Anther and pollen development in the lodgepole pine dwarf mistletoe (Arceuthobium americanum) staminate flower

Abstract

The lodgepole pine dwarf mistletoe, Arceuthobium americanum Nutt. ex Engelm., is a parasitic angiosperm that infects conifers in western Canadian forests. While production of viable pollen in anthers is critical to dwarf mistletoe reproduction, the few existing reports that examine staminate development in Arceuthobium are often incomplete or conflicting. The objective of this work was to investigate the developmental anatomy of anther and pollen of A. americanum using modern microscopy. We found that the microsporangium was toroidal from the outset and gave rise to a central peg-shaped sterile "columella" early in anther development. The endothecium was absent, the epidermis persisted as an "exothecium" fulfilling the role of the endothecium, and a primary parietal layer generated a secretory tapetum and middle layer. Thus, we suggest that a new category of anther wall development, the Arceuthobium type, be created. Microsporogenesis produced tetrahedral microspores via simultaneous cytokinesis and involved callose wall formation. Microgametogenesis resulted in round and atypical generative and vegetative nuclei. Additionally, the heterocolpate, echinate pollen grains, which were shed at the two-celled stage, were seen for the first time in their native state with environmental scanning electron microscopy. This work contributes to the understanding of A. americanum, the genus Arceuthobium, and angiosperms as a whole.Résumé : Le gui nain du pin lodgepole, Arceuthobium americanum Nutt. ex Engelm., est une angiosperme parasite qui infecte les conifères des forêts de l'Ouest du Canada. Alors que la production de pollen viable par les anthères est critique à la reproduction du gui nain, les rapports actuels, peu nombreux, qui se sont penchés sur le développement staminé chez Arceuthobium sont souvent incomplets ou contradictoires. L'objectif de ce travail était d'examiner l'anatomie développementale des anthères et du pollen d'A. americanum par microscopie moderne. Nous avons trouvé que le microsporange était en forme d'anneau dès le début et donnait naissance à une « columelle » centrale conoïde stérile tôt lors du développement de l'anthère. L'endothèque était absente, l'épiderme persistait comme « exothèque », remplissant le rôle de l'endothèque, et une couche pariétale primaire générait un tapetum sécrétoire et une couche intermédiaire; ainsi, nous suggérons qu'une nouvelle modalité de développement de la paroi de l'anthère soit créée, le type « Arceuthobium ». La microsporogenèse produisait des microspores tétraédriques par cytocinèse simultanée, et elle impliquait la formation d'une paroi calleuse. La microgamétogenèse résultait en noyaux génératifs et végétatifs ronds et atypiques. De plus, les grains de pollen hétérocolpés échinés qui étaient libérés au stade biloculaire ont été observés pour la première fois dans leur état natif par microscopie électronique à balayage environnemental. Ce travail contribue à mieux comprendre globalement A. americanum, le genre Arceuthobium, et les angiospermes.

Insular biogeography and population genetics of dwarf mistletoe (Arceuthobium americanum) in the Central Rocky Mountains

“…Given that entomophily is favored over anemophily in A. americanum during most of the growing season (Munro et al. ), our data support a hypothesis of pollination reliance on movement‐limited pollinators proposed by Penfield et al. ().…”

“…Individual plants are dioecious with the potential for hundreds of flowers per inflorescence, and the production of approximately 11,000 pollen grains per ovule (Gregor et al 1974, Penfield et al 1976, Hawksworth and Wiens 1996. Arceuthobium americanum, however, flowers very early in the spring when few insect pollinators are active (Munro et al 2014). As a result, when temperatures are low during anthesis, anemophily is likely to be the main pollen dispersal mechanism, whereas later in the growing season entomophily may be favored over anemophily (Munro et al 2014).…”

“…Arceuthobium americanum, however, flowers very early in the spring when few insect pollinators are active (Munro et al 2014). As a result, when temperatures are low during anthesis, anemophily is likely to be the main pollen dispersal mechanism, whereas later in the growing season entomophily may be favored over anemophily (Munro et al 2014).…”

“…As a result, when temperatures are low during anthesis, anemophily is likely to be the main pollen dispersal mechanism, whereas later in the growing season entomophily may be favored over anemophily (Munro et al. ).…”

Evolution of pollen morphology in Loranthaceae

“…Viscum is a large, widespread genus and its pollen is not only echinate, but shows various different types of SEM sculpturing (including rugulate, verrucate, baculate, clavate), forming the only lineage so far paralleling in this aspect the variation seen in Loranthaceae. Some species also bear pseudocolpi similar to those observed in Arceuthobium (Hawksworth & Wiens 1972; Feuer & Kuijt 1982; Feuer et al 1982; Munro et al 2014). …”