Late‐acting self‐incompatibility–the pariah breeding system in flowering plants

Abstract

Contents 717I.717II.718III.718IV.720V.722VI.722VII.728730References730 Summary It is estimated that around half of all species of flowering plants show self‐incompatibility (SI). However, the great majority of species alleged to have SI simply comply with ‘the inability of a fully fertile hermaphrodite plant to produce zygotes when self‐pollinated’ – a definition that is neutral as to cause. Surprisingly few species have been investigated experimentally to determine whether their SI has the type of genetic control found in one of the three established mechanisms, that is, homomorphic gametophytic, homomorphic sporophytic or heteromorphic SI. Furthermore, our knowledge of the molecular basis of homomorphic SI derives from a few species in just five families – a small sample that has nevertheless revealed the existence of three different molecular mechanisms. Importantly, a sizeable cohort of species are self‐sterile despite the fact that self‐pollen tubes reach the ovary and in most cases penetrate ovules, a phenomenon called late‐acting self‐incompatibility (LSI). This review draws attention to the confusion between species that show ‘self‐incompatibility’ and those that possess one of the ‘conventional SI mechanisms’ and to argue the case for recognition of LSI as having a widespread occurrence and as a mechanism that inhibits selfing and promotes outbreeding in many plant species.

Allelic incompatibility can explain female biased sex ratios in dioecious plants

“…Similar systems have been shown to repress the offspring generation from self crosses in several plant species (self incompatibity locus). Incompatibility can happen as late as at the moment of fertilization or even at the postzygotic stage (late-acting incompatibility [19]) raising the possibility of the abortion of incompatible haplotype combinations. Although incompatibility systems are usually designed to prevent the mating of relatives and the generation of inbred offspring, they may still occasionally be triggered in a mating of non-related parents just due to an unfortunate combination of a limited number of haplotypes/alleles.…”

“…Allelic/genotypic incompatiblitites can also possibly act prezygotically, preventing fusion of sperm and egg carrying certain alleles at key loci. A similar mechanism is the well-known phenomenon self-incompatibility in plants, which prevents self fertilization in co-sexual species and can, if acting late, even prevent the embryonal development of zygotes generated by self mating or by the mating of close relatives [19]. Furthermore, in species with heteromorphic sex chromosomes, the theoretical background to the ‘unguarded sex chromosome hypothesis’ predicts that mortality will be higher in the heterogametic sex compared to the homogametic sex because the heterogametic sex will be hemizygous for a large number of loci and will be unable to mask the expression of recessive deleterious alleles [8].…”

The Floral Biology, Breeding System and Pollination Efficiency of Schima superba Gardn. et Champ. (Theaceae)

“…This, together with the results of the pollen tube growth experiments, suggests that incompatibility may be late-acting self-incompatibility (LSI) [33], and the incompatibility may occur in the ovary. In the present study, pollen tubes reached the base of the style 36 h after self-pollination and cross-pollination, and the growth rate of the crossed tubes was slightly higher than that of the self-tubes, similar to what has been described for other plants of the same family, such as Camellia oleifera Abel.…”

Inbreeding depression maintained by recessive lethal mutations interacting with stabilizing selection on quantitative characters in a partially self-fertilizing population

“…Perennial plants often set relatively few seeds on experimental self-pollination in comparison to outcrossing, implying either a high early inbreeding depression (EID) causing embryo mortality or late-acting self-incompatibility (LSI) (Seavey and Bawa 1986;Gibbs 2014). LSI is often incomplete and/or variable in expression (Gibbs 2014), and in some species both EID and LSI may occur together (Dorken and Husband 1999;Hao et al 2012). Extremely high EID occurs in conifer trees that generally are self-compatible and easily studied because embryo mortality produces an empty seed.…”