The measure and mismeasure of reciprocity in heterostylous flowers

Abstract

SummaryThe goal of biological measurement is to capture underlying biological phenomena in numerical form. The reciprocity index applied to heterostylous flowers is meant to measure the degree of correspondence between fertile parts of opposite sex on complementary (inter-compatible) morphs, reflecting the correspondence of locations of pollen placement on, and stigma contact with, pollinators. Pollen of typical heterostylous flowers can achieve unimpeded fertilization only on opposite-morph flowers. Thus, the implicit goal of this measurement is to assess the likelihood of 'legitimate' pollinations between compatible morphs, and hence reproductive fitness.Previous reciprocity metrics fall short of this goal on both empirical and theoretical grounds. We propose a new measure of reciprocity based on theory that relates floral morphology to reproductive fitness. This method establishes a scale based on adaptive inaccuracy, a measure of the fitness cost of the deviation of phenotypes in a population from the optimal phenotype. Inaccuracy allows the estimation of independent contributions of maladaptive bias (mean departure from optimum) and imprecision (within-population variance) to the phenotypic mismatch (inaccuracy) of heterostylous morphs on a common scale.We illustrate this measure using data from three species of Primula (Primulaceae).

The evolvability of herkogamy: Quantifying the evolutionary potential of a composite trait

“…Carvallo and Medel (2010) reported selection of similar strength on M. luteus in the absence of pollinators (Table 3). The difference between our two measures of evolvability illustrates the consequences of the choice of measurement scale and standardization, two important but often neglected aspects of measurements in biological studies Houle et al 2011;Armbruster et al 2017). evolvability at our median of e R = 9.1%, herkogamy would be expected to be reduced by half in as few as t = [ln(0.5)/0.091 × 1] 7.6 generations (Fig.…”

Is sexual organ reciprocity related to legitimate pollen deposition in distylous Pulmonaria (Boraginaceae)?

“…For example, Brys and Jacquemyn (2015) recently showed strong deviations in reciprocity in forest populations of the distylous Primula veris, with the majority of plants of the longstyled ('L') morph having shorter styles than the average length of the corresponding anthers in the short-styled ('S') morph. However, quantifying these deviations has proven to be challenging and several measures (Richards and Koptur 1993, Eckert and Barrett 1994, Faivre and McDade 2001, Lau and Bosque 2003, Sánchez et al 2008, each with its own advantages and disadvantages, have been proposed to characterize the degree of spatial correspondence of compatible sexual organs in heterostylous flowers (reviewed by Armbruster et al 2017).…”

“…Recently, Armbruster et al (2017) developed a new metric based on theory that relates floral morphology to reproductive fitness. Essentially, it proposes a measure of inaccuracy, which assesses the fitness cost of the deviation of phenotypes from the optimal phenotype.…”

“…Essentially, it proposes a measure of inaccuracy, which assesses the fitness cost of the deviation of phenotypes from the optimal phenotype. Because pollen from one morph can only be optimally transported to the stigma of the opposite morph if they are positioned at the same level, this inaccuracy value is defined based on the positions of the anthers and stigmas in the whole population relative to this optimal position (Armbruster et al 2017). If all anthers and stigmas are at the same (optimal) position, inaccuracy will be zero.…”

“…If deviations from disassortative mating become too large, it can be expected that the necessary conditions needed to maintain a stable distylous system may no longer be met, and that the dimorphism may disappear over time (Lloyd and Webb 1992a). Whereas inaccuracy values have already been calculated for three species of the distylous genus Primula (Armbruster et al 2017), to the best of our knowledge there are no data available that have linked inaccuracy values to patterns of pollen transfer in heterostylous plant species and therefore it remains unclear whether inaccuracy of reciprocity is related to disassortative pollen deposition.…”