Secondary Pollen Presentation and a Curious Rupture of the Style in Spigelia (Spigeliaceae, Gentianales)

Abstract

In the genus Spigelia (Spigeliaceae, Gentianales) we report for the first time the phenomenon of secondary pollen presentation. In Spigelia anthelmia, S. marilandica, and S. splendens a deposition mechanism as found in some members of the Rubiaceae could be observed in living material. In Spigelia longiflora, S. sellowiana, and S. speciosa we can expect this mechanism from observation of pollen load on the style seen in herbarium material (K). Some time after anthesis the pistil shows curious behaviour in that the upper part of the style detaches from the lower part at a histologically differentiated area. In the allogamous Spigelia splendens this may promote prezygotic selection. We hypothesize that in the autogamous Spigelia anthelmia this event selects for the fittest male gametophytes by competition of the pollen tubes counteracting possible inbreeding depression caused by continual self-fertilisation. The fastest growing pollen tubes, which reach the lower style before detaching, are able to penetrate the ovules. Many germinated pollen grains whose pollen tubes grow slowly remain on the upper detached part of the style. Obviously Spigelia anthelmia accepts a lower seed set in favour of increased fitness of the progeny. Within the genus Spigelia, distributed in tropical and subtropical America, Spigelia anthelmia is the only species found on other continents. It is a successful pioneer plant and weed in tropical Africa and Asia, perhaps due to its “strategy of controlled autogamy”.

Key words

Secondary pollen presentation - autogamy - pollen tube competition - Spigeliaceae - Gentianales