A seven-year study of flower-color polymorphism in a Mediterranean annual plant

Flower-color polymorphisms are well-documented, but how color morph frequencies change over time is usually unknown. We studied this question using the milk thistle, Silybum marianum (Asteraceae). Purple flowers commonly occur in Israel, but an infrequent white morph has been recorded since the 1920s, at first from the south of the country. Over seven years, we recorded the frequencies of the two color morphs in 17 populations along a 156 km south-north transect with a steep rainfall gradient. The proportion of white inflorescences per population consistently decreased from south to north, and their mean proportion decreased from 24% to 12% over the study period. Four of the populations went extinct, but two were recolonized, and two additional populations decreased to <10 individuals. To test whether the color morphs differ in fitness, we collected the ripe inflorescences shortly before seed dispersal from all dimorphic populations in two flowering seasons. The numbers of developed, partially eaten and aborted seeds did not differ between color morphs. There was also no geographical trend in the numbers of developed, aborted and partially eaten seeds in either morph. Insect pollinators did not discriminate against the white morph. Thus, the decline of white-flowering plants over the course of the study cannot be attributed to negative interactions with pollinators or with seed herbivores, two potentially important agents affecting seed output. Our findings are compatible with genetic drift, mediated through population bottlenecks and founder effects, but this interpretation was not directly tested. Our study contributes to the ongoing discussion of the role of natural selection vs. random events in maintaining flower-color polymorphisms.