Covariance and decoupling of floral and vegetative traits in nine Neotropical plants: a re‐evaluation of Berg's correlation‐pleiades concept

Abstract

Nearly forty years ago R. L. Berg proposed that plants with specialized pollination ecology evolve genetic and developmental systems that decouple floral morphology from phenotypic variation in vegetative traits. These species evolve separate floral and vegetative trait clusters, or as she termed them, "correlation pleiades." The predictions of this hypothesis have been generally supported, but only a small sample of temperate-zone herb and grass species has been tested. To further evaluate this hypothesis, especially its applicability to plants of other growth forms, we examined the patterns of phenotypic variation and covariation of floral and vegetative traits in nine species of Neotropical plants. We recognized seven specific predictions of Berg's hypothesis. Our results supported some predictions but not others. Species with specialized pollination systems usually had floral traits decoupled (weak correlation; Canna and Eichornia) or buffered (relationship with shallow proportional slope; Calathea and Canna) from variation in vegetative traits. However, the same trend was also observed in three species with unspecialized pollination systems (Echinodorus, Muntingia, and Wedelia). One species with unspecialized pollination (Croton) and one wind-pollinated species (Cyperus) showed no decoupling or buffering, as predicted. While species with specialized pollination usually showed lower coefficients of variation for floral traits than vegetative traits (as predicted), the same was also true of species with unspecialized or wind pollination (unlike our prediction). Species with specialized pollination showed less variation in floral traits than did species with unspecialized or wind pollination, as predicted. However, the same was true of the corresponding vegetative traits, which was unexpected. Also in contrast to our prediction, plants with specialized pollination systems did not exhibit tighter phenotypic integration of floral characters than did species with generalized pollination systems. We conclude that the patterns of morphological integration among floral traits and between floral and vegetative traits tend to be species specific, not easily predicted from pollination ecology, and generally more complicated than R. L. Berg envisaged.

The quantitative-genetic and QTL architecture of trait integration and modularity in Brassica rapa across simulated seasonal settings

“…This theory predicts that plants with specialist pollinators will have highly correlated floral organ sizes, with correlations arising by selection for floral dimensions that fit the dimensions of the pollinator, whereas plants with generalist pollinators are predicted to have less strongly correlated floral traits. However, empirical studies have shown that the relationship between pollinator specificity and the level of covariation among floral traits is not as consistent as predicted by Berg (Conner and Sterling, 1995;Armbruster et al, 1999;Brock and Weinig, 2007), suggesting that other factors may also affect floral trait correlations.…”

“…Relative to plants with specialist pollinators, this theory predicts that plants with generalist pollinators will experience less selection for consistent floral organ sizes and will have less independent floral and vegetative traits (Berg, 1960). Empirical studies have shown that the degree of independence of floral and vegetative traits is not well predicted by pollination syndrome; modularity of floral and vegetative organs was equally pronounced in species with unspecialized vs specialized pollination syndromes (Armbruster et al, 1999). More generally, several empirical studies have found that floral and vegetative traits are modular, such that traits within floral or vegetative modules were correlated, whereas floral and vegetative traits were generally uncorrelated (Conner and Via, 1993;Conner and Sterling, 1996;Juenger et al, 2005;Ashman and Majetic, 2006; but see Brock and Weinig, 2007), and QTL mapping in a population of recombinant inbred lines (RILs) of Arabidopsis revealed that floral and vegetative traits had largely independent genetic architectures (Juenger et al, 2005).…”

Modularity and intra-floral integration in metameric organisms: plants are more than the sum of their parts

“…One potential explanation for the marked heterogeneity among taxa in levels of integration, and the unexpectedly weak integration observed in the flowers of some taxa, is that flowers are not single fully integrated units. Rather, the multiple functions of the flower are each performed by distinct suites of integrated characters that share a common function but are independent of sets of structures with alternative functions, that is, floral variation/covariation may be modular [46][47][48][49]. Independence among functional modules would reduce measures of overall integration for the flower as a whole.…”

Floral colour versus phylogeny in structuring subalpine flowering communities

“…Other studies, however, have found floral colour to be constrained (Chittka 1997), possibly when pigments have dual roles in pollinator attraction and herbivory defence (Chittka et al 2001). Depending on the level of sympatry between related species, there may be selection for divergence of floral colour among close relatives to attract different pollinators, or promote pollinator constancy, without sacrificing adaptive floral morphology traits (Armbruster et al 1999;Ashman & Majetic 2006;Smith & Rausher 2008). The rapid divergence in a trait between closely related species may erase any signal for phylogenetic divergence in communities (Brooks & McLennan 1991;Losos 1996;Sargent & Ackerly 2008;Cavender-Bares et al 2009).…”