This article was downloaded by: [Universita Di Pisa], [L. Peruzzi] On: 13 September 2012, At: 08:11 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Plant Biosystems - An International Journal Dealing with all Aspects of Plant Biology: Official Journal of the Societa Botanica Italiana Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tplb20 The evolution of apomixis in angiosperms: A reappraisal E. Hörandl a & D. Hojsgaard a a Department of Systematic Botany, Georg-August-University of Göttingen, Untere Karspüle 2, Göttingen, 37073, Germany Accepted author version posted online: 15 Aug 2012.Version of record first published: 13 Sep 2012. To cite this article: E. Hörandl & D. Hojsgaard (2012): The evolution of apomixis in angiosperms: A reappraisal, Plant Biosystems - An International Journal Dealing with all Aspects of Plant Biology: Official Journal of the Societa Botanica Italiana, 146:3, 681-693 To link to this article: http://dx.doi.org/10.1080/11263504.2012.716795 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Plant Biosystems, Vol. 146, No. 3, September 2012, pp. 681–693 NEW TRENDS IN PLANT CYTOGENETICS AND CYTOEMBRYOLOGY – DEDICATED TO THE MEMORY OF EMILIO BATTAGLIA The evolution of apomixis in angiosperms: A reappraisal E. HO¨ RANDL & D. HOJSGAARD Department of Systematic Botany, Georg-August-University of Go¨ttingen, Untere Karspu¨le 2, 37073 Go¨ttingen, Germany Abstract Apomixis, the asexual reproduction via seed, has long been regarded a blind alley of evolution. This hypothesis was based on the assumption that apomixis is an irreversible, phylogenetically derived trait that would rapidly lead to extinction of the respective lineages. However, recent updates of the taxonomic distribution of apomixis in angiosperms suggest an alternative evolutionary scenario. Apomixis is taxonomically scattered and occurs in both early and late branching lineages, with several reversals from apomixis to obligate sex along phylogeny. Genetic control of apomixis is based on altered expression patterns of the same genes that control sexual development; epigenetic changes following polyploidization and/or hybridization may trigger shifts from sexuality to apomixis. Mendelian inheritance confirms the facultative nature and possible reversibility of apomixis to sexual reproduction. Apomixis, therefore, could represent a transition period in the evolution of polyploid complexes, with polyspory in paleopolyploids being a remnant of lost apomixis. In neopolyploids, apomixis helps to overcome sterility and allows for geographical range expansions of agamic polyploid complexes. The facultative nature of apomixis allows for reversal to sexuality and further speciation of paleopolyploid lineages. Thus, apomixis may facilitate diversification of polyploid complexes and evolution in angiosperms. Keywords: Apomixis, angiosperm phylogeny, evolution, polyploidy, polyspory Phylogenetic patterns and evolution of reproductive tion is rare in mosses, hornworts, and liverworts, modes represent the most challenging questions in quite frequent in ferns (c. 10% of species), while in evolutionary biology. Sexual reproduction, i.e., the seed plants frequencies drop down to less than 1% of combined process of reductional cell division (meio- species (Burt 2000). The phylogenetic pattern of sis) and fusion of reduced gametes (mixis), is by far asexuality has been recognized as scattered in all the most frequent mode of reproduction in eukar- major groups (Burt 2000; Simon et al. 2003; Van yotes. It is nowadays well established that meiosis Dijk & Vijverberg 2005). first arose in protists and probably originated from In general, such a pattern can be explained by bacterial DNA repair mechanisms (Ramesh et al. three different phylogenetic models (Williams 1975): 2005; Schurko & Logsdon 2008). The prevalence of (1) sex changes irreversibly to asexuality, but Downloaded by [Universita Di Pisa], [L. Peruzzi] at 08:11 13 September 2012 sexual reproduction is striking because of its obvious increases the risk of rapid extinction; (2) sex changes high costs, the loss of gene combinations during occasionally to asexuality, but the reversal happens meiosis, and the need of two parental individuals. more readily; and (3) shifts from sexuality to Alternatively, asexual reproduction potentially avoids asexuality are rare. Initially, the first scenario was these costs and is expected to result in higher the most popular, as asexuality is thought to be quantities of offspring (Maynard Smith 1978; Bell hampered by the accumulation of deleterious muta- 1982). Despite its potential benefits, asexuality tions and by the loss of genetic variability. However, appears to be rare in higher eukaryotes; only in some evidence from long-term obligate asexuality in phyla of protists it is the only mode of reproduction. animals (e.g., Butlin 2002) and from the short-term In fungi, about 20% of species are estimated to be success of asexuals in geographical dimensions asexual, while in animals frequencies of asexuality (Kearney 2005; Ho¨randl 2006) questions the basic are less than 1%. In land plants, asexual reproduc- assumptions of this model. The second hypothesis Correspondence: E. Ho¨randl, Department of Systematic Botany, Albrecht-von-Haller Institute of Plant Sciences, Georg-August-University of Go¨ttingen, Untere Karspu¨ le 2, 37073 Go¨ttingen, Germany. Tel: þ49 551 397843. Fax: þ49 551 3922329. Email: [email protected] ISSN 1126-3504 print/ISSN 1724-5575 online ª 2012 Societa` Botanica Italiana http://dx.doi.org/10.1080/11263504.2012.716795 682 E. Ho¨randl and D. Hojsgaard regards asexuality as a complex trait, which is, also called adventitious embryony (Naumova 1992). according to Dollo’s law, not easily reversed; In the end, the seed contains both sexual and however, recent empirical evidence in animals apomictic embryos (polyembryony). We do not suggests that such reversals are possible (Domes consider vegetative propagation, which does not et al. 2007). The third hypothesis is indirectly involve development from a single-cell stage, and is supported by the recognition of several functional not regarded as a mode of apomictic reproduction, constraints for shifts to asexuality in animals and but rather as a mode of clonal growth (see Mogie plants (Engelsta¨dter 2008; Ho¨randl 2009a) and by 1992). Fertilization of the polar nuclei (pseudogamy) the complexity of regulatory mechanisms (see, for is retained in c. 90% of apomicts (Mogie 1992), example, Koltunow & Grossniklaus 2003). while pollen-independent endosperm development is Angiosperms are an interesting model system for rare (autonomous apomixis). Uniparental reproduc- studying evolution of asexuality in a highly late tion is possible for pseudogamous apomicts, since branched eukaryotic group because of their great they are usually self-fertile (Ho¨randl 2010). Thus, variety of reproductive pathways. The sexual mode the fundamental difference between sexual repro- follows the general life cycle in land plants, with the duction and apomixis in flowering plants is not alternation of a sporophytic (2n) and a gametophytic reproduction via a single or two parents, but is (n) generation, separated by meiosis and mixis, mostly based on meiotic versus apomeiotic develop- respectively (Figure 1). The four- to eight-celled ment. Interestingly, apomixis often occurs in the gametophyte can be produced from single spores same taxa that exhibit sexual polyspory, i.e., the (monospory) or from two or four spores (bispory and formation of embryo sacs out of more than one spore tetraspory, respectively) via mitotic divisions. (Carman 1997). Although meiosis–mixis cycles represent the typical Apomixis in angiosperms is heritable, but the elements of eukaryotic sex, some special features of genetic regulatory mechanisms are unexpectedly angiosperms need to be noted: (1) a late differentia- complex. For gametophytic apomixis, the two tion of archesporial cells within tissue of the adult components, apomeiosis and parthenogenesis, are sporophyte; (2) the potential to start developmental under different genetic control and can be uncoupled programs from both a reduced (haploid) and an (Ozias-Akins & Van Dijk 2007). In natural systems, unreduced (somatic) archespore; (3) the capacity for apomeiosis is due to temporal or spatial de-regula- parthenogenetic development in both reduced and tion of the genes controlling the sexual pathway unreduced egg cells; (4) the extreme reduction of rather than to an independent trait (e.g., Grimanelli