POLLEN INTERFERENCE AND CRYPTIC SELF‐FERTILITY IN CAMPSIS RADICANS

The effect of self pollen on the success of cross pollinations was studied in Campsis radicans (L.) Seem., a species previously considered self-sterile. The application of self pollen to stigmas in combination with or preceding the application of cross pollen markedly reduced the likelihood of fruit production. This effect was not due to the dilution of cross pollen nor to physical blocking of the stigmatic surface, since mixtures of talc and cross pollen were as effective as pure cross pollen in causing fruit production. Pollen tubes produced by self pollen grew at rates similar to those from cross pollen, and penetrated the ovary. The interference caused by self pollen appears to take place in the ovary, although it cannot be stated to what degree it is prezygotic or postzygotic. The marked protandry occurring in C. radicans is presumed to be due at least partly to selection for avoidance of these negative effects on female reproductive success. Up to 33% of viable seeds from mixed self + cross pollinations were sired by self pollen. The term "cryptic self-fertility" is coined to describe this phenomenon where pollination with loads of pure self pollen rarely or never yields fruit, but pollination using mixtures of self and cross pollen yields fruit containing considerable numbers of selfed seed. Estimates of selfing frequency that are based on pollination using loads of purely self pollen will be in error for species possessing cryptic self-fertility. SEVERAL REPRODUCTIVE traits of seed plants cause a separation of male and female sexual functions. Such traits include dichogamy, herkogamy, and breeding systems such as monoecy and dioecy. One possible advantage of such traits is that they reduce inbreeding, a reduction that would be favored in species wherein inbreeding depression yields offspring of low fitness. However, dichogamy, herkogamy and monoecy all occur in plant species having physiological self-incompatibility (Lloyd and Webb, 1986), indicating that prevention of inbreeding depression is not a complete explanation of these traits. An alternative explanation is that such traits are beneficial in reducing mutual interference of male and female reproductive functions, caused by self-pollination. Self-pollination could decrease female success even in a self' Received for publication 19 May 1987; revision accepted 4 November 1987. We thank C. Barnes, P. L. Angermeier, M. N. Melampy, and L. Wolfe for assistance with field work, and M. F. Willson for accommodations and lab space in Illinois. S. I. Guttman kindly allowed the use of his laboratory and provided advice concerning electrophoresis. G. J. Anderson, C. T. Philbrick and M. L. Ledbetter graciously allowed use of fluorescence microscopes and/or provided advice on the visualization of pollen tubes. S. C. H. Barrett, K. S. Bawa, R. N. Bowman, B. B. Casper, D. G. Lloyd and an anonymous reviewer kindly commented on earlier versions of this paper. This research was supported by the National Science Foundation (DEB 8206465) and by a Batchelor (Ford) summer fellowship to RIB. incompatible plant if self pollen on the stigma interferes with the deposition or performance of compatible pollen (discussion in Shore and Barrett, 1984). Self pollination would also decrease male success ofa self-incompatible plant because self-deposited pollen is unavailable for transport to other, compatible plants. The potential importance of selection for avoidance of pollen-stigma interference in the evolution of monoecy and dioecy has been noted by Bawa and Opler (1975), Zapata and Arroyo (1978), Bawa (1980) and Bawa and Beach (1981). Others, including Yeo (1975) and Lloyd and Yates (1982), have suggested that reduction of pollen-stigma interference could be important in the evolution of heterostyly. Most recently Lloyd and Webb (1986) and Webb and Lloyd (1 986) have argued for a central role of pollen-stigma interference in the evolution of dichogamy and herkogamy. Despite this growing theoretical literature, empirical studies of pollen-stigma interference are few. Crowe (1971) demonstrated interference of incompatible pollen with the performance of compatible pollen in Borago officinalis. Flowers receiving compatible pollen on one stigmatic lobe and incompatible on the other produced fewer than 5% as many seeds as flowers receiving compatible pollen on both lobes. In the distylous species Turnera ulmifolia, however, Shore and Barrett (1984) found a deleterious effect of self pollen on the performance of cross pollen only when a large (five