Pollen-limited seed set in pseudogamous blackberries (Rubus L. subgen. Rubus)

Summary Pseudogamous blackberry species are polyploid and usually exhibit meiotic irregularities causing severe reduction in pollen viability. When species means were compared, relative seed set was strongly correlated with the number of good pollen grains produced per flower divided by the number of ovules (good-pollen/ovule ratio).Partial correlation analysis for percentage viable pollen and relative seed set, controlling for good-pollen/ovule ratio, revealed an almost significant relationship, presumably because meiotic irregularities have a similar effect on both pollen viability and the viability of meiotically derived embryo sacs. Seed germination, on the other hand, showed no relationship with relative seed set or with pollen viability.The automatic selfing capacity exhibited by most of these blackberry species does not appear to be related to either pollen production or relative seed set.     

MEGASPOROGENESIS AND GAMETOPHYTE SELECTION IN PAEONIA CALIFORNICA

Walters , James L. (U. California, Goleta.) Megasporogenesis and gametophyte selection in Paeonia californica. Amer. Jour. Bot. 49(7): 787–794. Illus. 1962.—In the ovules of Paeonia californica, a massive archesporium produces numerous (estimated at 30–40) megasporocytes, many of which complete meiosis. Several continue into gametophyte development, which is of the Polygonum type, and at the time of fertilization there are from 1 to 4 gametophytes per ovule. Rarely does more than 1 seedling per seed appear in germination. This species is characterized by extensive translocation heterozygosity, and other meiotic irregularities, in its natural populations. It shows a complete range from plants forming only pairs to those with all their chromosomes in rings at meiosis. The latter types have as high as 90% bad pollen. The course of events in the ovules is compared with the “Renner-effect” found in Oenothera. The multiple megasporocytes and subsequent events are seen as a mechanism which insures each ovule a high probability of containing a viable egg in spite of meiotic behavior which can produce 90% sterility, and thus insures high seed set in the translocation heterozygotes.     

Ovary and Gametophyte Development Are Coordinately Regulated by Auxin and Ethylene following Pollination

The differentiation and development of ovules in orchid flowers are pollination dependent. To define the developmental signals and timing of critical events associated with ovule differentiation, we have examined factors that regulate the initial events in megasporogenesis and female gametophyte development and characterized its progression toward maturity and fertilization. Two days after pollination, ovary wall epidermal cells begin to elongate and form hair cells; this is the earliest visible morphological change, and it occurs at least 3 days prior to pollen germination, indicating that signals associated with pollination itself trigger these early events. The effects of inhibitors of ethylene biosynthesis on early morphological changes indicated that ethylene, in the presence of auxin, is required to initiate ovary development and, indirectly, subsequent ovule differentiation. Surprisingly, pollen germination and growth were also strongly inhibited by inhibitors of ethylene biosynthesis, indicating that male gametophyte development is also regulated by ethylene. Detailed characterization of the development of both the female and male gametophyte in pollinated orchid flowers indicated that pollen tubes entered the ovary and grew along the ovary wall for 10 to 35 days, at which time growth was arrested. Approximately 40 days after pollination, coincident with ovule differentiation as indicated by the presence of a single archesporial cell, the direction of pollen tube growth became redirected toward the ovule, suggesting a chemical signaling between the developing ovule and male gametophyte. Taken together, these results indicate that both auxin and ethylene contribute to the regulation of both ovary and ovule development and to the coordination of development of male and female gametophytes.     

Genetics of sex allocation in Mimulus (Scrophulariaceae)

Theoretical models of the evolution of resource allocation patterns to male and female function make the assumption that there are inherent trade-offs between the two. Here we use a quantitative genetic approach to quantify trade-offs between male and female function and to determine whether plant populations could readily respond to natural selection by quantifying the amount of genetic variation for pollen and ovule production. Both intra- and interspecific crossing designs were applied to two populations of the predominantly outcrossing Mimulus guttatus and two populations of the highly selfing congener, M. micranthus. The only significant correlations observed among pollen number, pollen size and ovule number were positive. Positive genetic correlations among the traits were sometimes reduced after removing the effect of flower size but still no significant negative correlations were detected. These results suggest that positive correlations between pollen and ovule production may be due to the joint positive correlation of these characters with the resource pool available for pollen and ovule production, as reflected by flower size. Heritabilities were moderate to high for ovule production but low for pollen number and pollen size and suggest that responses to selection would differ between the two traits. Crosses between the species revealed that there are additional genetic factors contributing to differences between the two species for corolla width, vs. pollen:ovule ratio. This is consistent with the hypothesis that genetic variation for resource acquisition may in part be responsible for the overall lack of a negative correlation between pollen and ovule production and provides a genetic explanation for little evidence of trade-offs between sexual functions in Mimulus.     

Pollen histochemistry and pollen : ovule ratios in Zingiberaceae

BACKGROUND AND AIMS: Pollen grains of 37 species from 11 genera in the family Zingiberaceae were examined to assess qualitatively starch or lipid contents; the pollen grain and ovule numbers per flower and pollen : ovule ratios were also counted and calculated. Pollen : ovule ratios were studied to reveal patterns of variation in the Zingiberaceae. METHODS: Freshly open flowers with dehiscing anthers were collected at random from plants growing in natural habitats or in botanical gardens. Presence of lipids or starch in pollen grains was tested by Sudan solution and IKI solution, respectively, and examined under a microscope. To estimate the pollen and ovule numbers per flower, one anther from each bud was carefully dissected and all pollen grains were counted; ovaries were carefully dissected out of each flower and counted. Whenever possible, at least 10-15 buds were used in the determination. KEY RESULTS: Thirty-three of all the 37 species examined had starchy pollen. Starch was not found in only four species and lipid was not found in only one species; among the four tribes in subfamily Zingiberoideae, all species of Zingibereae and Globbeae had pollen with no starch, Alpineae and Hedychieae had pollen with and without starch, whereas, all species of subfamily Costoideae had starchy pollen with abundant lipids. The mean pollen : ovule ratios in the members of the Zingiberaceae investigated range from 3.25 +/- 1.56 to 616.52 +/- 117.83. CONCLUSIONS: The pollen nutrition types seemed not related to mating systems. The pollen : ovule ratios in members of the Zingiberaceae with the same breeding system are noticeably lower than those recorded by previous authors. The lower pollen : ovule ratios in this family are presumed to be related to the highly efficient pollination systems, mediated by pollen which can be quite glutinous and the relatively large stigma area. In most of the Alpinia species the anaflexistylous flowers have much larger numbers of pollen grains and higher pollen : ovule ratios than the cataflexistylous flowers. There are significant differences in mean pollen grain numbers and pollen : ovule ratios between different life forms but ovule numbers are approximately the same.     

REPRODUCTIVE FEATURES OF DENTARIA LACINIATA AND D. DIPHYLLA (CRUCIFERAE), AND THE IMPLICATIONS IN THE TAXONOMY OF THE EASTERN NORTH AMERICAN DENTARIA COMPLEX

Reproductive features including ovule development, megasporogenesis, megagametogenesis, microsporogenesis, microgametogenesis, pollen tube growth, embryogeny, and natural seed germination were studied in a single population each of Dentaria laciniata Muhl. ex. Willd. and D. diphylla Michx. to test for possible agamospermy. The population of D. laciniata studied is sexual. The archesporial cell functions directly as the megasporocyte. It undergoes two meiotic divisions, but the micropylar cell of the dyad fails to undergo meiosis II, and a linear triplet of three cells is formed. The chalazal megaspore divides to form an eight-nucleate, seven-celled megagametophyte of the Polygonum type. Simultaneous cytokinesis follows the second meiotic division of the microsporocyte yielding a tetrahedral tetrad of microspores. A three-celled pollen grain is formed prior to anther dehiscence. Following apparent fertilization, the Capsella-variation of the Onagrad type of embryogeny results in a conduplicate embryo. Endosperm is initially nuclear, but eventually becomes cellular. Seeds readily germinate in nature. Similar events are documented in one population of D. diphylla up to the organization of the embryo-sac, which disintegrates before cellularization. These reproductive events and other data indicate that the eastern North American species of Dentaria may form a sexual polyploid complex with some sexual populations and some sterile ones.     

Evolution of mating system and the genetic covariance between male and female investment in Clarkia (onagraceae): selfing opposes the evolution of trade-offs

Sex allocation theory has assumed that hermaphroditic species exhibit strong genetically based trade-offs between investment in male and female function. The potential effects of mating system on the evolution of this genetic covariance, however, have not been explored. We have challenged the assumption of a ubiquitous trade-off between male and female investment by arguing that in highly self-fertilizing species, stabilizing natural selection should favor highly efficient ratios of male to female gametes. In flowering plants, the result of such selection would be similar pollen:ovule (P:O) ratios across selfing genotypes, precluding a negative genetic correlation (r(g)) between pollen and ovule production per flower. Moreover, if selfing genotypes with similar P:O ratios differ in total gametic investment per flower, a positive r(g) between pollen and ovule production would be observed. In outcrossers, by contrast, male- and female-biased flowers and genotypes may have equal fitness and coexist at evolutionary equilibrium. In the absence of strong stabilizing selection on the P:O ratio, selection on this trait will be relaxed, resulting in independence or resource-based trade-offs between male and female investment. To test this prediction, we conducted artificial selection on pollen and ovule production per flower in two sister species with contrasting mating systems. The predominantly self-fertilizing species (Clarkia exilis) consistently exhibited a significant positive r(g) between pollen and ovule production while the outcrossing species (C. unguiculata) exhibited either a trade-off or independence between these traits. Clarkia exilis also exhibited much more highly canalized gender expression than C. unguiculata. Selection on pollen and ovule production resulted in little correlated change in the P:O ratio in the selfing exilis, while dramatic changes in the P:O ratio were observed in unguiculata. To test the common prediction that floral attractiveness should be positively genetically correlated with investment in male function, we examined the response of petal area to selection on pollen and ovule production and found that petal area was not consistently genetically correlated with gender expression in either species. Our results suggest that the joint evolutionary trajectory of primary sexual traits in hermaphroditic species will be affected by their mating systems; this should be taken into account in future theoretical and comparative empirical investigations.     

羊草大、小孢子发生与雌、雄配子体发育的观察

利用常规石蜡制片技术研究了羊草大、小孢子发生及雌、雄配子体发育过程。主要结果是：(1)花药壁由4层结构组成,最外层为表皮,其次为药室内壁,1层中层,最内层绒毡层为分泌型;(2)小孢子母细胞减数分裂过程中的胞质分裂为连续型,四分孢子为左右对称型;(3)成熟花粉粒为3细胞型,具单萌发孔;(4)羊草为单子房、单胚珠,双珠被、薄珠心、倒生型胚珠,大孢子母细胞减数分裂形成线型或T型排列的4个大孢子;合点端大孢子具功能;(5)具有双孢原,双大孢子母细胞、双大孢子四分体和双胚囊的情况;(6)胚囊发育为蓼型,反足细胞经无丝分裂形成4～6个细胞的反足细胞群;(7)同一朵花中,前期雄蕊的发育早于雌蕊的发育,后期当花粉成熟时,雌配子体也达到成熟,雌雄蕊发育趋于同步。     

Arabidopsis HAP2 (GCS1) is a sperm-specific gene required for pollen tube guidance and fertilization

In flowering plants, sperm cells develop in the pollen cytoplasm and are transported through floral tissues to an ovule by a pollen tube, a highly polarized cellular extension. After targeting an ovule, the pollen tube bursts, releasing two sperm that fertilize an egg and a central cell. Here, we identified the gene encoding Arabidopsis HAP2, demonstrating that it is allelic to GCS1. HAP2 is expressed only in the haploid sperm and is required for efficient pollen tube guidance to ovules. We identified an insertion (hap2-1) that disrupts the C-terminal portion of the protein and tags mutant pollen grains with the beta-glucuronidase reporter. By monitoring reporter expression, we showed that hap2-1 does not diminish pollen tube length in vitro or in the pistil, but it reduces ovule targeting by twofold. In addition, we show that the hap2 sperm that are delivered to ovules fail to initiate fertilization. HAP2 is predicted to encode a protein with an N-terminal secretion signal, a single transmembrane domain and a C-terminal histidine-rich domain. These results point to a dual role for HAP2, functioning in both pollen tube guidance and in fertilization. Moreover, our findings suggest that sperm, long considered to be passive cargo, are involved in directing the pollen tube to its target.     

The ponticulus: a structure bridging pollen tube access to the ovule in Pistacia vera

The path of the pollen tube has been examined in pistachio (Pistacia vera), a chalazogamous species where the pollen tube penetrate the ovule via the chalaza. Special attention was paid to the way the pollen tube gains access to the ovule. A single anatropous ovule with a big funiculus occupies the entire ovary cavity. At anthesis, a physical gap exists between the ovule and the base of the style. However, upon pollen tube arrival a protuberance, the ponticulus, develops in the uppermost area of the funiculus between the style and ovule. This structure appears to facilitate access to the ovule by the pollen tube. The pollen tube penetrates the ovule via this ponticulus. Upon penetration, callose develops in the ponticulus cells surrounding the pollen tube. After pollen tube passage, the upper layer of the ponticulus lignifies and isolates the ovule from the style. This separation is further enlarged 2 weeks later when the ovary starts to develop without expansion of the ovule and a large gap develops separating the ovule from the style. Except for the induction of callose formation by the pollen tube in the funiculus, this process is independent of pollination and appears to be developmentally regulated since it occurs in the same way and at the same time in pollinated and unpollinated flowers. The ponticulus, although by a different mechanism, appears to be playing the role of an obturator regulating access of the pollen tube to the ovule. Furthermore, this access is restricted to a particular time during the development of the ovule.     

EFFECT OF MALE POLLEN DONOR AND FEMALE SEED PARENT ON ALLOCATION OF RESOURCES TO DEVELOPING SEEDS AND FRUIT IN CHAMAECRISTA FASCICULATA (LEGUMINOSAE)

In Chamaecrista fasciculata, fruit abortion levels are high and seed mass is highly variable, necessary preconditions for differential resource allocation of the female to seed and fruit sired by different males. This study investigated the relative role of pollen donor and seed parent on the allocation of resouces to developing seed and fruit, and assessed the role of genetic relatedness in contributing to any observed paternal effect in C. fasciculata. In addition, pollen donor effects were contrasted to within-seed parent sources of variation in resource allocation due to pollination date and ovule position in the pod. Plants collected from the field were brought to a greenhouse where single-donor crosses were conducted controlling for pollen donor source and interplant distance, a measure of genetic relatedness. Seed mass, number of seed/fruit, fruit maturation time, and fruit abortion rate were measured as indicators of resource allocation to developing seed and fruit. Variation in resource allocation was largely determined by the seed parent. Pollen donor effects were limited to differences between self vs. non-self pollinations, suggesting that inbreeding depression following mating events between related individuals is the source of any variation among pollen donors on differential resource allocation to developing seed and fruit. Once the effect of inbreeding was removed, however, pollination date and ovule position played larger roles than pollen source. Since there was no detectable variation among male pollen donors in their ability to accrue resources from the female seed parent apart from inbreeding effects, it is concluded that the opportunity for postzygotic mate choice is limited in C. fasciculata.     

新疆阿魏的胚胎学研究

采用常规石蜡制片技术,对新疆阿魏不同发育时期的花和果实进行了显微切片观察.结果表明:新疆阿魏小孢子母细胞的减数分裂为同时型,小孢子四分体为四面体型和十字交叉型,成熟花粉粒为3-细胞型.雌蕊2心皮合生成2室,中轴胎座,每子房室内产生上、下2个胚珠原基,其中,下方的原基正常发育,而上方的原基停止发育并最终解体,因此,每室仅产生一枚发育正常的倒生胚珠,单珠被,薄珠心,胚囊发育为蓼型;珠被绒毡层和珠孔塞发生于大孢子四分体时期,并于四核胚囊时分化完全,八核胚囊时珠被绒毡层细胞径向延长;3个成熟的反足细胞具双核.胚乳发育为核型,细胞壁较厚,细胞排列紧密,可保护胚免受机械损伤及防止胚失水.胚乳细胞中含有大量PAS染色呈正反应的物质,一些胚乳细胞异常生长形成细胞体积大、核及核仁均较大的巨形细胞.胚胎发生为茄型,四细胞原胚为直线形,十六细胞原胚的顶部由2排各4个细胞组成.成熟种子具胚乳.     

Pollen-ovule patterns in tribeTrifolieae (Leguminosae)

Relative pollen and ovule production in the genera of the legume tribeTrifolieae is explored particularly as to howMedicago with its explosive pollination mechanism compares with its allies.Medicago produces much larger, although much fewer pollen per ovule than the other five genera in the tribe; this is interpreted as a consequence of its highly specialized, irreversible pollination mechanism, which allows only one effective exchange of pollen with pollinators.Melilotus andTrifolium produce a comparatively large quantity of pollen; this can be related to their floral characteristics requiring abundant pollen to achieve fertilization. InMedicago andTrigonella, annuals convert a higher proportion of pollen grains and ovules into seeds than perennials.     

EFFICACY OF WIND POLLINATION: POLLEN LOAD SIZE AND NATURAL MICROGAMETOPHYTE POPULATIONS IN WIND-POLLINATED STABEROHA BANKSII (RESTIONACEAE)

A detailed examination was made of the numbers of pollen grains and microgametophytes occurring on stigmas of a dioecious, wind-pollinated species, Staberoha banksii. The number of pollen grains per pistil ranged from 0 to over 100; and the mean pollen load size per pistil per plant varied from 4.4 to 28. Distance to the fourth nearest male plant was used as a measure of local pollen availability and accounted for 63.5% of the between-plant variation in pollen load size. In contrast, the number of microgametophytes did not show a leptokurtic decline with increasing distance from a pollen source. On average there were 5.19 microgametophytes per ovule, which is higher than all comparable data for insect-pollinated taxa, and 66% of the stigmas had two or more microgametophytes. Given that there is a single ovule per flower, there is considerable potential for gametophytic competition. Seedset was very high, averaging 93%, and showed no significant decline in isolated female plants. We suggest that the quantity of pollen is not limiting fertilization in this population and that wind may therefore be highly effective as a pollen vector.     

Gamete fusion is required to block multiple pollen tubes from entering an Arabidopsis ovule

In double fertilization, a reproductive system unique to flowering plants, two immotile sperm are delivered to an ovule by a pollen tube. One sperm fuses with the egg to generate a zygote, the other with the central cell to produce endosperm. A mechanism preventing multiple pollen tubes from entering an ovule would ensure that only two sperm are delivered to female gametes. We use live-cell imaging and a novel mixed-pollination assay that can detect multiple pollen tubes and multiple sets of sperm within a single ovule to show that Arabidopsis efficiently prevents multiple pollen tubes from entering an ovule. However, when gamete-fusion defective hap2(gcs1) or duo1 sperm are delivered to ovules, as many as three additional pollen tubes are attracted. When gamete fusion fails, one of two pollen tube-attracting synergid cells persists, enabling the ovule to attract more pollen tubes for successful fertilization. This mechanism prevents the delivery of more than one pair of sperm to an ovule, provides a means of salvaging fertilization in ovules that have received defective sperm, and ensures maximum reproductive success by distributing pollen tubes to all ovules.     

Pollen-Ovule Ratio and Gamete Investment in Pedicularis （Orobanchaceae）

The Pedicularis species provides ideal materials to study floral evolution because of their substantial flower variation based on a narrow genetic basis, even though they are almost exclusively pollinated by bumblebee. These traits allow us to detect the evolutionary trends of floral parameters without considering genetic background and the difference of pollination vectors. The pollen-ovule ratio is widely used to estimate the pattern of resource investment in two sexual functions in flowering plants. Forty species representing all of the corolla types in Pedicularls were used to study pollen-ovule ratio, gamete investment, and their correlations. Results show that pollen-ovule ratio does not differ among both different corolla types and taxonomic groups. It is therefore suggested that pollen-ovule ratio should be a parallel evolution. The correlations between pollen-ovule ratio and pollen size （-）, and ovule size （＋） can be successfully explained in terms of sex allocation theory. The biological significance of such relationships was also discussed. Additionally, we analyzed the pattern of resource investment into female gamete, which has been somewhat neglected, and found that plants have different patterns of gamete investment between the two sexual functions.     

OVULE PACKAGING IN STOCHASTIC POLLINATION AND FERTILIZATION ENVIRONMENTS

The modular morphology of plants has important consequences for reproductive strategies. Ovules are packaged in discrete structures (flowers) that usually vary stochastically in pollen capture and ovule fertilization, because of the vagaries of pollen transfer by external agents. Different ovule packaging schemes may use limited reproductive resources more or less effectively, so that some number of ovules per flower may be optimal, given the prevailing probabilities of ovule fertilization. I derive a phenotypic model for ovule number per flower that maximizes the expected total ovule fertilizations on a plant when pollination and fertilization vary randomly among individual flowers. This model predicts that, except for small or inexpensive flowers, ovules should be “oversupplied” relative to the mean receipt of pollen tubes, so that pollen limitation of seed set should be common. Published data are congruent with this prediction. Additional hypotheses on the relation of ovule packaging to floral cost, plant size, and variance in pollen receipt are suggested by the model, but few data exist to evaluate these hypotheses.     

从柱头到胚囊——花粉管发育研究进展

花粉管的生长和发育是植物有性生殖过程中重要环节。本文首先介绍了花粉管的结构及其脉冲生长方式。然后介绍了花粉粒在柱头上萌发、花粉管在花柱道中生长及其进入胚囊的一些特点,同时还介绍了雌蕊在花粉管生长发育所起的重要作用。     

Genetic Evidence for a Long-Range Activity That Directs Pollen Tube Guidance in Arabidopsis

The fertilization process of plants is governed by different kinds of cell-cell interactions. In higher plants, these interactions are required both for recognition of the pollen grain by the female reproductive system and to direct the growth of the pollen tube inside the ovary. Despite many years of study, the signaling mechanisms that guide the pollen tube toward its target, the ovule, are largely unknown. Two distinct types of principles, mechanical and chemotropic, have been suggested to account for the directed growth of the pollen tube. The first of these two types of models implies that the guidance of the pollen tube depends on the architecture and chemical properties of the female reproductive tissues, whereas the latter suggests that the ovule provides a signal for the target-directed growth of the pollen tube. To examine such a role for the ovules, we analyzed the growth path of pollen tubes in mutants defective in ovule development in Arabidopsis. The results presented here provide unique in vivo evidence for an ovule-derived, long-range activity controlling pollen tube guidance. A morphological comparison of the ovule mutants used in this study indicates that within the ovule, the haploid embryo sac plays an important role in this long-range signaling process.     

The signals to trigger the initiation of ovule enlargement are from the pollen tubes: The direct evidence

In angiosperms, initiation of ovule enlargement represents the start of seed development, the molecular mechanism of which is not yet elucidated.It was previously reported that pollen tube contents,rather than double fertilization, can trigger ovule enlargement. However, it remains unclear whether the signal(s) to trigger the initiation of ovule enlargement are from the sperm cells or from the pollen tubes.Recently, we identified a mutant drop1- drop2-, which produces pollen tubes with no sperm cells. Taking advantage of this special genetic material, we conducted pollination assays, and found that the ovules pollinated with drop1- drop2- pollen could initiate the enlargement and exhibited significant enlarged sizes at 36 h after pollination in comparison with those unpollinated ovules. However, the sizes of the ovules pollinated with drop1- drop2- pollen are significantly smaller than those of the ovules pollinated with wildtype pollen. These results demonstrate that the pollen tube, rather than the sperm cells, release the signal to trigger the initiation of ovule enlargement, and that double fertilization is required for further enlargement of the seeds.