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Self-pollen in fruitless race for the ovule
In considering self-incompatibility (SI) mechanisms, most botanical text books focus on those that affect reception of pollen and/or growth of the pollen tube. However, angiosperms actually present us with a much wider range of SI mechanisms; we focus on two of these, in this commentary and the next. Sage and Sampson (Toronto, Canada and Wellington, New Zealand, pp. 807-816) draw attention to this range and suggest that the early evolutionary origin of several SI mechanisms contributed to ‘the early success of angiosperms’. They have investigated pollination and fertilization in Pseudowintera axillaris, a member of the Winteraceae; this family is regarded as primitive, based on features of its anatomy and on its long history in the fossil record. In their study of fertilization by either self or non-self pollen they first showed that the pollen tubes from the two types of pollen have very similar growth rates indicating that there are no stylar rejection mechanisms. But what happens then? Surprisingly, there appear to be no differences in the abilities of the two pollen types to penetrate the ovule or to achieve a double fertilization: thus a zygote is formed and an endosperm is initiated in both types of pollination event. However, it is at this stage that the self-fertilization fails: there is no division of the zygote and thus no further development of the embryo sac. This happens with all zygotes that arise from self-fertilization and not with just a subset as happens with post-zygotic inbreeding depression (a phenomenon in which embryos that are homozygous for deleterious recessive alleles are aborted). This careful piece of work has thus revealed an SI mechanism that operates very late in the fertilization process, but has left open the question of the timing and nature of the recognition and signalling mechanisms. This interesting system is ripe for further investigation.
j.a.bryant{at}exeter.ac.uk
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