Mixed pollen load and late‐acting self‐incompatibility flexibility in Adenocalymma peregrinum (Miers) L.G. Lohmann (Bignonieae: Bignoniaceae)

• Mixed cross and self‐pollen load on the stigma (mixed pollination) of species with late‐acting self‐incompatibility system (LSI) can lead to self‐fertilized seed production. This “cryptic self‐fertility” may allow selfed seedling development in species otherwise largely self‐sterile. Our aims were to check if mixed pollinations would lead to fruit set in LSI Adenocalymma peregrinum, and test for evidence of early‐acting inbreeding depression in putative selfed seeds from mixed pollinations.
• Experimental pollinations were carried out in a natural population. Fruit and seed set from self‐, cross and mixed pollinations were analysed. Further germination tests were carried out for the seeds obtained from treatments.
• Our results confirm self‐incompatibility, and fruit set from cross‐pollinations was three‐fold that from mixed pollinations. This low fruit set in mixed pollinations is most likely due to a greater number of self‐ than cross‐fertilized ovules, which promotes LSI action and pistil abortion. Likewise, higher percentage of empty seeds in surviving fruits from mixed pollinations compared with cross‐pollinations is probably due to ovule discounting caused by self‐fertilization. Moreover, germinability of seeds with developed embryos was lower in fruits from mixed than from cross‐pollinations, and the non‐viable seeds from mixed pollinations showed one‐third of the mass of those from cross‐pollinations.
• The great number of empty seeds, lower germinability, lower mass of non‐viable seeds, and higher variation in seed mass distribution in mixed pollinations, strongly suggests early‐acing inbreeding depression in putative selfed seeds. In this sense, LSI and inbreeding depression acting together probably constrain self‐fertilized seedling establishment in A. peregrinum.

     

PROTANDRY,INCOMPATIBILITY, AND SECONDARY POLLEN PRESENTATION IN CEPHALANTHUS OCCIDENTALS (RUBIACEAE)

Cephalanthus occidentalis L. is protandrous and presents pollen secondarily on the stigma surface. Because self-pollen is present on the stigma, the degree of selling vs. outcrossing in this species will depend on 1) the phenology of pollen presentation and stigma receptivity; 2) whether the species is self-incompatible; and 3) the rates of self vs. crossed pollen tube growth. This study describes floral morphology and phenology, self-incompatibility, and pollen tube growth rates in self- and crosspollinations of C. occidentalis. Light and scanning electron microscopy were used to study stigma morphology after flower opening, while controlled pollinations tested for incompatibility. Stigmas were unreceptive initially but became receptive by the second day after flower opening. Ninety-two percent of cross-pollinated flowers set fruit, compared to 12% fruit set in self-pollinations. Pollen tubes from selfed and out-crossed pollen initially had similar growth rates. Out-crossed pollen tubes began to grow rapidly ca. 5 hr after pollination of a receptive stigma, whereas selfed pollen tubes ceased growth or grew slowly after this time. Pollen tubes from out-crossed pollen grew the length of the style within 24 hr after pollination, while selfed pollen tubes were inhibited at the stigma-style junction. Our results indicate that C. occidentalis has selfincompatibility, in addition to protandry and secondary pollen presentation. Protandry allows removal of self-pollen from the unreceptive stigma, while self-incompatibility prevents fertilization by unremoved self-pollen.     

RELATIVE SUCCESS OF SELF AND OUTCROSS POLLEN COMPARING MIXED- AND SINGLE-DONOR POLLINATIONS IN AQUILEGIA CAERULEA

Flowers frequently receive both self (S) and outcross (OC) pollen, but S pollen often sires proportionally fewer seeds. Failure of S pollen can reflect evolved mechanisms that promote outcrossing and/or inbreeding depression expressed during seed development. The relative importance of these two processes was investigated in Aquilegia caerulea, a self-compatible perennial herb. In the field I performed single-donor (S or OC) and mixed-donor (S plus OC) pollinations to compare the relative success of both pollen types at various stages from pollen germination to seed maturity. Single-donor S pollinations produced significantly fewer and lighter seeds (x decrease = 12% and 3%, respectively) than OC pollinations. Abortion rates differed by an average of 38% whereas fertilization rates differed by only 5%, indicating that most differences in seed number arose postzygotically. This suggests that inbreeding depression was responsible for most failure of S pollen. One prezygotic effect measured was that 10% fewer S than OC pollen tubes reached ovaries after 42 hr, suggesting S pollen might fertilize proportionately fewer ovules after mixed pollination. Using allozyme markers, I found mixed-donor pollinations produced significantly more and heavier outcrossed than selfed seeds. However, the proportion of selfed seed, fertilized ovules, and aborted seeds for mixed-donor fruits were each predictable from pollen performance in single-donor fruits, suggesting that differential paternity is best explained by inbreeding depression during seed development. Even given these similarities between mixed- and single-donor fruits in the relative performance of S and OC pollen, both individual seed weight and seed set were significantly higher in multiply-sired fruits.     

AMONG- AND WITHIN-FLOWER COMPARISONS OF POLLEN TUBE GROWTH FOLLOWING SELF- AND CROSS-POLLINATIONS IN DIANTHUS CHINENSIS (CARYOPHYLLACEAE)

We compared the rate of pollen tube growth following self- and cross-pollinations both among and within flowers of two clones of Dianthus chinensis L. For among-flower comparisons, both styles of a flower were pollinated with either self- or cross-pollen. Within-flower comparisons were made between the two styles of the same flower, one of which was self-pollinated and the other cross-pollinated. Comparisons between flowers indicated that self-pollen grew slower than cross-pollen in both clones. However, differences in the growth rate of pollen tubes from self- and cross-pollinations were greater when comparisons were made between the two styles of the same flower than when pollinations were made in different flowers. These results suggest the existence of interstyle interactions in pollen tube growth.     

Effects of pollen quality and quantity on pollen limitation in Crataegus monogyna (Rosaceae) in NW Spain

Pollen limitation occurs when plants produce less fruits and/or seeds than they would with adequate pollen receipt. If the addition of cross-pollen to stigmas increases fruit/seed production, it is interpreted as an evidence of pollen limitation. Much of the limitation may be associated with the quality rather than quantity of pollen; however, most studies do not discriminate between the two, which may lead to misinterpretation of the results. We studied the effects of quality and quantity of pollen on the reproduction of a northern Spanish population of Crataegus monogyna. The treatments included self- and cross-pollination, and supplementation to open and bagged flowers. The response variables considered were number of pollen grains per stigma, pollen tubes per style, and initial and final fruit set. In the Cantabrian range, C. monogyna requires insect pollinators to set fruit and is partially self-incompatible. We found that the number of pollen tubes did not differ between cross- and self-pollination treatments; however, self-pollinated flowers set less fruits than flowers that received pure cross-pollen or were supplemented with both cross- and self-pollen. The experimental design allowed us to infer qualitative rather than quantitative pollen limitation. Comparison of the number of pollen grains and tubes, and initial and final fruit set among pollination treatments suggested post-zygotic embryo selection against selfed progeny.     

Is Eucalyptus Cryptically Self-incompatible?

BACKGROUND AND AIMS: The probability that seeds will be fertilized from self- versus cross-pollen depends strongly on whether plants have self-incompatibility systems, and how these systems influence the fate of pollen tubes. METHODS: In this study of breeding systems in Eucalyptus urophylla and Eucalyptus grandis, epifluorescence microscopy was used to study pollen tube growth in styles following self- and cross-pollinations. KEY RESULTS: Pollen tubes from self-pollen took significantly longer than those from cross-pollen to grow to the base of the style in both E. urophylla (120 h vs. 96 h) and E. grandis (96 h vs. 72 h). In addition, both species exhibited reduced seed yields following self-pollination compared with cross-pollination. CONCLUSIONS: The present observations suggest that, in addition to a late-acting self-incompatibility barrier, cryptic self-incompatibility could be a mechanism responsible for the preferential out-crossing system in these two eucalypt species.     

COMPETITION-DEPENDENT ABSCISSION OF SELF-POLLINATED FLOWERS OF PHORMIUM TENAX (AGAVACEAE): A SECOND ACTION OF SELF-INCOMPATIBILITY AT THE WHOLE FLOWER LEVEL?

The relative success of fruit from paired self- and cross-pollinations was examined in Phormium tenax when the contrasted pollinations were separated by different distances on the same and different inflorescences. We determined whether the retention of selfed fruits differed from that of crossed fruits and whether it depended on the level of competition with crossed fruit, the number of seeds per fruit, and/or the presence of earlier developing fruit. We found that the success of selfed fruits is determined by the degree of competition with crossed fruits and may be an expression of self-incompatibility. Competition-dependence of the abscission of selfed flowers has not been documented previously. It is parallel to cryptic self-incompatibility in which individual self-pollen grains are not as successful as cross-pollen when competing on the same pistil. The competition-dependent abscission of self-pollinations considered here, however, operates at the level of whole flowers. The phenomenon of competition-dependent abscission of selfed flowers in P. tenax also has implications for the measurement and interpretation of self-incompatibility in other species. Self-incompatibility is a quantitative phenomenon. The facultative success of selfing shows that the effective strength of self-incompatibility can be highly susceptible to the conditions of competition under which it is measured. The competition-dependent abscission of selfed flowers allows a high level of outcrossing to be achieved while it assures seed set when pollinations are scarce. Several other causes of intermediate selfing frequencies can also be explained by this “best-of-both-worlds” hypothesis.     

Do s genes or deleterious recessives control late-acting self-incompatibility in Handroanthus heptaphyllus (Bignoniaceae)? A diallel study with four full-sib progeny arrays

Background and AimsGenetically controlled self-incompatibility (SI) mechanisms constrain selfing and thus have contributed to the evolutionary diversity of flowering plants. In homomorphic gametophytic SI (GSI) and homomorphic sporophytic SI (SSI), genetic control is usually by the single multi-allelic locus S. Both GSI and SSI prevent self pollen tubes reaching the ovary and so are pre-zygotic in action. In contrast, in taxa with late-acting self-incompatibility (LSI), rejection is often post-zygotic, since self pollen tubes grow to the ovary, where fertilization may occur prior to floral abscission. Alternatively, lack of self fruit set could be due to early-acting inbreeding depression (EID). The aim of our study was to investigate mechanisms underlying the lack of selfed fruit set in Handroanthus heptaphyllus in order to assess the likelihood of LSI versus EID.MethodsWe employed four full-sib diallels to study the genetic control of LSI in H. heptaphyllus using a precociously flowering variant. We also used fluorescence microscopy to study the incidence of ovule penetration by pollen tubes in pistils that abscised following pollination or initiated fruits.Key ResultsAll diallels showed reciprocally cross-incompatible full sibs (RCIs), reciprocally cross-compatible full sibs (RCCs) and non-reciprocally compatible full sibs (NRCs) in almost equal proportions. There was no significant difference between the incidences of ovule penetrations in abscised pistils following self- and cross-incompatible pollinations, but those in successful cross-pollinations were around 2-fold greater.ConclusionsA genetic model postulating a single S locus with four S alleles, one of which, in the maternal parent, is dominant to the other three, will produce RCI, RCC and NRC full sib situations each at 33 %, consistent with our diallel results. We favour this simple genetic control over an EID explanation since none of our pollinations, successful or unsuccessful, resulted in partial embryo development, as would be expected under a whole-genome EID effect.     

The Role of Late-Acting Self-Incompatibility and Early-Acting Inbreeding Depression in Governing Female Fertility in Monkshood,Aconitum kusnezoffii

Reduced seed yields following self-pollination have repeatedly been observed, but the underlying mechanisms remain elusive when self-pollen tubes can readily grow into ovaries, because pre-, post-zygotic late-acting self-incompatibility (LSI), or early-acting inbreeding depression (ID) can induce self-sterility. The main objective of this study was to differentiate these processes in Aconitum kusnezoffii, a plant lacking stigmatic or stylar inhibition of self-pollination. We performed a hand-pollination experiment in a natural population of A. kusnezoffii, compared seed set among five pollination treatments, and evaluated the distribution of seed size and seed set. Embryonic development suggested fertilization following self-pollination. A partial pre-zygotic LSI was suggested to account for the reduced seed set by two lines of evidence. The seed set of chase-pollination treatment significantly exceeded that of self-pollination treatment, and the proportion of unfertilized ovules was the highest following self-pollination. Meanwhile, early-acting ID, rather than post-zygotic LSI, was suggested by the findings that the size of aborted selfed seeds varied continuously and widely; and the selfed seed set both exhibited a continuous distribution and positively correlated with the crossed seed set. These results indicated that the embryos were aborted at different stages due to the expression of many deleterious alleles throughout the genome during seed maturation. No signature of post-zygotic LSI was found. Both partial pre-zygotic LSI and early-acting ID contribute to the reduction in selfed seed set in A. kusnezoffii, with pre-zygotic LSI rejecting part of the self-pollen and early-acting ID aborting part of the self-fertilized seeds.     

Pollination and late-acting self-incompatibility in Cyrtanthus breviflorus (Amaryllidaceae): implications for seed production

Background and Aims

Animal pollination is typically an uncertain process that interacts with self-incompatibility status to determine reproductive success. Seed set is often pollen-limited, but species with late-acting self-incompatibility (SI) may be particularly vulnerable, if self-pollen deposition results in ovule discounting. Pollination is examined and the occurrence of late-acting SI and ovule discounting assessed in Cyrtanthus breviflorus.

Methods

The pollination system was characterized by observing floral visitors and assessing nectar production and spectral reflectance of flowers. To assess late-acting SI and ovule discounting, growth of self- and cross-pollen tubes, and seed set following open pollination or hand pollination with varying proportions of self- and cross-pollen, were examined.

Key Results

Native honeybees Apis mellifera scutellata pollinated flowers as they actively collected pollen. Most flowers (≥70 %) did not contain nectar, while the rest produced minute volumes of dilute nectar. The flowers which are yellow to humans are visually conspicuous to bees with a strong contrast between UV-reflecting tepals and UV-absorbing anthers and pollen. Plants were self-incompatible, but self-rejection was late-acting and both self- and cross-pollen tubes penetrated ovules. Seed set of open-pollinated flowers was pollen-limited, despite pollen deposition exceeding ovule number by 6-fold. Open-pollinated seed set was similar to that of the cross + self-pollen treatment, but was less than that of the cross-pollen-only treatment.

Conclusions

Flowers of C. breviflorus are pollinated primarily by pollen-collecting bees and possess a late-acting SI system, previously unknown in this clade of the Amaryllidaceae. Pollinators of C. breviflorus deposit mixtures of cross- and self-pollen and, because SI is late-acting, self-pollen disables ovules, reducing female fertility. This study thus contributes to growing evidence that seed production in plants with late-acting SI systems is frequently limited by pollen quality, even when pollinators are abundant.     

Early Inbreeding Depression and Pollen Competition inCalluna vulgaris(L.) Hull.

We investigated whether partial self-sterility inCalluna vulgarisresultsfrom abortion of selfed offspring owing to inbreeding depressionor a late-acting self-incompatibility mechanism, and whetherself-pollen interferes with normal functioning of cross-pollen.Self-pollination resulted in 75% less seed set than cross-pollination.Self-pollen tubes reached ovaries and penetrated ovules as oftenas those of cross-pollen. Following self-pollination, examinationof the size of undeveloped seeds showed that at least 70% resultedfrom ovule fertilization and arrest of development occurredat various stages. All self-pollinated plants produced seedsand self-fertility varied among plants. These results indicatethat the reduced seed set observed in self-pollination is morelikely the result of inbreeding depression rather than a late-actingself-incompatibility system. The fecundity component of inbreedingdepression was high (0.762). Seed set was reduced by an averageof 40% when self-pollen was mixed with cross-pollen, comparedto pure cross-pollination. Using genetic markers, we found about20% of seeds resulted from self-pollination in mixed-pollinatedfruits.C. vulgarisis likely to experience self-pollination innature and our data suggest this will reduce the number of ovulesthat might otherwise mature after cross-pollination.Copyright1999 Annals of Botany Company Calluna vulgaris(heather), self-pollination, pollen tube, ovule fertilization, early inbreeding depression, pollen interference.     

Early-acting inbreeding depression in three species of Vaccinium (Ericaceae)

Greenhouse pollinations were performed to determine whether early-acting inbreeding depression is contributing to low levels of self-fertility in three Vaccinium species: V. myrtilloides Michaux, V. angustifolium Aiton, and V. corymbosum L. All three species showed a significant reduction in self fruit set and in the proportion of fertilized ovules that developed into mature seed in self compared to outcross fruit. Reductions were more severe in V. myrtilloides and V. angustifolium than in V. corymbosum; however, early-acting inbreeding depression appeared to be the primary factor limiting self-fertility in all three species. Evidence for early- acting inbreeding depression included the presence and higher proportion of aborted ovules in self fruit than in outcross fruit, a correlation between levels of self and outcross seed set, and pollen chase experiments demonstrating that self-pollen does fertilize the ovules. Self-fertility in the three species was probably influenced by levels of genetic load, ploidy level and outcrossing rates. Received: 10 December 1999 / Revision accepted: 20 June 2000     

The reproductive biology of Cybistax antisyphilitica (Bignoniaceae), a characteristic tree of the South American savannah-like “Cerrado” vegetation

Bignoniaceous woody species are very important ecological components of neotropical forests, but the reproductive biology of many species, such as Cybistax antisyphilitica, remains virtually unknown. Most species of Bignoniaceae are characteristically self-sterile, despite typically exhibiting normal pollen tube growth throughout the style, combined with slow rates of ovule penetration, fertilisation and endosperm initiation in selfed pistils. Uniform abortion occurs within a few days of anthesis, indicating the occurrence of late-acting self-incompatibility (LSI). However, breeding system studies have been performed in fewer than 7% of species, and other types of breeding systems (e.g., self-compatibility and apomixis) have been reported in this family. In the present study, the reproductive biology of C. antisyphilitica was investigated by field observation of flower visitors and floral events. Moreover, reproductive biology of this species was examined through experimental pollinations, analyses of pollen tube growth and ovule penetration using fluorescence microscopy, verification of pistil longevity, and a histological analysis of unpollinated vs. self-pollinated pistils. Finally, morphological aspects, quantities and germination were investigated in seeds that resulted from different pollination treatments. Natural pollination was effected by large- and medium-sized bees, and their visiting behaviour favours a high proportion of geitonogamy and no pollen limitation. Self-pollinated flowers produced no fruits, and all of the characteristic post-pollination events cited above were verified, witnessing the occurrence of LSI with post-zygotic rejection of selfed pistils in C. antisyphilitica. Although some indications of extended pistil longevity were found in selfed pistils, this feature seemed to be affected by unidentified environmental factors. The seeds were always monoembryonic and with high viability. A larger variation in the number of viable seeds was found in fruits derived from natural pollination. A low fruit set was observed after both natural and cross-pollination, and most crossed fruits underwent abortion at several points during the juvenile phase, even when protected against herbivory. The formation of surplus flowers/juvenile fruits and the apparently wasteful selfing mechanism control implied in LSI are discussed in the context of the perennial life style of tropical woody plant species.     

Sexual reproduction in the cork oak (Quercus suber L). II. Crossing intra- and interspecific barriers

Intraspecific barriers promote outcrossing while interspecific mechanisms may contribute to the isolation of species; both control the exchange of genes between plants. In this paper we establish that post-pollination mechanisms promote outcross and act at different temporal and spatial levels to control seed set and quality in Quercus species. Controlled pollinations were performed to investigate intraspecific crossing barriers in Q. suber and pollen-pistil interactions following interspecific pollinations with some simpatric Quercus species. Cytological data of intraspecific crosses in Q. suber and Q. ilex have shown different kinetics on pollen tube growth after self and outcross pollination, but pollen tubes were able to reach the base of the styles in both species and seed set was successful. Although some pre-zygotic interaction is occurring at the style, the most important interaction takes place at ovary. The cross between Q. ilex and Q. suber is possible only in one direction, revealing a case of unilateral incongruity. We show that the lack of seed set observed in the cross Q. suber×Q. ilex is due to the inability of pollen tubes to penetrate the transmitting tissue after germination. With Q. suber mainly as female parent, pollinations with other simpatric Quercus species have shown different levels of constraint on pollen tube progression at stigma and style levels. Eventual hybridisation between Quercus species will depend on the compatibility of pollen-pistil interactions, on the competitive ability of pollen genotypes, and, most important, on the overlapping of geographical, phenological and ecological factors. Differences in seed set and seed allocation was evident in all crosses, and was particularly outstanding in interspecific and in self intraspecific crosses, determining the ultimate level of seed production and quality in Quercus species. Received: 4 April 2001 / Accepted: 8 May 2001     

Reproductive biology of a rare cactus, Opuntia spinosissima (Cactaceae), in the Florida Keys: why is seed set very low?

 Opuntia spinosissima (Martyn) Mill. (Cactaceae) is an extremely rare taxon with a single remaining wild population of 13 plants located on Little Torch Key, Florida. The plants rarely set viable seeds and most ovaries abscise without initiation of fruits. Pollination and breeding system were examined in Little Torch Key and in plant accessions located at Fairchild Tropical Garden. Four pollination treatments were carried out to test for apomixis, self-pollination, outcrossing, and autogamy; a control treatment was also monitored. Additionally, pollen viability, pollinators, and seed set and viability were investigated. On selfed, crossed, and open (control) stigmas pollen grains germinated, but the tubes usually did not reach the base of the style. This suggests that O. spinosissima has pre-zygotic self-incompatibility. None of the pairwise crosses set seed, so the extant plants were apparently not inter-compatible. Out of 173 manipulated and control flowers, only one set fruit. Although this flower was outcrossed, no pollen tubes germinated on the stigma; this suggests agamospermy, a process common in the Cactaceae. In those flowers where pollen tubes did reach the ovary they failed to penetrate ovules, suggesting ovarian inhibition or that this taxon has lost the ability to be fertilized. Most field-collected seeds were viable, but there is no seedling recruitment under natural conditions, and vegetative reproduction is common. Based on these findings, I hypothesize that O. spinosissima is a sterile polyploid and that the 13 extant plants are asexually derived from a single lineage. Received: 17 April 1998 / Revision accepted: 3 July 1998     

Self-incompatibility inCrocus vernus subsp.vernus (Iridaceae)

Outcross, self- and mixed pollinations were performed inCrocus vernus subsp.vernus, a species with bicellular pollen, dry stigmas and hollow style. No differences were noted among the above pollinations concerning the germination of pollen and the growth of pollen tubes until the top of ovary. Within 45 min after pollinations 62% of pollen grains germinated. Pollen tubes penetrated the papilla cuticle extending along the papilla wall; on entry into stigmatic lobes they continued growth in the stylar secretion to ovarian locules. Here, however, self-pollen tubes failed to reach or to enter the ovule micropyle; while pollen tubes from either outcross- or mixed pollinations grew until fertilizing ovules. These observations gave evidence of a self-incompatibility system inCrocus, which appeared to be neutralized by mentor effect. The ovary as site of incompatibility response is discussed.     

Ovarian self‐incompatibility in Narcissus papyraceus (Amaryllidaceae) is the result of a pre‐zygotic response

Ovarian self‐incompatibility, including pre‐ and post‐zygotic reactions, is a complex mechanism for which we still lack many details relating to its function and significance. The joint presence of ovarian self‐incompatibility with style polymorphism is a rare combination that is found in the genus Narcissus. Usually, style polymorphic species have heteromorphic (diallelic and linked to style length locus) incompatibility, which prevents fertilization between individuals of the same morph, thereby helping to maintain equal proportions of floral morphs in populations. However, when present, self‐incompatibility in Narcissus is not linked to style polymorphism and cross‐fertilization within each morph is possible. Hence, self‐incompatibility in Narcissus is of particular interest when attempting to unravel the nature of the rejection reaction and aiming to assess possible cryptic differences in the fertilization process in intra‐ and inter‐morph crosses, which might ultimately explain the wide variation of morph‐ratio in the field. We examined the breeding system of Narcissus papyraceus, a style‐dimorphic species that has biased morph ratios in most of its populations. We studied pollen‐tube growth in the pistil and ovule fate after experimentally controlled hand pollinations. The growth of pollen tubes in self‐ and intra‐ and inter‐morph crosses was similar up to the point of micropyle penetration in both morphs but, subsequently, a pre‐zygotic failure appeared to affect male and female gametophytes in selfed pistils. A high proportion of ovules from self‐pollinated flowers showed signs of collapse and self‐pollen tubes were blocked or behaved abnormally before entering the embryo sac. Self‐incompatibility was stronger in the long‐styled morph than in the short‐styled morph. We did not find any conclusive sign of differential functioning between intra‐ and inter‐morph cross‐pollinations in any morph. These results enable us to rule out the possible effects of pollen–pistil interactions in N. papyraceus as a cause of morph‐ratio biases and confirm the exceptional nature of the self‐incompatibility mechanism in this polymorphic species. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 629–643.     

Selection on pollen donors by Echium vulgare (Boraginaceae)

Fewer than 25% of the ovules of Echium vulgare in the field develop into viable seeds, even with no pollen limitation. The loss of ovules could enhance the fitness of the maternal parent, if less fit embryos are selectively aborted. Two pollination experiments were performed to examine the selectivity of maternal parents on self-pollen and different cross-pollen sources. Pollinated with one pollen genotype per flower, self-pollen was, on average, as successful in siring seeds as cross-pollen. However, the relative success of self-pollen compared to outcross-pollen differed significantly among the maternal parents. These results suggest that, under certain conditions, selfing can produce a greater number of seeds than cross-pollination. Pollen donors differed significantly in outcrossing success. The plants that were more successful in selfing were also more successful pollen donors in outcross-pollinations. No significant interaction between maternal parent and paternal genotype was found. Pollinations with a pollen mixture produced selfed and outcrossed seeds in the same ratios as the single-donor experiment. Overall, only slight differences were found between the single- and mixed-donor experiments. Pollen tube growth did not show a significant correlation with the success of the parental genotypes in the mixed-donor experiment, indicating that pollen tube growth is not the determining factor controlling the paternity of the seeds. These results are discussed with reference to possible mediating mechanisms.     

BREEDING SYSTEMS IN ASCLEPIAS INCARNATA L., A. SYRIACA L., AND A. VERTICILLATA L.

A series of experimental pollinations involving three sympatric species provided strong evidence for physiological and/or genetic barriers to hybridization in Asclepias. Pollen tubes were observed to penetrate the style in some interspecific crosses of all species pairs. Aniline blue fluorescence microscopy also demonstrated pollen tube penetration of foreign ovules following pollinations between A. incarnata and A. verticillata. However, none of the 279 total pollinations attempted between species yielded mature seed, indicating the presence of late pre-fertilization or early post-fertilization incompatibility. Intraspecific pollinations in greenhouse and field populations revealed greater crossability between populations than within populations of a single species. Self-pollinations of A. verticillata were unsuccessful, while 29% and 4% of those of A. incarnata and A. syriaca, respectively, yielded mature follicles. It is suggested that the potential for autogamy, combined with floral mechanisms requiring pollination by insects, insure the advantages of both the genetic variability promoted by outcrossing and the reproductive assurance of uniparental reproduction. Strong reproductive barriers between species reduce the change of intergradation where species occur sympatrically.     

Self-pollen interference is absent in wild radish (Raphanus raphanistrum, Brassicaceae), a species with sporophytic self-incompatibility

Explaining the diversity of mating systems and floral forms in flowering plants is a long-standing concern of evolutionary biologists. One topic of interest is the conditions under which self-pollination can interfere with seed set for flowering plants with a self-incompatibility system. We investigated the effect of self-pollen interference for wild radish, Raphanus raphanistrum, which has sporophytic self-incompatibility. We performed pollinations and determined seed set for plants grown in the greenhouse, using pollen mixtures representing either self- with outcross-pollen or outcross-pollen alone. Stigmas were collected for a subset of pollinated flowers to determine the number of pollen grains applied. Average seed set for the self/cross (5.13 seeds/pollination) and cross treatments (5.09 seeds/pollination) did not differ significantly. Stigmatic pollen loads averaged around 700 grains, an amount close to observed natural pollen loads on R. raphanistrum. We concluded that for R. raphanistrum in natural populations, self-pollen is unlikely to interfere with outcross-pollen success. This study is the first to investigate effects of self-pollen interference on seed set for a homomorphic species with sporophytic self-incompatibility where rejection occurs at the stigmatic surface.