Pollinator-mediated self-pollination and reproductive assurance in an isolated tree of Magnolia grandiflora L.
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RESEARCH
Open Access
Pollinator-mediated self-pollination and reproductive assurance in an isolated tree of Magnolia grandiflora L. Arun Sukumaran, Vinod Prasad Khanduri*
and Chandra Mohan Sharma
Abstract Background: The ability to produce seeds when pollinators or potential mates are scarce is one of the principal advantages of self-pollination in flowering plants. However, the role of pollinators mediating self-pollination to ensure seed set when pollen or potential mates are limited has received less attention. This study examined the reproductive consequences and involvement of pollinators in assuring seed set in an isolated tree of Magnolia grandiflora, a predominantly outcrossing species. Methods: We tested various aspects of reproduction such as flower density, floral rewards, stigma receptivity, pollinator abundance and behaviour, fruit set and seed set, in two successive reproductive years (2015–2016). Results: Flowers of M. grandiflora possess a suite of traits facilitating pollinator-mediated self-pollination (PMS), although the chances of autonomous self-pollination are reduced due to herkogamy, spatial separation of anther and stigma. The mean pollen production/flower was 5,152,289 ± 285,094 with a pollen-to-ovule ratio of 39430 ± 164. We found a significant positive correlation between number of visits and seed production for bees (r = 0.5099, p = 0.0007) and beetles (r = 0.7159, p = 0.00001), indicating these are effective at PMS. There was a significant negative correlation for thrips (r = – 0.3206, p = 0.044) and no correlation for flies or spiders. The percent fruit set was 100% and the seed set per ovule ranged between 19 and 20%. Conclusions: PMS will guarantee reproductive assurance and mitigate the effect of reduced mates or pollen limitation which is expected to increase the fecundity and establishment of individual trees in geographically isolated locations. Keywords: Pollinator-mediated self-pollination, Magnolia grandiflora, Autonomous self-pollination, Pollinator behaviour, Reproductive assurance
Background Since the time of Darwin (1876), researchers have been interested in why there are plants that are primarily selffertilizing even though the progeny produced by selffertilization (whether of normally outcrossing or normally selfing plants) are often less vigorous than progeny produced by cross-fertilization. Stebbins (1950) stated that the ecological conditions in which a given * Correspondence: [email protected] College of Forestry, VCSG Uttarakhand University of Horticulture and Forestry, Ranichauri, Tehri Garhwal, Uttarakhand 249199, India
species is found are supposed to determine whether it is predominantly self-fertilized or predominantly outcrossed. A high proportion of self-compatible species (or populations) are found in isolated locations (e.g. oceanic islands, isolated ponds; Baker 1955; Longhurst 1955). Self-pollinating morphs would be favoured by natural selection when population size or potential mates are limited. Hermaphrodite and monoecious species have the po
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