Abstract
BackgroundSelf-incompatibility (SI) systems prevent self-fertilization in several species of Poaceae, many of which are economically important forage, bioenergy and turf grasses. Self-incompatibility ensures cross-pollination and genetic diversity but restricts the ability to fix useful genetic variation. In most inbred crops it is possible to develop high-performing homozygous parental lines by self-pollination, which then enables the creation of F1 hybrid varieties with higher performance, a phenomenon known as heterosis. The inability to fully exploit heterosis in outcrossing grasses is partially responsible for lower levels of improvement in breeding programmes compared with inbred crops. However, SI can be overcome in forage grasses to create self-compatible populations. This is generating interest in understanding the genetical basis of self-compatibility (SC), its significance for reproductive strategies and its exploitation for crop improvement, especially in the context of F1 hybrid breeding.ScopeWe review the literature on SI and SC in outcrossing grass species. We review the currently available genomic tools and approaches used to discover and characterize novel SC sources. We discuss opportunities barely explored for outcrossing grasses that SC facilitates. Specifically, we discuss strategies for wide SC introgression in the context of the Lolium–Festuca complex and the use of SC to develop immortalized mapping populations for the dissection of a wide range of agronomically important traits. The germplasm available is a valuable practical resource and will aid understanding the basis of inbreeding depression and hybrid vigour in key temperate forage grass species.ConclusionsA better understanding of the genetic control of additional SC loci offers new insight into SI systems, their evolutionary origins and their reproductive significance. Heterozygous outcrossing grass species that can be readily selfed facilitate studies of heterosis. Moreover, SC introduction into a range of grass species will enable heterosis to be exploited in innovative ways in genetic improvement programmes.
Highlights
Self-incompatibility (SI) is the most widespread mechanism for promoting outcrossing in hermaphrodite flowering plants
In most inbred crops it is possible to develop high-performing homozygous parental lines by self-pollination, which enables the creation of F1 hybrid varieties with higher performance, a phenomenon known as heterosis
The inability to fully exploit heterosis in outcrossing grasses is partially responsible for lower levels of improvement in breeding programmes compared with inbred crops
Summary
Self-incompatibility (SI) is the most widespread mechanism for promoting outcrossing in hermaphrodite flowering plants. Because of the breeding behaviour and the self-incompatible nature of the plants, forage crop breeding is predominantly based on lengthy population improvement programmes using recurrent selection, achieving modest genetic gains compared to self-compatible crop species (Laidig et al, 2014; McDonagh et al, 2014) This might be because in outcrossing species it is extremely challenging to produce the inbred lines necessary to fully exploit heterosis. This can be achieved by recurrent self-pollination of plants showing high performance at each selfing generation, until fully homozygous plants are created with a similar fitness to the original, highly heterozygous parent This is important in perennial species, like many of the forage grasses, that regularly reproduce asexually, accumulating recessive deleterious somatic mutations (Zhang et al, 2019). The availability of well-diverged heterotic groups, coupled with advancement in sequencing technologies will allow the identification of genomic heterotic loci, for a better understanding of the genetic regulation of this complex phenomenon
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