Retha Edens-Meier

What We Think We Know vs. What We Need to Know About Orchid Pollination and Conservation: Cypripedium L. as a Model Lineage

While Darwin (1862, 1877) showed that reproductive success in orchid populations depended on adaptive floral morphology coupled with pollinator visitation a more recent review of the literature (Tremblay et al., 2005) confirmed that many out-breeding species are pollinator-limited because most orchid species showing low fecundity also lack rewards. The absence of rewards depresses both pollinator fidelity and the frequency of pollinator visits to an orchid population even though orchid flowers that lack rewards retain the same interlocking floral structures for precise pollinia removal and deposition found in related species that offer rewards. Using the genus, Cypripedium, as a model lineage of non-rewarding flowers this study also shows that the correlation between low fruit set in a Cypripedium sp. and its specific pollinator(s) is insufficient to predict specific frequencies of low fecundity. Annual rates of fruit set often vary broadly between populations of the same species and within the same population over several seasons. We speculate that fruit-set rates also decline when orchid demography and additional biotic and abiotic factors interrupt rates of pollinator activity (pre-zygotic) and fertilization/fruit maturation (post-zygotic). We suggest that that traditional field studies on pollination ecology and breeding systems be combined with data sets recording genetic variation and orchid flower demography in relation to seasonal variation in climate. We also propose that the same information be collected in regard to genetic variation, demography and phenology of populations of known orchid pollinators and co-blooming angiosperm species native to orchid habitats.

The Pollination Biology of an Annual Endemic Herb, Physaria filiformis (Brassicaceae), in the Missouri Ozarks Following Controlled Burns1

Abstract Physaria filiformis, a threatened species of calcareous glade communities, blooms profusely within the Rocky Barrens Conservation Area (Missouri) under a program of controlled burns. Flowers visited by insect foragers in 2008 and 2009 had a mean > 35 and > 50 germinating grains on their stigmas respectively and a mean of 1.96–2.0 pollen tubes entering their ovaries (mean of 4 ovules/ovary). Results of controlled hand-pollinations conducted in situ in 2009 showed that this species had a negligible rate of self-pollination (mechanical autogamy) and showed classic Brassica-type, early-acting self-incompatibility. Each dissected flower had nectar glands and each seasonal population was visited by insects representing four insect orders; Coleoptera, Diptera, Lepidoptera and Hymenoptera. The most commonly collected floral foragers found to carry the pollen of the host flower were male and female native bees representing 38 species in the Andrenidae, Apidae, Colletidae, Halictidae, Megachilidae and the naturalized Apis mellifera. Only four of these species (11%) were collected in all three years, but these four accounted for slightly more than a third of all individual bees collected. None of the bees were known specialists on flowers of the Brassicaceae and only two species were habitat (glade) specialists. As P. filiformis is an obligate out-breeder but a generalist entomophile we suggest that future management plans include conservation and management of adjacent non-glade habitats to sustain the diverse range of pollinators.

POLLEN-PISTIL INTERACTIONS IN NORTH AMERICAN AND CHINESE CYPRIPEDIUM L. (ORCHIDACEAE)

Fluorescence microscopy is used to compare frequencies of pollen tube penetration in in situ populations of Cypripedium bardolphianum W.W. Smith et Farrer, Cypripedium flavum W.W. Smith, Cypripedium montanum Dougl. ex Lindl., Cypripedium parviflorum Salisbury var. pubescens (Wildenow) O.W. Knight, Cypripedium reginae Walter, and Cypripedium tibeticum Schltr. The average natural (insect‐mediated) pollination rates measured over five seasons are wide ranging among the six species (0.08–0.74). However, the pollination rate of hand‐manipulated populations (self and/or cross) is significantly greater than the rate of insect‐mediated pollinations in all species studied. A few pollen tubes in both self‐ and cross‐pollinations display aberrant growth in the styles and/or ovaries, but their numbers are too small to suggest evidence of self‐incompatibility. Pollen tubes germinate and grow up to the bases of styles within 48 h in C. bardolphianum, C. flavum, and C. tibeticum. Pollen tubes remain at the bases of the styles in C. montanum for 5 d after pollination. In C. parviflorum, pollen tubes penetrate ovaries at 7 d. Pollen tube penetration of ovaries is observed within 15 d after hand pollination in all six species but remains incomplete at this time, with the greatest number of ovule penetrations observed in C. reginae (which has the shortest floral life span). Therefore, we suggest there are additional factors aside from low pollinator visitation for low conversion rates of flowers into fruits in these Cypripedium species. These include inadequate pollen loads deposited on receptive stigmas (pollen limitation), coupled with environmental stress and/or predation disrupting or destroying the slow processes of fertilization and/or fruit maturation.

Global Collaborative Research on the Pollination Biology of Rare and Threatened Orchid Species (Orchidaceae)1

Abstract Charles Darwin (1862, 1877) conducted field and lab research on orchid pollination mechanisms before any protocols or laws protected orchid species or individual populations. Information on the reproductive ecology of rare and threatened orchids remains intrinsic to their conservation as populations continue to diminish during the Anthropocene. We provide case studies of species representing five genera (Calanthe R. Br., Corunastylis Fitzg., Cypripedium L., Spiranthes Rich., and Thelymitra J. R. Forst. & G. Forst.) in three countries (the United States, Australia, and China). We compare differences in their pollination systems and seasonal variations in their fertility rates. Declining rates of fruit set and/or seed development are compared to breeding systems, pollinator life histories/foraging behaviors, and climate cycles. We conclude that, while some orchid species are pollinator limited, several interrelated factors influence rates of fecundity in terrestrial/lithophytic orchids.

Reproductive ecology of Asclepias meadii Torr. ex A.Gray (Apocynaceae), a federally threatened species1

Abstract Reproductive success in the federally threatened species Asclepias meadii is limited by several factors, including the increasing age and fragmentation of populations, low fruit set, and potential disease. We investigated the reproductive ecology of three isolated populations of A. meadii over five seasons (2010–14) in Missouri and Kansas. Experimental hand-pollinations showed that flowers at all sites were incapable of automatic self-pollination (autogamy) and lacked early acting self-incompatibility. The average number of ovaries with pollen tubes in open (insect-mediated) pollination varied from 0.10 to 0.81 depending on site and season. The conversion rate of ovaries into fruits never exceeded more than two fruits per umbel with insect-mediated fruit set varying from 0% to 50% according to site and season. Nectar was the only reward with a per-flower volume as high as 5 μL containing dissolved sugars from 21% to ≥ 50%. The nectar was hexose dominant, and the amino acid content was limited to aspartic acid and arginine. Nectar-drinking insects were common visitors at all sites, but only three taxa in the family Apidae (Anthophora abrupta, Apis mellifera, and Bombus species) were consistent carriers of A. meadii pollinaria on their legs. Density and diversity of these three pollinaria carrying taxa also varied according to site and season. Although all three taxa were successful pollinaria vectors, the nonnative A. mellifera does not appear to be as efficient as the native species in the two other bee genera. Our findings offer evidence that Bombus species are especially successful in vectoring pollinaria between flowers of A. meadii; that pollen tubes are successfully reaching the ovaries in insect-mediated, hand-self, and hand-cross pollination events; and that automatic self-pollination (autogamy) does not occur.

Pollination Ecology of Polylepis tomentella (Rosaceae), an Andean Anemophilous Tree Presenting a Potential Floral Fungal Infection

Premise of research. Anemophily is considered to be a mechanism that evolved to promote pollination success. However, reproductive output can decrease if pollen loads are scarce due to low abundance of compatible mates or floral infections. Here we analyze the effects of breeding system, pollen limitation, and a potential floral fungal infection on the reproductive output of the Andean tree Polylepis tomentella (Rosaceae). Methodology. We performed pollen supplementation experiments in the field to assess the extent of the effect of pollen limitation on fruit and seed set and to identify any pre- or postzygotic self-incompatibility responses within receptive pistils. Supplementary scanning electron microscopy and epifluorescence microscopy were used to observe the possible effects of a potential floral fungal infection on sexual reproduction. Pivotal results. Pollen supplementation did not increase fruit and seed set, suggesting weak evidence of pollen limitation. Most stigmas from self- and cross-pollinated flowers showed germinated pollen grains and pollen tubes in the style, indicating that the species exhibits prezygotic self-compatibility. The presence of three Ascomycota genera in stigmas and styles appears to have negatively affected the germination of pollen grains by inhibiting pollen tube growth, but we did not find evidence suggesting that hyphae are able to penetrate the ovary. Conclusions. This study showed that P. tomentella possesses a prezygotic self-compatible system and that fruit and seed set are not affected by pollen limitation. Therefore, reproductive assurance is achieved through pollen transfer between plants by wind and the role played by self-compatibility that allows selfing in the species. However, it is possible that the potential fungal infection could reduce the fecundity of its tree host, as hyphae are able to penetrate pollen grains.

Potential Fungal Pathogens of Mead's Milkweed (Asclepias meadii: Apocynaceae)

ABSTRACT: In 2012, cleared floral organs of Meads milkweed, a federally threatened species, were found to contain fungal hyphae. All fresh flowers (n = 10) collected and analyzed from Rockhill Prairie (Missouri) contained Metschnikowia spp., Cladosporium cladosporiodes, Alternaria spp., Thanatephorus sp., Aspergillus sp., and Cryptococcus sp. Of the ten fresh flowers collected from the Anderson County Prairie Preserve (Kansas), 90% (n = 10) of the flowers contained fungi, including C. cladosporiodes, Alternaria spp., and Metschnikowia spp. A dried stem collected from the Anderson County Prairie Preserve revealed the presence of Fusarium spp., Alternaria spp., Metschnikowia sp., Lewia infectoria, Stemphylum solani, and Filobasidium floriforme. In addition, we analyzed pinned insects (n = 13) captured previously over a three-year period. These insects were all caught while they foraged on flowers of A. meadii. The fungal genus, Alternaria, was the only fungal taxa isolated at least once from our samples of flowers, dried stems, and some anthophilous insects.