Pin Stigmas Research Articles

Developmental variation and the evolution of distyly in Hedyotis caerulea (Rubiaceae).

The development of distyly is thought to arise from differential growth patterns in the pin and thrum morphs. However, few detailed studies exist on the early floral development of distylous flowers, and fewer still look at variation in these traits among populations. Buds at multiple stages of development were collected from five populations of Hedyotis caerulea to quantify how pins and thrums diverge with respect to the initiation, rate, and termination of growth between the stamens and stigmas. The growth rate of anthers varied little spatially across five populations and temporally in both pins and thrums, although thrum anthers grew faster than pin anthers. Dimorphy in stigma height was more complex. Pin stigmas first grew at a faster rate than those of thrums, and late in bud development, growth of thrum styles slowed. These rate changes varied among populations, and they differed from the congeneric H. salzmanii. Similar differences between morphs are known in other heterostylous species, and such variation in growth pattern among related species has been used to infer independent evolution of distylous systems.

Distyly in Polygonum jucundum Meisn. (Polygonaceae)

Distyly has been clearly confirmed in Polygonum jucundum Meisn. The distylous species is dimorphic for tepals, styles, stigmas, and pollen size and number. Long (L)-morph flowers are slightly larger than short (S)-morph flowers. The stigma width of thrum flowers is larger than that of pin flowers, and pin stigmas have longer and fewer papillae than the thrum ones. Flowers of the L-morph produced significantly smaller pollen grains in greater numbers than flowers of the S-morph. We found that Polygonum jucundum is not an obligate cross-pollination plant, and a reliable pollinator of the species is often available in the field. Our study revealed that flower flies are the most effective pollinators of Polygonum jucundum, whereas ants and thrips probably behave as “nectar thieves”; during the foraging process, however, thrips may effect pollination. In addition, other insects visited Polygonum jucundum flowers. The study supports the model proposed by Barrett that heterostyly is a strategy evolved to reduce sexual interference and inhibit self fertilization.

Three‐dimensional reciprocity of floral morphs in wild flax (Linum suffruticosum): a new twist on heterostyly

Here, we studied the floral morphology and pollination of the distylous plant Linum suffruticosum (Linaceae) in southern Spain. We observed a previously unreported form of distyly that involved twisting and bending of styles and stamens during floral development to achieve three-dimensional reciprocity of anthers and stigmas in the long-styled (pin) and short-styled (thrum) morphs. This developmental pattern causes pin pollen to be placed on the underside of pollinating Usia flies (Bombyliidae), and thrum pollen to be placed on the top of the thorax and abdomen. The pin stigmas contact the flies on the dorsum, apparently picking up predominantly thrum pollen, and the thrum stigmas contact the flies on the ventral surface, apparently picking up predominantly pin pollen. This form of heterostyly would appear on morphological grounds to be far more efficient in dispersing pollen between compatible morphs than the typical pin-thrum system. If so, this plant fits Darwin's prediction of efficient pollen flow between heterostylous morphs more closely than anything Darwin himself reported. Molecular phylogenetic analyses indicate that this form of heterostyly evolved in a lineage that already had typical heterostyly. The analyses also indicate that there have been several independent origins of heterostyly in Linum and at least one reversal to stylar monomorphism.

Stigma morphology in distylous and non-heterostylous species ofVillarsia (Menyanthaceae)

Stigma morphology was examined with the SEM in 14 of the 16 species ofVillarsia. In nine of the ten distylous species studied, stigmas of the floral morphs were strongly dimorphic in length, shape, configuration of the receptive surface, and in the size and density of their papillae. Thrum stigmas ofVillarsia, in contrast to those of most other distylous species, are not simply smaller versions of the conspecific pin stigmas, but generally exhibit an array of morph-specific characters. Thrum stigma lobes may be broader than those of pins, they may have undulate margins, lobes subdivided into secondary lobes, papillae more extensively distributed than in pins, and various combinations of these traits occur. The traits that distinguish thrum from pin stigmas achieve an increase in the receptive area and may enhance more efficient pollen capture by the shorter and less accessible thrum stigmas. The morphogenesis of the stigma shape dimorphism appears to involve processes more complex than inhibition of elongation in thrum styles. InVillarsia, the stigma dimorphisms are species-specific. No correlations were found between morphologies of the stigma and the different breeding systems in distylous species. Stigmas of the four non-heterostylous species examined resemble the thrum stigma type found in most distylousVillarsia species.

Pollen Flow, Fecundity, and the Adaptive Significance of Heterostyly in Palicourea padifolia (Rubiaceae)1

ABSTRACT Palicourea padifolia is a distlous shrub visited primarily by hummingbirds and bees in mid‐elevation rain forests in Costa Rica. At Las Cruces Biological Station, the population of P. padifolia is composed of equal numbers of pin and thrum plants and morphs are randomly distributed. Like the majority of distylous species, P. padifolia exhibits a self and intramorph‐incompatible mating system. The analysis of stigmatic pollen loads obtained from emasculated flowers shows that pollen flow between floral morphs is disassortative, which supports the Darwinian hypothesis concerning the adaptive significance of heterostyly. Pins experience greater disassortative pollination than thrums. Pins also set more seed than thrums, suggesting that the higher frequency of compatible pollen deposited on pin stigmas results in greater fecundity; alternatively, greater seed set in pins could be due to the differential allocation of resources to male and female function between morphs. Overall, seed set for the Las Cruces popularion of P. padifolia is much lower than the potential maximum. Factors which could contribute to seed set limitation include the frequency and/or spatial pattern of pollinator visits, pollen availability, and resource availability.

Sequential variation in the components of reproductive success in the distylousJasminum fruticans (Oleaceae).

A 2-year study of three natural populations of the distylousJasminum fruticans showed that mean fruit and seed production were significantly greater in shortstyled plants (thrums) than in long-styled plants (pins). In this study, we investigated the role of four sequential factors which may differentially influence fruit and seed set in the two floral morphs: (1) differences in flowering phenology, (2) a limitation of pollen transfer towards pins, (3) a differential capacity of the two morphs to act through famale and male function and (4) differential fruit abortion in the two morphs. Fruit set was significantly influenced by differences in flowering phenology although there were no differences in flowering time between the two morphs. supplementary pollinations in a natural population significantly increased fruit set and reduced the difference in fruit set between the two morphs in relation to controls, indicating a limitation on pollen transfer which was most severe towards pin stigmas. In reciprocal crosses, seed set was significantly dependent on the paternal and maternal identity of the pin parent. There was no significant variation among thrums for their performance as male or female parent. Furthermore, individual pin plants with relatively high percentage seed set as female parents gave poor seed set as male parents and vice-versa. Whereas fruit removal had no effect on seed number in thrum plants, a greater proportion of viable seeds were produced on pin plants which were left to naturally mature their fruits than on pins which had fruits artificially removed, suggesting the occurrence of selective fruit abortion in pins but not in thrums. The initially greater maternal fitness of thrums due to their greater success as pollen recipients may thus be opposed by increased viable seed set in the pins due to factors acting after the pollination stage. The relative reproductive success of floral morphs in the distylousJ. fruticans is thus differentially influenced by ecological factors occurring at different stages of the reproductive process.

Four New Distylous Species of Waltheria (Sterculiaceae) and a Key to the Mexican and Central American Species and Species Groups

Four new distylous species of Waltheria endemic to Mexico are described and illus- trated: W. bicolor from Colima, W. fryxellii from Mexico, Guerrero, Michoacan, and Jalisco, W. lundelliana from Nayarit, and W. tridentata from Guerrero, Puebla, and Oaxaca. Stigmatic and pollen dimorphisms between pin and thrum flowers are described for the new species and placed within the context of those dimorphisms found in other Mexican and Central American Waltheria species. Three species groups, based partially on stigma types (pin-clavate, fan-plumose, and elon- gate-plumose), are newly described and all Mexican and Central American species delineated to them. All four new species belong to the newly designated crest-flowered waltherias, characterized by possession of crest-like inflorescences, clavate pin stigmas, ovoid chartaceous capsules with partially loculicidal dehiscence, and bracteal trichomes that are exclusively stellate. A key to the 15 Mexican and Central American species is provided and their geographical distributions are illus- trated. Waltheria is a primarily neotropical genus of approximately 50 species. Although one species is herbaceous, most are shrubs or subshrubs that flower for several months, producing a few open flowers at each node that remain open up to one day. In Mexico, species usually occur in semiarid areas in the Caribbean Floristic Region (Rzedowski 1978). The two centers of diversity for the genus are Mexico, with 15 species (Fig. 1), and Brazil, with approximately 25 species. The genus consists of many species with local- ized or regional distribution, with approxi- mately 20 species endemic to Brazil, and nine species endemic to Mexico (Table 1): W. aca- pulcensis, W. acuminata, W. bicolor, W. conzattii, W. fryxellii, W. lundelliana, W. preslii, W. pringlei, and W. tridentata. Waltheria detonsa A. Gray is an en- demic to the biogeographic province connect- ing northern Mexico and southeastern Arizona. Except for the pantropical W. indica, the distri- butions of the remaining species found in Mex- ico extend into Central or South America (Table

The control of incompatibility in distylous Pulmonaria affinis Jordan (Boraginaceae)

In P. affinis, pin pollen is shorter on average than thrum pollen. Pins have more coronal hooks on stigmatic papillae than thrums, but gaps between papillae are relatively smaller for thrums. Pin stigmas receive more pollen than thrum stigmas. Thrum stigmas receive more (dissortive) pin pollen, but pin stigmas are assortively pollinated. Pollen only germinates when trapped below papilla coronas. On thrum stigmas, most trapped pollen is pin. Pollen germination is better on thrum stigmas than pin stigmas, and thrum stigmas show a close relationship between numbers of legitimate pollen grains, numbers of germinating grains, and numbers of pollen tubes in the style. There is no inhibition of illegitimate pollen germination. Illegitimate pollen tubes are inhibited in the style. Incompatibility operates by a combination of dissortive pollination, dissortive pollen trapping, and stylar pollen tube inhibition. All heteromorphic features differing between pins and thrums are implicated in the inhibition of within-morph fertilization in thrums.

Fine Structure and Cytochemistry of the Stigma Surface and Incompatibility in some Distylous Linum Species

The receptive surface of the stigma in distylous Linum grandiflorum and L. pubescens was studied by electron microscopy and cytochemical techniques. In both floral morphs a proteinaceous-lipoidal coating is present on the papilla surface. In thrum stigmas the cuticle is highly irregular and pitted at the papilla tip. The cuticle is dislodged and torn at anthesis and an osmiophilic secretion product is released within a pectinaceous matrix. The secretion product stains for proteins and lipids and contributes to adhesion of pollen. In the larger pin papillae the cuticle is wavy, continuous, thicker than in thrum papillae and adjoins the cell wall. In both species the surface of the two types of pollen grains is coated with lipids and protein. A similar behaviour of the male gametophyte is observed in incompatible pollinations of L. pubescens, L. mucronatum and L. grandiflorum. In intramorph thrum pollinations pollen tubes are arrested within the stigma. In intramorph pin pollinations the majority of pollen grains fail to adhere to the stigma. Low permeability to water in pin papillae, as determined by the neutral red test, may be a factor preventing imbibition of the few adhering grains. Tubes of the few germinated grains are inhibited inside the stigma. On the part of the stigma, the difference in the major site of inhibition in the two intramorph incompatible combinations may be accounted for by the dissimilar properties of the papillae, i.e. the occurrence of wet thrum stigmas and dry pin stigmas. Functionally, the unusual association of sporophytic incompatibility with wet thrum stigmas is attributed to retention of the secretory material on individual papillae.

Differential localization of enzymes in the stigmatic exudates ofPrimula obconica

The stigmatic dimorphisms in the distylous speciesPrimula obconica, which has been dealt with in an earlier paper, has been described further. An ultrastructural localization of various enzymes in the exudates is hereby revealed, and evidence is given for intermorph differences. Esterases are confined to the lipid phase of the exudates, including the lipid globuli in the papillae walls of both morphs, but they are not found in the pellicle of the dry thrum stigma. However, this pellicle exhibits acid phosphatase activity, as does the lumen of the blisters in the viscous exudate on pin stigmas. The blisters are presumed to hold the watery phase of the pin secretion. From the present findings and from previous results with LM cytochemical methods it is suggested that the sites of peroxidase activity resemble those of acid phophatases.

POLLEN FLOW IN DIMORPHIC TURNER A SUBULATA (TURNERA CEAE).

Turner a subulata J. E. Smith is heterostylous with pin and thrum flower forms and is self-incompatible but cross-compatible. Pin and thrum flowers produce the same number of pollen grains. Only 3·07% of the pollen produced participates in the pollination. Pin stigmas receive an average of 425 pollen grains and thrums 327. Stigmas of pin flowers receive more thrum than pin pollen grains. Thus in compatible pollinations, pollen flow from the thrum to the pin form was greater than in the reverse direction.

The dry and wet stigmas ofPrimula obconica: Ultrastructural and cytochemical dimorphisms

The stigmatic papillae of the distylous speciesPrimula obconica are studied by means of cytochemical, light and electron microscopic techniques. The papillae on thrum stigmas are smaller than those on pin stigmas. At the bud stage, secretory vesicles are not a conspicious part of the cytoplasm, although certain signs of secretory activity are present. The young papillae bear the thin, superficial pellicle typical to dry stigmas. Small vesicles are numerous in mature papillae of both morphs, and seem to originate from the ER. A layer of closely packed, osmiophilic globuli is present in the outermost part of mature walls of pin papillae. At sites with cuticle disruption, the globuli seem to migrate outwards to be incorporated with the copious, blistery exudate. Due to this exudate the pin stigma is characterized as wet. Cytochemical tests suggest that the exudate contains mainly lipids, and different carbohydrates and protein are detected. It reacts positively in tests for peroxidase, acid phosphatase, and non-specific esterases. The thrum stigma remains dry at maturity, with a distinct pellicle also reacting positively to the enzyme tests. Only a few, scattered osmiophilic globuli, sized and situated as those in the pin papillae walls, are found in the thrum walls, and they do not form a proper layer.

The distyly in Primula elatior (L.) Hill (Primulaceae), with a study of flowering phenology and pollen flow

Flowers of Primula elatior exhibit all the dimorphic traits typical of distyly. The difference in pollen production between pin and thrum flowers is not thought to be caused by the pollen size dimorphism. The percentage of pin pollen which is probably viable exceeds that of thrum pollen. In the population studied the morph ratio is 1:1, and plants are randomly dispersed with regard to morph type. Bumblebees are the main pollinators, and analysis of the stigmatic pollen load in naturally |xjllinated flowers shows that pin stigmas receive many more pollen grains than thrum stigmas. Intramorph pollination is considerable in pins, while thrums are subject to some differential pollination. Emasculation experiments have shown that intraflower pollination in the thrum morph is negligible. Aging of pin flowers has a reducing effect on their total pollen loads, while in thrums there is no change in total pollen load after the first days of anthesis.

Pollen flow inPrimula veris (Primulaceae)

Observations on two natural and one artificial distylous populations ofPrimula veris in Britain revealed very high proportions of intramorph pollen on pin stigmas and moderate proportions of intermorph pollen on thrum stigmas. The average ratio of pin: thrum pollen production per flower was 2.4:1. Large amounts of pollen remained in dehisced anthers, perhaps reflecting low levels of pollinator activity. Population size and density had no clear effect on pollen flow patterns, which are asymmetrical for the two morphs.

Pollen flow in a population of Primula vulgaris Huds.

A British population of Primula vulgaris was visited four times between 1971 and 1976 for study of the nature and functioning of distyly. The stainability and production of pollen from pins exceeded those of thrums. About 20% of pin pollen and 48% of thrum pollen was removed from anthers by insect visitors. Pins and thrums were present in a 1:1 ratio in 1971 but pins outnumbered thrums in 1976. Thrums produced more seeds per flower than pins though the number of ovules produced by each was similar. Two methods for collecting stigmas from open flowers and analyzing them for pollen loans produced somewhat different results. For pin stigmas, the pollen load consisted of 2–23% thrum pollen; for thrum stigmas, the pollen load consisted of 0–71% pin pollen with most stigmas having less than 50% pin pollen. In general, intermorph pollen flow is less than would be expected if pollen flow were random. It is probable that most intramorph pollen on stigmas is a result of self- or geitonogamous pollination. The extensive literature concerning the natural pollinators of the primrose is reviewed. Although Darwin's hypothesis concerning the functional significance of distyly in promoting intermorph pollination was never quantified, the pollen flow patterns observed in P. vulgaris are unexpectedly deviant and are similar to those patterns observed in several unrelated heterostylous species in other plant families.

Pollen flow in distylous populations of Amsinckia (Boraginaceae)

Stigmatic pollen loads were analyzed from naturally pollinated pin and thrum form flowers of Amsinckia douglasiana and A. vernicosa var. furcata. Pin stigmas captured more total pollen than thrum stigmas. Pins experienced either net self-pollination or random pollination. Thrum stigmas experienced significant disassortative pollination. Comparing pollen loads from intact and emasculated thrum flowers of A. douglasiana indicated that self-pollination and geitonogamy were relatively unimportant in the pollination of the thrum form. The level of disassortative pollination of A. vernicosa var. furcata does not appear to be high enough to account for the level of disassortative mating observed by progeny testing, suggesting that this species may possess an incomplete stylar self-incompatibility system such as has been reported in A. grandiflora.

The Reproductive System of Amsinckia grandiflora, a Distylous Species

The flowers of Amsinckia grandiflora are distylous and the pollen is dimorphic in size. The single knozwn population of this annual species contained a 1: 1 representation of the two morphs during two seasons, but a 2: 1 pin: thrum ratio in a third season. The chief insect visitor to the flowers is Anthophora edwardsii, a bee that preferentially removes thrum pollen. Pin stigmas receive more pollen grains than thrum stigmas and also a greater proportion of illegitimate pollen than thrums. Seasonal fluctuations in the representation of the two morphs have no striking influence on pollen-flow patterns. Two sympatric, aggressive homostylous weeds of the same genus produce more seeds per flower than does A. grandiflora. It is possible that the greater reproductive success of these zweedy species has allowed them to displace A. grandiflora over much of the latter's former range. Amsinckia grandiflora Kleeberger (Boraginaceae) is an annual herb now restricted to a single, small population in San Joaquin County, California. The flowers are distylous, and the two floral forms differ conspicuously in the position of the anthers and the length of the style (Fig. 1-4). The floral morphology and highly restricted distribution of this species con- trast sharply with those of a number of other members of the genus, such as A. tessellata Gray and A. intermedia Fischer c Amsinckia is of particular interest because of the occurrence within it of distyly and an array of derived homostylous systems. In view of the con- trasts between the floral heteromorplhism and restricted distribution of A. grandiflora, a primitive species of the genus (Ray & Chisaki, 1957b), and those of its more successful homostylous congeners, I have studied the reproductive system of this species. This paper presents the results of this study and suggests that the present restricted distribution of the species is perhaps associated with its archaic and relatively inefficient reproductive system.

The Breeding System of Lithospermum caroliniense: Adaptation and Counteradaptation

The breeding system of the perennial herb Lithospermum caroliniense was analyzed in a population located near Zion, Lake County, Illinois. The species is both heterostylic and functionally cleistogamic, its populations containing plants which are solely chasmogamic, solely cleistogamic, or partially cleistogamic. Heterostyly is accompanied by pollen dimorphism so that levels of legitimate pollen flow may be accurately measured by examining stigmas and noting the size of the legitimate pollen loads. Legitimate pollen flow was measured at Zion, and it was found that 63% of the pin stigmas bore legitimate grains as compared with 42% of the thrum stigmas. The average numbers of such grains on all pin and thrum stigmas were 4.88 and 2.75, respectively, the range being one to 45 in the former and one to 43 in the latter. The distribution of legitimate loads on both stigmas was markedly leptokurtic. Average pollen loads per plant were related to the proximity of legitimate pollen donor. Illegitimate loads ranged...