Insect Pollination of Cycads 9 10 Alicia Toon1, L
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1 2 DR. ALICIA TOON (Orcid ID : 0000-0002-1517-2601) 3 4 5 Article type : Invited Review 6 7 8 Insect pollination of cycads 9 10 Alicia Toon1, L. Irene Terry2, William Tang3, Gimme H. Walter1, and Lyn G. Cook1 11 12 1The University of Queensland, School of Biological Sciences, Brisbane, Qld, 4072, 13 Australia 2 14 University of Utah, School of Biological Sciences, Salt Lake City, UT 84112, USA 15 3 USDA APHIS PPQ South Florida, P.O.Box 660520, Miami, FL 33266, USA 16 17 Corresponding author: Alicia Toon 18 [email protected] Ph: +61 (0) 411954179 19 Goddard Building, The University of Queensland, School of Biological Sciences, Brisbane, 20 Qld, 4072, Australia. 21 22 23 24 25 26 27 28 29 30 Manuscript Author 31 This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/AEC.12925 This article is protected by copyright. All rights reserved 32 33 Acknowledgements 34 We would like to thank Dean Brookes for discussions about genetic structure in cycad 35 pollinating thrips populations. Also, thanks to Mike Crisp for discussions about plant 36 diversification and Paul Forster for information on Australian cycads. This work was funded 37 by ARC Discovery Grant DP160102806. 38 39 Abstract 40 Most cycads have intimate associations with their insect pollinators that parallel those of 41 well-known flowering plants, such as sexually-deceptive orchids and the male wasps and 42 bees they deceive. Despite this, the mistaken belief that cycads are mostly wind-pollinated is 43 still commonly expressed. Perhaps as a consequence, cycad-pollinator systems are rarely 44 exemplified in studies of the role of pollinators in plant evolution and diversification. 45 Although first recognized more than a century ago, specialised associations between cycads 46 and their insect pollinators have been elucidated experimentally only in the past few decades. 47 This review covers the history of understanding pollination in cycads, the advances that have 48 been made since the 1980s using field observations and experiments, and analyses of 49 molecular data from the population to phylum level. We outline areas for future research to 50 address how such interactions might have affected speciation and extinctions. We stress that 51 inclusion of cycads in broader considerations of the role of pollinators in plant diversification 52 is important because they are phylogenetically distant from flowering plants and their 53 pollination systems might have evolved independently of one another. This review is timely 54 because cycads are a globally threatened group that might be vulnerable to co-extinction with 55 pollinator loss. 56 57 58 59 60 61 Author Manuscript Author 62 63 Key words: diversification, cycads, insect pollination, push-pull 64 Terms: This article is protected by copyright. All rights reserved 65 Cospeciation: parallel speciation among interacting taxa. 66 Phylogenetic tracking: pattern of cospeciation where speciation in one taxon is a consequence 67 of the dependence on another taxon. 68 Coevolution: the process of parallel speciation among multiple taxa, as a consequence of 69 reciprocal selection on each other. 70 Brood-site pollination: the offspring of pollinators complete part of their life cycle within 71 reproductive tissues of their host that developed as a consequence of host fertilisation 72 facilitated by that pollinator. 73 74 Introduction 75 Until recently, it was generally taught that all gymnosperms are wind pollinated. This 76 is not the case, with most species of cycads (Terry et al. 2012) and gnetales (Ickert-Bond and 77 Renner 2016) being pollinated by insects. Thus, two of the four extant phyla of gymnosperms 78 are insect, rather than wind, pollinated. 79 Despite almost all cycads being pollinated by insects, they are rarely mentioned in 80 reviews of plant-pollinator diversification and cospeciation (but see Dufaÿ and Anstett 2003). 81 This lack of recognition might be because cycads are not as speciose as their angiosperm 82 cousins, their reproductive organs are not as showy as those of many flowering plants, and 83 their dependence on insects has only recently been accepted among the broader scientific 84 community (Terry et al. 2012). Given that cycads form a divergent lineage within the 85 gymnosperms, they represent a distant and independent case of association with insect 86 pollinators, and could add substantial breadth to our understanding of the effects on plant 87 populations of pollination interactions. 88 In this review, we outline the history of discovery of insects as pollinators of cycads 89 and speculate as to why it took so long before early reports were verified. We then explain 90 the pollination system in cycads and its variations, and what is not yet understood. We cover 91 the potential ecological consequences of cycads being reliant on insects as pollinators, and in 92 particular how insects might affect gene flow, population structure and long-term resilience 93 of cycads, and propose future directions for cycad-pollinator research. 94 Author Manuscript Author 95 Pollination of seed plants 96 Interactions with animals are central to understanding the diversification of seed plants, 97 i.e., angiosperms and gymnosperms. For many seed plants, animals are involved in This article is protected by copyright. All rights reserved 98 pollination and the dispersal of seeds, as well as being herbivores and the vectors of disease. 99 Some of these types of interactions likely date to the very origin of seed plants, around 300 100 million years ago (Linkies et al. 2010) when early insects were diversifying (Grimaldi and 101 Engel 2005), including the beetles (Zhang et al. 2018), hemipterans (Johnson et al. 2018) and 102 flies (Wiegmann et al. 2011). While many seed plants are wind-pollinated, such as the 103 species-rich grasses and pines, the vast majority of seed plants are pollinated by animals, 104 mostly insects (Ollerton et al. 2011). 105 The astounding diversity of flowering plants and insects is often attributed to their biotic 106 interactions (Ehrlich and Raven 1964; Grant and Grant 1965; Suchan and Alvarez 2015). 107 Interactions with animals have greatly affected the ecology and diversification of plants, 108 especially by seed dispersal and pollen transport that might lead to plant isolation and or 109 affect gene flow (Ballesteros-Mejia et al. 2016; Ghazoul 2005; Krauss et al. 2017). For 110 example, fleshy fruits that are dispersed by animals have evolved repeatedly in the 111 Myrtaceae, and the switch to fleshy fruit is generally accompanied by dramatic increases in 112 the diversification rate of the plant lineage compared with lineages without fleshy fruit 113 (Biffin et al. 2010). In contrast, evolutionary shifts among animal pollinators (e.g., from 114 using insects to using birds, or between insects with different feeding styles (guilds)) are 115 correlated with increased diversification rates or, in some cases, with decreases (Kay and 116 Sargent 2009; Serrano-Serrano et al. 2017; Smith 2010; Toon et al. 2014). While other 117 factors probably play a more important role in the huge species richness of angiosperms 118 compared with other land plants (e.g., Brodribb and Feild 2010; Amborella Genome Project 119 2013), the situation in gymnosperms still needs to be clarified (e.g., Bolinder et al. 2016). 120 Only a minority of animal-pollinated plant species are pollinated by a single species of 121 animal: instead, pollination typically involves guilds of relatively generalist pollinators (e.g., 122 Myrtaceae and flies, beetles and butterflies) (Waser et al. 1996) or there has been diffuse 123 coevolution with a particular guild of pollinator (Lunau 2004), e.g, buzz-pollination bees and 124 Solanum (Buchmann and Cane 1989). Of the more specialised pollination systems, the 125 obligate mutualisms Yucca—yucca moth (Pellmyr 2003), fig—fig wasp (Herre et al. 2008) 126 and Glochidion—leafflower moth are the most well studied. In Yucca (Pellmyr et al. 1996) Author Manuscript Author 127 and Glochidion (Kato et al. 2003), pollination is achieved by the female moth actively 128 collecting pollen and placing it on the style of another flower, then laying her eggs within the 129 flower of the plant where the larvae will feed on a subset of the developing seeds. Even in This article is protected by copyright. All rights reserved 130 such ecologically specialised mutualisms, co-pollinators are known and one-to-one 131 relationships are rare (Hembry and Althoff 2016; Herre et al. 2008). 132 There has been considerable research investigating co-diversification of host and 133 pollinator in obligate pollination systems. Phylogeographic concordance among host and 134 pollinators, e.g., European globeflower, Trollius europaeus, and their pollinating flies, 135 Chiastocheta spp. (Espíndola et al. 2014), and Ficus and their pollinating wasps (Rodriguez 136 et al. 2017; Tian et al. 2015), supports the idea that strong ecological associations might 137 affect co-genetic structure, at least within species. Although evidence for coevolution is 138 weak and phylogenetic congruence at the species level is rare (Hembry and Althoff 2016), 139 Yucca—yucca moth (Althoff et al. 2012), fig—fig wasp (Yang et al. 2015) and some clades 140 of Glochidion—leafflower moth (Hembry and Althoff 2016) show significant cophyletic 141 structure across phylogenies. This pattern of phylogenetic congruence at the clade level, 142 might in part be explained by conservation of the strong ecological association between host 143 and pollinator, combined with host-switching (Hembry and Althoff 2016). Although there 144 are obligate pollination systems within gymnosperms, little is currently known of the extent 145 to which pollinators and hosts affect each other’s ecology or evolution.