Characterizing the interaction between the bogus yucca moth and yuccas: do bogus yucca moths impact yucca reproductive success?

Yucca moths are most well known for their obligate pollination mutualism with yuccas, where pollinator moths provide yuccas with pollen and, in exchange, the moth larvae feed on a subset of the developing yucca seeds. The pollinators, however, comprise only two of the three genera of yucca moths. Members of the third genus, Prodoxus, are the bogus yucca moths and are sister to the pollinator moths. Adult Prodoxus lack the specialized mouthparts used for pollination and the larvae feed on plant tissues other than seeds. Prodoxus larvae feed within the same plants as pollinator larvae and have the potential to influence yucca reproductive success directly by drawing resources away from flowers and fruit, or indirectly by modifying the costs of the mutualism with pollinators. We examined the interaction between the scape-feeding bogus yucca moth, Prodoxus decipiens, and one of its yucca hosts, Yucca filamentosa, by comparing female reproductive success of plants with and without moth larvae. We determined reproductive success by measuring a set of common reproductive traits such as flowering characteristics, seed set, and seed germination. In addition, we also quantified the percent total nitrogen in the seeds to determine whether the presence of larvae could potentially reduce seed quality. Flowering characteristics, seed set, and seed germination were not significantly different between plants with and without bogus yucca moth larvae. In contrast, the percent total nitrogen content of seeds was significantly lower in plants with P. decipiens larvae, and nitrogen content was negatively correlated with the number of larvae feeding within the inflorescence scape. Surveys of percent total nitrogen at three time periods during the flowering and fruiting of Y. filamentosa also showed that larval feeding decreased the amount of nitrogen in fruit tissue. Taken together, the results suggest that although P. decipiens influences nitrogen distribution in Y. filamentosa, this physiological effect does not appear to impact the female components of reproductive success.     

Genetic consequences of specialization: yucca moth behavior and self-pollination in yuccas

Reciprocal specialization in interspecific interactions, such as plant-pollinator mutualisms, increases the probability that either party can have detrimental effects on the other without the interaction being dissolved. This should be particularly apparent in obligate mutualisms, such as those that exist between yucca and yucca moths. Female moths collect pollen from yucca flowers, oviposit into floral ovaries, and then pollinate those flowers. Yucca moths, which are the sole pollinators of yuccas, impose a cost in the form of seed consumption by the moth larvae. Here we ask whether there also is a genetic cost through selfish moth behavior that may lead to high levels of self fertilization in the yuccas. Historically, it has been assumed that females leave a plant immediately after collecting pollen, but few data are available. Observations of a member of the Tegeticula yuccasella complex on Yucca filamentosa revealed that females remained on the plant and oviposited in 66% of all instances after observed pollen collections, and 51% of all moths were observed to pollinate the same plant as well. Manual cross and self pollinations showed equal development and retention of fruits. Subsequent trials to assess inbreeding depression by measuring seed weight, germination date, growth rate, and plant mass at 5 months revealed significant negative effects on seed weight and germination frequency in selfed progeny arrays. Cumulative inbreeding depression was 0.475, i.e., fitness of selfed seeds was expected to be less than half that of outcrossed seeds. Single and multilocus estimates of outcrossing rates based on allozyme analyses of open-pollinated progeny arrays did not differ from 1.0. The discrepancy between high levels of behavioral self-pollination by the moths and nearly complete outcrossing in mature seeds can be explained through selective foreign pollen use by the females, or, more likely, pollen competition or selective abortion of self-pollinated flowers during early stages of fruit development. Thus, whenever the proportion of pollinated flowers exceeds the proportion that can be matured to ripe fruit based on resource availability, the potential detrimental genetic effects imposed through geitonogamous pollinations can be avoided in the plants. Because self-pollinated flowers have a lower probability of retention, selection should act on female moths to move among plants whenever moth density is high enough to trigger abortion. Received: 18 March 1996 \Accepted: 30 July 1996     

THE INFLUENCE OF THE MATING SYSTEM ON SEED SIZE VARIATION IN CRINUM ERUBESCENS (AMARYLLIDACEAE)

We present evidence that extreme seed size variation in fruits of Crinum erubescens (range: 0.1 to 66.5 g per seed) occurs when mating pairs are inbred, either from selfing or biparental inbreeding. Several relatively uniform seeds of intermediate size are produced when pollen from several pollen donors is applied simultaneously to a flower. Selfed fruits and some fruits pollinated with a single pollen donor produce both large and small seeds, although selfed fruits produce fewer seeds than outcrossed fruit. These results are contrary to the hypothesis that variation in seed size is attributable to either pollen competition or differential allocation of maternal resource to seeds of different genotypes.     

The role of abiotic and biotic factors in determining coexistence of multiple pollinators in the yucca–yucca moth mutualism

The determinants of a species' geographic distribution are a combination of both abiotic and biotic factors. Environmental niche modeling of climatic factors has been instrumental in documenting the role of abiotic factors in a species' niche. Integrating this approach with data from species interactions provides a means to assess the relative roles of abiotic and biotic components. Here, we examine whether the high host specificity typically exhibited in the active pollination mutualism between yuccas and yucca moths is the result of differences in climatic niche requirements that limit yucca moth distributions or the result of competition among mutualistic moths that would co‐occur on the same yucca species. We compared the species distribution models of two Tegeticula pollinator moths that use the geographically widespread plant Yucca filamentosa. Tegeticula yuccasella occurs throughout eastern North America whereas T. cassandra is restricted to the southeastern portion of the range, primarily occurring in Florida. Species distribution models demonstrate that T. cassandra is restricted climatically to the southeastern United States and T. yuccasella is predicted to be able to live across all of eastern North America. Data on moth abundances in Florida demonstrate that both moth species are present on Y. filamentosa; however, T. cassandra is numerically dominant. Taken together, the results suggest that moth geographic distributions are heavily influenced by climate, but competition among pollinating congeners will act to restrict populations of moth species that co‐occur.     

The evolution of obligate pollination mutualisms: senita cactus and senita moth

We report a new obligate pollination mutualism involving the senita cactus, Lophocereus schottii (Cactaceae, Pachyceereae), and the senita moth, Upiga virescens (Pyralidae, Glaphyriinae) in the Sonoran Desert and discuss the evolution of specialized pollination mutualisms. L. schottii is a night-blooming, self-incompatible columnar cactus. Beginning at sunset, its flowers are visited by U. virescens females, which collect pollen on specialized abdominal scales, actively deposit pollen on flower stigmas, and oviposit a single egg on a flower petal. Larvae spend 6 days eating ovules before exiting the fruit and pupating in a cactus branch. Hand-pollination and pollinator exclusion experiments at our study site near Bahia Kino, Sonora, Mexico, revealed that fruit set in L. schottii is likely to be resource limited. About 50% of hand-outcrossed and open-pollinated senita flowers abort by day 6 after flower opening. Results of exclusion experiments indicated that senita moths accounted for 75% of open-pollinated fruit set in 1995 with two species of halictid bees accounting for the remaining fruit set. In 1996, flowers usually closed before sunrise, and senita moths accounted for at least 90% of open-pollinated fruit set. The net outcome of the senita/senita moth interaction is mutualistic, with senita larvae destroying about 30% of the seeds resulting from pollination by senita moths. Comparison of the senita system with the yucca/yucca moth mutualism reveals many similarities, including reduced nectar production, active pollination, and limited seed destruction. The independent evolution of many of the same features in the two systems suggests that a common pathway exists for the evolution of these highly specialized pollination mutualisms. Nocturnal flower opening, self-incompatible breeding systems, and resource-limited fruit production appear to be important during this evolution. Received: 19 August 1997 / Accepted: 24 November 1997     

Florivore impacts on plant reproductive success and pollinator mortality in an obligate pollination mutualism

Florivores are present in many pollination systems and can have direct and indirect effects on both plants and pollinators. Although the impact of florivores are commonly examined in facultative pollination mutualisms, their effects on obligate mutualism remain relatively unstudied. Here, we used experimental manipulations and surveys of naturally occurring plants to assess the effect of florivory on the obligate pollination mutualism between yuccas and yucca moths. Yucca filamentosa (Agavaceae) is pollinated by the moth Tegeticula cassandra (Lepidoptera: Prodoxidae), and the mutualism also attracts two florivores: a generalist, the leaf-footed bug Leptoglossus phyllopus (Hemiptera: Coreidae), and a specialist, the beetle Hymenorus densus (Coleoptera: Tenebrionidae). Experimental manipulations of leaf-footed bug densities on side branches of Y. filamentosa inflorescences demonstrated that feeding causes floral abscission but does not reduce pollen or seed production in the remaining flowers. Similar to the leaf-footed bugs, experimental manipulations of beetle densities within individual flowers demonstrated that beetle feeding also causes floral abscission, but, in addition, the beetles also cause a significant reduction in pollen availability. Path analyses of phenotypic selection based on surveys of naturally occurring plants revealed temporal variation in the plant traits important to plant fitness and the effects of the florivores on fitness. Leaf-footed bugs negatively impacted fitness when fewer plants were flowering and leaf-footed bug density was high, whereas beetles had a positive effect on fitness when there were many plants flowering and their densities were low. This positive effect was likely due to adult beetles consuming yucca moth eggs while having a negligible effect on floral abscission. Together, the actions of both florivores either augmented the relationship of plant traits and fitness or slightly weakened the relationship. Overall, the results suggest that, although florivores are always present during flowering, the impact of florivores on phenotypic selection in yuccas is strongly mitigated by changes in their densities on plants from year to year. In contrast, both florivores consistently influenced pollinator larval mortality through floral abscission, and H. densus beetles additionally via the consumption of pollinator eggs.     

Reproductive consequences of interactions between Yucca glauca (Agavaceae) and Tegeticula yuccasella (Lepidoptera) in Colorado

For over 100 years the association between Yucca (Agavaceae) and Tegeticula (Lepidoptera: Incurvariidae) has been accepted as a quintessential example of an obligate mutualism. The yucca moth is purported to be the sole pollinator of Yucca, while Yucca flowers provide courtship and mating arenas, and Yucca seeds provide food for developing Tegeticula larvae. We studied Yucca glauca in northern Colorado, comparing the reproductive ecology and breeding systems of Yucca in plains populations, the “preferred” habitat of Yucca, with “marginal” sites at the edge of the local elevational distribution. Tegeticula are abundant at plains sites, and fruit set is significantly higher than in the foothills, where fruit set is limited by the paucity of moths. The low frequency of moths at high elevation, coupled with behaviors of adult female Tegeticula which lead to self-pollination, failure to pollinate, and periodic overloading of fruits with eggs, may help explain why Yucca glauca appears to maintain alternative pollinators. Some fruits lack evidence of Tegeticula infestation, suggesting that larvae die before completing development, or that fertilization of Yucca sometimes occurs without the intervention of Tegeticula. Biochemical analyses of nectar and observations of floral visitors revealed that it is highly likely that the fly Pseudocalliope sp. nov. (Lauxaniidae), which congregates and mates on Yucca glauca blossoms, acts as a secondary pollinator. Autogamy appears to occur infrequently in natural populations. We therefore propose that the yucca-yucca moth symbiosis be viewed as a facultative mutualism.     

Pollen dispersal in Yucca filamentosa (Agavaceae): the paradox of self-pollination behavior by Tegeticula yuccasella (Prodoxidae)

We investigated pollen dispersal in an obligate pollination mutualism between Yucca filamentosa and Tegeticula yuccasella. Yucca moths are the only documented pollinator of yuccas, and moth larvae feed solely on developing yucca seeds. The quality of pollination by a female moth affects larval survival because flowers receiving small amounts of pollen or self-pollen have a high abscission probability, and larvae die in abscised flowers. We tested the prediction that yucca moths primarily perform outcross pollinations by using fluorescent dye to track pollen dispersal in five populations of Y. filamentosa. Dye transfers within plants were common in all populations (mean ± 1 SE, 55 ± 3.0%), indicating that moths frequently deposit self-pollen. Distance of dye transfers ranged from 0 to 50 m, and the mean number of flowering plants between the pollen donor and recipient was 5 (median = 0), suggesting that most pollen was transferred among near neighbors. A multilocus genetic estimate of outcrossing based on seedlings matured from open-pollinated fruits at one site was 94 ± 6% (mean ± 1 SD). We discuss why moths frequently deposit self-pollen to the detriment of their offspring and compare the yucca-yucca moth interaction with other obligate pollinator mutualisms in which neither pollinator nor plant benefit from self-pollination.     

Seed germination patterns of Salvia mellifera in fire-prone environments

Summary I examined the effects of pollen loads containing pollen from one, three and five donors on fruit production and fruit quality in Campsis radicans. Number of pollen donors had no significant effect on % fruit production, seed number, seed weight or seed germination. In singledonor pollinations the identity of the donor did have a strong effect on the above parameters. Furthermore, the best single donor sired fruits with more seeds and heavier seeds than any mixture containing this donor. This pattern indicates interference of pollens or preemption of some ovules by the inferior pollen. In Campsis, therefore, the number of pollen donors contributing to a pollen load is less important than the identity of these donors in determining fruit production and fruit quality. Seeds from fruits resulting from mixed pollination were slightly more variable than seeds from fruits resulting from single-donor pollinations.     

NONRANDOM MATING IN WILD RADISH: VARIATION IN POLLEN DONOR SUCCESS AND EFFECTS OF MULTIPLE PATERNITY AMONG ONE- TO SIX-DONOR POLLINATIONS

Nonrandom mating in plants may involve differential success of self- vs. outcross pollen, of pollen from relatives vs. nonrelatives, and of pollen from several compatible donors. Of these, the last is least understood. Wild radish plants were crossed with six pollen donors in all possible combinations to investigate differences in pollen donor performance and to compare multiply sired fruits with different numbers and identities of fathers. Pollen donors differed in number of seeds sired, position of seeds sired within fruits, and mass of seeds sired. Performance in single pollination was not a good predictor of performance in mixed pollination. Performance in two-donor pollinations was a better predictor of the outcome of multidonor pollinations. Multiply sired fruits were selectively filled, but not all multiply sired fruits were equal. Both the number and identity of donors affected fruit filling. These results and others with wild radish suggest that variation in pollen donor ability to sire seeds is relatively common in wild radish. Since wild radish plants in the field typically receive pollen from several donors, there may be considerable opportunity for selection at this stage of the life cycle in wild radish.     

Facultative non-mutualistic behaviour by an “Obligate” mutualist: “Cheating” by Yucca moths

The interaction between yucca moths (Tegeticula spp., Incurvariidae) and yuccas (Yucca spp., Agavaceae) is an obligate pollination/seed predation mutualism in which adult female yucca moths pollinate yuccas, and yucca moth larvae feed on yucca seeds. In this paper we document that individual yucca moths, which are capable of acting as mutualists, facultatively cheat by ovipositing in yucca pistils without attempting to transfer pollen. Additionally, a high proportion of flowers are unlikely to receive pollen even when pollination is attempted, because many yucca moths carry little or no pollen. The probability of occurrence of non-mutualistic behaviour is not affected by the amount of pollen a moth carries: moths with full pollen loads are just as likely to act non-mutualistically as moths carrying little or no pollen. We propose four hypotheses that could explain facultative non-mutualistic behaviour in yucca moths.Present address: Department of Biology, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada     

Pollination ecology of Yucca elata

The pollination biology of a population of 250 Yucca elata (Liliaceae) plants was studied in southern New Mexico. Yucca elata and the prodoxid yucca moth Tegeticula yuccasella have a mutualistic association that is essential for the successful sexual reproduction of both species. However, a wide range of other invertebrate species visit flowers during the day and at night. Our aim was to quantify the role of yucca moths and other invertebrate visitors in pollination and fruit set, using manipulative field experiments. Inflorescences were bagged during the day or night (N=12 inflorescences) to restrict flower visitors to either nocturnal or diurnal groups. Yucca moths were active exclusively nocturnally during the flowering period and thus did not visit inflorescences that were unbagged during the day. None of the 4022 flowers exposed only to diurnal visitors set fruit, whereas 4.6% of the 4974 flowers exposed only to nocturnal visitors (including yucca moths) produced mature fruit. The proportion of flowers producing fruit in the latter treatment was not significantly different from unbagged control inflorescences. In a series of experimental manipulations we also determined that: (1) flowers opened at dusk and were open for two days on average, but were only receptive to pollen on the first night of opening; (2) pollen must be pushed down the stigmatic tube to affect pollination; and (3) most plants require out-cross pollination to produce fruit. The combination of these results strongly suggests that yucca moths are the only species affecting pollination in Y. elata, and that if another species was to affect pollination, it would be a rare event.     

Inbreeding depression affects pollen performance in Cucurbita texana

Most studies of the effects of inbreeding focus on vegetative vigor and reproductive output through the female (fruit and seed) function. This study not only examines the effects of inbreeding on the female function but it also examines the effects of inbreeding on pollen performance both in vitro and in vivo. This study used Cucurbita texana, a wild gourd, and was performed under field conditions. In vivo performance was assessed by placing equal amounts of pollen from either the inbred or outcrossed plants onto a stigma together with pollen from a tester line. As with other studies, we found that outcrossed plants had greater reproductive output (male flowers and fruits) than plants produced from self pollinations. Unlike most studies of inbreeding depression, which mostly ignore the male function of plants, we also found that the pollen produced by outcrossed plants had faster pollen tube growth in vitro than the pollen produced on selfed plants. Moreover, pollen from selfed plants sired significantly fewer seeds than pollen from outcrossed plants under conditions of pollen competition (i.e. the number of pollen grains deposited onto the stigmas was larger than the number of available ovules). These findings indicate that inbreeding affects the performance of the resulting sporophytic generation and the microgametophytes they produce.     

Pollinator attraction to volatiles from virgin and pollinated host flowers in a yucca/moth obligate mutualism

The classic obligate pollination–seed consumption mutualism between yuccas and yucca moths has been thought to be mediated by chemical cues, but empirical data on pollinator attraction to host floral volatiles in this association have been lacking. Here we show that the scent from virgin flowers of the host Yucca glauca is sufficient to attract its obligate pollinator Tegeticula yuccasella in Y‐tube olfactometer tests. Interestingly, both sexes of moths were attracted to the scent stimulus. Because yucca moths mate inside host flowers, the attraction of both females and males to host floral volatiles is likely to increase encounter rates. In a second test, female moths did not discriminate between virgin and hand‐pollinated flowers, indicating no post‐pollination change in scent production by the host that would lead to a reduction in pollinator attraction and thereby limit exploitation of the available seeds in host flowers. However, other mechanisms that could stabilise the mutualism between T. yuccasella and its yucca hosts have already been documented, i.e. selective abortion of heavily infested flowers, and a female‐derived host‐marking pheromone. Headspace collection and GC–MS were used to identify the blend of floral volatiles emitted by Y. glauca, which was found to be very similar to those of two other allopatric capsular‐fruited species, Y. elata and Y. filamentosa, revealing strong conservation of this trait within Yucca section Chaenocarpa.     

Short-distance pollen dispersal in a protogynous Annonaceae tree species from the Brazilian Cerrado

Annona crassiflora (Annonaceae) is a protogynous beetle-pollinated savannah tree species, widely distributed in the savannahs of the Cerrado biome. Studies on the mating system and pollen dispersal of protogynous species are very scarce. Here, we used six microsatellite loci to assess the mating system and pollen dispersal of A. crassiflora in a savannah remnant in Central Brazil. We mapped and sampled leaves of 112 adult trees and collected 74 fruits from 20 mother trees (1–4 fruits per plant) to obtain the seeds used (460) for mating system and parentage analyses. Annona crassiflora has predominantly allogamous mating systems, with a high multilocus outcrossing rate (t_m?=?0.974, SE?=?0.011) that did not differ among mother trees (F?=?1.32, p?=?0.165). However, t_m–t_s was variable among seed trees, indicating that some seeds were produced by mating among relatives. Our results also showed multiple paternity within fruits. Multilocus correlation of outcrossed paternity was high (r_p?=?0.302, SE?=?0.045), indicating that for each mother tree, the probability that the same pollen donor sired two random sibs was 30.2%, and the mean number of pollen donors per mother tree was high (6.3). We detected a maximum pollen dispersal distance of 360.7 m and an average of 124.3 m (SD?=?80 m), but most pollination events (73%) occurred at shorter distances (<?160 m), indicating short-distance pollen dispersal, most likely due to the pollinator behaviour.     

Regulation of the mutualism between yuccas and yucca moths: intrinsic and extrinsic factors affecting flower retention

Yuccas initiate far more flowers than they can mature as fruit, thereby providing opportunities for them to mature flowers of the highest quality. Flower quality in yuccas has both intrinsic and extrinsic components. Intrinsic components relate to flower morphology and inflorescence architecture. Yucca moths (Tegeticula spp., Incurvariidae), the sole pollinators and primary seed predators of most yuccas (Yucca spp., Agavaceae), mediate extrinsic components of flower quality through their ovipositions in flowers, and the quantity and quality of pollen that they transfer. In addition, intrinsic and extrinsic components interact as a function of flowering phenology and moth activity within inflorescences.
We investigated selective abscission of flowers in Y. kanabensis with respect to various combinations of intrinsic and extrinsic factors. First, we considered the effect of high and low pollen loads delivered to different subsets of flowers and in different presentation orders. In the absence of moth ovipositions, Y. kanabensis is sensitive to the amount of pollen that moths deliver and tends to retain high pollen flowers, even when all flowers receive sufficient pollen for full fertilization. However, pollen delivery sequence and the position of flowers with an inflorescence modify this high pollen effect. We then considered the interplay between high and low pollen combined with moth ovipositions and found that the number of ovipositions dominated the pollen effect. Finally, we considered number of ovipositions in conjunction with different flowers in the blooming sequence while controlling pollen levels and found that the clear effect of ovipositions on flower fate can be tempered by where the flower is in the blooming sequence.
These results have implications for the regulation of the mutualism between yuccas and yucca moths, indicating that yuccas are capable of regulating costs, retaining flowers of relative high quality and selectively abscising the rest. Yucca sensitivity to several intrinsic and extrinsic factors allows the plant to respond flexibly to the pollination environment and several species of moths.     

Infertile seeds ofYucca schottii: A beneficial role for the plant in the yucca-yucca moth mutualism?

Summary The yucca-yucca moth interaction is a classic case of obligate mutualism. Female moths pollinate and oviposit in the gynoecium of the flower; however, maturing larvae eat a fraction of the developing seeds. We studied within-fruit distributions of four seed types (fertile, infertile, eaten and uneaten seeds) in order to evaluate costs and benefits in aYucca schottii population in southeastern Arizona. We focused on how the spatial arrangement of seeds affected larval behaviour and, hence, the costs of the mutualism to the yucca. Infertile seeds were distributed throughout both infested and uninfested locules. Additionally, moth larvae feeding in a single locule preferred fertile seeds and even avoided infertile seeds and left the fruit significantly more often when they encountered infertile seeds. We suggest that, regardless of the cause of infertile seeds, they function as blocking units within seed locules and therefore reduce seed predation by moth larvae. We also suggest that, together with certain other fruit traits, the presence of infertile seeds promotes the evolutionary stability of this pollination mutualism.     

Variation in the costs and benefits of mutualism: the interaction between yuccas and yucca moths

Summary Yucca moths are both obligate pollinators and obligate seed predators of yuccas. I measured the costs and net benefits per fruit arising for eight species of yuccas from their interaction with the yucca moth Tegeticula yuccasella. Yucca moths decrease the production of viable seeds as a result of oviposition by adults and feeding by larvae. Oviposition through the ovary wall caused 2.3–28.6% of ovules per locule to fail to develop, leaving fruit with constrictions, and overall, 0.6–6.6% of ovules per fruit were lost to oviposition by yucca moths. Individual yucca moth larvae ate 18.0–43.6% of the ovules in a locule. However, because of the number of larvae per fruit and the proportion of viable seeds, yucca moth larvae consumed only 0.0–13.6% of potentially viable ovules per fruit. Given both oviposition and feeding effects, yucca moths decreased viable seed production by 0.6–19.5%. The ratio of costs to (gross) benefits varied from 0% to 30%, indicating that up to 30% of the benefits available to yuccas are subsequently lost to yucca moths. The costs are both lower and more variable than in a similar pollinator-seed predator mutualism involving figs and fig wasps.There were differences between species of yuccas in the costs of associating with yucca moths. Yuccas with baccate fruit experienced lower costs than species with capsular fruit. There were also differences in costs between populations within species and high variation in costs between fruit within populations. High variability was the result of no yucca moth larvae being present in over 50% of the fruit in some populations, while other fruit produced up to 24 larvae. I present hypotheses explaining both the absence and high numbers of larvae per fruit.     

Preferential outcrossing in the complex species Persoonia mollis R. Br. (Proteaceae)

The breeding system of the extremely diverse species Persoonia mollis (Proteaceae) was characterized to, firstly, assess its importance as a mechanism promoting diversity and, secondly, to investigate the mode of control over selective fruit abortion. Fruit quantity and quality was assessed following self-and outcross-pollination manipulations. Twenty percent of outcrossed flowers set fruit, compared to only 1% of flowers fertilized with self-pollen. Fruits produced by self-fertilization were 72% of the weight of cross-fertilized fruits. Fruits produced by self-fertilization were significantly fewer in number and lighter than fruits following natural pollination of unmanipulated flowers (17% fruit set), but outcrossed and naturally pollinated fruits were equivalent. Flower to fruit demography suggested that a post-zygotic mechanism may be preferentially selecting the most vigorous zygote genotypes, as ovary abscission occurs mostly between 4 and 30 weeks after pollination, regardless of pollen source. Self-pollen tube growth was found to be inhibited within the styly, while pollen tubes were found in the ovary for 50% of all outcrossed flowers. These data suggest that a pre-zygotic pseudo self-incompatibility mechanism is the cause of low fruit set following self-pollination. The breeding system of P. mollis was found to promote outbreeding, with an emphasis on flexibility and post-zygotic choice following pre-zygotic pseudo self-incompatibility.Publication no. 120 from the Ecology and Genetics Group of the University of Wollongong     

Herkogamy and mate diversity in the wild daffodil Narcissus longispathus: beyond the selfing–outcrossing paradigm in the evolution of mixed mating

Spatial separation of male and female reproductive structures (herkogamy) is a widespread floral trait that has traditionally been viewed as an adaptation that reduces the likelihood of self‐pollination. Here we propose that increased herkogamy may also influence another important aspect of plant mating: the diversity of pollen donors siring seeds within fruits. We test this hypothesis in Narcissus longispathus, a wild daffodil species with extensive variation in anther–stigma separation. To study the morphological basis of variation in herkogamy, floral measurements were undertaken in 16 populations of N. longispathus. We then quantified multilocus outcrossing rates and the correlation of outcrossed paternity in three of these populations sampled over several years. Mating system estimates were calculated for each population and year, and also separately for groups of plants that differed markedly in herkogamy within each population and year. In N. longispathus herkogamy was much more variable than other floral traits, and was more closely related to style length than to anther position. Averaged across populations and years, plants with high herkogamy had similar outcrossing rates (0.683) to plants with intermediate (0.648) or low herkogamy (0.590). However, a significant linear trend was found for correlation of outcrossed paternity, which increased monotonically from high herkogamy (0.221), through intermediate herkogamy (0.303) to low herkogamy (0.463) plants. The diversity of pollen donors siring seeds of high herkogamy Narcissus flowers was thus consistently greater than the diversity of pollen donors siring seeds of low herkogamy flowers. Results of this study contribute to the emerging consensus that floral traits can simultaneously influence several aspects of plant mating system in complex ways, thus extending the traditional focus centred exclusively on patterns and relative importance of self‐ and cross‐fertilisation.