The advancements and novel interests in reproductive assurance and delayed selfing, including genetic and ecological mechanisms for the evolution of selfing and its theoretical models, reproductive assurance hypothesis, the exemplifications of testing if selfing provides the reproductive assurance, the types of delayed selfing, and the positive or negative exemplifications of delayed selfing supporting the reproductive assurance, were introduced in this review.
Though in the face of selection against self-pollination, mixed mating systems are maintained in many plants. Many theoretical and empirical efforts have been made to understand the evolution of selfing. Researches on genetic mechanisms have focused on the avoidance of selfing if inbreeding depression is high. Recently, researches have shifted on the ecological mechanisms, which oppose the evolution of selfing, including pollen discounting and seed discounting. More recently, there are two novel hypotheses on the evolution of self-fertilization: reproductive assurance hypothesis and automatic selection hypothesis. However, up to now, there are a few convincing studies showing selfing is adaptive in species with mixed mating. In many species the selfing component of mixed mating may represent a non-adaptive cost which is associated with the large floral displays required to attract animal pollinators; and it is also difficult to determine which selfing is from the automatic selection or the reproductive assurance.
Reproductive assurance hypothesis, where self-fertilization ensures seed production when pollinators and/or potential mate are scarce, is the most longstanding and most widely accepted explanation to the evolution of selfing, and it is one of the central concerns of plant reproductive ecology and evolutionary biology. According to this hypothesis, the fitness advantage of selfing comes from increased success with respect to outcrossing. Selection for the reproductive assurance provided by selfing may be especially important in explaining the widespread occurrence of partial selfing in species where the fitness value of producing selfed progeny is greatly reduced by inbreeding depression. However, despite the exemplifications of selfing supporting the reproductive assurance hypothesis had been reported in some plant species, only two of them have combined experimental measures to test if selfing provides the reproductive assurance with estimates of pollen discounting, seed discounting, selfing rates and inbreeding depression in different populations in different years, and few exemplifications in the level of individual or flower test the reproductive assurance hypothesis.
For delayed selfing, self-pollination is delayed until the opportunity for outcrossing has missed. This is regarded as a reproductive adaptability because it apparently ensures seed production when pollinators are absent or scare, yet, it allows outcrossing to predominate when they are abundant. Delayed selfing in many plant occurs from three suites of floral morphological traits has been shown to modify the degree of self-pollination within flowers, including dichogamy, transient self-incompatibility, and floral parts movement (eg. pollen slide, the late curling of stylar branches, stamens bending upwards late in flowering, stamens progressively elongate towards the exerted stigma, corolla abscission and corolla closing, etc.). Although delayed selfing were reported in many plant species, few studies have quantitatively measured its impact on reproductive assurance. As an only exemplification, report in Nature by Kalisz showed that delayed selfing of Collinsia verna supports the reproductive assurance, which combined reproductive assurance coefficient, selfing rate, pollinator failure rate and inbreeding depression in different populations in different seasons of different years.
The study on the reproductive assurance and delayed selfing in China is on the way in recent years. A simple evolutionarily stable strategy model of resource allocation in partially selfing plants was developed, and delayed selfing in some plants were reported by Chinese scholars, but the benefits of these delayed selfing were not further clarified.
According to the developments and present studies on the reproductive assurance and delayed selfing, we believe that the research on the reproductive assurance is shifting from single season, single population and single factor to comprehensive research phase of multi-seasons, multi-populations and multi-factors (selfing modes and its relative proportion, pollen discounting, seed discounting, selfing rate and inbreeding depression), and from traditional and classical approaches to modern experimental technologies (e.g. the molecular markers of SSR, SNP, etc.) with advanced apparatus. We also put forward our insights into the study for future.