Abstract
Breeding systems vary widely in plants, but mostly cluster towards outbreeding or inbreeding extremes. Conifers, which are woody and generally long-lived perennials, are almost all classical outbreeders, but include occasional species characterised as inbreeders. The latter include some very narrow endemics, but narrow endemics include outbreeders. The inbreeders show high self-fertility, minimal inbreeding depression, typically low DNA polymorphism, and modest functional genetic variation, but self-fertilisation rates can be low. Seven such species are reviewed. It is widely (if often tacitly) assumed that inbreeders arise through severe population bottlenecks. It is proposed, more specifically, that inbreeders could arise through recessive resistance alleles of large effects being expressed, during biotic crises, through some inbreeding in the typically mixed mating systems of outbreeders. Such a crisis might produce not only a population bottleneck but also the conditions for recessive alleles to operate beneficially to make inbreeding advantageous. Avenues for probing the recessive-alleles hypothesis are: quantitative modelling to identify what are plausible conditions, examining genomic signatures of inbreeders, and empirical observation. Appropriate modelling appears challenging, as does seeking informative genomic signatures. Empirical observation, however, may be facilitated by biotic crises promoted by current global migration of pathogens and animal pests.