Floral traits influence the opportunity for selection among male gametophytes: independent and combined effects of style length and petal area

Abstract

Premise Strong correlations between traits can obscure their independent effects on components of reproduction. Style length (SL) and petal area (PA) vary within species, for example, but their independent effects on the opportunity for selection among pollen genotypes are poorly understood. Previous work in Clarkia detected a positive effect of SL on pollen receipt, potentially intensifying selection. However, this apparent effect of SL may be influenced by a correlated trait, such as PA. Here, we examine the independent effects of these two traits on pollen receipt and performance. Methods We collected petals and styles from wild populations of two insect‐pollinated Clarkia taxa and estimated the independent and combined effects of SL and PA on pollen receipt and performance. Results In both taxa, SL and PA are positively correlated. In C. unguiculata, both traits positively and independently affect pollen receipt, but in C. xantiana ssp. xantiana, the two traits act only in combination to affect pollen receipt. In both taxa, pollen receipt positively affects the numbers of pollen tubes entering and penetrating the style, as well as pollen tube attrition. Conclusions The effects of SL and PA on pollen receipt and performance are taxon specific. In C. unguiculata, both traits may be independent targets of selection due to their effects on pollen receipt. In C. xantiana ssp. xantiana, by contrast, the combined (but not independent) effects of SL and PA influence pollen receipt. Ecological differences between these taxa require exploration to understand the mechanisms by which these traits affect pollinator behavior.

Sex‐specific floral attraction traits in a sequentially hermaphroditic species

“…By contrast, in the current study, individual flowers were removed from intact, greenhouse‐grown plants and then displayed to pollinators in the field, with stigmas harvested after two hours of exposure to pollinators. The difference between the Mazer et al () study and the field experiment described here with respect to the effect of petal area on pollen receipt may be due in part to this methodological difference. The amount of pollen received during the first two hours of a stigma's exposure to pollinators may not be strongly correlated with the total amount of pollen received by receptive stigmas.…”

“…A previous study (Mazer, Chellew, & Peach, ) revealed that field‐collected, senesced stigmas of C. unguiculata sampled from individuals with relatively large petals receive significantly more pollen than those sampled from small‐petaled individuals in some, but not all, populations. The stigmas sampled by Mazer et al () were harvested from fully intact individual plants after the flowers had senesced; receptive stigmas were therefore exposed to pollinators for more than 24 hr. By contrast, in the current study, individual flowers were removed from intact, greenhouse‐grown plants and then displayed to pollinators in the field, with stigmas harvested after two hours of exposure to pollinators.…”

“…Alternatively, the difference between the two studies may reflect geographic variation in pollinator preference or the strength of pollinator-mediated selection (Gómez, Perfectti, Bosch, & Camacho, 2009). In addition, petal area in C. unguiculata may be positively correlated with one or more other floral traits not measured by Mazer et al (2019), but which may have promoted pollen receipt. Glaettli and Barrett (2008), Dudash Hassler, Stevens, & Fenster, (2011), and Eckhart (1991 similarly found that in Sagittaria latifolia, Silene virginica, and Phacelia linearis, respectively, flower/display size is positively correlated with pollinator visitation, but these studies did not examine the independent effects of additional components of floral color and pattern (that may be correlated with petal area) on pollen receipt or female reproduction.…”

Polyploidy‐induced floral changes lead to unexpected pollinator behavior in Arabidopsis arenosa

“…Also, we did not test for different foraging behavior between visitors as we did not identify pollinator species in detail, and it could be that specific visitor species were more attracted to tetraploids than others. Alternatively, the size of floral organs like the style has been shown to influence the efficiency of pollen transfer from the pollinator body to the stigma (Mazer et al 2019), and it could be that the larger floral organs (including the pistil) of synthetic polyploids improved the pollen transfer and, consequently, increased fruit set. More generally, we studied visitors and not actual pollinators, and differences in fruit set between cytotypes could be due to cytotype‐specific foraging behaviors/pollen transfer efficiency of certain insect visitors.…”