sugar transport experiments with isolated seed tissues or detached kernels have not unequivocally demonstrated how sugar transport occurs. In the present study, Tassel-seed Tunicate (Ts-5 Tu) maize kernels were investigated as a model system for introducing test solutions into the pedicel apoplast with minimal wounding. Transpiration in leafy glumes drew ¹⁴C-sugar solutions up the 8- to 10-millimeter-long pedicel stalks into the basal endosperm transfer cell region. ¹⁴C from fructose was incorporated into starch for 8 days. Sugar uptake into endosperm and embryo tissue showed specificity and inhibitor sensitivity. In particular, p-chloromercuribenzene sulfonate partially inhibited fructose uptake into the endosperm but had no effect on the metabolic conversion of that fructose that entered the endosperm. These results are consistent with active, carrier-mediated sugar transport, but a definitive determination would require more detailed tissue analysis. We propose that further refinement of the incubation solution may allow long-term kernel growth without cob tissue and thus provide a more precise determination of which maternal factors influence seed development.     

水稻亚种间杂交粒重及加工品质性状的遗传效应分析

用4个广亲和粳型品种和5个籼型品种为材料,按NCⅡ设计配制杂交组合,获得同一环境下的亲本及F_1植株上的籽粒群体(F_2),对其稻谷千粒重、糙米千粒重、出糙率、总精米率以及整精米率等粒重和加工品质性状进行测定,并按胚乳性状遗传模型和混合线性模型的分析方法对籼粳亚种间杂交稻粒重及加工品质性状的遗传效应进行了研究,结果表明:籼粳交籽粒的粒重及各加工品质性状同时受到胚乳直接基因效应、母体基因效应以及微弱的细胞质效应的影响,但其主要受制于母体加性效应,并且存在一定的胚乳杂种优势和母体杂种优势;不同亲本品种对于粒重及加工品质性状的遗传改良具有不同的作用.     

Increased Maternal Genome Dosage Bypasses the Requirement of the FIS Polycomb Repressive Complex 2 in Arabidopsis Seed Development

Seed development in flowering plants is initiated after a double fertilization event with two sperm cells fertilizing two female gametes, the egg cell and the central cell, leading to the formation of embryo and endosperm, respectively. In most species the endosperm is a polyploid tissue inheriting two maternal genomes and one paternal genome. As a consequence of this particular genomic configuration the endosperm is a dosage sensitive tissue, and changes in the ratio of maternal to paternal contributions strongly impact on endosperm development. The FERTILIZATION INDEPENDENT SEED (FIS) Polycomb Repressive Complex 2 (PRC2) is essential for endosperm development; however, the underlying forces that led to the evolution of the FIS-PRC2 remained unknown. Here, we show that the functional requirement of the FIS-PRC2 can be bypassed by increasing the ratio of maternal to paternal genomes in the endosperm, suggesting that the main functional requirement of the FIS-PRC2 is to balance parental genome contributions and to reduce genetic conflict. We furthermore reveal that the AGAMOUS LIKE (AGL) gene AGL62 acts as a dosage-sensitive seed size regulator and that reduced expression of AGL62 might be responsible for reduced size of seeds with increased maternal genome dosage.     

The significance of genic balance to endosperm development in interspecific crosses

Summary The endosperm has played a significant role in the evolution of angiosperms because of its physiological and genetic relationships to the embryo. One manifestation of this evolutionary role is its abnormal development in interploidy crosses. It is now established that the endosperm develops abnormally in interploidy-intraspecific crosses when the maternal: paternal genome ratio deviates from 21 in the endosperm itself. We propose an Endosperm Balance Number (EBN) hypothesis to explain endosperm development in both interploidy-intraspecific and interspecific crosses. Each species is assigned an EBN on the basis of its crossing behavior to a standard species. It is the EBN which determines the effective ploidy in the endosperm of each species, and it is the EBNs which must be in a 21, maternal:paternal ratio. The EBN of a species may be determined by a few genes rather than the whole genome. This hypothesis brings most intraspecific-interploidy and interspecific crossing data under a single concept with respect to endosperm function. The implications of this hypothesis to isolating mechanisms, 2n gametes, the evolution of disomic polyploids, and reciprocal differences in seed development are discussed.     

Basis of difference between reciprocal crosses involving Triticum boeoticum and T. urartu

Summary The boeoticum () X urartu () F₁ hybrids gave small, plump and viable seeds while the reciprocal crosses with T. urartu as the female parent had long, shrivelled and non-viable seeds. Reciprocal nuclear-substitution lines comprising the nucleus of one species into the cytoplasm of the other were developed through repeated backcrossing and were crossed as female parents with respective non-recurrent parents (the cytoplasm donors). The difference between the reciprocal crosses was presumably attributable to different boeoticum urartu genomic ratios in the triploid endosperm rather than to the cytoplasmic difference between the diploid wheats. The endosperm with two doses of the boeoticum and one of the urartu genome resulted in small, plump and viable seed while the endosperm of the reciprocal crosses with two doses of the urartu and one of the boeoticum genome led to large but shrivelled and non-viable seeds irrespective of the cytoplasmic type. One dose of the paternal genome in the triploid endosperm is probably not expressed in the presence of two doses of the maternal genome thereby leading to the difference between the reciprocal crosses. The results reported here indicate that difference between reciprocal crosses may not always be attributed to cytoplasmic difference between the parental species.     

Two new B-10 translocations involved in the control of nondisjunction of the B chromosome in maize

A B-A translocation, TB-10(18), has been established involving breakpoints in the proximal region of the long arm of chromosome 10 and the minute short arm of the maize B chromosome. TB-10(18) differs in its nondisjunctional behavior at the second microspore division from TB-10(19), which has a breakpoint in the same region of 10 but in the heterochromatic region of the long arm of B, in the following ways: (1) Nondisjunction of the B¹⁰ chromosome of the TB-10(18) translocation occurs in the absence of the reciprocal element (10^B), albeit at low frequency. (2) Presence of 10^B increases the frequency of B¹⁰ nondisjunction but not to the level found for TB-10(19) and certain other translocations. (3) The frequency of B¹⁰ nondisjunction varies among closely related sublines both when 10^B is present and when it is absent. It is inferred that the B¹⁰ of TB-10(18) carries all the components of B necessary for nondisjunction but that expression is weak in the absence of 10^B, suggesting the existence in the B chromosome short arm of a factor influencing efficient nondisjunction.     

Genetic control of seed proteins in wheat

Summary Electrophoretic profiles of crude protein extracts from seed of F₁ hybrids and reciprocal crosses among diploid, tetraploid and hexaploid wheats were compared with those of their respective parental species. The electrophoretic patterns within each of three pairs of reciprocal crosses, T.boeoticum X T.urartu, T.monococcun X T. urartu and T.dicoccum X T. araraticum, were different from one another but were identical with those of their respective maternal parents. Protein bands characteristic of the paternal parents were missing in F₁ hybrid seed suggesting that the major seed proteins in wheat were presumably regulated by genotype of the maternal parent rather than by the seed genotype. However, in another three pairs of reciprocal crosses, T.boeoticum X T. durum, T.dicoccum X T.aestivum and T. zhukovskyi x T. aestivum, protein bands attributable to the paternal parents were present in the F₁ hybrid seeds indicating that the seed proteins were not always exclusively regulated by the maternal genotype. The expression of paternal genomes is presumably determined by dosage and genetic affinity of the maternal and paternal genomes in the hybrid endosperm. The maternal regulation of seed protein content is probably accomplished through the maternal control over seed size. The seed protein quality may, however, depend upon the extent of expression of the paternal genome.     

Genomic imprinting and endosperm development in flowering plants

Genomic imprinting, the parent-of-origin-specific expression of genes, plays an important role in the seed development of flowering plants. As different sets of genes are imprinted and hence silenced in maternal and paternal gametophyte genomes, the contributions of the parental genomes to the offspring are not equal. Imbalance between paternally and maternally imprinted genes, for instance as a result of interploidy crosses, or in seeds in which imprinting has been manipulated, results in aberrant seed development. It is predominantly the endosperm, and not or to a far lesser extent the embryo, that is affected by such imbalance. Deviation from the normal 2m:1p ratio in the endosperm genome has a severe effect on endosperm development, and often leads to seed abortion. Molecular expression data for imprinted genes suggest that genomic imprinting takes place only in the endosperm of the developing seed. Although far from complete, a picture of how imprinting operates in flowering plants has begun to emerge. Imprinted genes on either the maternal or paternal side are marked and silenced in a process involving DNA methylation and chromatin condensation. In addition, on the maternal side, imprinted genes are most probably under control of the polycomb FIS genes.     

Extensive maternal DNA hypomethylation in the endosperm of Zea mays

A PCR-based genomic scan has been undertaken to estimate the extent and ratio of maternally versus paternally methylated DNA regions in endosperm, embryo, and leaf of Zea mays (maize). Analysis of several inbred lines and their reciprocal crosses identified a large number of conserved, differentially methylated DNA regions (DMRs) that were specific to the endosperm. DMRs were hypomethylated at specific methylation-sensitive restriction sites upon maternal transmission, whereas upon paternal transmission, the methylation levels were similar to those observed in embryo and leaf. Maternal hypomethylation was extensive and offers a likely explanation for the 13% reduction in methyl-cytosine content of the endosperm compared with leaf tissue. DMRs showed identity to expressed genic regions, were observed early after fertilization, and maintained at a later stage of endosperm development. The implications of extensive maternal hypomethylation with respect to endosperm development and epigenetic reprogramming will be discussed.     

Abscisic Acid inhibition of endosperm cell division in cultured maize kernels

The response of developing maize (Zea mays L.) endosperm to elevated levels of abscisic acid (ABA) was investigated. Maize kernels and subtending cob sections were excised at 5 days after pollination (DAP) and placed in culture with or without 90 micromolar (±)-ABA in the medium. A decreased number of cells per endosperm was observed at 10 DAP (and later sampling times) in kernels cultured in medium containing ABA from 5 DAP, and in kernels transferred at 8 DAP to medium containing ABA, but not in kernels transferred at 11 DAP to medium containing ABA. The number of starch granules per endosperm was decreased in some treatments, but the reduction, when apparent, was comparable to the decreased number of endosperm cells. The effect on endosperm fresh weight was slight, transient, and appeared to be secondary to the effect on cell number. Mature endosperm dry weight was reduced when kernels were cultured continuously in medium containing ABA. Endosperm (+)-ABA content of kernels cultured in 0, 3, 10, 30, 100, or 300 micromolar (±)-ABA was measured at 10 DAP by indirect ELISA using a monoclonal antibody. Content of (+)-ABA in endosperms correlated negatively (R = −0.92) with endosperm cell number. On the basis of these studies we propose that during early kernel development, elevated levels of ABA decrease the rate of cell division in maize endosperm which, in turn, could limit the storage capacity of the kernel.