Life-History Differentiation and the Maintenance of Monoecy and Dioecy in Sagittaria Latifolia (Alismataceae)

Abstract

The existence of monoecious and dioecious populations within plant species is rare. This limits opportunities to investigate the ecological mechanisms responsible for the evolution and maintenance of these contrasting sexual systems. In Sagittaria latifolia, an aquatic flowering plant, monoecious and dioecious populations exist in close geographic proximity but occupy distinct wetland habitats differing in the relative importance of disturbance and competition, respectively. Life-history theory predicts contrasting evolutionary responses to these environmental conditions. We propose that the maintenance of monoecy and dioecy in S. latifolia is governed by ecological selection of divergent life-history strategies in contrasting habitats. Here we evaluate this hypothesis by comparing components of growth and reproduction between monoecious and dioecious populations under four conditions: natural populations, a uniform glasshouse environment, a common garden in which monoecious and dioecious populations and their F 1 progeny were compared, and a transplant experiment using shaded and unshaded plots in a freshwater marsh. Plants from dioecious populations were larger in size and produced heavier corms in comparison with monoecious populations. Monoecious populations flowered earlier and produced more flowers, clonal ramets, and corms than dioecious populations. The life-history differences between the sexual systems were shown to have a quantitative genetic basis, with F 1 progeny generally exhibiting intermediate trait values. Survival was highest for each sexual system in field plots that most closely resembled the habitats in which monoecious (unshaded) and dioecious (shaded) populations grow. These results demonstrate that monoecious and dioecious populations exhibit contrasting patterns of investment in traits involved with growth and reproduction. Selection for divergent life histories between monoecious and dioecious populations of S. latifolia appears to be the principal mechanism maintaining the integrity of the two sexual systems in areas of geographic overlap.

Influences of clonality on plant sexual reproduction

“…However, this prediction was rejected in S. latifolia because genetic estimates of clone size indicated the opposite pattern, with larger clones in monoecious than dioecious populations (46). Monoecious populations are adapted to temporary aquatic habitats (e.g., ditches, farm ponds), which probably explains why they have higher vegetative growth rates and corm production than dioecious populations, which are more commonly found in permanent wetlands (48). This difference in life history may explain the observed difference in clone size between the sexual systems.…”

Darwin's legacy: the forms, function and sexual diversity of flowers

“…Monoecious populations commonly occupy ephemeral aquatic habitats such as ditches and farm ponds, whereas dioecious populations are found in more stable wetland habitats associated with large lakes and extensive river systems (Dorken et al 2002). Common garden and transplant studies have demonstrated that populations of the two sexual systems are differentiated from one another, possessing a suite of life-history traits associated with adaptation to their contrasting wetland habitats (Dorken & Barrett 2003). Crossing studies indicate no barriers to inter-fertility between the sexual systems and patterns of genetic differentiation at neutral loci suggest that ecological differentiation limits extensive gene flow between the sexual systems (Dorken et al 2002).…”

Analysis of sexual phenotype and prezygotic fertility in natural populations ofFucus spiralis, F. vesiculosus(Fucaceae, Phaeophyceae) and their putative hybrids

“…Analysing variation in reproductive strategies in hybridizing taxa provides an excellent opportunity for directly studying the factors that influence the evolutionary pathway between different reproductive systems (Barrett, 1998;Charlesworth, 1999;Hewitt, 2001;Dorken et al, 2002;Dorken & Barrett, 2003). While changes in the mating system can influence the degree of reproductive isolation of the parental species, reproductive isolation may also evolve due to divergent selective pressures in different habitats (e.g.…”