Higher seed number compensates for lower fruit set in deceptive orchids

Abstract

Summary 1 1. Floral deception is widespread in orchids, with more than one third of the species being 2 pollinated this way. The evolutionary success of deceptive orchids is puzzling, as species 3 employing this strategy are thought to have low reproductive success (less flowers yielding 4 fruits) because of low pollination rates. However, direct measurements of seed production in 5 orchids are scarce due to the extremely small size of their seeds. 6 2. Here, we quantified seed numbers in 1,015 fruits belonging to 48 orchid species from the 7 Pannonian ecoregion (central Europe) and obtained fruit-set and thousand-seed weight data 8 for these species from the literature. We used phylogenetic comparative methods to test the 9 hypothesis that deceptive species should compensate for their lower fruit-set by having either 10 larger seeds or more seeds in a fruit. 11 3. Similarly to previous studies, we found that deceptive orchids have substantially lower 12 fruits-set than nectar-rewarding ones. Also, we found that deceptive species have more seeds 13 in a fruit but not larger seeds compared to nectar-rewarding ones. Based on our results, 14 deceptive species compensate for their lower fruit-set by having higher seed numbers per 15 fruit, thus their seed numbers per shoot do not differ from that of nectar-rewarding ones. 16 4. Together with other benefits of deceptive pollination (e.g. lower energy expenditure due to 17 the lack of nectar production and higher genetic variability due to decreased probability of 18 geitonogamous pollination), our results can explain why deceptive strategies are so 19 widespread in the orchid family. 20 5. Synthesis. Our study provides new seed number data for 48 terrestrial orchid species. 21Using these data we have tested the hypothesis that deceptive species should compensate for 22 their lower fruit-set by having either larger seeds or more seeds in a fruit than nectar-23 rewarding ones. Our results suggest that deceptive species have more seeds in a fruit but not 24 larger seeds compared to nectar-rewarding ones. As a consequence, there are no significant 25 3 differences in seed numbers per shoot between different pollination types. 1 2

Are pollinators the agents of selection on flower colour and size in irises?

“…Consistently with previous studies on I. lutescens (Imbert et al 2014a, b), and other studies on rewardless species (Aragón andAckerman 2004, Jersáková et al 2006b), we did not detect any difference in fruit-set between colour morphs for both naturally and supplementary-pollinated plants, regardless of morph frequency at either site. Furthermore, as commonly observed in rewardless plants (Aragón and Ackerman 2004, Tremblay et al 2005, Dormont et al 2010a, Sletvold et al 2016, Sonkoly et al 2016, we documented a low natural fruiting rate and a high pollinator limitation level for both studied species. For comparison, in I. tuberosa, a Mediterranean species producing nectar, natural fruiting rate is greater than 60% (Pellegrino 2015).…”

Factors affecting reproductive success in three entomophilous orchid species in Hungary

“…Seed numbers per fruit ('capsular seed number') were determined under light microscope following the method of Sonkoly et al [35]. Seed numbers per specimens were given by the mean of the three determined capsular seed numbers of a certain individual multiplied by the number of matured fruits of the same individual.…”

Novel Computed Tomography-based tools reliably quantify plant reproductive investment

“…Ovule numbers strongly differ between species with rewarding and deceptive flowers. Deceptive flowers contain more ovules than rewarding flowers ((Sonkoly et al , 2016); this study), which is possibly an adaptation that enables deceptive inflorescences to have the same seed set as rewarding inflorescences, despite lower fertilization rates (Sonkoly et al , 2016). Ovule numbers do not show any phylogenetic signal, possibly due to high homoplasy in this character state, i.e.…”

“…Significant differences in variance of P:O between deceptive and rewarding flower and the well-supported strong phylogenetic correlation of these two characters (P:O variance and pollination strategy) highlight the differences in constraints acting on the reproductive investment in deceptive versus rewarding species. Given that deceptive flowers contain significantly more ovules than rewarding flowers ((Sonkoly et al , 2016); this study), and given that usually only the lower flowers on the inflorescences of deceptive flowers are fertilized (Jersáková and Kindlmann, 1998; Nilsson, 1980; Vogel, 1993), it seemed likely that deceptive inflorescences are under strong constraint to stringently decrease ovule numbers from bottom to top of the inflorescence in order to efficiently allocate resources. In rewarding flowers there are much fewer ovules, and fertilization occurs across the whole inflorescence.…”

“…At the ovule stage, counting is manual after dissection, either directly or on photographs (Nazarov, 1989), either on the whole ovary or an a stretch of the latter (ovules per mm are then extrapolated to the length of the whole gynoecium). At the seed stage, methods fall into two groups: (1) extrapolations based on counting an aliquot of known weight (Salisbury, 1942), or on a portion of a line of dry seeds (Darwin, 1862), or on the surface of a liquid (Burgeff, 1936), or within a suspension of seeds (Proctor and Harder, 1994; Sonkoly et al , 2016). (2) use of particle counting devices (DuBois, 2000).…”