A new look at incompatibility relationships in higher plants

Summary Pollen tube growth was evaluated using an 18-step scale after both intra- and interspecific pollinations of genotypically widely differing diploid potato species and potato dihaploids expressing monofactorial gametophytic incompatibility. The results obtained account for a wide array of types of pollen tube growth resulting from crossing partners with distinct incompatibility behavior. Based on the assumption that inhibition of pollen tubes is the rule and solely prevented by the pollen itself, a model proposing one common cause underlying different mechanisms for both intraspecific selfincompatibility and interspecific incompatibility in diploids is put forward. Data supporting the model are presented from the experimental results of this study and from the literature. The strength of pollen-style interaction depends on particular S-alleles in combination with recognition and activity properties of the basic S-genotypes. The model is suitable to explain all former observations on incompatibility in diploids with a gametophytic system of incompatibility and, with modifications of the manner of phenotypic expression, also in plants with sporophytic incompatibility. The proposed scheme of pollen tube growth phenotypes permits prediction of pollen tube growth behavior in an intended cross combination. The model is based on both classical Mendelian genetics and recent molecular genetic insight.     

Genetic analysis of incompatibility in the diploid Ipomoea species closely related to the sweet potato

Summary In order to identify the genotypic constitutions of incompatibility in the diploid species, Ipomoea leucantha Jacq. (K221), which is most closely related to the sweet potato, the progenies derived from the reciprocal crosses, backcrosses and testcrosses were analysed. All the plants examined were self-incompatible, and pollen germination was inhibited on the stigma after incompatible pollinations. No reciprocal differences were found in the incompatibility reactions. In the progenies three incompatibility groups were observed which showed the rather simple segregation ratios. The homozygous plants for incompatibility alleles were obtained in the progenies. The experimental results demonstrated a sporophytic type of incompatibility controlled by a single locus with multiple S-alleles exhibiting a dominance relationship in both the pollen and the stigma. The plants obtained in the progenies had the following genotypes: S ₁ S ₂, S ₁ S ₃, S ₂ S ₂, S ₂ S ₃ and S ₃ S ₃.     

Biological containment of potato (Solanum tuberosum): outcrossing to the related wild species black nightshade (Solanum nigrum) and bittersweet (Solanum dulcamara)

The biological containment of the potato (Solanum tuberosum) was assessed by establishing the crossability of this tuberous crop with the related wild non-tuberous species in The Netherlands, black nightshade (S. nigrum) and bittersweet (S. dulcamara). To circumvent crossability barriers, genotypes with different ploidy number were employed and crosses were performed under different environmental conditions. S. dulcamara was shown to be incongruent with potato at all ploidy levels, while S. nigrum displayed unilateral incompatibility. If S. nigrum was emasculated and used as female, fertilization by potato pollen resulted in berry set and seed development. Emasculation of S. nigrum was essential in this cross, because analysis of the fertilization process demonstrated that this species is highly self-compatible and potato pollen was outcompeted by pollen of S. nigrum. The hybrid seeds derived from this cross did not mature and appeared not to be viable. By application of the technique of embryo rescue of immature embryos, hybrid plants could be obtained. However, these hybrid plants proved to be sterile. These data demonstrate that gene flow by pollen dispersal from potato to its most common wild relatives in Western Europe is highly unlikely. The potato is thus a naturally contained species in this part of the world.     

Allelism of Endosperm Balance Number (EBN) in Mexican tuber-bearing Solanum species

Summary Endosperm Balance Number (EBN) is a genetic, dose-dependent crossability system functioning in tuber-bearing Solanum species. Each species has been assigned 1EBN, 2EBN, or 4EBN. Species thus designated cross only within their EBN group. Doubling of chromosome number also doubles the EBN. The ploidy: EBN ratio is not consistent among Solanum species. Some diploids are 2EBN while others are 1EBN. Some tetraploids are 4EBN while others are 2EBN. Species from Mexico typically have EBNs that are one-half of their ploidy [e.g. 2x(1EBN), 4x(2EBN)]. Hybrids of Mexican species and a South American species, 2x(1EBN) S. Commersonii, and its 4x(2EBN) colchicine derivative were made and crossed to 1, 2, and 4EBN standard testers to determine the relationship of the genetic organization of EBN among and within these species. Diploid hybrids crossed only to 1EBN standard testers. Hybrids of 4x(2EBN) S. commersonii and 4x(2EBN) Mexican species crossed almost exclusively to 2EBN standard testers. Complex tetraploid hybrids involving S. commersonii, S. stenophyllidium (a Mexican diploid), and Mexican tetraploids of series Longipedicellata also crossed only to 2EBN testers. The apparent lack of recombination and segregation for EBN in these hybrids indicates that the genomes of the Mexican diploid and tetraploid species carry EBN in a way genetically similar to that of the South American species S. Commersonii.Cooperative investigation of the U.S. Department of Agriculture, Agricultural Research Service and the Wisconsin Experiment Station. Supported in part by the USDA/Cooperative States Research Service Competitive Grant No. 83-CRCR-1-1253     

杜鹃花属植物杂交不亲和与败育分布研究

该研究以杜鹃花属5亚属3组17亚组38种,计109个不育组合及91个可育组合的杂交结果为依据,对4个杂交不亲和指标与败育系数及败育频度参数进行了分析。结果表明:(1)杜鹃花属不同类群间杂交的不育组合比例约为54.5%,其不亲和与败育包括不能坐果(capsul aborted,Cab型)、坐果但不能形成种子(seed aborted,Sab型)和能形成种子但不能发芽(seed not germinated,Sng型) 3种情况,其中Cab与Sab类型均可能是前合子期不亲和与后合子期不亲和的复合表征,Sng型则可以肯定为后合子期种子发育阶段败育的情况,Cab∶Sab∶Sng=81∶13∶15;而可育组合中部分败育苗无疑属于"杂种不活hybrid inviability"的败育类型。(2)有关不育类型的分布与杜鹃花属植物亲本类群及其分类与亲缘关系具有明显关联,从同一亚组内、同一亚属内到不同亚属间杂交的不育类型的分布呈Sng型→Sab型→Cab型增加的趋势,杂交双亲分类上亲缘关系越密切Sng型的频度越高,反之关系越疏远Cab型的频率越高,亚属间远交往往止于Cab型。(3)"杂种不活"为败育苗的表现形式之一,从同一亚组、同一亚属到不同亚属间杂交,其败育等级频度分布通常也从无或轻度败育到严重败育方向发展,常绿杜鹃亚属内杂交的败育情况明显低于杜鹃亚属内杂交,这与后者多倍体亲本的介入相关,而上述2亚属间杂交的苗木败育则更加严重,但映山红(R. simsii)分别与百合花杜鹃(R. liliiflorum)和毛肋杜鹃(R. augustinii)的亚属间杂交未出现败育苗分布。     

Localization of adenylate cyclase activity in Populus: its relation to pollen-pistil recognition and incompatibility

Summary Adenylate cyclase has been localized cytochemically in female and male parents as well as during the pollen-stigma interaction with an original technique employing strontium as the capture ion and adenyl imidodiphosphate as the specific substrate. The specificity of the reaction was checked by using several controls. No final specific reaction product was detected in unpollinated P. deltoides stigmas or in the P. deltoides or P. alba pollen grains used for compatible and incompatible pollinations. In the compatible cross between P. deltoides × P. deltoides, fine dense precipitates were observed in the dictyosomes and the plasma membrane and exterior to the exine of hydrated pollen grains adhering to the stigma surface. Labeling of the stigmatic pellicle was also observed after pollen adhesion and hydration. This was accompanied by a strong reactivity of the cell wall and plasmalemma of the stigma papillae at the sites of pollen tube germination on the stigma surface and at the sites of penetration of pollen tubes between adjacent papillae. In the incompatible cross between P. deltoides x P. alba, adenylate cyclase activity was still present but reduced at the stigma surface following adhesion, hydration, and germination of P. alba pollen. This activity was completely abolished after the penetration of pollen tubes between stigma papillae. These findings suggest that in Populus, adenylate cyclase activity is correlated to pollen adhesion, hydration, and germination at the stigma surface, and that the abolition of this enzyme activity could be one of the cellular events governing the gametophytic phenotype of incompatibility in the cross between P. deltoides and P. alba.     

Occurrence of 2n pollen and ps gene frequencies in cultivated groups and their related wild species in tuber-bearing Solanums

Summary The gene frequency for parallel spindles (ps) was estimated from the frequency of plants producing 2n pollen in three cultivated groups: 2x Phureja (phu), 2x Stenotomum (stn), and 4x Andigena (adg), as well as in four related wild taxa: 2x Solanum brevicaule (brc), 2x S. sparsipilum (spl), 4x S. gourlayi (grl) and 4x S. gourlayi-S. infundibuliforme hybrids (grl-ifd). Plants with more than 1% large pollen were considered as 2n pollen producers. Observations of meiosis in a sample of 2n pollen-producing plants indicated that parallel spindles is the mechanism of 2n pollen formation. The number of plants with 2n pollen among the total examined was 228 plants (15.5%) of 1,473 in 2x spl, 31 (26.7%) of 116 in 2x brc, 92 (17.4%) of 528 in 2x stn, 665 (22.1%) of 3,008 in 2x phu, 731 (51.4%) of 1,421 in 4x adg, 591 (41.2%) of 1,436 in 4x grl, and 36 (64.3%) out of 56 in 4x grl-ifd. The ps gene frequencies assuming Hardy-Weinberg equilibrium were: 0.393 for 2x spl, 0.462 for 2x brc, 0.417 for 2x stn, 0.470 for 2x phu, 0.847 for 4x adg, 0.801 for 4x grl, and 0.895 for 4x grl-ifd. Twenty-five adg clones were randomly selected from a large population and were crossed with 2x clone W5295.7, which produces 2n pollen by parallel spindles (ps). The 4x progenies from 4x×2x crosses were used to determine the genotypes at the ps locus by screening 10–20 plants in each family for 2n pollen. Based on chromosome segregation at the ps locus, 9, 14, 1, and 1 clones were nulliplex, simplex, simplex or duplex, and duplex, respectively. The frequency of the ps gene in the adg population was estimated to be 0.825 and 0.815 for chromosome and chromatid segregation, respectively. The high frequencies of 2n pollen and the ps gene in cultivated 2x and 4x groups, and in wild taxa closely related to them, provide evidence for sexual polyploidization in the tuber-bearing Solanums.Paper No. 3032 from the Laboratory of Genetics. Research supported by the College of Agriculture and Life Sciences; International Potato Center; USDA, SEA, CGRO 84-CRCR-1-1389; and Frito Lay, Inc.     

Irregular meiosis in a somatic hybrid between S. bulbocastanum and S. tuberosum detected by species-specific PCR markers and cytological analysis

A system of randomly amplified polymorphic DNA (RAPD) markers was developed to facilitate the transfer of S. bulbocastanum (blb) genes into the S. tuberosum (tbr) genome by hybridization and backcrossing. DNA from tbr, blb and the hexaploid hybrid was used as a template for polymerase chain reaction (PCR) amplification. Polymorphic RAPD products, originating from 10-mer primers, specific for blb were cloned and sequenced at their ends to allow the synthesis of 18-mer primers. The 18-mer primers allowed a more reproducible assay than the corresponding RAPDs. Of eight 18-mer primer pairs, four amplified the expected products specific for blb. However, the stringency of the primer annealing conditions needed to be carefully optimized to avoid amplification of the homeologous tbr product, suggesting that the original RAPD polymorphisms were due to single base-pair changes rather than deletions or insertions. Two primers used for amplification of backcross 2 progeny segregated in a 11 (presence:absence) ratio; the other two were unexpectedly absent. The most likely explanation for the loss of these markers is irregular meiosis in the original hexaploid hybrid and subsequent elimination of chromosomes. Cytological analysis of the meiosis in the hybrid demonstrated widespread irregular pairing and the presence of lagging univalents. In addition, the first backcross individual used as the parent for the second backcross had 54 chromosomes instead of the predicted 60. In conclusion, our results demonstrate that PCR technology can be used for the efficient isolation of taxon-specific markers in Solanum. Furthermore, by the use of these markers we detected the loss of chromosomes that was subsequently shown by cytological analysis to be caused by irregular meiosis of the somatic hybrid.     

Plastid DNA inheritance and plastome-genome incompatibility in interspecific hybrids of Zantedeschia (Araceae)

Plastid DNA (ptDNA) probes were used in RFLP analysis to determine ptDNA inheritance in interspecific hybrids in Zantedeschia. Biparental and maternal ptDNA inheritance was found in albino hybrids between the evergreen species Z. aethiopica and several winter-dormant species. From two albino hybrids, different types of ptDNA were detected in shoots derived from different parts of an embryo. This result indicates that plastids were sorted out during embryo development. Only maternal ptDNA was detected in the hybrids of Z. aethiopica × Z. odorata (a summer-dormant species) but paternal, biparental, and maternal ptDNA were found in the hybrids of the reciprocal cross. Z. odorata × Z. aethiopica. By correlating these ptDNA inheritance patterns with the leaf colour (albino, pale-green, and green) of the hybrids, it is suggested that the Z. odorata plastome is incompatible with the Z. aethiopica genome. The Z. aethiopica plastome is partially compatible with the Z. odorata genome but the development of Z. aethiopica plastids appears to be blocked by the presence of the Z. odorata plastids.     

Self-incompatibility and interspecific incompatibility: relationships in intra- and interspecific crosses of Zinnia elegans Jacq. and Z. angustifolia HBK (Compositae)

Summary Intraspecific and reciprocal interspecific crosses involving Zinnia angustifolia clones and Z. elegans lines showed that in both species, sporophytic self-incompatibility (SI) systems were present. Intensity of SI varied among clones and lines, and high self seed set was associated with a concomitant decrease in callose fluorescence in papillae and pollen tubes. Incomplete stigmatic inhibition of pollen germination and tube growth was observed in reciprocal interspecific crosses and associated with callose synthesis, suggesting S-gene activity. Seed set and progeny obtained following Z. angustifolia×Z. elegans matings was comparable to intraspecific compatible matings of Z. angustifolia although the rate of pollen tube growth through the style was slower. In Z. elegans × Z. angustifolia matings, additional prezygotic barriers were present and acted between pollen tube penetration of the stigma and syngamy. SI X SI interspecific incompatibility was essentially unilateral, with no embryos or progeny obtained when Z. elegans was the pistillate parent. It was hypothesized that nonfunctioning of Z. elegans × Z. angustifolia crosses was due to S-gene expression at the stigmatic surface and to other isolating mechanisms in the stylar or ovarian transmitting tissue.Scientific Article No. A-4448, Contribution No. 7439 of the Maryland Agricultural Experiment Station, Department of HorticultureA portion of this paper was presented in the report: Boyle TH, Stimart DP (1986) Incompatibility relationships in intra- and interspecific crosses of Z. elegans Jacq. and Z. angustifolia HBK (Compositae). In: Mulcahy D (ed) Biotechnology and ecology of pollen. Springer, New York     

Hybridization and fertility of hybrid derivatives of Solatium melongena L. and Solanum macrocarpon L.

Summary Eleven genotypes of Solanum melongena L. and one genotype tentatively identified as Solanum macrocarpon were reciprocally intercrossed. Three patterns of the crossability were determined: a) reciprocally crossable, b) reciprocally non-crossable, and c) unidirectionally crossable. In toto 524 F₁ interspecific hybrids were grown during one season under open pollination conditions in the field. A large proportion of the F₁ hybrids produced seed set. The highest degree of seed set was recorded in the reciprocal F₁ hybrid of S. melongena (cv. Burpee Hybrid) and S. macrocarpon (Acc. 21–73). In addition, a limited number of back-cross progeny have been produced. The germinating seeds produced an F₂ generation of which some recombinants showed a considerably higher degree of fertility than the F₁. This finding suggests the possibility of the transfer of genes for resistance to two-spotted spider mite from S. macrocarpon to S. melongena.This research was performed as part of NJAES Project No. 99201 (NE-9) supported by the New Jersey Agricultural Experiment Station and Regional Hatch Funds. Paper of the Journal Series of the New Jersey Agricultural Experiment Station. Please address reprint requests to Dr. C. Boyer     

Organization of nuclear ribosomal DNA and species-specific polymorphism in closely related Fraxinus excelsior and F. oxyphylla

The ribosomal DNA repeat units of two closely related species of the genus Fraxinus, F. excelsior and F. oxyphylla, were characterized. The physical maps were constructed from DNA digested with BamHI, EcoRI, EcoRV and SacI, and hybridized with three heterologous probes. The presence or the absence of an EcoRV restriction site in the 18s RNA gene characterizes two ribosomal DNA unit types found in both species and which coexist in all individuals. A third unit type appeared unique to all individuals of F. oxyphylla. It carries an EcoRI site in the intergenic spacer. Each type of unit displayed length variations. The rDNA unit length of F. excelsior and F. oxyphylla was determined with EcoRV restriction. It varied between 11kb and 14.5kb in F. excelsior and between 11.8kb to 13.8kb in F. oxyphylla. Using SacI restriction, at least ten spacer length variants were observed in F. excelsior, for which a detailed analysis was conducted. Each individual carries 2–4 length variants which vary by a 0.3-kb step multiple. This length variation was assigned to the intergenic spacer. By using the entire rDNA unit of flax as probe in combination with EcoRI restriction, each species can be unambiguously discriminated. The species-specific banding pattern was used to compare trees from a zone of sympatry between the two species. In some cases, a conflicting classification was obtained from morphological analysis and the use of the species-specific rDNA polymorphism. Implications for the genetic management of both species are discussed.     

Genetics of self incompatibility in diploid Ageratum houstonianum mill

Summary Diallels and backcrosses among self-incompatible (SI) clones and progeny of Ageratum houstonianum Mill. could be organized into intra-incompatible classes. Four of 5 progenies segregated in expected ratios of S genotypes. Ageratum expressed a one-locus incompatibility system of the sporophytic type with a linear dominance series of multiple alleles and complete allelic dominance in both pollen and stigma. In the second part of the study, a high percentage of self-seed set was observed during the first flowering of a progeny from a pseudo-self compatible (PSC) seed source. Six progenies were derived from the PSC seed source. Five of the 6 segregated PSC SI plants, 4 of which fit a 3 1 ratio of PSC SI plants. All plants of the sixth progeny were SI. Two F₁ progenies with the same PSC pollen parent produced significantly different segregations of PSC SI plants. It appeared that PSC acted as a major gene when the most recessive S allele was also present, but PSC was not expressed when the most dominant S allele was present. Clones propagated from PSC plants were SI and cross incompatible with a related S-allele tester. Thus, PSC was transient in that it was apparent in seed-propagated plants but not in plants clonally propagated from the PSC individuals.Scientific Journal Series Paper Number 12,299 of the Minnesota Agricultural Experiment Station     

A critical evaluation of reproductive barriers between closely related species using DNA markers - a case study in Pinus

Former controlled crosses between twelve Pinus montana var. rostrata (Pinus mugo complex) and eight P. sylvestris clones revealed that only two P. sylvestris had efficiently fertilised P. montana. Two species-diagnostic chloroplast DNA markers were applied to verify the species purity of the parental clones. All maternal P. montana were unambiguously confirmed to belong to the P. mugo complex at both chloroplast DNA marker loci. Six P. sylvestris clones carried the `sylvestris' haplotypes. However, the same two P. sylvestris clones that had efficiently fertilised P. montana displayed the chloroplast haplotypes diagnostic to the P. mugo complex. The patterns of highly polymorphic cpDNA microsatellite markers in parents and offspring ruled out contamination by foreign pollen. We concluded that the two clones successful in the crosses represent fertile hybrids between the two species with P. mugo as the pollen donor. Consequently, DNA markers are proposed for verifying or falsifying the success of artificial fertilisation in general. The existence of crossing barriers between the two Pinus species, meaningful to the postulated natural hybridisation and the evolution of their populations in sympatric stands, was indicated and is newly discussed.     

Reproductive seasonality and nest-site differentiation in three closely related armoured catfishes (Siluriformes: Callichthyidae)

Synopsis Three sympatric, closely related armoured catfishes showed a similar, bimodal breeding season in coastal plain swamps in Suriname (South America). The bimodal pattern of reproduction inCallichthys callichthys,Hoplosternum littorale andHoplosternum thoracatum was correlated to the annual distribution of rainfall. Floating bubble nests were constructed throughout the period of swamp inundation (rainy season). Close inspection of the timing of reproduction ofH. littorale in northern South America revealed the relationship between reproduction and rainfall, but also unexpected differences in the length of the breeding season. Strong interspecific competition among the three species was probably avoided through differentiation of nest sites with respect to water depth, distance to the nearest tree, distance to the edge of the swamp, and cover above the nest. Nests ofH. littorale were built in herbaceous swamps, whileC. callichthys andH. thoracatum build their nests in swamp-forest. Nests ofC. callichthys were observed in extremely shallow water or in holes. Few nests of the three species were observed in canals. Over large parts of their geographical rangeC. callichthys andH. thoracatum are found in small rainforest streams. Several aspects of the reproductive ecology and behaviour of both species are probably related to the unpredictability of the stream habitat.     

Quantitative inheritance and barriers to crossability in Cicer arietinum L.

Summary Three different diallel crosses were studied in Cicer arietinum; two of size 6×6, one within each of the two botanical groups macrosperma and microsperma of the cultivated subspecies, and one of 9×9 involving lines covering most of the morphological variation of chickpea. Barriers to crossability present neither a botanical nor a geographical pattern, being probably a direct consequence of interactions between genotypes. The genetic systems of twelve quantitative characters were analysed. Full dominance in a negative sense (small values dominant) is shown by leaflet length, width and shape index, rachis length, leaflet density on the rachis and pod length. Full dominance in a positive sense is shown by seeds per pod. Overdominance (in a positive sense) is evident for pods, seeds and yield per plant. Weak reciprocal differences were manifested by pod length, and pods, seeds and yield per plant. The system controlling number of leaflets per leaf is not clear. Dominance of primitive over selected characters seems to be the rule. As far as the environmental effects have permitted the analysis, no differences in genetic systems were observed between botanical groups.     

Allelism of endosperm balance number (EBN) in Solanum acaule Bitt. and other wild potato species

The inheritance of endosperm balance number (EBN), a genetic, dose-dependent crossability system functioning in tuber-bearing Solanum (potato) species, was investigated for certain wild potato species having an EBN equal to one half of their ploidy. The EBN of Solanum acaule, a disomic 4(2EBN) South American species, was investigated by producing F₁ and F₂ hybrids with artificial 4x(2EBN) S. commersonii. This allowed assessment of recombination among the two genomes of disomic S. acaule and that of S. commersonii. When crossability of the hybrids with 1EBN, 2EBN and 4EBN standards was tested, no variation for EBN was detected. The apparent lack of recombination and segregation for EBN in these hybrids indicates that the genomes of S. acaule and S. commersonii carry EBN in a genetically-similar way. Combined with previous reports, these data indicate that the inheritance of EBN is similar in widely-separated taxa from South America and Mexico.     

Evidence for intergeneric incompatibility in ferns

The archegonial mucilage ofAthyrium filix-femina andA. distentifolium paralyses spermatozoids ofDryopteris filix-mas (and in one caseD. inaequalis) before they penetrate the archegonial venter. The archegonial mucilage ofDryopteris filix-mas has a weak positive chemotactic influence on the spermatozoids of the twoAthyrium species. The spermatozoids ofDryopteris were never observed in the archegonia ofAthyrium. Incompatibility was not observed within and between the twoAthyrium species, withinDryopteris filix-mas or betweenAthyrium filix-femina and twoAsplenium species.Contribution No. 327.     

The PELPIII glycoproteins in Solanaceae: stylar expression and transfer into pollen tube walls

The class III pistil-specific extensin-like proteins (PELPIII) of Nicotiana tabacum accumulate in the intercellular matrix (IM) of the style transmitting tissue (TT). After pollination, the 110–140 kDa PELPIII is translocated from the IM into the pollen tube walls. PELPIII-like sequences have been found in several solanaceous species. These sequences are expressed in mature non-pollinated styles at both RNA and protein level. Of the genus Nicotiana, the species N. alata, N. x sanderae and N. sylvestris (section Alatae), and N. tomentosiformis and N. otophora (section Tomentosae) showed an expression level of PELPIII homologues similar to that in mature styles of N. tabacum. PELPIII genes were absent in the most ancient species studied, namely N. trigonophylla (section Trigonophyllae). To study the species dependence of the translocation of PELPIII into the pollen tube wall in tobacco, interspecific pollinations on N. tabacum pistils were carried out with pollen from the incongruous species N. rustica, N. trigonophylla and Petunia hybrida, where PELPIII homologues are absent in the style. Immunocytological tests showed that the N. tabacum PELPIII is translocated into the pollen tube walls of all three species. Thus, the pollen tube walls of these species do not form a barrier for IM compounds such as the 110–140 kDa PELPIII and the absence of any possible effect of PELPIII on pollen tube growth cannot be due to failure of PELPIII transport through the wall. The importance of these findings is discussed with respect to the evolutionary origin of PELPIII, the pollen pistil interaction, the function of style TT-specific proteins and the physical properties of pollen tube walls.