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Delayed fertilization and pollen-tube growth in pistils of Fagus japonica (Fagaceae)¹

Department of Botany, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

Received for publication February 23, 2006. Accepted for publication October 9, 2006.

ABSTRACT

In contrast to most angiosperms, in which fertilization occurs 1 or 2 days after pollination, in some plant orders, including the Fagales, fertilization is delayed from 4 days to more than 1 year, raising questions regarding why fertilization is delayed and where and how pollen tubes remain in the pistil during the delay. To answer these questions, we investigated pollen-tube growth in pistils of Fagus japonica (Fagaceae), which are tricarpellate and have six ovules, using light, fluorescence, and scanning electron microscopy. The ovules were immature at the time of pollination and required 5 weeks to become fully developed. During this 5 weeks, pollen tubes grew from the stigma to the embryo sac in association with the development of ovules and intermittently in three steps with two growth-cessation sites, i.e., on the funicle and near the micropyle. The number of pollen tubes was gradually reduced from many to one at the two growth-cessation sites, and fertilization occurred in one ovule that apparently developed earlier than the others in the pistil. Thus, delayed fertilization plays an important role in gametophyte competition and selection leading to nonrandom fertilization. Intermittent pollen-tube growth is also likely widespread in angiosperms because it is known in other Fagales and an unrelated order Garryales.

Key Words: delayed fertilization • embryo sac • Fagaceae • Fagus • fertilization • micropyle • pollen tube

In angiosperms, in general, pollen grains (male gametophytes) that arrive on the stigma outnumber the total ovules in an ovary (for review, see Erbar, 2003 ). After germinating on the stigma, numerous pollen tubes grow downward within the style, but only a single pollen tube eventually reaches an embryo sac (female gametophyte), fertilizing the ovule within 24 to 48 h or even less (Maheshwari, 1950 ; Gao et al., 1992 ; Higashiyama et al., 1997 ; Tian and Russell, 1997 ; Faure et al., 2002 ). In some species, however, fertilization is delayed for approximately 1 month.

A delay in fertilization was first recorded over a century ago in several species of Fagales (Alnus glutinosa, Betula alba, Carpinus betulus, Corylus avellana, and Fagus sylvatica; Benson, 1894 ). Since then, delayed fertilization has been reported in different forms, such as the reduction of pollen-tube growth rate in the style, retarded development of ovules at the time of pollination, or a lapse in time between pollination and fertilization from over 4 days to over 1 year, as exemplified in additional Fagales species (Conrad, 1900 ; Berridge, 1914 ; Woodroof, 1928 ; Langdon, 1934 , 1939 ; Nast, 1935 ; McKay, 1947 ; Swamy, 1948 ; Yen, 1950 ; Hjelmqvist, 1953 ; Håkansson, 1955 ; Barlow, 1958 ; Stairs, 1964 ; Thompson, 1979 ; Dahl and Fredrikson, 1996 ; Cecich, 1997 ; Boavida et al., 1999 ; Williams et al., 1999 ; Borgardt and Nixon, 2003 ; Sogo et al., 2004a , b ; Sogo and Tobe, 2005 , 2006a , c ). Species with delayed fertilization have also been reported in a number of unrelated plant groups, including species of Asparagales (Wirth and Withner, 1959 ; Arditti, 1992 ), Brassicales (Foster, 1943 ), Buxales (Endress and Igersheim, 1999 ), Ericales (Palser et al., 1989 ), Garryales (Palm and Rutgers, 1917 ; Hallock, 1930 ; Tang, 1962 ; Sogo and Tobe, 2006b ), Gunnerales (Endress and Igersheim, 1999 ), Laurales (Sedgley, 1979 ), Malpighiales (Igersheim and Endress, 1998 ), Piperales (Igersheim and Endress, 1998 ), Proteales (Bretzler, 1924 ; Endress and Igersheim, 1999 ), Ranunculales (Lonay, 1907 ; Endress, 1969 ; Hu et al., 1990 ; Endress and Igersheim, 1999 ), Rosales (Polito and Pimienta, 1982 ; Pimienta and Polito, 1983 ; Pimienta et al., 1983 ; Herrero and Arbeloa, 1989 ), Sapindales (Ton and Krezdorn, 1967 ; Yamashita, 1978 ; Wunnachit et al., 1992 ), and Saxifragales (Shoemaker, 1905 ; Schmitt, 1965 ; Endress, 1977 ; Endress and Igersheim, 1999 ) (Table 1). Delayed fertilization thus occurs more widely in angiosperms than previously thought, although the biological function of fertilization delay in these plant groups, and where and how pollen tubes remain within a pistil during the delay remain unknown in fertilization research.

Here, we document the growth of pollen tubes in the pistils of Fagus japonica Maxim. (Fagaceae), which has a similar but distinct mode from plants in the Casuarinaceae, Betulaceae, and Myricaceae. Unlike the Casuarinaceae, Betulaceae, and Myricaceae in which we have reported the occurrence of chalazogamy or pseudoporogamy (Sogo and Tobe, 2006a ), members of the Fagaceae (eight genera, ca. 700 species) exhibit porogamy (Benson, 1894 , F. sylvatica; Hjelmqvist, 1953 , Quercus robur). Earlier studies have shown that the ovules are still rudimentary or immature at the time of pollination in Castanopsis chrysophylla (Berridge, 1914 ), F. sylvatica (Benson, 1894 ), and several Quercus species (Langdon, 1939 ; Boavida et al., 1999 ; Borgardt and Nixon, 2003 ). In Quercus species, pollen-tube growth ceased at the base of the style and resumes 5–6 weeks or 1 year later (Cecich, 1997 ). In F. japonica, however, the cessation sites of pollen-tube growth in the pistil differed completely from those in Quercus and even included a different cessation site from those in other families of Fagaes. We need to explain this mode of pollen-tube growth not only to increase our knowledge of Fagaceae and Fagales, but also to understand biological significance of both delayed fertilization and intermittent pollen-tube growth across different plant groups.

MATERIALS AND METHODS

Plant materials
Female inflorescences of Fagus japonica were collected from two trees weekly from the middle of April to the end of May from 2001 to 2003 at the Ashiu Forest Research Station of Kyoto University, Kyoto Prefecture, Japan. They were fixed in FAA (5 parts stock formalin; 5 parts glacial acetic acid; 90 parts 50% ethanol). The trees were about 7–8 m tall and bore flowers on branches at a height that prevented us from conveniently conducting artificial crosses.

Observation of pollen-tube growth
We collected pistils and ovules by dissecting female inflorescences in a solution of 50% ethanol. The pistils and ovaries were decolorized overnight in a diluted (1.0%) solution of sodium hypochlorite (NaClO) at room temperature. After rinsing two to three times in water, they were macerated in 1 N sodium hydroxide (NaOH) at 60°C for 2 h, then stained with 0.5% aniline blue in 0.1 N K₃PO₄ for 3 to 12 h. The pistils or ovules were mounted in a few drops of staining medium on two glass coverslips on a glass slide and were then covered with another glass cover slip (Herr, 1971 ). Pollen-tube growth in pistils or ovules was observed with a fluorescence microscope (Zeiss LSM410, Germany) using a UV filter set (model no. 01) with excitation filter (365 nm, band pass 12 nm), dichroic mirror (FT395), and barrier filter (LP397).

Pollen grains on stigmas and pollen-tube growth in ovarian locules were observed using scanning electron microscopy (SEM). The pistils were dissected to expose an ovule, dehydrated through an ethanol series, and then critical-point dried. Finally, the pistils were coated with gold and observed with a scanning electron microscope (JSM-5800LV; Jeol Datum, Tokyo, Japan).

Observations of ovule and embryo-sac development
We examined ovules and embryo sacs in microtome sections of pistils collected from the inflorescences. The pistils were dehydrated through a t-butyl alcohol series and embedded in Paraplast (Tyco Healthcare Group LP, Mansfield) with a melting point of 57–58°C for subsequent microtome sectioning. Serial sections (6 or 7 µm thick) were stained with Heidenhains hematoxylin, safranin, and fast green FCF, mounted in Entellan (Merck, Darmstadt, Germany) and observed with a bright field microscope (Olympus BX-51, Tokyo, Japan).

Observations of pollen-tube growth and embryo-sac development in ovules
To compare the relative positions of pollen tubes at different developmental stages of the ovule or its embryo sac, the ovules were macerated and cleared in hydrogen peroxide solution at 60°C for 2 days and in Hoyer's solution (1.5 g gum arabic, 20 g chloral hydrate, and 1.0 mL glycerol in 6.0 mL of water) after rinsing two to three times in water, and their nucellus was observed using differential interference microscopy (Olympus BX-51). The ovules were later stained with aniline blue and observed with a fluorescence microscope as described.

We also made serial sections of ovules for this purpose. The ovules were dissected from pistils, and serial sections (7 µm thick) were cut using a rotary microtome. The sections were stained with 0.5% toluidine blue in 0.1% sodium carbonate (Na₂CO₃) solution, and embryo-sac development was observed using the light microscope. The same sections were stained with 0.5% aniline blue in 0.1 N potassium phosphate (K₃PO₄), and pollen-tube growth was observed using the fluorescence microscope.

RESULTS

Pollination of Fagus japonica began in mid-April at Ashiu Forest Research Station. This species has separate male and female inflorescences on the same tree. To avoid self-pollination, the anthers (male flowers) mature before the pistils (female flowers), so that pollen grains from one tree are dispersed before the pistils of the same tree become receptive. At the time of pollination, each female flower has a pistil, composed of three carpels terminated by three separate elongate stigmas (Fig. 1A). The ovary is about 1 mm wide and is divided by septa into three locules, each of which has two ovules (Fig. 1B). The ovule is pendulous, with a short, thick funicle (Fig. 1C). The nucellus faces slightly upward and is surrounded by the recently initiated inner integument. Sporogenous cells have not yet differentiated in the nucellus. In F. japonica, fertilization was delayed for about 5 weeks after pollination. Table 2 lists the number of ovules that had the pollen-tube tip at particular positions around or in the ovule after pollination, showing where and when the pollen tubes were present in growing pistils.

View larger version (184K): [in this window] [in a new window]   Fig. 1. Fagus japonica. Pollen-tube growth and ovary development at the start (A–G), 2 wk after (H), and 3 wk after (I–L) pollination. Microtome sections (B, C, L); scanning electron micrographs (D, J, K); fluorescence micrographs (E–H). (A) Pistil; scale bar = 1 mm. (B) Transverse section of an ovary, cut at the dashed line in (A); bar = 100 µm. (C) Longitudinal section (LS) of the portion of the ovary enclosed by the bottom rectangle (***) in (A); bar = 100 µm. (D) Stigma with pollen grains on the adaxial surface; bar = 100 µm. (E) Pollen tubes growing in the portion of the stigma enclosed by the top rectangle (*) in (A); bar = 100 µm. (F) Pollen tubes (arrows) growing in the portion of the style enclosed by the middle rectangle (**) in (A); bar = 200 µm. (G) Pollen tubes (arrows) entering the locules of the ovary after pollination, meandering along the surface of the septa or on the thick funicle; bar = 200 µm. (H) Pollen tubes in the locules of the ovary 2 wk after pollination, meandering on the thick funicle; bar = 100 µm. (I) Pistil; bar = 1 mm. (J) Developing ovules, with many pollen tubes among them; bar = 100 µm. (K) Magnified view of the portions of the ovules enclosed by the rectangle in (J), showing branched pollen tubes (arrows) or pollen tubes in a zigzag pattern (arrowhead) on the ovule surface; bar = 50 µm. (L) LS of a developing ovule with a megaspore mother cell; bar = 50 µm. Figure abbreviations: f, funicle; h, hair; ii, inner integument; mmc, megaspore mother cell; nu, nucellus; oi, outer integument; ova, ovary; ovu, ovule; se, septum; sg, stigma; sy, style

The stigma appeared to remain receptive for more than 1 week after the initiation of pollination, but thereafter started to wither from the tip. Pollen grains that arrived later in the receptive period also germinated, resulting in a large number of pollen tubes (at least 10 per locule) in ovaries (Fig. 1H: 2 weeks after pollination).

For about 3 weeks after the initiation of pollination, the ovary enlarged to about 2.5 mm in width (Fig. 1I). Each ovule developed two integuments (outer and inner) around the nucellus (Fig. 1J, K), although the micropyle had not yet formed. The nucellus contained a megaspore mother cell (Fig. 1L). The apical part of the outer integument was strongly pressed to the inner wall of the locule (Fig. 1J, K), so that the locule was divided into the upper and lower spaces (Fig. 1L). In the lower space, many narrow hairs, similar in thickness to the pollen tubes, grew from the bottom of the locule (Fig. 1J, K). The six ovules in the pistil appeared to be at nearly the same stage of development. In six ovaries with such ovules, we observed that pollen tubes meandered on the funicle with their tips extending on the surface of the ovules (Fig. 1J, K; Table 2). However, they never grew down beyond the ovule into the lower space of the locule. The pollen tubes meandering in the locule were approximately 500–800 µm long.

About 4 weeks after the beginning of pollination, the ovary was approximately 3 mm wide (Fig. 2A). During the fourth week, each anatropous ovule developed two well-formed integuments (Fig. 2B). We examined 23 such ovules obtained from 15 pistils and found that, even within the same ovary, they spanned a range of developmental stages. Of the 23 ovules, eight had a tetrad of megaspores formed by meiosis of the megaspore mother cell (Fig. 2C), five had a 1-nucleate embryo sac developed from the deepest megaspore, and 10 had a 2-nucleate embryo sac. In the eight ovules with a tetrad of megaspores, the inner integument was closed at the apex to form a micropylar canal, the endostome, whereas the outer integument still had a relatively wide opening (Fig. 2B, arrowhead).

View larger version (155K): [in this window] [in a new window]   Fig. 2. Fagus japonica. Pollen-tube growth and ovary and ovule development 4 wk after the start of pollination. Microtome sections (B, C) and fluorescence micrographs (D, E). (A) Pistil; scale bar = 1 mm. (B) Longitudinal section of an older ovule. The inner integument is closed to form the endostome; the outer integument remains open (arrowhead); bar = 100 µm. (C) Magnified view of the portion of the ovule enclosed by the rectangle in (B) showing a tetrad of megaspores (arrows); bar = 20 µm. (D, E) Pollen tubes (arrows) growing and branching in the space between the inner and outer integuments; bars = 100 µm (D), 50 µm (E). (F) Diagram illustrating pollen-tube growth in an older pistil. Figure abbreviations: as in Fig. 1 and ens, endostome; es, embryo sac

About 5 weeks after the start of pollination, the ovary had maintained its size and was still approximately 3.0 mm wide. We examined 32 ovules obtained from 17 pistils, and the ovules spanned a range of developmental stages. Of the 32 ovules, two were at a 4-nucleate embryo-sac stage, four at an 8-nucleate embryo-sac stage (more exactly, at telophase of nuclear division when eight sets of chromosomes are countable), and 26 at a mature embryo-sac stage or a stage soon after fertilization. In ovules with a mature embryo sac, the outer integument was closed at its apex to form a micropylar canal, the exostome (Fig. 3A). We observed that in all ovules with a 4- or 8-nucleate embryo sac, and even in four ovules with a mature embryo sac, the pollen tubes still remained near the micropyle (Table 2). However, in 22 ovules with a mature embryo sac (including ovules just after fertilization), a single pollen tube reached the embryo sac through the endostome (Fig. 3B) and nucellus (Fig. 3C, D; Table 2). This indicates that the pollen tube entered the endostome to cause fertilization soon after the ovule developed a mature embryo sac. Of the six ovules in each pistil, only the one that apparently developed earlier than the other five was fertilized; the remaining five ovules were not fertilized and later degenerated. The ovary that contained the sole fertilized ovule developed into a one-seeded nut.

View larger version (64K): [in this window] [in a new window]   Fig. 3. Fagus japonica. Pollen-tube growth and ovule development 5 wk after the start of pollination or at the time of fertilization. Microtome sections (A and C) and fluorescence micrographs (B). (A) Longitudinal section of ovule with a mature embryo sac; bar = 100 µm. (B) The portion of the ovule enclosed by the top rectangle (*) in A. Note that a single pollen tube (arrows) grows toward the nucellus; bar = 50 µm. (C) The portion of the ovule enclosed by the bottom rectangle in (A). A single pollen tube (arrows) reaches the mature embryo sac; scale bar = 20 µm. (D) Diagram illustrating pollen-tube growth in the mature ovule. Figure abbreviations: as in Figs. 1, 2, exs, exostome

Mode of pollen-tube growth in Fagus japonica
In our samples, the ovary was not yet mature at the time of pollination, requiring an additional 5 weeks before it was fully developed. As we have documented, pollen tubes lost their directional growth and stayed on the funicle in the locule for 4 weeks, and subsequently, remained near the micropyle (or at the space between the inner and outer integuments) for another week. One may question whether the pollen tubes kept growing while they were on the funicle in the locule for 4 weeks and near the micropyle for another week. Considering that in most angiosperms, fertilization occurs in 24 to 48 h, or even less, after pollination (e.g., Maheshwari, 1950 ; Gao et al., 1992 ; Higashiyama et al., 1997 ; Tian and Russell, 1997 ; Faure et al., 2002 ), pollen tubes usually grow from a stigma to an embryo sac in this short time. Tian and Russell (1997) noted that pollen tubes grew through a 4-cm style of tobacco for 2 days (i.e., at 0.8 mm/h); Higashiyama et al. (1997) reported that pollen tubes grew at 2.3 mm/h in a style of Torenia fournieri.

Therefore, if pollen tubes had continued to grow in the pistils of Fagus japonica at a similar rate to those of tobacco or Torenia for the 4- or 1-week period, they should have become much longer than we observed in the locule or near the micropyle. In the ovary 3 weeks after pollination (Fig. 1J, K), pollen tubes meandered on the funicle in the upper space of the locule as if they had continued to grow there during the 3 weeks. However, their length (approximately 500–800 µm) suggests that the pollen tubes grew for only 1–2 h after reaching the locule. Thus, although we did not measure the length of pollen tubes in pistils of F. japonica with the passage of time, it is very likely that their growth stopped in the locule for at least 2–3 weeks. Likewise, with regard to the pollen tubes that grew near the micropyle, their length (400 µm at the longest, see Fig. 2E) indicates that they grew there for less than 1 h and had remained there without growing for most of the week.

Thus, the mode of pollen-tube growth in F. japonica is such that pollen tubes grow intermittently in three steps associated with the development of the ovules and embryo sacs (Fig. 4). Many pollen tubes (at least 10 per locule) stopped growing on the funicles of the ovules in the ovarian locule (the first step) when the ovules were between the primordial and megaspore tetrad stages. After resuming growth, some pollen tubes (range = 1–4; mean = 1.89) stopped growing once more near the micropyle (or at the space between the inner and outer integuments of the ovule; the second step). This developmental cessation occurred at a time when the ovules were still at immature embryo-sac stages. Thereafter, when an ovule reached maturity, a single pollen tube resumed growth and passed through the micropyle, more strictly the endostome, to reach the embryo sac (the third step).

View larger version (32K): [in this window] [in a new window]   Fig. 4. Fagus japonica. Diagram illustrating the three steps of pollen-tube growth during the 5 wk between pollination and fertilization. Growth in the first, second, and third steps is denoted by open, gray, and solid lines, respectively

Why does pollen-tube growth cease at these sites? The funicle and chalaza in C. equisetifolia and A. sieboldiana are massive and are supplied by thick vascular tissue. The cells of the nucellar apex in M. rubra are rich in starch grains (Sogo et al., 2004b; Sogo and Tobe, 2005 , 2006c ). Accordingly, the growth-cessation sites are thought to have a nutritive role for pollen tubes that are awaiting the maturation of the embryo sac (Sogo and Tobe, 2006a , c ). In F. japonica as well, the funicle was thick and massive. In regard to the site near the micropyle, Endress (1977 , p. 341) suggested, on the basis of its appearance, that an elongate outer integument in the Fagaceae (and Gunneraceae and Hamamelidaceae) "may partly function as an obturator." Although we did not find any particular structure that was a likely nutrient source, the site near the micropyle (or the space between the two integuments) may play a nutritive role for pollen tubes in F. japonica.

Although several pollen tubes (mean = 1.89) reached the region near the micropyle, it was always a single pollen tube that grew beyond the micropyle (or endostome) to reach the embryo sac. Pollen-tube attraction by the synergid cell (Higashiyama et al., 2001 , Torenia fournieri) and specific interaction between the synergid cell and the pollen tube (Mogensen, 1978 , Proboscidea louisiana) are known. In addition, recent studies of Arabidopsis mutants suggest that the synergid cell impedes the entry of extra pollen tubes after the first has arrived or prevents discharge (Huck et al., 2003 ; Rotman et al., 2003 ). Thus, in F. japonica the site near the micropyle provides the pollen tubes with an environment in which to await the maturation of the embryo sac and in collaboration with the synergid cell(s), contributes to selecting a single pollen tube. According to Tian et al. (2005) , receptive gametes in tobacco need to reach G2 receptivity, i.e., cell cycle congruity. Although cell cycle characteristics of F. japonica have not yet been examined, it is certainly possible that egg cell receptivity is an important last criterion for success and that phase synchrony needs to be met in at least one of the six retarded ovules while the pollen tubes stay near the micropyle.

Knowing that pollen tubes grow intermittently in the pistil will contribute to our understanding of the mechanisms of pollen-tube guidance in angiosperms (Sogo and Tobe, 2005 ). The Arabidopsis POP2 gene is suggested to control the concentration of GABA (-amino butyric acid) and regulate pollen-tube growth toward the micropyle (Palanivelu et al., 2003 ; corresponding to the second step in Fagus japonica). We are interested to know whether a homolog of the Arabidopsis POP2 gene or GABA functions near the micropyle in F. japonica as well.

Evolutionary implications of delayed fertilization and intermittent growth of pollen tubes in pistils
We found that, like other members of the Fagales, fertilization was delayed and pollen-tube growth in the pistils was intermittent in F. japonica. A delay in fertilization occurs in the Fagales because of the retarded development of ovules and/or embryo sacs. This retarded development may provide all pollen tubes with an equal start to the ovules, regardless of their arriving at different times at the stigmatic surface (Dahl and Fredrikson, 1996 ). In F. japonica the stigma appeared to remain receptive for more than 1 week after the initiation of pollination, so that the ovary continued to receive more pollen tubes to the ovules. However, this period does not explain the entire length (i.e., 5 weeks) of the delay in fertilization after pollination. Delayed fertilization must have additional purposes.

It is uncertain whether all plant groups with delayed fertilization also have intermittent growth of pollen tubes in pistils. We recently reported that pollen tubes grew intermittently in a style in three steps in Eucommia ulmoides (Eucommiaceae, Garryales; Sogo and Tobe, 2006b ). The mode of pollen-tube growth in Eucommia, in which pollen tubes stopped growing within the placenta and at an enlarged integumental tip, was different from those in Fagales, but suggests that intermittent pollen-tube growth is likely to be widespread in angiosperms. At least in the Fagales and Garryales examined, which were all wind-pollinated, delayed fertilization concomitant with intermittent pollen-tube growth plays a role in gametophyte competition and selection leading to nonrandom fertilization by reducing the number of pollen tubes from many to one (all genera studied: Casuarina, Alnus, Myrica, Fagus, and Eucommia) and the number of ovules to be fertilized from two (Casuarina and Alnus) or six (Fagus) to one (Sogo et al., 2004b ; Sogo and Tobe, 2005 , 2006b , c ). In addition, one may consider a competitive advantage that may be gained in being able to compete for pollination, even when the female gametes themselves have not been formed, because this mechanism prevents a further allocation of reproductive effort in cases of pollination failure. In fact, it is known that in Corylus avellana (Betulaceae) ovule primordia are not formed in the ovary when a flower is not pollinated, although it is uncertain that this is also the case in F. japonica (for review, see Germain, 1994 ).

How often and where pollen tube growth ceases in the pistil differs among species and families. Furthermore, among the plant groups with delayed fertilization (Table 1), a wide range of times is observed in which fertilization is delayed after pollination, from a few days (e.g., Juglans [Langdon, 1934 ; Nast, 1935 ], Carya [McKay, 1947 ], Persea [Sedgley, 1979 ], Anacardium [Wunnachit et al., 1992 ]) up to over 1 year (e.g., Quercus [Conrad, 1900 ; Berridge, 1914 ; Langdon, 1939 ; Stairs, 1964 ; Cecich, 1997 ]). Short-term delays are usually accomplished by the cessation of pollen-tube growth at the base of the style (or on the obturator; Hormaza and Herrero, 1994 ). In contrast, as in the Fagales, long-term delays appear to be accomplished by repeated cessation and resumption of pollen-tube growth at more sites within the pistil, particularly within the ovule, i.e., at the chalaza (Sogo et al., 2004a , b ; Sogo and Tobe, 2005 ), at the nucellar surface (Sogo and Tobe, 2006c ), or near the micropyle. Most early researchers were not aware of the occurrence of sites of pollen-tube growth cessation in the ovule itself, probably because of technical difficulties in the clearing of ovules. The exact sites of pollen-tube growth cessation should be determined to confirm the widespread occurrence of intermittent pollen-tube growth and elucidate its evolutionary implications in individual plants or throughout plant groups with delayed fertilization.

Editor's note (3 Jan 07): This online article differs from the print version; see press erratum for print journal in January vol. 94(1).

FOOTNOTES

¹ The authors thank H. Setoguchi, H. Azuma, S. Nishida, S. Fujii, A. Naiki, and J. S. Kim for assistance in collecting materials, and D. E. Boufford for help on an earlier draft of this manuscript. This study was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (no. 18370036) and a grant for the Biodiversity Research of the 21st Century COE (A14).

² Author for correspondence (e-mail: tobe{at}sys.bot.kyoto-u.ac.jp )

LITERATURE CITED

Arditti J.. 1992. Fundamentals of orchid biology John Wiley, New York, New York, USA.

Barlow B. A.. 1958. Heteroploid twins and apomixis in Casuarina nana. Australian Journal of Botany 6: 204-219.[CrossRef]

Benson M.. 1894. Contribution to the embryology of the Amentiferae. Part I. Transactions of the Linnean Society of London, 2nd series, Botany 3: 409-424.

Berridge E. M.. 1914. The structure of the flower of Fagaceae, and its bearing on the affinities of the group. Annals of Botany 28: 509-526.

Boavida L. C. Varela M. C. Feijo J. A.. 1999. Sexual reproduction in the cork oak (Quercus suber L.). I. The progamic phase. Sexual Plant Reproduction 11: 347-353.[CrossRef][ISI]

Borgardt S. J. Nixon K. C.. 2003. A comparative flower and fruit anatomical study of Quercus acutissima, a biennial-fruiting oak from the Cercis group (Fagaceae). American Journal of Botany 90: 1567-1584.[Abstract/Free Full Text]

Bretzler E.. 1924. Beiträge zur Kenntnis der Gattung Platanus. Botanisches Archiv 7: 388-417.

Cecich R. A.. 1997. Pollen tube growth in Quercus. Forest Science 43: 140-146.[ISI]

Conrad A. H.. 1900. A contribution to the life history of Quercus. Botanical Gazette 29: 408-418.[CrossRef]

Dahl Å. E. Fredrikson M.. 1996. The timetable for development of maternal tissues sets the stage for male genomic selection in Betula pendula (Betulaceae). American Journal of Botany 83: 895-902.[CrossRef][ISI]

Endress P. K.. 1969. Gesichtspunkte zur systematischen Stellung der Eupteleaceen (Magnoliales). Berichte der Schweizerischen Botanischen Gesellschaft 79: 229-278.

Endress P. K.. 1977. Evolutionary trends in the Hamamelidales–Fagales group. Plant Systematics and Evolution (Supplement) 1: 321-347.

Endress P. K. Igersheim A.. 1999. Gynoecium diversity and systematics of the basal eudicots. Botanical Journal of the Linnean Society 130: 305-393.[CrossRef]

Erbar C.. 2003. Pollen tube transmitting tissue: place of competition of male gametophytes. International Journal of Plant Sciences 164: S265-S277.[CrossRef]

Faure J.-E. Rotman N. Fortuné P. Dumas C.. 2002. Fertilization in Arabidopsis thaliana wild type: developmental stages and time course. Plant Journal 30: 481-488.[CrossRef][ISI][Medline]

Foster L. M.. 1943. Morphological and cytological studies on Carica papaya. Botanical Gazette 105: 116-126.[CrossRef]

Gao X. Francis D. Ormrod J. C. Bennett M. D.. 1992. An electron microscopic study of double fertilization in allohexaploid wheat Triticum aestivum L. Annals of Botany 70: 561-568.[Abstract/Free Full Text]

Germain E.. 1994. The reproduction of hazelnut (Corylus avellana L.): a review. Acta Horticulturae 351: 195-209.

Håkansson A.. 1955. Endosperm formation in Myrica gale L. Botaniska Notiser 108: 6-16.

Hallock F. A.. 1930. The relationship of Garrya. The development of the flowers and seeds of Garrya and its bearing on the phylogenetic position of the genus. Annals of Botany 44: 771-812.

Herr J. M.. 1971. A new clearing-squash technique for the study of ovule development in angiosperms. American Journal of Botany 58: 785-790.[CrossRef][ISI]

Herrero M. Arbeloa A.. 1989. Influence of the pistil on pollen tube kinetics in peach (Prunus persica). American Journal of Botany 76: 1441-1447.[CrossRef][ISI]

Higashiyama T. Kuroiwa H. Kawano S. Kuroiwa T.. 1997. Kinetics of double fertilization in Torenia fournieri based on direct observations of the naked embryo sac. Planta 203: 101-110.[CrossRef][ISI]

Higashiyama T. Yabe S. Sasaki N. Nishimura Y. Miyagishima S. Kuroiwa H. Kuroiwa T.. 2001. Pollen tube attraction by the senergid cell. Science 293: 1480-1483.[Abstract/Free Full Text]

Hjelmqvist H.. 1953. The embryo sac development of Quercus robur L. Phytomorphology 3: 377-384.[ISI]

Hormaza J. I. Herrero M.. 1994. Gametophytic competition and selection. In E. G. Williams, A. E. Clarke, R. B. Knox, editors Generic control of self-incompatibility and reproductive development in flowering plants 372-400 Kluwer, Norwell, USA.

Hu Z.-H. Yang J. Jing R.-Q. Dong Z.-M.. 1990. Morphological studies on Circaeaster agrestis. II. Morphology and anatomy of flower, fruit and seed. Cathaya 2: 77-88.

Huck N. Moore J. M. Federer M. Grossniklaus U.. 2003. The Arabidopsis mutant feronia disrupts the female gametophytic control of pollen tube reception. Development 130: 2149-2159.[Abstract/Free Full Text]

Igersheim A. Endress P. K.. 1998. Gynoecium diversity and systematics of the paleoherbs. Botanical Journal of the Linnean Society 127: 289-370.[CrossRef]

Langdon L. M.. 1934. Embryogeny of Carya and Juglans, a comparative study. Botanical Gazette 96: 93-117.[CrossRef]

Langdon L. M.. 1939. Ontogenetic and anatomical studies of the flower and fruit of the Fagaceae and Juglandaceae. Botanical Gazette 101: 301-327.[CrossRef][ISI]

Lonay H.. 1907. Structure anatomique du péricarpe et du spermoderme chez les Renonculacées. Recherches complémentaires. Mémoires de la Société Royale des Sciences, Liège, séries 3 7: 1-34.

Maheshwari P.. 1950. An introduction to the embryology of angiosperms McGraw-Hill, New York, New York, USA.

McKay J. W.. 1947. Embryology of pecan. Journal of Agricultural Research 74: 263-283.

Mogensen H. L.. 1978. Pollen tube–synergid interaction in Proboscidea louisianica (Martineaceae). American Journal of Botany 65: 953-964.[CrossRef][ISI]

Nast C. G.. 1935. Morphological development of the fruit of Juglans regia. Hilgardia 9: 345-381.

Palanivelu R. Brass L. Edlund A. F. Preuss D.. 2003. Pollen tube growth and guidance is regulated by POP2, an Arabidopsis gene that controls GABA levels. Cell 114: 47-59.[CrossRef][ISI][Medline]

Palm B. Rutgers A. A. L.. 1917. The embryology of Aucuba japonica. Recueil des Travaux Botaniques Néerlandais 14: 119-126.

Palser B. F. Rouse J. L. Williams E. G.. 1989. Coordinated timetables for megagametophyte development and pollen tube growth in Rhododendron nuttallii from anthesis to early postfertilization. American Journal of Botany 76: 1167-1202.[CrossRef][ISI]

Pimienta E. Polito V. S.. 1983. Embryo sac development in almond [Prunus dulcis (Mill.) D. A. Webb] as affected by cross-, self-, and non-pollination. Annals of Botany 51: 469-479.[Abstract/Free Full Text]

Pimienta E. Polito V. S. Kester D. E.. 1983. Pollen tube growth in cross- and self-pollinated ‘nonpareil’ almond. Journal of American Society of Horticultural Science 108: 643-647.

Polito V. S. Pimienta E.. 1982. A rapid and accurate fluorescence method using ovule whole mounts to assess fertilization in plants. Mikroskopie 39: 32-34.[ISI][Medline]

Rotman N. Rozier F. Boavida L. Dumas C. Berger F. Faure J.-E.. 2003. Female control of male gamete delivery during fertilization in Arabidopsis thaliana. Current Biology 13: 432-436.[CrossRef][ISI][Medline]

Schmitt D.. 1965. The pistillate inflorescence of sweetgum (Liquidambar styraciflua L.). Silvae Genetica 15: 33-35.

Sedgley M.. 1979. Light microscopy study of pollen tube growth, fertilization, and early embryo and endosperm development in the avocado varieties Fuerte and Hass. Annals of Botany 44: 353-359.[Abstract/Free Full Text]

Shoemaker D. N.. 1905. On the development of Hamamelis virginiana. Botanical Gazette 39: 248-266.[CrossRef]

Sogo A. Jaffré T. Tobe H.. 2004a. Pollen-tube growth and fertilization mode in Gymnostoma (Casuarinaceae): their characteristics and evolution. Journal of Plant Research 117: 249-251.[ISI][Medline]

Sogo A. Noguchi J. Jaffré T. Tobe H.. 2004b. Pollen-tube growth pattern and chalazogamy in Casuarina equisetifolia (Casuarinaceae). Journal of Plant Research 117: 37-46.[CrossRef][ISI][Medline]

Sogo A. Tobe H.. 2005. Intermittent pollen-tube growth in pistils of alders (Alnus). Proceedings of the National Academy of Sciences, USA 102: 8770-8775.[Abstract/Free Full Text]

Sogo A. Tobe H.. 2006a. The evolution of fertilization modes independent of the micropyle in Fagales and ‘pseudoporogamy.'. Plant Systematics and Evolution 259: 73-80.[CrossRef][ISI]

Sogo A. Tobe H.. 2006b. Mode of pollen-tube growth in pistils of Eucommia ulmoides (Eucommiaceae, Garryales). International Journal of Plant Sciences 167: 933-941.[CrossRef]

Sogo A. Tobe H.. 2006c. Mode of pollen-tube growth in pistils of Myrica rubra (Myricaceae): a comparison with related families. Annals of Botany 97: 71-77.[Abstract/Free Full Text]

Stairs G. R.. 1964. Microsporogenesis and embryogenesis in Quercus. Botanical Gazette 125: 115-121.[CrossRef]

Stevens P. F.. 2006. Angiosperm phylogeny website, version 7 Website http://www.mobot.org/MOBOT/research/APweb/ [accessed October 9, 2006].

Swamy B. G. L.. 1948. A contribution to the life history of Casuarina. Proceedings of the American Academy of Arts and Sciences 77: 1-32.

Tang S. H.. 1962. Sporogenesis and gametophyte development in Eucommia ulmoides Oliv. Acta Botanica Sinica 10: 29-34 (in Chinese with English summary).[Medline]

Thompson M. M.. 1979. Growth and development of the pistillate flower and nut in ‘Barcelona’ filbert. Journal of the American Society for Horticultural Science 104: 427-432.[ISI]

Tian H.-Q. Russell S. D.. 1997. Calcium distribution in fertilized and unfertilized ovules and embryo sacs of Nicotiana tabacum L. Planta 202: 93-105.[CrossRef][ISI]

Tian H.-Q. Yuan T. Russell S. D.. 2005. Relationship between double fertilization and the cell cycle in male and female gametes of tobacco. Sexual Plant Reproduction 17: 243-252.[CrossRef][ISI]

Ton L. D. Krezdorn A. H.. 1967. Growth of pollen tubes in three incompatible varieties of Citrus. Proceedings of American Society of Horticultural Science 89: 211-215.

Williams J. H. Friedman W. E. Arnold M. L.. 1999. Developmental selection within the angiosperm style: using gamete DNA to visualize interspecific pollen competition. Proceedings of the National Academy of Sciences, USA 96: 9201-9206.[Abstract/Free Full Text]

Wirth M. Withner C. L.. 1959. Embryology and development in the Orchidaceae. In C. L. Withner, editor The orchids, a scientific survey 155-188 Ronald Press, New York, USA.

Woodroof N. C.. 1928. Development of the embryo sac and young embryo of Hicoria pecan. American Journal of Botany 15: 416-421.[CrossRef][ISI]

Wunnachit W. Pattison S. J. Giles L. Millington A. J. Sedgley M.. 1992. Pollen tube growth and genotype compatibility in cashew in relation to yield. Journal of Horticultural Science 67: 67-75.[ISI]

Yamashita K.. 1978. Studies on self-incompatibility of Hyuganatsu, Citrus tamurana Hort. ex Tanaka. Journal of Japanese Society of Horticultural Science 47: 188-194.

Yen T. K.. 1950. Structure and development of the flower and the fruit of Myrica rubra Sieb. & Zucc. Peking Natural History Bulletin 19: 1-20.

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