An increase in atypical petal numbers during a shift to autogamy in a coastal sand verbena and potential evolutionary mechanisms

PremiseThe evolution of variation in reproductive traits is of longstanding interest in biology. In plants, meristic traits, such as petal and sepal numbers, are usually considered invariant within taxa. However, certain species consistently exhibit great variability in these traits, though the factors contributing to “atypical” counts are not well-known. The sand verbenas, Abronia (Nyctaginaceae), usually have five perianth lobes (‘petals’) in their fused corollas and are self-incompatible, thus departures from either of these norms in populations, varieties, or species are of evolutionary interest.MethodsTo characterize and understand an increase in atypical petal numbers during a transition from xenogamy (outcrossing) to autogamy (selfing) in the coastal sand verbena Abronia umbellata, we integrated common garden studies with analysis of over 11,000 photographed flowers from iNaturalist, a citizen science project. Here we evaluate several adaptive and nonadaptive explanations for the production of these ‘atypical’ flowers.Key resultsOur photo analysis and common garden show that the nominate xenogamous variety has 5 petals with very little variation, however, an autogamous, geographically separated variety, A. u. var. breviflora has a high preponderance of four-petalled morphs. Flower morph did not affect successful autogamy, and petal numbers were not related to environmental factors, hybridization, or flower size in the ways hypothesized.ConclusionsWe conclude that this loss of petals is consistent with relaxation of selection on petal number in selfers, inbreeding leading to a loss of developmental stability, or correlated selection on another trait. This study strongly demonstrates the power of data available from public citizen databases for easily scored traits, such as petal number.