Diversity of styles and mechanisms of secondary pollen presentation in basal Asteraceae—New insights in phylogeny and function

Abstract

Based on new studies on style morphology of basal subfamilies and tribes of Asteraceae (Barnadesioideae, Famatinanthoideae, Mutisioideae, Stifftioideae, Wunderlichioideae, Gochnatioideae, Hecastocleidoideae), we establish 15 style types in view of shape, bifurcation and distribution of stylar hairs and stigmatic tissue. From the stylar characters and observing different anthetic stages of the proterandrous flowers, we inferred the mechanisms of secondary pollen presentation. The high diversity of stylar shapes in the basal groups of Asteraceae is remarkable. In the most basal subfamily Barnadesioideae, the stylar surface is smooth or, at most, papillate and secondary pollen presentation is either a deposition/primitive brushing or a pump mechanism. Considering Calyceraceae (deposition mechanism) as sister group, we conclude that presumably a deposition/primitive brushing mechanism is the beginning of all secondary pollen presentations in Asteraceae. Whereas in the higher Asteraceae both the brushing and pump mechanisms function perfectly, in some lower Asteraceae, the mechanisms work less efficiently regarding pollen release. The subfamily Mutisioideae is well-defined by the occurrence of true stylar hairs and the tribe Mutisieae by its stigmatic tissue on the margins of the stylar arms, confluent at their tips. The early step from a continuous stigma covering the whole inner surface of the stylar branches (as in most basal members) to a stigma arrangement in two separate lines (as in most Asteroideae) is realized in two ways: either by adhesion of the median ventral tissues of the stylar branches (e.g. Onoseris, Wunderlichia) or by receptivity loss of the median tissues (e.g. Mutisia). Our study supports further DNA-based clades by style morphology (e.g. Famatinanthoideae).