Selection on reproductive characters: floral morphology in Asclepias syriaca

Abstract

Pollinia insertions and removals represent a convenient index of male and female fertilities in the Asclepiadaceae. This fact, in conjunction with regression analysis, allows estimation and comparison of gender-specific selection acting on floral characters in Asciepias syriaca. Univariate regression analysis indicates statistically significant selection on minute aspects of floral morphology through the male and especially female reproductive pathways. Analyses using multiple regression to correct for overall plant size do not qualitatively change the univariate results. The relatively large number of statistically significant selection gradients suggests active adaptation of floral morphological features to the pollinator fauna. Selection through female function differs significantly from selection on the same trait through male function for four traits, emphasizing an antagonism between genders in hermaphrodites. This antagonism and the absence of selection of the antagonistic traits through net fertility are consistent with an equilibrium quantitative genetic perspective, while greater opportunity for selection through female function is counter to expectations based on Bateman's principle of sexual selection.

Variation in resource limitation of plant reproduction influences natural selection on floral traits of Asclepias syriaca

“…Floral landmarks used to measure corpusculum length and stigmatic slit width are not shown. All floral landmarks follow Morgan and Schoen (1997) USA (41°42.3¢N, 92°47.2¢W). Because A. syriaca can be highly clonal (Wyatt and Broyles 1994), we systematically sampled plants throughout the population to maximize the probability that individuals included in the study were independent of each other.…”

“…1 for floral landmarks used for these measurements). These traits were chosen because they are under significant selection in other populations of A. syriaca (Morgan and Schoen 1997). We averaged the five measurements of hood, horn, and stigmatic slit characters for each flower.…”

“…Only ten flowers were collected if the inflorescence contained <50 flowers. All flowers were preserved in jars that contained a sponge soaked in 25 ml of a 1:3 solution of acetic acid and ethanol (as in Morgan and Schoen 1997). Because A. syriaca flowers are small, all floral morphology traits were measured using a digital imaging system (NIH Image for the Macintosh and a Leica MZ8 microscope w/digital camera).…”

“…Unlike most plant species, members of the Asclepiadaceae such as A. syriaca package their pollen grains into discrete packets known as pollinia (Woodson 1954) whose removal can be used as a proxy for male fitness (Broyles and Wyatt 1990). In addition, significant selection on floral traits of A. syriaca via both male and female fitness has been documented (Morgan and Schoen 1997). Finally, the reproduction of Asclepias spp.…”

Intra-locus sexual conflict and sexually antagonistic genetic variation in hermaphroditic animals

“…For example, Morgan & Schoen [27] found opposing selection pressures via male and female sex function on the same trait for four traits (corolla pigment, hood width, horn length and slit length) in the hermaphroditic common milkweed plant, Asclepias syriaca. Although concordant selection pressures on floral traits via each sex function seem to be common in plants [28], other examples of antagonistic selection pressures via male and female fitness function have also been found [29,30].…”