Adult survival is considered to be a major factor determining the evolution of reproductive schedules. High adult mortality has been implicated in the evolution of early and condensed reproductive schedules in a number of species, including vertebrates, insects and plants (Harvey & Zammuto 1985; Franco & Silvertown 1996; Reznick et al. 1996; Dudycha & Tessier 1999; Stearns et al. 2000; Lesica & Young 2005). Resource trade-offs between adult survival and reproduction may exist, such that prior reproduction compromises subsequent survival and reproduction, and they have been the focus of many studies of the evolution of reproductive strategies (Williams 1966; Schaffer & Rosenzweig 1977; Reznick 1985; Obeso 2002). Functional trade-offs at the adult stage may also be important determinants of reproductive timing (Price & Schluter 1991; Endler 1995), as demonstrated in sticklebacks, in which high fecundity during a single reproductive bout compromised swimming performance, thereby favouring smaller, more frequent reproductive bouts (Foster, Baker & Bell 1992; Ghalambor, Reznick & Walker 2004).
However, selection also occurs at juvenile stages. As such selection necessarily occurs before selection at adult stages, and as organisms must survive early stages to express adaptive traits at later stages, adaptation of juvenile stages is hypothesized to be especially important in certain contexts, especially during colonization, range expansion or changing environments (Schluter, Price & Rowe 1991; Stratton 1992; Maun 1994; Miriti 2006; Poorter 2007; Donohue et al. 2010; Huang et al. 2010). If certain physiological or developmental traits influence both early survival and adult reproduction, natural selection acting on those traits at the juvenile stage would also affect the evolutionary dynamics of adult reproductive strategies (Lande 1982; Partridge et al. 1991). Thus, the developmental context of life-history variation must be evaluated to predict how selection across the entire life cycle will influence life-history evolution.
Semelparity is a widespread reproductive strategy in both animal and plant kingdoms. Semelparous (monocarpic) organisms have a single reproductive episode followed by the death of the organism, while iteroparous (polycarpic) organisms reproduce repeatedly throughout their lifetime (Young & Augspurger 1991; Roff 1992; Stearns 1992). A major hypothesis regarding the evolution of semelparity is that, when adult mortality is high compared with juvenile mortality, a semelparous genotype with a higher reproductive output would have greater fitness than an iteroparous genotype with a lower reproductive output per reproductive episode (Schaffer & Rosenzweig 1977; Young 1981; Orzack & Tuljapurkar 1989).
Interestingly, variation of semelparity/iteroparity in plant species is often associated with their architecture. For instance, many long-lived semelparous plant taxa form a single apical rosette, while closely related iteroparous taxa produce multiple rosettes and/or branches from the apical rosette (Young & Augspurger 1991). In iteroparous species, only a subset of rosettes contributes to a particular reproductive episode by developing determinate inflorescences; nonreproductive rosettes remain vegetative during reproduction and are able to produce inflorescences in future growing seasons. In contrast, semelparous plants frequently produce indeterminate inflorescences from their single apical rosette, and vegetative tissues of the apical rosette degenerate during reproduction. Notably, rosettes that are necessary for iteroparity develop at the juvenile stage. Therefore, at least for some rosette plant species, the production of additional vegetative rosettes at the juvenile stage is likely to be a developmental prerequisite for an iteroparous strategy as an adult (Silvertown 1989).
In such species, natural selection on rosette production at the juvenile stage has the potential to influence the evolution of parity expressed at the adult stage. A more general question concerning the evolution of semelparity/iteroparity is whether the morphological traits required for iteroparous reproduction are subjected to natural selection at earlier life stages. If juvenile traits are prerequisites for adult reproductive strategies, then selection on those traits at the prereproductive stage may influence the evolution of adult life histories.
Western wallflowers (Erysimum capitatum, Brassicaceae) in the Colorado Rocky Mountains exhibit altitudinal variation in iteroparity (Price 1987), and such variation is manifest in a common greenhouse environment (Table 1). Erysimum capitatum, especially those from high-elevation populations, produce multiple rosettes at the axils of leaves on the apical rosette (i.e. axillary rosettes; Fig. 1). Notably, in both field and greenhouse environments, plants from low-elevation semelparous populations produced significantly fewer rosettes at the prereproductive stage than plants from high-elevation iteroparous environments (Kim & Donohue 2011a). In addition, plants with more rosettes at the prereproductive stage had more vegetative rosettes after reproduction, which in turn positively influenced survival after reproduction and the opportunity for future reproductive episodes (Kim & Donohue 2011b). Thus, in E. capitatum, production of multiple rosettes at the juvenile stage is a morphological prerequisite for iteroparity.
|Region||Population||Elevation (m a.s.l.)||Life history||Post-reproductive survival|
|1 (Gunnison county, CO)||H1||3191||Iteroparous||86·9 (2·4)||84·1 (5·5)|
|L1||2630||Intermediate||17·0 (9·4)||63·4 (7·5)|
|2 (Clear Creek county, CO)||H2||3636||Iteroparous||71·8 (11·7)||90·0 (4·7)|
|L2||2234||Semelparous||0·0 (0·0)||53·3 (7·4)|
|3 (Boulder county, CO)||H3||3505||Intermediate||20·1 (9·4)||76·3 (6·9)|
|L3||1831||Semelparous||3·5 (3·5)||51·2 (7·8)|
Not only are populations of E. capitatum differentiated across altitude in rosette production and iteroparity, but plants from low elevation have higher fitness at low elevation than plants from high elevation (E. Kim, unpublished data). When plants from high-elevation populations were transplanted into low-elevation sites, they produced more rosettes and suffered higher mortality during the summer than those from low-elevation populations. Water availability at the low-elevation site was extremely low during the summer (Kim & Donohue 2011a), suggesting drought may have imposed selection on rosette production, resulting in lower survival of high-elevation populations at low-elevation sites. Here, we investigated selection on rosette production at the juvenile stage in the western wallflower under different conditions of water availability.
First, we tested whether natural variation in rosette production was associated with juvenile survival under low-water conditions (drought stress) in the greenhouse. We then manipulated the number of rosettes by physically removing rosettes to examine whether rosette number influenced survival under drought conditions. Specifically, the following questions were addressed: (i) Do populations from different altitudes show differential survivorship in drought conditions in the greenhouse? (ii) Is the production of axillary rosettes associated with survivorship under drought stress? (iii) Does the removal of axillary rosettes improve survivorship under drought stress? Given that plants living in drought conditions have fewer rosettes (and are more semelparous) than those living in nondrought conditions, we predicted that plants with fewer rosettes would have higher juvenile survival under drought stress, but not under conditions of abundant water.