Floral Morphometrics and the Evolution of Sexual Dimorphism in Lycium (Solanaceae)

Abstract

Plants of Lycium californicum, L. exsertum, and L. fremontii produce flowers that are either male-sterile (female) or hermaphroditic, and populations are morphologically gynodioecious. As is commonly found in gynodioecious species, flowers on female plants are smaller than those on hermaphrodites for a number of floral traits. Floral size dimorphism has often been hypothesized to be the result of either a reduction in female flower size that allows reallocation to greater fruit and seed production, or an increase in hermaphroditic flower size due to the increased importance of pollinator attraction and pollen export for hermaphroditic flowers. We provide a test of these two alternatives by measuring 11 floral characters in eight species of Lycium and using a phylogeny to reconstruct the floral size shifts associated with the evolution of gender dimorphism. Our analyses suggest that female flowers are reduced in size relative to the ancestral condition, whereas flowers on hermaphrodites have changed only slightly in size. Female and hermaphroditic flowers have also diverged both from one another and from ancestral cosexual species in several shape characteristics. We expected sexual dimorphism to be similar among the three dimorphic taxa, as gender dimorphism evolved only a single time in the ancestor of the American dimorphic lineage. While the floral sexual dimorphism is broadly similar among the three dimorphic species, there are some species-specific differences. For example, L. exsertum has the greatest floral size dimorphism, whereas L. fremontii had the greatest size-independent dimorphism in pistil characters. To determine the degree to which phylogenetic uncertainty affected reconstruction of ancestral character states, we performed a sensitivity analysis by reconstructing ancestral character states on alternative topologies. We argue that investigations such as this one, that examine floral evolution from an explicitly phylogenetic perspective, provide new insights into the study of the evolution of floral sexual dimorphism.

Association of Ploidy and Sexual System in Lycium Californicum (Solanaceae)

“…Relatively few empirical studies have focused on polyploidy as a possible cause for transition to a dimorphic sexual system. This trigger of gender dimorphism was proposed in studies of North American Lycium (Miller andVenable 2000, 2002;Miller 2002). Of the approximately eighteen North American species, three (L. exsertum, L. fremontii, and L. californicum) are polyploid (Lewis 1961;Chiang-Cabrera 1981), gender dimorphic (Miller and Venable 2000), and selfcompatible (SC; Miller and Venable 2002).…”

Constraints on the Evolution of Males and Sexual Dimorphism: Field Estimates of Genetic Architecture of Reproductive Traits in Three Populations of Gynodioecious Fragaria Virginiana

“…Meagher (1999) assumed the G matrix was constant and used a retrospective simulation to determine how strong selection would have had to be to generate the degree of dimorphism seen in dioecious Silene latifolia. Miller and Venable (2003), in contrast, took a phylogenetic approach when comparing flower size of gynodioecious and hermaphrodite Lycium species. Interestingly, both concluded that dimorphism was the result of evolutionary reduction in flower size in the smaller-flowered morph (males in Silene, Meagher 1999; females in Lycium, Miller and Venable 2003).…”

“…Miller and Venable (2003), in contrast, took a phylogenetic approach when comparing flower size of gynodioecious and hermaphrodite Lycium species. Interestingly, both concluded that dimorphism was the result of evolutionary reduction in flower size in the smaller-flowered morph (males in Silene, Meagher 1999; females in Lycium, Miller and Venable 2003). However, neither of these approaches can determine whether the sex morphs are currently at their optimal flower size-this requires information on the selective environment.…”

Mapping Uncertainty and Phylogenetic Uncertainty in Ancestral Character State Reconstruction: An Example in the Moss Genus Brachytheciastrum

“…Phylogenies are, however, rarely known without error. Different estimates of the phylogenetic tree can return different answers to the comparative question (Miller and Venable, 2003;Ober, 2003;Thompson and Oldroyd, 2004), causing a second source of uncertainty known as 'phylogenetic uncertainty' (Pagel and Lutzoni, 2002;Ronquist, 2004;. In this context, Markov chain Monte Carlo methods offer a formal framework to sample phylogenies according to their posterior probabilities.…”