Repeated loss of floral scent in independent transitions from bee to hummingbird pollination in spiral gingers

Abstract

The repeated evolution of the same trait in different lineages provides powerful natural experiments to study the phenotypic and genotypic predictability of how traits are gained and lost. A fascinating example of this is the repeated evolution of hummingbird pollination in plant lineages in the Americas, a widespread and often unidirectional phenomenon. The spiral gingers in the genus Costus are ancestrally bee-pollinated, and hummingbird pollination has evolved multiple times independently in the tropical Americas. These pollinator transitions are accompanied by predictable morphological and color changes, but the changes in floral scent have not been described. In this study, we describe the floral scent composition of 30 species of Costus sampled across the phylogeny to understand how floral scent has evolved across the genus with respect to pollinator transitions. We then combine transcriptomics and genomics to identify genetic expression differences and gene family evolution associated with pollinator transitions. We show that hummingbird-pollinated species have mostly lost their floral scent, whereas bee-pollinated species exhibit either floral scent maintenance or in some cases, gains of more diverse scent profiles. We find the floral scent loss appears to be due to gene downregulation rather than pseudogenization. The remarkable consistency of scent loss in hummingbird-pollinated species highlights the shared strong selection pressures experienced by these lineages. Even species with more recent transitions from bee to hummingbird pollination exhibit scent loss, highlighting the rapid breakdown of scent production following pollinator transitions. This research highlights the capacity for rapid changes when selection pressures are strong through downregulation of floral scent genes. ResumenLa evolucion independiente de un rasgo en diferentes linajes ofrece la oportunidad de estudiar la predictibilidad de fenotipos y sus bases geneticas. Un ejemplo fascinante es la evolucion repetida de polinizacion por colibries en varios linajes de plantas en las Americas, un fenomeno generalizado y tipicamente unidireccional. Las especies de cana agria del genero Costus son polinizadas ancestralmente por abejas y la polinizacion por colibries ha evolucionado varias veces de forma independiente en America tropical. Cada transicion de polinizador ha sido acompanada de varios cambios morfologicos y colores predecibles, pero los cambios en el aroma floral no han sido descritos. En este estudio describimos la composicion del aroma floral de 30 especies de Costus para comprender como el aroma floral ha evolucionado con respecto a las transiciones de polinizadores. Combinamos transcriptomica y genomica en un clado de Costus para identificar diferencias en la expresion genetica y la evolucion de la familia de genes asociadas con las transiciones de polinizadores. Nuestros resultados muestran que las especies polinizadas por colibries han perdido en su mayoria su aroma floral, mientras que las especies polinizadas por abejas ya sea mantienen el aroma floral y, en algunos casos, muestran ganancias de perfiles de aroma mas diversos. La consistencia en la perdida de olor en especies polinizadas por colibries resalta las fuertes presiones de seleccion natural que experimentan estos linajes. Incluso las especies con transiciones mas recientes de polinizacion de abejas a colibries exhiben perdida de olor, lo que destaca la rapida interrupcion de la produccion de olores despues de las transiciones de los polinizadores. Tambien encontramos que la perdida de aroma floral parece estar controlada por regulacion genica negativa y no por la perdida de genes de produccion de aromas (e.g. pseudogenizacion). Esta investigacion destaca la rapidez de cambios a traves de la regulacion negativa de genes que controlan aromas florales cuando las presiones de seleccion son fuertes. Significance StatementThe independent evolution of the same trait in different lineages provides a natural experiment to investigate the predictability of evolution. A fascinating example is the evolution of hummingbird pollination, which has occurred many times across flowering plants, and often involves convergence in multiple traits. In this study, we combine comparative chemical ecology with genomics and transcriptomics to study floral scent evolution in the genus Costus. We show that species exhibit loss of floral scent following transitions to hummingbird pollination suggesting a shared strong selection pressure favoring scent loss. We demonstrate the floral scent loss appears to be due to gene downregulation. This research highlights the capacity for rapid changes when selection pressures are strong through downregulation of scent genes.