Heritability of stigma position and the effect of stigma‐anther separation on outcrossing in a predominantly self‐fertilizing weed, Datura stramonium (Solanaceae)

Abstract

A polymorphism for anthocyanin production was used as a genetic marker to document the relationship between anther-stigma separation and outcrossing rate in the predominantly self-fertilizing weed Datura stramonium. White-flowered plants that differed in anther-stigma separation were placed into populations consisting exclusively of purple-flowered plants. Self vs. outcross origin of progeny was evident in the hypocotyl color of the seedlings. Outcrossing rates measured for single flowers were significantly positively correlated with anther-stigma separation, albeit with some scatter around the regression line, especially for flowers with exserted stigmas. We also performed an 8 × 8 diallel cross to determine whether anther-stigma separation is genetically determined. Heritability in two field plots was ∼0.3 and in the greenhouse was ∼0.2. Maternal effects, epistasis, and dominance appeared to be relatively unimportant. Genotypes performed consistently across the three environments, although total plant size varied more than fivefold. It appears that the mixed-mating system of D. stramonium has a heritable basis and would be capable of responding to selection.

Anther-Stigma Separation Is Associated With Inbreeding Depression in Datura Stramonium, a Predominantly Self-Fertilizing Annual

“…In addition, for D. stramonium, anther-stigma distance is not susceptible to inbreeding depression. Genetic variance in anther-stigma separation is characterized by additive effects (Motten and Stone 2000).…”

“…Population-level estimates of outcrossing rate range from 0% to about 10% (Motten and Antonovics 1992). Anther-stigma separation has a narrow-sense heritability of about 0.3 in a range of environments, and maternal effects and epistasis appear to be unimportant (Motten and Stone 2000). Variation in anther-stigma separation attributable to dominance variance is only exhibited in the richest environments, with exserted stigmas being recessive (Motten and Stone 2000).…”

“…Flowers having the stigma at or below the level of the anthers typically produce only selfed seed. For flowers having the stigma exserted beyond the level of the anthers, outcrossing increases in proportion to the degree of exsertion (r 2 ϭ 0.32), with a maximum of about 30-50% outcrossing achieved very infrequently (Motten and Stone 2000). Population-level estimates of outcrossing rate range from 0% to about 10% (Motten and Antonovics 1992).…”

“…Differences among families in levels of inbreeding depression can only be interpreted as evidence for lineage-specific inbreeding depression if levels of inbreeding depression are correlated with variation in the mating system. Numerous studies have reported significant levels of additive genetic variance in traits such as antherstigma separation that influence outcrossing rates (e.g., Breese 1959;Schoen 1982;Holtsford and Ellstrand 1992;Fenster and Ritland 1994;Chang and Rausher 1998;Motten and Stone 2000). In spite of the ubiquity of heritable variation in traits influencing the mating system, lineage-specific inbreeding depression has rarely been observed, even when directly sought (Byers and Waller 1999).…”

“…Local populations of Datura stramonium have population-level outcrossing rates that are typically less than 10% (Motten and Antonovics 1992), permitting the development of genetically isolated lineages. Another important attribute of this species as a study organism is that outcrossing rate is correlated with the degree of anther-stigma separation, which shows heritable variation in the study populations (Motten and Stone 2000). We measured inbreeding depression in lifetime female fitness for field-grown plants from 35 families showing a range of anther-stigma separation.…”