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Having it both ways: outbreeding and inbreeding
When it comes to sex, angiosperms present a wonderful array of breeding systems ranging from apomixis (no sex at all) to obligate outbreeding with all possible variants in between. Indeed, individual genera may contain species exemplifying several types of breeding system. One such genus is Silene, highlighted by Buide and Guitián (University of Santiago de Compostela, pp. 691-699). Their paper focuses on S. acutifolia and particularly on the phenomenon of dichogamy, the separation in time of anther dehiscence (and hence pollen availability) and stigma receptivity. It occurs in many species possessing hermaphrodite flowers in which it is perceived to be a barrier to inbreeding. In individual flowers of S. acutifolia, anthers dehisce before stigmas are receptive, with pollen viability declining to a very low level within 48 h of their release. Thus, it would be expected that flowers would not be self-pollinated. However, the flowers within an inflorescence do not open synchronously: within one inflorescence there is enough overlap between pollen availability and stigmatic receptivity to allow some self-pollination. Furthermore, as the authors show, there is no self-incompatibility barrier to pollen germination, fruit set being as efficient with self- or non-self-pollination. The disadvantage of self-pollination becomes apparent at seed set which declines to about one-third of the level achieved in non-self-pollinations. Thus, although the self-compatibility index is high (0.98 %), the self-fertilization index is rather lower (0.36 %), and this difference is seen both in natural and forced self-pollinations. The authors could detect no evidence of inbreeding depression, at least in respect of seed fitness. Thus, the overall effect of dichogamy in S. acutifolia is to reduce the probability of seed set from self-pollination without eliminating it completely. The selective advantage of this ‘both ways’ breeding system may well be to ensure some seed set in small or scattered populations.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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