Reproductive Biology of Asclepias Exaltata

Shannon, T. Randolph; Wyatt, Robert

doi:10.1002/j.1537-2197.1986.tb09675.x

Cited by 28 publications

(18 citation statements)

References 33 publications

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“…Opposite double insertions, however, are more common than adjacent double insertions in A. syriaca (Sparrow andPearson, 1948) andin A. tuberosa (Wyatt, 1976). Furthermore, Shannon (1984) observed that only 5% and 18% of the flowers in a population of A. exaltata studied in two different years contained two or more inserted pollinia. On the other hand, Lynch (1977) reported that 70% ofthe flowers ofA.…”

Section: Discussionmentioning

confidence: 99%

Paternity Analysis in a Natural Population of Asclepias exaltata: Multiple Paternity, Functional Gender, and the "Pollen-Donation Hypothesis"

Broyles¹,

Wyatt²

1990

Evolution

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175

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Recently, some evolutionary biologists have argued that selection on the male component of fitness shapes the evolution of reproductive characters in angiosperms. Floral features, such as inflorescence size, that lead to increased insect visitation without a concomitant increase in seed production are viewed as adaptations to enhance the probability offathering seeds on other plants. In tests of this "pollen donation hypothesis," male reproductive success has usually been measured indirectly by flower production, pollinator visitation, or pollen removal. We tested the pollen donation hypothesis directly by quantifying the number of seeds sired by individual genotypes in a natural population of poke milkweed, Asclepias exaltata, in southwestern Virginia. Multiple paternity was low within fruits, a fact which allowed us to use genotypes ofprogeny arrays to identify a unique pollen parent for 85% of the fruits produced in the population. Seeds sired (male success)and seeds produced (female success)were significantly correlated with flower number per plant (for male success, r = 0.32, P < 0.05; for female success, r = 0.66, P < 0.001). While the number of pollinaria removed, the usual estimator of male success in milkweeds, was highly correlated with numbers of seeds sired (r = 0.47; P < 0.001), it was even more highly correlated with numbers of seeds produced (r = 0.71, P < 0.001). Analysis of functional gender indicated that plants with many flowers did not behave primarily as males. In fact, individuals with the highest total reproductive success contributed equally as males and females. Furthermore, estimates of gender based on numbers of flowers produced or pollinaria removed overestimated the number of functional males in the population. In pollen-limited species, such as many milkweeds, proportional increases in both male and female reproductive success indicate the potential for selection to shape the evolution of large floral displays through both male and female functions.

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Section: Discussionmentioning

confidence: 99%

Paternity Analysis in a Natural Population of Asclepias exaltata: Multiple Paternity, Functional Gender, and the "Pollen-Donation Hypothesis"

Broyles¹,

Wyatt²

1990

Evolution

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138

175

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“…By focusing on the total RS of individuals that vary in size of total floral display, Wyatt, Broyles, and others concluded that selection on inflorescence size through male and female function is equally important (Lynch 1977;Wyatt 1980;Shannon and Wyatt 1986;Broyles and Wyatt 1990a). By emphasizing per flower or per umbel RS of individuals varying in umbel size, other investigators have concluded that the importance of selection on umbel size is greater through male than female function (Willson and Rathcke 1974;Chaplin and Walker 1982;Queller 1983Queller , 1985Bell 1985;Broyles and Wyatt 1995).…”

Section: The Importance Of Male Function and The Evolution Of "Inflormentioning

confidence: 99%

Evolution of Inflorescence Design: Theory and Data

Fishbein

Venable

1996

Evolution

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Abstract.-Very low fruit set in milkweeds and other flowering plants often has been attributed to greater sexual selection on inflorescence size via male, rather than female, reproductive success. Although this explanation has been generally accepted, alternate explanations have been presented, and recently the "male function" or "pollen donation" hypothesis has been sharply criticized. In this paper, we make the distinction between selection on total flower number and on the size of inflorescence units, both of which have been termed "inflorescence size." We present an ESS model for the evolution of inflorescence design that considers reproductive success through male and female function. The model predicts that selection will balance the proportional changes in female and male reproductive success resulting from changes in inflorescence-unit size. We conducted a field study of selection on the size of inflorescence units (umbels) by manipulating umbel size and number in a natural population of Asclepias tuberosa, in southeastern Arizona, during two reproductive seasons. We found that the male fitness function reached a maximum at an intermediate umbel size in both years (although not significantly different from the smallest umbel size in either year), whereas the female fitness function was highest for the smallest umbel size in one year, but was constant across umbel sizes in the other year. We also found that pollinator visitation rate corresponded well with male, but not female, function, and that between-year variation in the male reproductive success of different umbel sizes corresponded with variation in the composition of the pollinator pool. Our empirical results, when inserted in the model, predict ESS umbel sizes similar to those observed in the study population and the species throughout its range.

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“…Like all species of Asclepias, A. exaltata is diploid, with n ϭ 11 (Woodson, 1954). Mature plants typically produce one to three stems, each of which typically bears one to six umbels of 10 to 25 flowers (Shannon and Wyatt, 1986). Pollen in milkweeds is produced in discrete sacs termed ''pollinia.''…”

Section: Methodsmentioning

confidence: 99%