Puzzles of pollen performance
In the infamous words of Donald Rumsfeld, the current US Defence Secretary ‘there are things we know that we do not know’. Being aware of gaps in our knowledge is a major driver for our research. Thus the multinational team led by Tammy Sage (Koehl et al. Toronto, New Orleans, USA and Glen Osmond, Australia, pp. 43–50) states that ‘to date, the site and timing of self-incompatibility within the carpel of Illicium floridanum remain unidentified’. Why is this important? Illicium floridanum is a relictual species, belonging to an ancient family that can be traced back to the basal branches of angiosperm phylogeny. Study of its self-incompatibility (SI) mechanisms may therefore yield information on the evolution of breeding systems in the earliest flowering plants. The flowers of I. floridanum are bisexual and the anthers mature before the stigmatic surface is receptive. This phenomenon (protogyny) prevents selfing in an individual flower. However, flowers on a single plant are at different stages and pollinators can (and do) transfer self-pollen to other flowers. How then is SI mediated? In a very careful study, this group has examined all possible stages of pollen growth and fertilization. The results show clearly that there are no differences between self- and non-self pollen in terms of germination, pollen tube growth, entry into the ovule or participation in the double fertilization. The situation becomes even more puzzling after fertilization: there are no differences in the appearance of the zygote, nor in the formation of the endosperm, following self- or non-self fertilizations (although, interestingly, endosperm formation is very variable in both selfed and non-selfed ovules: at 30 days after pollination, cellular endosperm varies between four and 300 cells). Overall then, SI relies on a very late mechanism, most likely to be, according to the authors, post-zygotic inbreeding depression. Whether this is the most primitive form of SI however remains unclear. j.a.bryant{at}exeter.ac.uk
