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For more than two centuries, the diversity of plant sexual systems has captured the curiosity and imagination of biologists and stimulated fruitful research by both ecologists and population geneticists (Cheptou & Schoen, 2007). In angiosperms, only 6% of species are dioecious, but their scattered taxonomic positions within the group suggest that shifts in sexual systems, mostly towards dioecy from monoecious ancestors, occurred independently multiple times (Renner & Ricklefs, 1995). The evolutionary transition from sexual monomorphism to dimorphism is governed by three factors, which are the optimal allocation of resources to female and male function, the genetic control of sex expression, and the fitness consequences of selfing and outcrossing (Barrett, 2002).
Dioecious species may benefit from reduced inbreeding depression, but, as only females produce seeds, dioecious females must produce more or better seeds than their hermaphrodite or monecious counterparts and/or disperse those seeds to an equivalent number of suitable sites to ensure an equivalent number of progeny (Charlesworth & Charlesworth, 1978). In fact, theoretical work suggests that, in order for dioecy to persist, dioecious individuals need a substantial fitness and/or dispersal advantage over cosexuals, even when the number of seeds per female is twice that of cosexuals (Heilbuth et al., 2001).
In bryophytes, where sexual reproduction depends on sperm swimming to eggs via free water, separation of sexes can present serious problems for successful sexual fusion and dioecious species therefore often fail to reproduce sexually (Vanderpoorten & Goffinet, 2009). In a study on the reproductive biology of British mosses, Longton (1997) found that 87% of the species for which sporophytes are unknown are dioecious, whereas sporophytes are commonly found in 83% of the monoecious species. Despite this, c. 70% of liverworts and 60% of mosses are dioecious (Vanderpoorten & Goffinet, 2009). Along with the frequent independent evolution of dioecy among angiosperm lineages and other major groups of land plants (Renner & Ricklefs, 1995; Sakai et al., 1995), this suggests that separation of sexes is not necessarily associated with increased extinction risks. In fact, the uncertainty of sexual reproduction in dioecious bryophyte species is thought to have selected for a series of life-history traits that promote dispersal (Wilson & Harder, 2003; During, 2007). In particular, a strong association between dioecy and the ability to produce specialized asexual propagules has long been known to exist (Longton & Schuster, 1983; Hedderson & Longton, 1995; During, 2007). Crawford et al. (2009), however, failed to find support for a phylogenetic correlation between the production of specialized propagules and separate sexes in a recent comparative study across mosses. That study revealed that organisms with separate sexes tend to be significantly larger than monoecious ones and characterized by strong, competitive vegetative growth. This correlation, also observed in angiosperms (de Jong, 2000), is consistent with the hypothesis that dioecious taxa compensate for the low production of spores or seeds by increased size and life expectancy.
These trade-offs between sexual systems and other life-history traits have led to the concept of life-history strategies, which can be defined as a recurrent combination of life-history traits that are predicted to occur in response to particular ecological conditions (During, 1992). For example, given the presumed ephemeral nature of their substrates, and the importance of dispersal and colonization (Snäll et al., 2005), epiphytes are typically hermaphroditic (van Dulmen, 2001) and expected to undergo substantial inbreeding to ensure reproductive success (Hooper & Haufler, 1997; Chiou et al., 2002). In fact, a survey of 15 vascular montane cloud forest epiphytes in Costa Rica showed that all were self-compatible, which represents ‘the highest proportion of self-compatible species documented within any life form at any tropical site’ (Bush & Beach, 1995).
A number of studies have employed phylogenies to investigate evolutionary transitions in plant sexual systems (see Case et al., 2008 for a review). The aims of those studies were primarily to estimate the number of times particular traits arose, to identify their life-history trait correlates, and to determine the local direction of changes. In practice, however, mapping traits onto trees is not necessarily straightforward, particularly for those with a high degree of lability that erodes phylogenetic signal (Weller & Sakai, 1999). Maximum parsimony has been the most widely used principle for inferring character-state transformations from a phylogenetic tree (Smith, 2010). Under the parsimony criterion, which implicitly involves transition rates being equal among states, the solution requiring the minimum amount of changes on a tree is singled out, regardless of branch lengths. A range of alternative reconstructions on the same tree, however, exists, creating a source of error known as ‘within-tree uncertainty’ (Pagel et al., 2004). Model-based methods, by contrast, directly use branch lengths to determine the probability of change along a branch, allow transition rates between states to vary, and enable an assessment of the accuracy of reconstructions. Because they implement actual transition rates, model-based methods further enable one to make and test hypotheses regarding evolutionary processes (Pagel, 1994). Looking across multiple shifts, it is, in particular, possible to test for significant correlations among traits and determine the contingency and order of acquisition of those traits independently from the ancestral state reconstructions themselves (Pagel, 1994). Most recently, Markov chain Monte Carlo methods have offered a formal framework to sample model parameters and phylogenies according to their posterior probabilities, thereby taking both within-tree and among-tree uncertainty into account and increasing the statistical power as compared with single-tree approaches (Pagel & Lutzoni, 2002).
In the present study, we take advantage of the leafy liverwort genus Radula to investigate the evolution of sexual systems and explore trade-offs in life-history traits involved in those transitions. The genus includes c. 200 species (Yamada, 2003) and has an almost world-wide distribution, with the majority of the species occurring in humid, tropical or warm-temperate regions. The species usually grow as epiphytes on bark or living leaves, and less commonly on rock, in a variety of habitats varying from rainforest to alpine tundra, and from sea level to over 4000 m in elevation. Phylogenetically, Radula has a rather isolated position within the leafy liverworts and is classified in its own family and order (Crandall-Stotler et al., 2009). Morphologically, the genus stands out by virtue of its unique, Radula-type branching system, rhizoids being produced on leaf lobules rather than on stems, the total absence of underleaves, the possession of very large, often single, brown oil bodies in leaf cells, and the development of a tubular or trumpet-like, dorsiventrally flattened perianth which surrounds and protects the developing sporophyte. The genus exhibits a wide range of reproductive characteristics in terms of sexual condition and production of asexual propagules. The genus has both obligate and facultative epiphytes and therefore allows the unique opportunity to investigate possible correlations between this particular trait and sexual systems.
Using a molecular species-level phylogeny of the genus, we studied the evolution of its sexual systems in relation to four life-history traits (reproductive effort, asexual reproduction, gametophyte size and facultative vs obligate epiphytism), implementing competing explicit models of trait evolution (Barker et al., 2007). We then tested phylogenetic correlations among those traits and addressed the following questions. What is the ancestral sexual system in Radula? Are shifts from one sexual system to another random or directional, thereby suggesting their adaptive potential? To what extent are shifts in sexual system correlated with shifts from a facultative to an obligate epiphytic condition; that is, is monoecy favoured in strictly epiphytic lineages in response to the necessity to disperse in a dynamic environment? Do dioecious species compensate for the reduced levels of sporophyte production by increasing life-span and the production of asexual gemmae?