Pollen Tetrads Research Articles

Comment on “The ‘seed-fern’ Lepidopteris mass-produced the abnormal pollen Ricciisporites during the end-Triassic biotic crisis” by V. Vajda, S. McLoughlin, S. M. Slater, O. Gustafsson, and A. G. Rasmusson [Palaeogeography, Palaeoclimatology, Palaeoecology, 627 (2023), 111,723

Recently, Ricciisporites Lundblad and Cycadopites Wodehouse (=Monosulcites Cookson) pollen types have been found cooccurring in Antevsia zeilleri (Nathorst) Harris pollen sacs and it was assumed that the tetrads of Ricciisporites resulted from atypical pollen developmental pathway in which the pollen grains failed to separate. The pollen grains of Cycadopites type were interpreted as normal pollen grains of a peltasperm with Antevsia zeilleri pollen sacs and Lepidopteris ottonis (Göppert) Schimper foliage, and Ricciisporites, as abnormal pollen tetrads of this parent plant. However, if it were true, the exines in the pollen grains of the two types should have something in common in the ultrastructure. Examples of aberrant pollen in living seed plants show that changes occurring in the predetermined succession of events of exine development are minor and even if they finally lead to a significantly altered mature exine, the original structural plan remains recognizable. Contrariwise, the two pollen types are too dissimilar by their exine ultrastructure as well as by the general morphology and exine sculpture. Comparison with ontogenetic data on suitable modern analogues also allows one to suppose that exines of Ricciisporites and Cycadopites developed differently, in unrelated parent plants. Another explanation should be found for the presence of Ricciisporites tetrads in these pollen sacs.

Pollinium development and sporoderm structure of Dendrobium officinale

The developmental characteristics of pollen in plants that produce pollinia are not well understood. Here, pollinium development in the orchid Dendrobium officinale was observed using electronic microscopy and light microscopy coupled with calcium localization analyses. The anther wall of D. officinale is composed of a layer of epidermis cells, two layers of endothecium cells, a layer of middle layer cells, and a layer of tapetal cells. Before anthesis, the tapetal cells degenerate and the endothecium cells develop fibrous bands that arise from the inner tangential walls. The microspore mother cells have no obvious callose wall structure before meiosis. The microspore mother cells of D. officinale show simultaneous-type cytokinesis during meiosis. After meiosis, the four microspores in the tetrad do not separate from each other. Instead, they sequentially develop as a pollen tetrad, whose components stick together to form the pollinium. During microspore development, sporopollenin covers the surface of the whole pollinium to form the exine, but the pollen tetrads inside the pollinium have no exine.

Pollen Germination and Tube Growth in Northern Highbush Blueberry are Inhibited by Extreme Heat

The increasing intensity and frequency of extreme heat events threaten crop productivity globally. Certain phases of plant reproduction necessary for fertilization are highly sensitive to extreme heat, particularly during pollen development, germination, and tube elongation. However, few studies have assessed the effects of extreme heat on pollen performance in perennial crop plants. To understand how northern highbush blueberry pollen responds to high temperatures, we quantified pollen germination and pollen tube growth in vitro using four commercially relevant cultivars (Bluecrop, Elliott, Jersey, and Liberty) in climate-controlled chambers. We also tested recovery from high heat in ‘Bluecrop’ to determine whether pollen tubes can still germinate and grow after short bursts of extreme heat. We found the highest proportion of germinated pollen tetrads and the greatest pollen tube growth at 20 and 30 °C, and the lowest levels at 10 and 40 °C, with nearly complete inhibition at 40 °C. Exposure to between 30 and 40 °C revealed significant reduction in pollen germination and tube growth above 35 °C across all cultivars and assessment times. Exposure to 37.5 °C for only 4 hours resulted in substantial reductions in pollen germination and pollen tube growth, even after pollen was moved to optimal conditions of 25 °C. Extreme heat exposure, even for a short duration, significantly limits blueberry pollen germination and tube development. This is expected to have cascading effects on fruit set and crop yield. The nonreversibility of the effects on pollen highlights the need to prevent fields reaching damaging temperatures by developing crop monitoring and management strategies to protect crops during bloom.

Callose wall and sporoderm characteristics during the anther development stage of Dendrobium officinale

The pollen developmental characteristics in plants with pollinia remain unknown. In this study, Dendrobium officinale (Orchidaceae) pollinium development was investigated on the basis of a histochemical examination of polysaccharides and lipids. The D. officinale anther wall is composed of one layer of epidermal cells, two layers of endothecial cells, one layer of middle layer cells, and one layer of tapetal cells. The microspore mother cells, which have no obvious callose wall structure before meiosis, undergo simultaneous microsporogenesis. After meiosis, four microspores in a tetrad do not separate and subsequently develop as pollen tetrads, which ultimately form two pollinia. During microspore development, sporopollenin covers the pollinium surface and forms the exine of the pollinium, but the pollen tetrads inside the pollinium lack the exine structure. A pollen germinal pore was not detected in the pollinium exine. An in vitro analysis of pollen germination revealed that pollen grains at the surface of pollinia covered with an exine layer do not germinate. Furthermore, pollen tubes are mainly produced by the pollen grains inside the pollinium.

Discovery of a novel mitochondrial DNA molecule associated with tetrad pollen sterility in potato

BackgroundTetrad sterility in potato is caused by a specific cytoplasm, called TSCsto, derived from the Mexican wild tetraploid species Solanum stoloniferum. Different S. stoloniferum accessions crossed as females with S. tuberosum resulted in 12 fertile hybrids and 27 sterile hybrids exhibiting tetrad sterility.ResultsWhole-mitochondrial-genome sequencing was performed for two fertile hybrids and three hybrids exhibiting tetrad sterility. Two to seven contigs, with the total assembly lengths ranging from 462,716 to 535,375 bp, were assembled for each hybrid. Unlike for the reference mitochondrial genome (cv. Désirée), two different recombinant-type contigs (RC-I and RC-II) were identified. RC-I featured by the rpl5-ψrps14 gene joined to the nad6 gene, generating a novel intergenic region. Using a PCR marker (P-3), we found that this intergenic region occurred exclusively in interspecific hybrids exhibiting tetrad sterility and in their parental S. stoloniferum accessions. A part of this intergenic sequence was expressed in the pollen. From a large survey in which P-3 was applied to 129 accessions of 27 mostly Mexican wild species, RC-I was found in diploid S. verrucosum and polyploid species. From eight accessions of S. verrucosum used as females, 92 interspecific hybrids were generated, in which only those carrying RC-I exhibited tetrad sterility.ConclusionsRC-I was clearly associated with tetrad sterility, and the RC-I-specific intergenic region likely contains a causal factor of tetrad sterility.

Knock-down of gene expression throughout meiosis and pollen formation by virus-induced gene silencing in Arabidopsis thaliana.

SUMMARYThrough the inactivation of genes that act during meiosis it is possible to direct the genetic make‐up of plants in subsequent generations and optimize breeding schemes. Offspring may show higher recombination of parental alleles resulting from elevated crossover (CO) incidence, or by omission of meiotic divisions, offspring may become polyploid. However, stable mutations in genes essential for recombination, or for either one of the two meiotic divisions, can have pleiotropic effects on plant morphology and line stability, for instance by causing lower fertility. Therefore, it is often favorable to temporarily change gene expression during meiosis rather than relying on stable null mutants. It was previously shown that virus‐induced gene silencing (VIGS) can be used to transiently reduce CO frequencies. We asked if VIGS could also be used to modify other processes throughout meiosis and during pollen formation in Arabidopsis thaliana. Here, we show that VIGS‐mediated knock‐down of FIGL1, RECQ4A/B, OSD1 and QRT2 can induce (i) an increase in chiasma numbers, (ii) unreduced gametes and (iii) pollen tetrads. We further show that VIGS can target both sexes and different genetic backgrounds and can simultaneously silence different gene copies. The successful knock‐down of these genes in A. thaliana suggests that VIGS can be exploited to manipulate any process during or shortly after meiosis. Hence, the transient induction of changes in inheritance patterns can be used as a powerful tool for applied research and biotechnological applications.

High-Throughput Fluorescent Pollen Tetrad Analysis Using DeepTetrad.

Meiotic recombination initiates from ~100-200s of programmed DNA double stranded breaks (DSBs) in plants. Meiotic DSBs can be repaired using homologous chromosomes to generate a crossover . Meiotic crossover is critical for chromosomal segregation and increasing genetic variation. The number of crossovers is limited to one and three per chromosome pair in most plant species. Genetic, epigenetic, and environmental factors control crossover frequency and distribution. Due to the limited number of crossovers it is challenging to measure crossover frequency along chromosomes. We adapted fluorescence-tagged lines (FTLs ) that contain quartet1 mutations and linked transgenes expressing dsRed, eYFP, and eCFP in pollen tetrads into the deep learning-based image analysis tool, DeepTetrad. DeepTetrad enables the measurement of crossover frequency and interference by classifying 12 types of tetrads from three-color FTLs in a high-throughput manner, using conventional microscope instruments and a Linux machine. Here, we provide detailed procedures for preparing tetrad samples, tetrad imaging, running DeepTetrad, and analysis of DeepTetrad outputs. DeepTetrad-based measurements of crossover frequency and interference ratio will accelerate the genetic dissection of meiotic crossover control.

Chromosome Doubling in Genetically Diverse Bilberry (Vaccinium myrtillus L.) Accessions and Evaluation of Tetraploids in Terms of Phenotype and Ability to Cross with Highbush Blueberry (V. corymbosum L.)

To expand the gene pool and introduce new traits to the tetraploid cultivars of Vaccinium corymbosum from wild diploid species V. myrtillus, it is necessary to double the chromosome number in diploid species in order to overcome a post zygotic crossing barrier and a strong triploid block, existing within the genus Vaccinium. Five genetically diverse bilberry genotypes were selected from 21 accessions taken from the breeding collection of the National Institute of Horticultural Research (Skierniewice, Poland) for this study. The bilberry genotypes were derived from the Polish locations of Bolimów Landscape Park, Budy Grabskie and forest complex Zwierzyniec (Łódź Province), and habitats in Norway. The selection of genotypes was made based on the analysis of amplified fragment length polymorphism (AFLP-PCR). Analysis of the Jaccard similarity indexes and the UPGMA method revealed that the examined accessions formed two main groups on the dendrogram. The first group consisted of accessions from Norway, while the second group agglomerated Polish accessions. A further two classes were distinguished in the Polish group: the first included accessions from Budy Grabskie and the second from Zwierzyniec, located ca. 9 km from Budy Grabskie. In order to obtain plant material for in vitro polyploidisation, in vitro shoot cultures of the selected accessions were initiated and multiplied. Both antimitotics used, colchicine and APM, induced tetraploids for all of the accessions. The obtained tetraploids were multiplied, rooted ex vitro and grown in a greenhouse and then in a field. The first flowering was observed in 1.5-year-old plants, either diploid or tetraploid. Diploids bloomed slightly earlier and more profusely than tetraploid plants. Compared to diploids, autotetraploids had significantly larger flowers by ca. 64% and larger pollen tetrads by ca. 35%. The germination capacity of pollen tetrads was high in tetraploids (87.8%), although slightly lower than in diploids (94.3%). After pollinating the flowers of three highbush blueberry cultivars with pollen from the bilberry tetraploid accession, J-4-4x, the plants formed fruits, some of which contained properly formed seeds. The effectiveness of interspecific crossing between V. corymbosum and tetraploid V. myrtillus, defined as the percentage of obtained seedlings in relation to the number of pollinated flowers, was highest (53.3%) in the blueberry ‘Liberty’, and lower in ‘Bluecrop’ and ‘Northland’, 14.8% and 10.0%, respectively. Before using the seedlings for further breeding, their hybridity will be confirmed by molecular markers and the phenotype will be evaluated.

Cr and Ni simultaneous phytotoxicity and mutagenicity assay

For genotoxicity study simultaneous phytotoxicity and mutagenicity assay with Vicia sativa L. var. Klára was used. For phytotoxicity the following rank orders of growth inhibition can be arranged: for roots: Ni(II) > Cr(VI) > Cr(III); for shoots: Ni(II) > Cr(VI) ≥ Cr (III). For mutagenicity assay root tips of V. sativa were used and chromosome aberrations were determined at least in 500-anatelophases. All tested metals exerted in V. sativa a significant increase of chromosomal aberration rate in applied concentrations. Maximum of aberrations invoked Cr(VI) and the rank order of aberrations fall was: Cr(VI) > Ni(II) > Cr(III). Genotoxic effects of metals were determined by analysis of micronuclei frequency in the pollen tetrads of Tradescantia plants. None of tested metal significantly stimulated micronuclei frequency and genotoxic effect was decreased in order: Cr(VI) ≥ Ni(II) > Cr(III).

Menatanthus mosbruggeri gen. nov. et sp. nov. \u2013 A flower with in situ pollen tetrads from the Paleocene maar lake of Menat (Puy-de-D\xf4me, France)

The Paleocene Fossil-Lagerstätte Menat in France is well known for its wealth of outstandingly well preserved fossil insects and plants. Despite being known for more than a century, the palaeoflora, which is regarded as typical for the late Thanetian by some authors, has largely been neglected since the 1940s. New excavations and surveys yielded exceptionally well-preserved plant material, including a minute, heptamerous flower bearing in situ pollen tetrads, comparable to tetrads of the modern ericacean genus Kalmia L, in its anthers. The only known modern ericacean genus which is characterised by heptamerous flowers is Bejaria Mutis ex L., a basal relative of the tribe Phyllodoceae within Ericaceae, which also includes the genus Kalmia in a relatively basal position. However, heptamerous flowers also occur very rarely (mostly interpreted as teratologies) in a number of other modern Ericaceae, but also in various other modern angiosperm families. Due to the unique combination of a heptamerous flower with Kalmia-type pollen tetrads within the anthers, the new taxon Menatanthus mosbruggeri gen. nov. et sp. nov. is erected. The lack of morphological data from the flower itself and the fact that comparable pollen tetrads can be produced by a number of modern families, however, prevent an assignment of the new taxon to any known angiosperm family.

Development of the permanent tetrad wall in Juncus L. (Juncaceae, Poales).

Juncaceae, a cosmopolitan family, belong to the cyperid clade of Poales together with Cyperaceae and Thurniaceae. Pollen grain of Juncaceae, as in Thurniaceae, is dispersed in a permanent tetrad, and knowledge about the ontogeny of its wall is still incipient, based on data from only one species. This study aims to analyze the formation of the pollen wall of seven Juncus species in order to characterize the timing and the ontogenetic events that lead to the cohesion of the four pollen grains in a permanent tetrad. Anthers at different developmental stages were submitted to techniques of light microscopy and transmission electron microscopy; dehiscent anthers with mature pollens were also analyzed in scanning electron microscopy. In all the species here studied, callose deposits around each microsporocyte, with dissolution prior to meiosis. Microspore wall starts at the end of the second meiotic division with formation of primexine. Exine comprises tectum, columellae, and foot layer. During cytokinesis, cell plates form the internal wall of the pollen tetrad. Mature permanent tetrad is enveloped externally by both the exine and intine and internally by the intine and the foot layer, which forms the continuous internal wall. Callose was detected in the early stages of microsporocytes, although reported to be absent in Juncaceae. Our data confirm the variation in Juncaceae cytokinesis and the occurrence of simple cohesion due to the presence of a continuous tectum along the pollen tetrad.

ASY1 acts as a dosage-dependent antagonist of telomere-led recombination and mediates crossover interference in Arabidopsis

During meiosis, interhomolog recombination produces crossovers and noncrossovers to create genetic diversity. Meiotic recombination frequency varies at multiple scales, with high subtelomeric recombination and suppressed centromeric recombination typical in many eukaryotes. During recombination, sister chromatids are tethered as loops to a polymerized chromosome axis, which, in plants, includes the ASY1 HORMA domain protein and REC8-cohesin complexes. Using chromatin immunoprecipitation, we show an ascending telomere-to-centromere gradient of ASY1 enrichment, which correlates strongly with REC8-cohesin ChIP-seq data. We mapped crossovers genome-wide in the absence of ASY1 and observe that telomere-led recombination becomes dominant. Surprisingly, asy1/+ heterozygotes also remodel crossovers toward subtelomeric regions at the expense of the pericentromeres. Telomeric recombination increases in asy1/+ occur in distal regions where ASY1 and REC8 ChIP enrichment are lowest in wild type. In wild type, the majority of crossovers show interference, meaning that they are more widely spaced along the chromosomes than expected by chance. To measure interference, we analyzed double crossover distances, MLH1 foci, and fluorescent pollen tetrads. Interestingly, while crossover interference is normal in asy1/+, it is undetectable in asy1 mutants, indicating that ASY1 is required to mediate crossover interference. Together, this is consistent with ASY1 antagonizing telomere-led recombination and promoting spaced crossover formation along the chromosomes via interference. These findings provide insight into the role of the meiotic axis in patterning recombination frequency within plant genomes.

DeepTetrad: high-throughput image analysis of meiotic tetrads by deep learning in Arabidopsis thaliana.

Meiotic crossovers facilitate chromosome segregation and create new combinations of alleles in gametes. Crossover frequency varies along chromosomes and crossover interference limits the coincidence of closely spaced crossovers. Crossovers can be measured by observing the inheritance of linked transgenes expressing different colors of fluorescent protein in Arabidopsis pollen tetrads. Here we establish DeepTetrad, a deep learning-based image recognition package for pollen tetrad analysis that enables high-throughput measurements of crossover frequency and interference in individual plants. DeepTetrad will accelerate the genetic dissection of mechanisms that control meiotic recombination.

Reproductive Biology of Wild Blueberry (Vaccinium angustifolium Aiton)

Wild blueberry, Vaccinium angustifolium Aiton, is a native forest understory plant that is managed as a fruit crop. Over the past 51 years, experiments have been conducted to investigate its reproduction. A model was developed that predicts bloom to begin at 100° days (base 4.4 °C) after 1 April and to end at 500° days for a period of three to four weeks. Flower stigmas are only receptive to pollen deposition for eight to 10 days, and the rate of fruit set declines rapidly after four days. Placement of pollen upon receptive stigmas suggests that fruit set occurs with as little as a single pollen tetrad. Twelve tetrads result in 50% fruit set. Several years of exploratory fruit set field experiments show viable seeds per berry, which result from pollination with compatible genotype pollen, is associated with larger berry mass (g). Decomposition of the total variance in fruit set shows that stem variation explains 65% to 79% of total variance in the fruit set. To a lesser extent, the field, year, and clone also explain the percent fruit set variation. Variation between stems may be due to variation in the number of flowers. Fruit set tends to decrease as the flower density increases, possibly due to the limitation of pollinators.

A Novel Pollen Transfer Mechanism by Honey Bee Foragers on Highbush Blueberry (Ericales: Ericaceae).

Honey bees (Apis mellifera L. (Hymenoptera: Apidae)) are effective pollinators of many crops but are thought to be inefficient in pollinating blueberries (Vaccinium corymbosum L.) due to their inability to buzz pollinate. Nonetheless, commercial growers rent honey bee hives for pollination, resulting in the dominance of honey bee workers visiting flowers during bloom. The objective of this study was to examine where on the honey bee pollen is carried and how it is transferred from flower to bee to the stigma of other flowers. Examination of 90 honey bee workers foraging on blueberry flowers documented that blueberry pollen tetrads were present on all body parts. Relative amounts were as follows: head 12%, body 6%, legs 19%, and tarsi 63%. Quantities on the body subregions ranged from an average of 400 tetrads on the basitarsi to 16 on the fore tarsal claws. Thus, a single contact between many of the pollen-carrying body parts and a stigma has the potential to transfer significant amounts of pollen. The study also revealed that bee behaviors unrelated to the intentional collection of nectar and pollen, such as the frequent touching of stigmas by the claws, tarsi, or legs, while foraging, grooming, and walking across flower clusters, could result in pollen transfer. These contacts occurred 65.9% of the time a honey bee landed on a flower cluster. These findings have broad implications for future assessments of the efficiencies of various bee species in pollinating diverse crops and plants.

Native pollinator management may be a key to improving fruit set in Tasmanian Mountain Pepper, Tasmannia lanceolata (Winteraceae), an emerging spice resource

ABSTRACTTasmanian Mountain Pepper is a new crop that produces a valuable flavoring agent derived from its fruit. However, annual yields are unpredictable and its commercial prospects depend upon a reliable fruit set. As poor pollination is a possible cause, the pollination biology of this dioecious plant was examined in natural and plantation settings using direct observations, sticky traps, gas chromatography, and microscopy. A community of small insects visits the flowers, and the insect species represent at least seven families of small flies, three families of beetles, and various wasps and moths. Pollen grains were observed on the bodies of bibionid flies (Bibionidae), syrphid flies (Syrphidae), and snail-flies (Sciomyzidae). The tetrahedral tetrad pollen grains are unlikely to be suitable for dispersal by wind. Similar volatiles were emitted by both males and females; several ocimene compounds are attractants for insect pollen vectors. The female flowers also emitted indole, a known fly attractant. It is therefore likely that Tasmannia lanceolata is dependent on the services of diverse insects, especially Diptera, for promoting fruit set. Plantations should be established in the vicinity of biodiverse native vegetation that supports a wide variety of native flying insects capable of transferring pollen.

Pollen transfer efficiency of <i>Apocynum cannabinum</i> (Apocynaceae): a comparative perspective

Pollen transfer efficiency (PTE), the percentage of removed pollen delivered to conspecific stigmas, has been implicated in the morphological evolution, population dynamics, and lineage diversification of flowering plants. Pollinia, the aggregated contents of pollen sacs, present in Apocynaceae subfamilies Asclepiadoideae (milkweeds), Secamonoideae, and Periplocoideae and orchids (Orchidaceae), are the pre-eminent example of a plant trait that elevates PTE (to ca. 25%). However comparison of species with pollinia to “average” flowers (PTE ca. 1%) may over-estimate the gains from pollinia. We hypothesize that elevated PTE evolved in Apocynaceae prior to pollinia. We measured PTE and pollen to ovule ratio, a possible correlate of PTE, in Apocynum cannabinum, a milkweed relative with pollen tetrads (instead of pollinia) and simple bands of style head adhesive (instead of complex pollinium-carrying translators), comparing them to reports of other species collated from the literature. PTE of A. cannabinum is 7.9%, in the 24th percentile of reports for 35 milkweed species, but more than twice the highest PTE reported for a species with monads (3.4%). The bands of style head adhesive are functionally equivalent to the translators of milkweeds. The pollen to ovule ratio of A. cannabinum, at 19.8, is in the 94th percentile of ratios reported for milkweeds (mean 9.6). Our results are consistent with the hypothesis that floral novelties of Apocynaceae that evolved prior to pollinia also promote aggregated pollen transport and elevated PTE.

Sporogenesis, gametogenesis and pollen morphology of Solanum japonense and S. septemlobum (Solanaceae)

Solanum japonense Nakai and S. septemlobum Bunge are medicinal plants used chaotically in traditional Chinese medicine because they have the same Chinese name, Shuyangquan. In this study, anther wall development, microsporogenesis, male gametophyte development, megasporogenesis and female gametophyte development of S. japonense and S. septemlobum were studied using traditional paraffin section technology for the first time, and their pollen morphologies were compared using scanning electron microscopy. The results showed that both species exhibit dicotyledonous anther wall development, dual tapetum origination, secretory tapetum development, simultaneous microsporocyte cytokinesis, tetrahedral tetrad, coexistent 2-celled and 3-celled mature spheroidal pollen grains that are circular along the equatorial view and have tricolporate groves, 2 locules per ovary, axial placenta, anatropous, unitegmic and tenuinucellate ovule, linear megaspore tetrad, as well as monosporic Polygonum type of embryo sac, where the chalazal megaspore develops into the functional megaspore and the other three megaspores degenerate. However, the pollen grains of S. japonense are circular along the polar view, while those of S. septemlobum are triangular; the pollen surface ornamentation of S. japonense is granulate-verrucate-punctate, while that of S. septemlobum is granulate-punctate-fossula. These results enrich the embryological data of Solanum and provide palynological bases for the classification of these two species. https://doi.org/10.2298/ABS170724048L Received: July 24, 2017; Revised: October 20, 2017; Accepted: November 9, 2017; Published online: November 22, 2017 How to cite this article: Liu Y, Gu L, Liu J. Sporogenesis, gametogenesis and pollen morphology of Solanum japonense and S. septemlobum (Solanaceae). Arch Biol Sci. 2018;70(2):313-20.

The fossil pollen tetrad Quadripollis krempii Drugg 1967; morphology, palaeogeography and botanical affinity

ABSTRACTQuadripollis krempii was described by Drugg 1967 from Maastrichtian deposits in California and has been reported from the Albian to Paleocene of the western United States and Canada. A revised description is supported by illustrations of specimens found in outcrops of the Straight Cliffs Formation in southern Utah. Q. krempii is unique among fossil tetrads and the tetrahedral tetrad is apparently inaperturate. We observe that the thinning and occasional rupturing of the exine observed by Drugg is a vestigial aperture, herein described as a tenuitas. Q. krempii has been thought to be either gymnospermous or angiospermous. No modern gymnosperms produce pollen in obligate tetrads but at least 23 angiosperm families do. Our review revealed possible families with some similarities to Q. krempii, mainly members of the Juncaceae which exhibit an ulcerated or thinned exine in the distal areas of the monads. A relationship to bryophytes cannot be ruled out.

AN OVERVIEW OF POLLEN AND ANTHER WALL DEVELOPMENT IN Catalpa bignonioides WALTER (BIGNONIACEAE)

Anther development in Catalpa bignonioides Walter was investigated from the sporogenous cell to the mature pollen grain stages to determine whether the pollen and anther wall development follows the basic scheme in angiosperms. In order to follow pollen ontogeny through successive stages of pollen development, anthers at different developmental stages were embedded in epon according to the usual method, and semi-thin sections, taken from the epon embedded anthers, were stained with toluidine blue for general histological observations under light microscopy. The young anther wall of C. bignonioides consists of four layers; from the exterior, the epidermis, endothecium, middle layer, and a secretory tapetum. The tapetum is dual in origin and dimorphic. Ubisch bodies were observed on the inner tangential walls of the tapetal cells. The number of the anther wall layers changes depending on the developmental stage and region of the anther. In contrast to the other anther wall layers , epidermis and endothecium layers remain intact until anthesis. Endothecial cells enlarge and develop thickenings at maturity. During microspore development , meiocytes undergo meiosis and simultaneous cytokinesis leading to the formation of permanent tetrahedral, isobilateral and rarely linear tetrads. Pollen tetrads are shed from the anther as compound pollen grains. Results of the study revealed that pollen and anther wall development in C. bignonioides follows the basic scheme in angiosperms.