POLLEN‐COLLECTING HAIRS OF CAMPANULA (CAMPANULACEAE), I: HISTORICAL REVIEW

Abstract

Summary The literature on the unique pollen‐collecting hairs of Campanula (Campanulaceae) is reviewed as background for new pollination studies, with particular reference to the nature and function of the collecting‐hair mechanism in the breeding system. The main features of the floral mechanism, including the pronounced dichogamy (protandry) and insect adaptation, were first described and depicted accurately by Sprengel in 1793, although pollination observations date back at least to Linnaeus in 1738. Numerous other workers have since added details but little that is basically new to our knowledge of the floral mechanism. Brief reference is made to some of the significant contributions to cytology, cytogenetics, and pollen morphology in recent years. Campanula is basically allogamous, although cases of limited autogamy are known. Experimental studies have shown that campanulas range from complete self‐sterility to complete self‐compatibility, but self‐fertilization usually is accomplished only with difficulty under experimental conditions, and there is little evidence of spontaneous self‐pollination in nature. The pollen is presented from the hairs on the style, which collect the pollen from the anthers while in the bud. The hairs have been known since 1839 to possess the unusual capacity to retract into the style, but the adaptive significance of this mechanism has yet to be explained satisfactorily. The collecting hairs, since before they were known to invaginate, have been interpreted by some investigators as part of a mechanism either to effect fertilization directly through the style without the stigmas or as a back‐up to insure autogamy when cross‐pollination breaks down. Although little evidence exists to support either view, there is still no definitive explanation of their function in the breeding system. Among campanulaceous families, the genera of the Campanulaceae s.str. are believed to have the least advanced (“naked”) form of stylar pollen presentation.

No inbreeding depression in an outcrossing alpine species: The breeding system of Campanula thyrsoides

“…This kind of temporal separation of male and female maturation, dichogamy, is usually regarded as an outbreeding mechanism (Bhardwaj and Eckert, 2001;Silvertown and Charlesworth, 2001). In our case this was truly so, since the results show us that C. thyrsoides is allogamous and selfincompatible like it has been recorded to be the most common reproductive mode of the genus (Nyman, 1993;Shetler, 1979). Moreover, the SI system of C. thyrsoides proved to be of gametophytic origin since the flowers pollinated with pollen from plants from the same seed family (sister mating) produced as many seeds as outcrossed flowers (called semi-compatibility), but the same has been documented for other species in the Campanulaceae (Richards, 1997;Steinbachs and Holsinger, 2002;Stephenson et al, 1992).…”

“…The strong SI system and the allogamous habit of C. thyrsoides should not be very surprising since most Campanula species are SI and allogamous (Shetler, 1979;Nyman, 1993). Since strict self-incompatibility is rare among alpine and arctic plants (Brochmann and Steen, 1999;Grundt et al, 2005; exceptions: e.g., Kelso, 1987;Molau, 1993), a weaker SI system in C. thyrsoides is logically expected.…”

“…Most Campanula species are self-incompatible (SI) and allogamous (Nyman, 1993;Shetler, 1979), but complete self-compatibility has also been recorded, e.g. in the arctic Campanula uniflora (AEgisdo´ttir and Tho´rhallsdo´ttir, 2006).…”

Reproductive attributes of Campanula populations from Mt Olympos, Greece

“…C. spatulata and C. lingulata, which were also examined for self-compatibility, are essentially obligate outcrossers. Only very few instances of autogamy have been found in Campanula species (Shetler 1979;Inoue and Amano 1986), and in most cases the seed content per fruit was very low. Our results offer additional evidence to the general pattern of lack of autogamy for the genus.…”

Secondary Pollen Presentation in Angiosperms and Its Biological Significance

“…Secondary pollen presentation was first described in 1793 when Sprengel published his observations on Canparzula (Shetler 1979). Earlier, in 1738, Linnaeus had noted pollen on the style of Canzpanula but proposed that this was a form of autogamy (Shetler 1979).…”