Abstract
Variation in plant reproductive success is affected by ecological conditions including the proximity of potential mates. We address the hypothesis that spatial distribution of sexes affects female reproductive success (RS) in the dioecious cycad, Zamia portoricensis. Are the frequencies of males, operational sex ratios, and distances to the nearest mate associated with RS in females? We studied the spatial distribution of sexes in two populations in Puerto Rico and compared RS of target females with the number of males and operational sex ratios. Population structure suggests regular successful recruitment. Adults, males, and females were randomly distributed with respect to one another. Reproductive success of females was highly variable, but was higher in neighborhoods with more males than females and generally decreased with increasing distance to the nearest male, becoming statistically significant beyond 190 cm. This possible mate-finding Allee effect indicates that pollinator movement among plants may be limited for this mutually dependent plant-pollinator interaction. Yet being close to male plants is a matter of chance, perhaps a factor generating the high intra-population genetic diversity in Z. portoricensis.
Highlights
Sexual systems, mating systems, and pollination systems influence the dispersal of alleles and play a major role in the evolution of plants (Barrett, 2003; Barrett, 2013; Devaux, Lepers & Porcher, 2014)
Biased operational sex ratios are common in populations of dioecious species despite theoretical expectations of an equilibrium 1:1 rato (Field, Pickup & Barrett, 2012), and the populations of Z. portoricensis we studied are no exception
Proches & Johnson (2009), in their study of Stangeria eriopus pollination, found no effect of distance to the nearest male on fecundity (r2 = 0.043, df = 61, P = 0.114), whereas we found that seed set in Z. portoricensis is significantly associated with the proximity to males, revealing mate-finding Allee effects when distances to males reached 1.9 m and beyond
Summary
Sexual systems (e.g., dioecy or monoecy), mating systems (e.g., selfing or outcrossing), and pollination systems (e.g., generalist or specialist vector, reward or deception mediated pollination) influence the dispersal of alleles and play a major role in the evolution of plants (Barrett, 2003; Barrett, 2013; Devaux, Lepers & Porcher, 2014). Obligate outcrossing species (e.g., dioecious species) are affected by access to mates, which is influenced by the pollination system and the spatial distribution of individuals (Barrett & Thomson, 1982; Heilbuth, Ilves & Otto, 2001; Ghazoul, 2005) resulting in limitations to the reproductive success that may have constrained the spread of dioecy among plants (Barrett, 2013). These are mate-finding Allee effects, which can be most prevalent in dioecious species where both population size and sex ratios can affect reproductive success (Cabral & Schurr, 2010; Xia et al, 2013).
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