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Sexual Expression and Reproductive Output in the Ephemeral Geranium Transversale Are Correlated with Environmental Conditions1

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Sexual Expression and Reproductive Output in the Ephemeral Geranium Transversale Are Correlated with Environmental Conditions1 RESEARCH ARTICLE AMERICAN JOURNAL OF BOTANY Sexual expression and reproductive output in the ephemeral Geranium transversale are correlated with environmental conditions1 Aysajan Abdusalam 2,3 , Dunyan Tan 2,5 , and Shu-Mei Chang 4,5 PREMISE OF THE STUDY: Abiotic environmental factors are often considered to be important in the distribution and maintenance of variation in sexual systems in fl owering plants. Associations between sexes and abiotic factors are well documented in dioecious systems, but much less is known about this relationship in other sexually polymorphic systems. Species that are highly variable in sexual expression and habitat distribution can provide insights into the role of abiotic factors in maintaining variation in sexual expression. METHODS: Focusing on a sexually polymorphic species, Geranium transversale , we measured sexual expression at both the fl ower and the plant level and examined vegetative and fl oral traits, pollen deposition, and reproductive success. We also tested for correlations between sexual expression and other traits and examined whether and how these traits covaried with abiotic environmental conditions. KEY RESULTS: We identifi ed unique variation of sexual expression in G . transversale . There are four sexual morphs that display diff erent combinations of the three fl ower types (pistillate, staminate, and perfect). Sexual morphs that are phenotypically more female (i.e., female and gynomonoecious morphs) are found in wetter and milder environments, and fl ower earlier than morphs that are more male (i.e., hermaphroditic and andromonoecious morphs). Addi- tionally, fl oral organ size and reproductive success are infl uenced not only by the fl ower type but also by the sexual morph of the plant. CONCLUSIONS: Environmental conditions are likely to cause some of the variation in sexual expression found in G . transversale . Both genetic and ecological factors likely contribute to the maintenance of sexual variation in this species. KEY WORDS andromonoecy; breeding-system variation; cold desert; fl oral variation; Geraniaceae; gynomonoecy. S e x u a l e x p r e s s i o n i n fl owering plants is amazingly diverse. Species (e.g., androdioecy and gynodioecy) individuals are found across range from having individuals with perfect fl owers only (hermaph- diff erent lineages ( Barrett and Hough, 2013 ; Elzinga and Varga, roditism) to having distinct individuals that produce either male/ 2017 ). Considering the direct impact that sexual expression has on staminate or female/pistillate fl owers only (dioecy). Many addi- the genetic fi tness of individuals, variation of sexual expression tional polymorphic systems with diff erent combinations of fl owers within and across populations can signifi cantly infl uence evolution- within (monoecy, andromonoecy, and gynomonoecy) or among ary processes such as adaptation and speciation (e.g., Wolfe, 1998 ; Calviño and Galetto, 2010 ; Yakimowski and Barrett, 2014 ). Hence, identifying the conditions and mechanisms through which varia- tion in sexual expression evolves and is maintained is a fundamen- 1 Manuscript received 1 July 2017; revision accepted 10 November 2017. tal question in plant evolution (see reviews by Barrett, 2002 ; 2 Xinjiang Key Laboratory of Grassland Resources and Ecology and Ministry of Education Charlesworth, 2013 ; Zemp et al., 2015 ). Key Laboratory for Western Arid Region Grassland Resources and Ecology, College of Grassland and Environment Sciences, Xinjiang Agricultural University, Ürümqi 830052, Th e evolution of dioecy from hermaphroditism is a well-recognized PR China; evolutionary transition that has been documented in >100 cases 3 Key Laboratory of Ecology and Biological Resources in Yarkand Oasis, College of Biology across distantly related taxa in angiosperms ( Bawa, 1980 ; Renner and Geography Sciences, Kashgar University, Kasghar 844006, PR China; and and Ricklefs, 1995 ; Webb, 1999 ; Weiblen et al., 2000 ; Charlesworth, 4 Department of Plant Biology, 2502 Plant Sciences Building, University of Georgia, Athens, Georgia 30602, USA 2002 ; Renner, 2014 ). Th is repeated evolutionary pattern across a 5 Authors for correspondence (e-mail: [email protected]; [email protected]) substantial number of evolutionary lineages raises the possibility https://doi.org/10.3732/ajb.1700258 that the causes or conditions favoring the selection of separate sexes 1920 • AMERICAN JOURNAL OF BOTANY 104 (12): 1920 – 1929 , 2017; http://www.amjbot.org/ © 2017 Botanical Society of America DECEMBER 2017 , VOLUME 104 • ABDUSALAM ET AL. —VARIATION IN SEXUAL EXPRESSION OF GERANIUM TRANSVERSALE • 1921 may be ubiquitous in fl owering species. A few important factors size of a population may also cause variation in sex ratios that could select for dioecy include inbreeding depression ( Charles- through genetic drift. Small populations are more likely to be worth and Charlesworth, 1978 , 1979 ; Charlesworth, 2006 ; Eppley strongly affected by random fluctuations in allele frequencies, and Pannell, 2009 ), trade-off s between allocation to male and fe- and this has been proposed to explain why smaller populations male functions when resources are limited ( Charnov et al., 1976 ; of the gynodioecious Lobelia siphilitica often have higher female Charnov, 1979 ; Givnish, 1980 ; Campbell, 2000 ; Parachnowitsch frequency ( Caruso and Case, 2007 ). and Elle, 2004 ), and sexual selection ( Willson, 1979 ; Th omson and Finally, abiotic factors can interact with endogenous characteris- Barrett, 1981 ; Moore and Pannell, 2011 ; Delph and Herlihy, 2012 ). tics, such as life-stage and plant size, to infl uence sexual expression Unfortunately, it is diffi cult to infer what factors facilitated the evo- in some plants. For example, in sequentially hermaphroditic spe- lution of separate sexes in extant dioecious species because recre- cies, smaller and younger individuals tend to produce male sexual ating the ancestral ecological or genetic conditions of a species is structures but can switch to producing either perfect or female oft en diffi cult, if not impossible. Th is problem, however, can be cir- structures when they grow larger (e.g., Givnish, 1980 ; Doust and cumvented by examining ecological and genetic conditions associ- Cavers, 1982 ; Matsui, 1995 ; Renner et al., 2007 ). A similar ated with diff erent sexual morphs found in species that exhibit correlation between plant size and sex expression or allocation to within-species variation in sexual expression ( Barrett, 1992 ; Barrett diff erent sexual functions can be found in other sexually polymor- and Hough, 2013 ; Yakimowski and Barrett, 2014 ). phic species such as gynodioecious ( Ashman et al., 2001 ), andro- Abiotic environmental conditions, such as soil moisture, soil fer- monoecious ( Wolfe, 1998 ; Liao and Zhang, 2008 ), monoecious tility, and annual temperature, are oft en correlated with the preva- ( Torices and Mendez, 2011 ), and hermaphroditic ( Peruzzi et al., lence of a particular sex in plant populations and are likely to be 2012 ; Lankinen and Green, 2015 ) species. Th e eff ect of abiotic fac- important factors involved in the evolution of separate sexes (e.g., tors on plant size could also infl uence sex expression ( Delesalle, Maurice, 1999 ; Sakai and Weller, 1999 ; Case and Barrett, 2004 ). For 1989 ; Zhang et al., 2014 ). For example, larger plants may have ac- example, in dioecious species, male individuals are more oft en cess to more available resources (both above and below ground) found in lower-quality sites, compared to female individuals, which and tend to have larger fl owers, which could aff ect the number of likely refl ects the outcome of selection acting on the relative re- pollen grains received (e.g., Philipp and Hansen, 2000 ). Th is can, in source demands of pollen production vs. seed production ( Meagher, turn, indirectly shift resource allocation to seed production rather 1984 ; Th omas and LaFrankie, 1993 ; Korpelainen, 1998 ; Queenborough than to pollen function ( Dudash, 1991 ; Wolfe and Shmida, 1997 ). et al., 2007 ; Peruzzi et al., 2012 ). By contrast, in gynodioecious To better understand how environmental conditions may infl u- species that contain both female and hermaphroditic individuals, ence sexual expression in wild populations, we investigated sex ex- populations with females are oft en found in hotter and dryer envi- pression and sex ratios in natural populations of a wild geranium ronments than populations that contain only hermaphrodites species, Geranium transversale , which grows in stony slopes, sandy ( Sakai and Weller, 1999 ; Case and Barrett, 2004 ; Bishop et al., 2010 ; deserts, and sandy steppes in central Asia. Geranium transversale ex- Barrett and Hough, 2013 ; Ruff atto et al., 2015 ). One hypothesis to hibits variation in both within-fl ower sexual expression and within- explain this pattern is that, when faced with limited resources, her- plant fl ower-type combinations. To distinguish between variation at maphrodites may emphasize pollen production, allowing seed pro- these two levels, we refer to sex expression of individual fl owers as duction in females to surpass that of hermaphrodites and thus one of three “fl ower types”—perfect, pistillate, and staminate—and enabling the establishment of females in the population ( Delph, refer to sex expression of individual plants as one of four “sexual 1990 , 2003 ; Bishop et al., 2010 ). Th ough the associations of sex ra- morphs” that refl ect the combinations of fl owers within
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