The transition from outcrossing to selfing involve convergent patterns of flower trait covariation

Flower evolutionary transitions related to mating or pollination systems are usually described as changes in combinations of mean trait values. However, associations among traits also represent major components of complex adaptation, yet these have been an almost unexplored aspect of plants' multivariate phenotypic evolution. To fill this gap, we illustrate the use of covariance space analysis to examine whether associations among morphometric corolla and sexual organs traits during an evolutionary transition from large to small flowers, presumably associated with a shift from outcrossing to selfing, show a convergent pattern of traits associations.

We used three independent evolutionary transitions in the section Androceras (Solanaceae) to examine changes in variances and covariances among floral traits. Controlled crosses were performed to estimate the extent of self-compatibility and autonomous self-fertilization in large and small flowered species. Mean and covariance space approximation combined with multivariate analyses were performed to describe the multivariate changes during the evolutionary transition.

As expected, small-flowered taxa showed a higher capacity for autonomous self-pollination than large-flowered species. The extent of divergence bewteen small- and large-flowered species within clade differ suggesting that major differences were detected in the older clade. Two patterns emerged from the covariance space analyses: (1) the relaxation of the connection between the attraction (corolla) and the efficiency (sexual traits) modules during the transition to small-flowers, and (2) the increase in the associations among sexual traits in small-flowered taxa.

The covariance space approach provided new pieces of evidence to describe how multiple flower traits reconfigure their associations during the evolution of selfing and can help to propose functional hypotheses. The transition to selfing in Solanum section Androceras modularize the flower through the reconfiguration of the variance-covariance structure of morphological flower traits.