Teosinte crossing barrier1, a locus governing hybridization of teosinte with maize

A teosinte gene or gene cluster, Teosinte crossing barrier1 (Tcb1), that restricts crossability with maize mapped 6 centiMorgans distal to sugary-1 on chromosome 4. Tcb1 is loosely linked with the gametophyte-1 locus whose Ga1-s allele, present in many popcorns, confers nonreceptivity to the pollen of other maize varieties (ga1). Full-strength Tcb1 (positive modifiers present) was nonreceptive to Ga1-s as well as to ga1 pollen. Attenuated Tcb1 (positive modifiers absent) was detectably more receptive to Ga1-s than to ga1, suggesting cross recognition between the two systems of incompatibility. Reciprocally, homozygous Ga1-s was unreceptive both to Tcb1 and tcb1 pollen, but heterozygous Ga1-s/ga1 plants were somewhat more receptive to Tcb1 than to tcb1. Discrimination by Tcb1/− females against tcb1 pollen is prezygotic, accomplished without the loss of viable ovules. When introduced into maize, Tcb1 incompatibility may be useful for isolating one category of commercial varieties from another. Received: 25 September 2000 / Accepted: 24 November 2000     

A selfish gene governing pollen-pistil compatibility confers reproductive isolation between maize relatives

Some populations of maize's closest relatives, the annual teosintes of Mexico, are unreceptive to maize pollen. When present in the pistil (silk and ovary) a number of maize genes discriminate against or exclude pollen not carrying the same allele. An analogous gene Tcb1-s was found in some teosinte populations but not in sympatric or parapatric maize. It was polymorphic among populations of teosinte growing wild, but regularly present in populations growing in intimate association with maize as a weed. Introduction of Tcb1-s into maize substantially to fully restored compatibility with Tcb1-s carrying teosintes. Although Tcb1-s pollen can fertilize tcb1 tcb1 maize, it is at a competitive disadvantage relative to tcb1 pollen. Hence, the influence of Tcb1-s on crossability is bidirectional. In the absence of maize, Tcb1-s can increase in teosinte populations without improving their fitness. In the presence of maize, Tcb1-s appears to have been co-opted to provide reproductive isolation for adaptation to a cultivated habitat.     

Pollination between maize and teosinte: an important determinant of gene flow in Mexico

Gene flow between maize [Zea mays (L.)] and its wild relatives does occur, but at very low frequencies. Experiments were undertaken in Tapachula, Nayarit, Mexico to investigate gene flow between a hybrid maize, landraces of maize and teosinte (Z. mays ssp. mexicana, races Chalco and Central Plateau). Hybridization, flowering synchrony, pollen size and longevity, silk elongation rates, silk and trichome lengths and tassel diameter and morphology were measured. Hybrid and open-pollinated maize ears produced a mean of 8 and 11 seeds per ear, respectively, when hand-pollinated with teosinte pollen, which is approximately 1–2% of the ovules normally produced on a hybrid maize ear. Teosinte ears produced a mean of 0.2–0.3 seeds per ear when pollinated with maize pollen, which is more than one-fold fewer seeds than produced on a maize ear pollinated with teosinte pollen. The pollination rate on a per plant basis was similar in the context of a maize plant with 400–500 seeds and a teosinte plant with 30–40 inflorescences and 9–12 fruitcases per inflorescence. A number of other factors also influenced gene-flow direction: (1) between 90% and 95% of the fruitcases produced on teosinte that was fertilized by maize pollen were sterile; (2) teosinte collections were made in an area where incompatibility systems that limit fertilization are present; (3) silk longevity was much shorter for teosinte than for maize (approx. 4 days vs. approx. 11 days); (4) teosinte produced more pollen on a per plant basis than the landraces and commercial hybrid maize; (5) teosinte frequently produced lateral branches with silks close to a terminal tassel producing pollen. Collectively these factors tend to favor crossing in the direction of teosinte to maize. Our results support the hypothesis that gene flow and the subsequent introgression of maize genes into teosinte populations most probably results from crosses where teosinte first pollinates maize. The resultant hybrids then backcross with teosinte to introgress the maize genes into the teosinte genome. This approach would slow introgression and may help explain why teosinte continues to co-exist as a separate entity even though it normally grows in the vicinity of much larger populations of maize.     

Pollen–pistil barriers to crossing in maize and teosinte result from incongruity rather than active rejection

Many popcorn strains cannot be fertilized by pollen of dent and flint strains although the reciprocal crosses are successful. Similarly, plants of some annual teosinte populations can fertilize maize but do not accept its pollen. Single genes or gene complexes govern these two unilateral barriers to crossing. Failure of fertilization could reflect active rejection by the pistil of pollen containing a contrasting allele (incompatibility). Alternatively, the pistil could require presence of a matching allele in pollen (congruity). To distinguish between these possibilities genetically, the receptivity to pollen having both alleles was determined. If there is active rejection, heteroallelic pollen would not be accepted; if presence of a matching allele is required, heteroallelic pollen would be accepted. In both the popcorn and teosinte crossing barrier systems, heteroallelic pollen functioned, consistent with the congruity model.     

In vitro inhibition of incompatible pollen tubes in <Emphasis Type="Italic">Nicotiana alata</Emphasis> involves the uncoupling of the F-actin cytoskeleton and the endomembrane trafficking system

In the S-RNase-based self-incompatibility system, subcellular events occurring in the apical region of incompatible pollen tubes during the pollen rejection process are poorly understood. F-actin dynamics and endomembrane trafficking are crucial for polar growth, which is temporally and spatially controlled in the tip region of pollen tubes. Thus, we developed a simple in vitro assay to study the changes in the F-actin cytoskeleton and the endomembrane system at the apical region of incompatible pollen tubes in Nicotiana alata. Growth but not germination of pollen tubes of S _c10 -, S ₇₀ -, and S ₇₅ -haplotypes was selectively inhibited by style extracts carrying the same haplotypes. Pollen F-actin cytoskeleton and endomembrane system, visualized by fluorescent markers, were normal during the initial 60 min of pollen culture in the presence of compatible and incompatible style extracts. Additional culture resulted in complete growth arrest and critical alterations in the integrity of the F-actin cytoskeleton and the endomembrane system of incompatible pollen tubes. The F-actin ring and the V-shaped zone disappeared from the apical region, while distorted F-actin cables and progressive formation of membrane aggregates evolved in the subapical region and the shank. The vacuolar network of incompatible pollen tubes invaded the tip region, but vacuolar membrane integrity remained mostly unaffected. The polar growth machinery of incompatible pollen tubes was uncoupled, as evidenced by the severe disruption of colocalization between the F-actin cytoskeleton and the endomembrane compartments. A model of pollen rejection integrating the main subcellular events occurring in incompatible pollen is discussed.     

Interspecific crosses in the genus Tulipa L.: identification of pre-fertilization barriers

 Pollen tube growth in the pistil and pollen tube penetration of ovules have both been studied in crosses between cultivars from Tulipa gesneriana L. and 12 tulip species from all eight sections of the genus Tulipa to identify pre-fertilization barriers. Depending on the cross, pollen tubes grew as far as the stigma or the style or continued growing down into the ovary. Pollen tubes penetrated none or only a few percent of the ovules of some crosses, despite the presence of many pollen tubes in the ovary. In other crosses, from which no or only a few hybrids have been obtained after seed maturation on the plant, pollen tube penetration was found in up to 79% of the ovules. Apparently, various kinds of barriers preventing fertilization or normal embryogenesis occur in interspecific tulip crosses. Received: 26 July 1996 / Revision accepted: 31 January 1997     

The Importance of Light Intensity for Pollen Tube Growth and Embryo Survival in Wheat x Maize Crosses

The success of Triticum aestivumxZea mays crosses, used to producewheat doubled haploids, is influenced by light intensity. Toexamine the basis for this response, pollen tube growth, embryosurvival and indicators of photosynthetic rate were measuredin two wheat cultivars (‘Karamu’ and ‘Kotuku’)crossed with maize at two irradiance levels (250 or 750 µmolm^-2s^-1, PAR). Pollen tube growth was significantly affectedby light intensity in ‘Karamu’ plants but not in‘Kotuku’ plants, despite both cultivars being pollinatedby the same maize source. The percentage of pollen tubes reachingthe cavity between the ovarian wall and integuments, or in themicropyle of ‘Karamu’ plants at high light intensity(65%) was nearly three-times greater than that at low lightintensity (22%). Thus, either low light intensity can affectthe maternal wheat plant in a way that inhibits pollen tubegrowth and/or high light intensity may promote pollen tube growthin ‘Karamu’ plants. Significant differences in ratesof electron transport in plants grown at the two light intensitiesindicated that the rate of photosynthesis may also have an effecton pollen tube growth. These results have importance for improvingthe efficiency of wheat x maize crosses and other wide cerealcrosses. Copyright 2001 Annals of Botany Company Intergeneric hybridization, light intensity, pollen tube growth, embryo survival, Triticum aestivum, wheat,Zea mays , maize     

Unilateral and bilateral hybridization barriers in inter-series crosses of 4x 2EBN Solanum stoloniferum, S. pinnatisectum, S. cardiophyllum, and 2x 2EBN S. tuberosum haploids and haploid-species hybrids

Wild Mexican potato species are an important untapped source of useful variation for potato improvement. Introgression methods such as 2n gametes, chromosome doubling, and crossing with disomic 4x 2 endosperm balance number (EBN) bridge species have been used to overcome post-zygotic endosperm failure according to the EBN hypothesis. Stylar barriers can prevent zygote formation, bilaterally when zygote formation is blocked in both directions of the cross or unilaterally when zygote formation is blocked in self incompatible (SI) × self compatible (SC) crosses. In several Solanaceae species, the S-locus for SI has been implicated in interspecific incompatibility. The objectives of this research were to determine if: (1) disomic 4x 2EBN Solanum stoloniferum can be used as a bridge species for introgression of the Mexican 2x 1EBN species Solanum cardiophyllum and Solanum pinnatisectum, (2) pre- and/or post-zygotic barriers limit hybridization among EBN compatible Solanum inter-series crosses, and (3) reproductive barriers act unilaterally or bilaterally. Fruit formation and seed set was recorded for inter-pollinations of S. stoloniferum, 4x 2EBN chromosome doubled S. cardiophyllum and S. pinnatisectum, and 2x 2EBN S. tuberosum haploids (HAP) or haploid-species hybrids (H-S). In vivo pollen tube growth was analyzed for each cross combination with fluorescence microscopy. Attempts to create bridge hybrids between S. stoloniferum, and S. cardiophyllum or S. pinnatisectum were not successful. Pre- and post-zygotic barriers prevented seed formation in crosses involving S. cardiophyllum and S. pinnatisectum. Self compatibility in S. stoloniferum and S. pinnatisectum suggests that the S-locus does not contribute to the stylar barriers observed with these species. Alternatively, the presence of functional and nonfunctional (SC) S-alleles may explain interspecific incompatibility in intra- and inter-ploidy crosses. A non-stylar unilateral incongruity was discovered in H-S/HAP × S. stoloniferum crosses, indicating either a post-zygotic barrier, or a pre-zygotic barrier acting at or within the ovary. Furthermore, lack of S. stoloniferum pollen rejection may occur through absence of S. stoloniferum pollen-active genes needed to initiate pollen rejection, or through competitive interaction in S-locus heterozygous S. stoloniferum pollen. Introgression strategies using these species would benefit potato breeding by introducing genetic diversity for several traits simultaneously through co-current introgression.     

A single silk- and multiple pollen-expressed PMEs at the Ga1 locus modulate maize unilateral cross-incompatibility

The Gametophyte factor1 (Ga1) locus in maize confers unilateral cross-incompatibility (UCI), and it is controlled by both pollen and silk-specific determinants. Although the Ga1 locus has been reported for more than a century and is widely utilized in maize breeding programs, only the pollen-specific ZmGa1P has been shown to function as a male determinant; thus, the genomic structure of the Ga1 locus and all the determinants that control UCI at this locus have not yet been fully characterized. Here, we used map-based cloning to confirm the determinants of UCI at the Ga1 locus and maize pan-genome sequence data to characterize the genomic structure of the Ga1 locus. The Ga1 locus comprises one silk-expressed pectin methylesterase gene (PME) (ZmGa1F) and eight pollen-expressed PMEs (ZmGa1P and ZmGa1PL1-7). Knockout of ZmGa1F in Ga1/Ga1 lines leads to the complete loss of the female barrier function. The expression of individual ZmGa1PL genes in a ga1/ga1 background endows ga1 pollen with the ability to overcome the female barrier of the Ga1 locus. These findings, combined with genomic data and genetic analyses, indicate that the Ga1 locus is modulated by a single female determinant and multiple male determinants, which are tightly linked. The results of this study provide valuable insights into the genomic structure of the Ga2 and Tcb1 loci and will aid applications of these loci in maize breeding programs.     

AtLURE1/PRK6-mediated signaling promotes conspecific micropylar pollen tube guidance

Reproductive isolation is a prerequisite to form and maintain a new species. Multiple prezygotic and postzygotic reproductive isolation barriers have been reported in plants. In the model plant, Arabidopsis thaliana conspecific pollen tube precedence controlled by AtLURE1/PRK6-mediated signaling has been recently reported as a major prezygotic reproductive isolation barrier. By accelerating emergence of own pollen tubes from the transmitting tract, A. thaliana ovules promote self-fertilization and thus prevent fertilization by a different species. Taking advantage of a septuple atlure1null mutant, we now report on the role of AtLURE1/PRK6-mediated signaling for micropylar pollen tube guidance. Compared with wild-type (WT) ovules, atlure1null ovules displayed remarkably reduced micropylar pollen tube attraction efficiencies in modified semi-in vivo A. thaliana ovule targeting assays. However, when prk6 mutant pollen tubes were applied, atlure1null ovules showed micropylar attraction efficiencies comparable to that of WT ovules. These findings indicate that AtLURE1/PRK6-mediated signaling regulates micropylar pollen tube attraction in addition to promoting emergence of own pollen tubes from the transmitting tract. Moreover, semi-in vivo ovule targeting competition assays with the same amount of pollen grains from both A. thaliana and Arabidopsis lyrata showed that A. thaliana WT and xiuqiu mutant ovules are mainly targeted by own pollen tubes and that atlure1null mutant ovules are also entered to a large extent by A. lyrata pollen tubes. Taken together, we report that AtLURE1/PRK6-mediated signaling promotes conspecific micropylar pollen tube attraction representing an additional prezygotic isolation barrier.

A modified ovule targeting assay revealed that AtLURE1/PRK6-mediated signaling promotes micropylar guidance of Arabidopsis thaliana pollen tubes while discriminating tubes of related Arabidopsis lyrata.     

Genetic analysis and fine mapping of the Ga1-S gene region conferring cross-incompatibility in maize

Cross-incompatibility genes known as gametophyte factors (ga) are numerous in maize. Many popcorn strains carry these genes and cannot be fertilized by pollen of dent and flint maize strains although the reciprocal crosses are successful. A Chinese popcorn strain SDGa25 carries the strongest allele of Ga1 (Ga1-S) and the majority of Chinese dent and flint maize germplasm are incompatible with SDGa25. The incompatibility is due to pollen tube growth obstruction 2 h after pollination. The pollen tube is arrested in the silk segment 5.5 cm distal to the pollination area and never reaches the ovule. The Ga1-S carried by SDGa25 behaves as a single dominant gene. This gene was mapped between markers SD3 on BAC AC200747 0.827 cM apart on the telomere side and SD12 on BAC AC204382 0.709 cM apart on the centromere side. The genetic region mapped spanning the Ga1-S locus was estimated to be 1.5 cM in length and the physical distance is 2,056,343 bp on ctg156 based on the B73 RefGen_v2 sequence. Gametophyte factors influence gene flow direction and the strongest Ga1-S allele is useful for isolating one category of commercial varieties from another. The eight tightly linked markers to Ga1-S developed in this study would greatly improve marker-assisted introgression efficiency and the fine mapping would facilitate the isolation of the Ga1-S.     

Molecular analysis of crosses between Tripsacum dactyloides and Zea diploperennis (Poaceae)

 DNA fingerprinting verified hybrid plants obtained by crossing Eastern gamagrass, Tripsacum dactyloides L., and perennial teosinte, Zea diploperennis Iltis, Doebley & R. Guzmán. Pistillate inflorescences on these hybrids exhibit characteristics intermediate to the key morphological traits that differentiate domesticated maize from its wild relatives: (1) a pair of female spikelets in each cupule; (2) exposed kernels not completely covered by the cupule and outer glumes; (3) a rigid, non-shattering rachis; (4) a polystichous ear. RFLP analysis was employed to investigate the possibility that traits of domesticated maize were derived from hybridization between perennial teosinte and Tripsacum. Southern blots of restriction digested genomic DNA of parent plants, F₁, and F₂ progeny from two different crosses were probed with RFLP markers specifically associated with changes in pistillate inflorescence architecture that signal maize domestication. Pairwise analysis of restriction patterns showed traits considered missing links in the origin of maize correlate with alleles derived from Tripsacum, and the same alleles are stably inherited in second generation progeny from crosses between Tripsacum and perennial teosinte. Received: 11 October 1996/Accepted:8 November 1996     

Genetic Variation in Sorghum for the Inhibition of Maize Pollen Tube Growth

Aniline blue fluorescence was used to study the growth of maizepollen tubes in the stigmas of 13 diverse sorghum accessions.In 12, only short maize pollen tubes were formed, but in thesingle exception (Sorghum nervosum Nr481) maize pollen tubesgrew at least as far as the base of the style. The S. bicolorgenotypes S9B and CMS (a cytoplasmic male sterile line) werehybridized with Nr481, and analysis of maize pollen tube growthin F₁ plants, and BC₁ plants using Nr481 as the recurrent parent,suggested that differences in inhibition of pollen tube growthwere due to variation at a single locus, which we propose todesignate lap (Inhibition of alien pollen tubes). AccessionNr481 appears to be homozygous for a recessive allele permittingmaize pollen tube growth. Attempts were made to produce sorghumx maize hybrids using Nr481 and CMS derivatives which were knownto allow maize pollen tube growth to the base of the style.A putative hybrid endosperm was obtained in one Nr481 x Seneca60 maize cross, but this was not repeatable and no hybrid plantswere produced. A fundamental problem may be the large size ofthe maize pollen tube, which could have difficulty growing throughthe sorghum ovary and in entering the micropyle. Sorghum bicolor spp. bicolor (L.) Moench, Zea mays L, sorghum, maize, pollen tube growth, hybridization barriers     

Intergeneric hybridization between Diplotaxis siifolia,a wild species and crop brassicas

Summary Attempts were made to obtain intergeneric hybrids between Diplotaxis siifolia, a wild species, and cultivars of Brassica (B. campestris, B. juncea, and B. napus). The crosses showed unilateral incompatibility. When the wild species was used as female parent, pollen germination and pollen tube growth were normal, but hybrid seeds aborted due to post-fertilization barriers. Reciprocal crosses (cultivars as female parent) showed strong pre-fertilization barriers; although pollen grains showed germination, pollen tubes failed to enter the stigma. Hybrids were realized in two of the crosses, D. siifolia x B. juncea and D. siifolia x B. napus, through ovary culture. The hybrids were multiplied in vitro by multiplication of axillary shoots, or somatic embryogenesis. Detailed studies were carried out on the hybrid D. siifolia x B. juncea. F₁ hybrids had shrivelled anthers and were pollen sterile. Amphiploids of this hybrid showed 60% pollen fertility and produced seeds upon self-pollination as well as backcross pollination with the pollen of B. juncea.     

Unilateral incongruity in crosses involvingLycopersicon pennellii andL. esculentum is distinct from self-incompatibility in expression,timing and location

Both interspecific and intraspecific mechanisms restrict the exchange of genes between plants. Much research has focused on self incompatibility (SI), an intraspecific barrier, but research on interspecific barriers lags behind. We are using crosses betweenLycopersicon esculentum andL. pennellii as a model with which to study interspecific crossing barriers. The crossL. esculentum×L. pennellii is successful, but the reciprocal cross fails. Since the cross can be successfully made in one direction but not the other, gross genomic imbalance or chromosomal abnormality are precluded as causes. We showed that the lack of seed set observed in the crossL. pennellii×L. esculentum is due to the inability of pollen tubes to grow more than 2–3 mm into the style, whereas S1 crosses show continued slow pollen tube growth but, also, fail to set seed. These results indicate that the unilateral response is a barrier distinct from SI, differing from SI in the timing and location of expression in the style. We therefore suggest that this unilateral response in theL. pennellii×L. esculentum cross is more accurately referred to as unilateral incongruity (UI) rather than interspecific incompatibility. Periclinal chimeras were used to determine the tissues involved in UI. The results of crosses with the available chimeras indicate that the female parent must beL. pennellii at either LI (layer 1) or both LI and LII (layer 2) and the male parent must beL. esculentum at either LII or both LI and LII to observe UI similar to that seen in theL. pennellii×L. esculentum cross. Pollinations with a mixture of pollen fromL. pennellii and from transgenicL. esculentum plants harboring a pollen-specific GUS reporter gene marker were used to ascertain whether the growth of the pollen tubes of either species was modified as a possible means of overcoming UI. We found no evidence of communication between the two types of pollen tubes to either enhance or restrict all pollen tube growth.     

Post-pollination hybridization barriers in <Emphasis Type="Italic">Nicotiana</Emphasis> section <Emphasis Type="Italic">Alatae</Emphasis>

Nicotiana section Alatae contains eight species with variable flower sizes and morphologies. Section members readily hybridize in the glasshouse, but no hybrids have been observed in natural sympatric and parapatric populations. To investigate interspecific crossing relationships with respect to mechanisms preventing hybridization, all members of section Alatae were intercrossed in a complete diallel. We found positive correlation between the pistil length of the pollen donor and interspecific seed set relative to the conspecific cross. Pollen tube growth rate and pollen donor pistil length were positively correlated as well. Furthermore, pollen from short-pistil members of section Alatae could only grow a maximum distance proportional to, but greater than, their own pistil lengths. Our results show that pollen tube growth capacity (i.e., rate and distance), provides a hybridization barrier in long-pistil species × short-pistil species crosses. We also found another hybridization barrier not specifically related to pollen tube growth capacity in short-pistil species × long-pistil species. Taken together, these barriers can generally be described by a ‘pistil-length mismatch’ rule; in section Alatae, pollen has the most success fertilizing ovules from species with pistil lengths similar to their own. This rule could contribute to hybridization barriers in Section Alatae because the species display dramatically different pistil lengths. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of seasons and hormones on pre- and postfertilization barriers of crossability andin vitro hybrid development betweenVigna unguiculata andV. mungo crosses

Effect of two different seasons and hormones on pre- and postfertilization barriers in crossability were studied inVigna mungo, V. unguiculata and their crosses. Significant differences between the two seasons for pollen fall, pollen germination, tube growth and pod set were observed. In kharif season with an average temperature of 26 to 30 °C and 75 to 85 % humidity all the characters showed higher values than in the spring season with a higher temperature of 32 to 36 °C and low humidity of 40 to 50 %. Application of gibberellic acid (GA) alone and a combination of gibberellic acid, naphthaleneacetic acid (NAA) and kinetin (KIN) significantly increased the pollen fall by 7–13 %, pollen germination by 10–12 % and pollen tube growth by 27–30 %. More pollen tube abnormalities in interspecific crosses only indicates partly incompatible reaction of pollen tube in the stylar tisue. Hormonal treatments, especially GA, significantly increased the pod set and pod harvest by 20 % and 34 %, respectively. Higher pod harvest was seen in crosses withV. unguiculata as female parent. GA treatment enhanced pollen germination and pollen tube growth, and by partially overcoming embryo abortion for 10 to 12 d, immature embryos were successfully rescued forin vitro production of hybrid plantlets.     

Heterostyly inPrimula. 2. Sites of pollen inhibition,and effects of pistil constituents on compatible and incompatible pollen-tube growth

Summary In incompatible (intramorph) pollinations of the heterostylousPrimula vulgaris, pollen germination or tube growth may be partially inhibited in several sites associated with the stigma or style. Blockage may occur, a) on the stigma surface through the failure of germination or of pollen tube penetration after germination, b) in the stigma head during the passage of the tube through the specialized transmitting tissue of the head, or c) in the transmitting tract of the style. None of the barriers is complete, and the prohibition of selfing or intramorph crossing depends upon the cumulative screening effect of one following upon the other. In both morphs, the germination of incompatible pollen on the stigma is enhanced in high ambient relative humidity, but many tubes still fail to penetrate the stigma. Those that do are retarded or blocked in their growth in the transmitting tissues of the stigma head and style. Crude extracts from the tissues of the stigma head and style show some differential effect on the growth of pollen tubesin vitro, and dialysates of extracts containing high molecular weight fractions show a consistent differential effect, those from thrum tissues retarding thrum tubes while having a lesser effect on pin tubes, and those from pin tissues retarding pin tubes while having lesser effect on thrum. It is suggested that the factors influencing tube growth are present in the intercellular secretions of the transmitting tract.     

Establishment of the male germline and sperm cell movement during pollen germination and tube growth in maize

Two sperm cells are required to achieve double fertilization in flowering plants (angiosperms). In contrast to animals and lower plants such as mosses and ferns, sperm cells of flowering plants (angiosperms) are immobile and are transported to the female gametes (egg and central cell) via the pollen tube. The two sperm cells arise from the generative pollen cell either within the pollen grain or after germination inside the pollen tube. While pollen tube growth and sperm behavior has been intensively investigated in model plant species such as tobacco and lily, little is know about sperm dynamics and behavior during pollen germination, tube growth and sperm release in grasses. In the March issue of Journal of Experimental Botany, we have reported about the sporophytic and gametophytic control of pollen tube germination, growth and guidance in maize.1 Five progamic phases were distinguished involving various prezygotic crossing barriers before sperm cell delivery inside the female gametophyte takes place. Using live cell imaging and a generative cell-specific promoter driving α-tubulin-YFP expression in the male germline, we report here the formation of the male germline inside the pollen grain and the sperm behaviour during pollen germination and their movement dynamics during tube growth in maize.Key words: male gametophyte, generative cell, sperm, pollen tube, tubulin, fertilization, maize